changes in snowpack and snowmelt runoff for key mountain regions

17
HYDROLOGICAL PROCESSES Hydrol. Process. 23, 78–94 (2009) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.7128 Changes in snowpack and snowmelt runoff for key mountain regions Iris T. Stewart* Santa Clara University, Environmental Studies Institute, Santa Clara, CA, USA Abstract: Mountain snowpack and spring runoff are key components of surface water resources, and serve as important, regionally integrated indicators of climate variability and change. This study examines whether mountain snowpack and snowmelt have manifested a consistent hydrologic response to global climatic changes over the past several decades. Prior findings are compared to identify spatial and temporal patterns of trends in the volume, extent, and seasonality of snowpack and melt for key mountain regions. Evidence suggests that both temperature and precipitation increases to date have impacted mountain snowpacks simultaneously on the global scale; however, the nature of the impact is, among other factors, strongly dependent on geographic location, latitude, and elevation. Warmer temperatures at mid-elevations have decreased snowpack and resulted in earlier melt in spite of precipitation increases, while they have not affected high-elevation regions that remain well below freezing during winter. At high elevations, precipitation increases have resulted in increased snowpack. Not all local responses are consistent with the general findings, possibly because of local climatic trends, atmospheric circulation patterns, record lengths, or data quality issues. With continued warming, increasingly higher elevations are projected to experience declines in snowpack accumulation and melt that can no longer be offset by winter precipitation increases. There is a continued research need for hydroclimatic trend detection and attribution in mountains owing to the length, quality, and sparseness of available data from monitoring stations not directly impacted by human activity. Copyright 2008 John Wiley & Sons, Ltd. KEY WORDS streamflow; snowmelt; climate change; cryosphere; mountain; snow Received 27 January 2008; Accepted 15 July 2008 THE SENSITIVITY OF MOUNTAIN SNOWPACK AND SNOWMELT RUNOFF TO CLIMATIC CHANGES Reductions in mountain snowpack and changes in snowmelt-derived streamflow timing from higher eleva- tions are of considerable concern in arid regions where human water demand already equals or exceeds the renewable water supply today. In these areas, even greater pressures on water resources are projected with growing populations and warmer temperatures. For many arid and semi-arid lowland regions, the surface water is derived from precipitation falling as rain or snow on neigh- bouring higher elevations. While rain immediately con- tributes to streamflow, the mountain snowpack serves as a natural reservoir for cold-season precipitation stor- age, releasing water during the warmer months to the drier and much hotter valleys below. Therefore, moun- tain snowpack and snowmelt is an important predictor of summer streamflow and constitutes the primary source of water for large populations (Singh and Bengtsson, 2004), and the natural release of water from snowmelt in conjunction with constructed reservoirs is supplying human and ecosystems demands during the season of greatest need. Economically, the seasonal snowpack is more valuable than glacier ice (Krishna, 2005). Thus, for * Correspondence to: Iris T. Stewart, Santa Clara University, Environ- mental Studies Institute, Santa Clara, CA, USA. E-mail: [email protected] the regional mountain to dry valley natural water supply systems, understanding the processes of snow accumu- lation, snowmelt, and streamflow is key to managing scarce water resources, and questions about the hydro- logic impacts of climate variability and change are of particularly great importance. For example, in the west- ern United States, a region with very high urban and agricultural water demands, 50–70% of the precipitation falls as snow (Serreze et al., 2001), and spring/early sum- mer snowmelt runoff accounts for 50–80% of the total annual runoff for snowmelt-dominated basins (Stewart et al., 2004). Snowpack represents the cumulative effects of snow accumulation and ablation during the preceding months. Both mountainous and low-elevation seasonal snow cover are playing an important role in the Earth’s hydroclimatic system owing to its low thermal conductivity, large spa- tial extent, seasonal amplitude, and latitudinal variation. Snow cover acts as a control on summer soil-water stor- age, and variations in snowpack are a key component in the global heat budget. Fresh snow has a high albedo (e.g. Groisman et al., 1994), and reductions in snow cover are expected to contribute to polar amplification of global warming (Serreze et al., 2000). The timing, volume, and extent of mountain snow- pack, and the associated snowmelt runoff, are intrinsically linked to seasonal climate variability and change. Warmer cold season temperatures reduce snow accumulation, as a greater fraction of the precipitation comes as rain (lower Copyright 2008 John Wiley & Sons, Ltd.

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Page 1: Changes in snowpack and snowmelt runoff for key mountain regions

HYDROLOGICAL PROCESSESHydrol. Process. 23, 78–94 (2009)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.7128

Changes in snowpack and snowmelt runoff for key mountainregions

Iris T. Stewart*Santa Clara University, Environmental Studies Institute, Santa Clara, CA, USA

Abstract:

Mountain snowpack and spring runoff are key components of surface water resources, and serve as important, regionallyintegrated indicators of climate variability and change. This study examines whether mountain snowpack and snowmelt havemanifested a consistent hydrologic response to global climatic changes over the past several decades. Prior findings arecompared to identify spatial and temporal patterns of trends in the volume, extent, and seasonality of snowpack and melt forkey mountain regions. Evidence suggests that both temperature and precipitation increases to date have impacted mountainsnowpacks simultaneously on the global scale; however, the nature of the impact is, among other factors, strongly dependenton geographic location, latitude, and elevation. Warmer temperatures at mid-elevations have decreased snowpack and resultedin earlier melt in spite of precipitation increases, while they have not affected high-elevation regions that remain well belowfreezing during winter. At high elevations, precipitation increases have resulted in increased snowpack. Not all local responsesare consistent with the general findings, possibly because of local climatic trends, atmospheric circulation patterns, recordlengths, or data quality issues. With continued warming, increasingly higher elevations are projected to experience declines insnowpack accumulation and melt that can no longer be offset by winter precipitation increases. There is a continued researchneed for hydroclimatic trend detection and attribution in mountains owing to the length, quality, and sparseness of availabledata from monitoring stations not directly impacted by human activity. Copyright 2008 John Wiley & Sons, Ltd.

KEY WORDS streamflow; snowmelt; climate change; cryosphere; mountain; snow

Received 27 January 2008; Accepted 15 July 2008

THE SENSITIVITY OF MOUNTAIN SNOWPACKAND SNOWMELT RUNOFF TO CLIMATIC

CHANGES

Reductions in mountain snowpack and changes insnowmelt-derived streamflow timing from higher eleva-tions are of considerable concern in arid regions wherehuman water demand already equals or exceeds therenewable water supply today. In these areas, even greaterpressures on water resources are projected with growingpopulations and warmer temperatures. For many arid andsemi-arid lowland regions, the surface water is derivedfrom precipitation falling as rain or snow on neigh-bouring higher elevations. While rain immediately con-tributes to streamflow, the mountain snowpack servesas a natural reservoir for cold-season precipitation stor-age, releasing water during the warmer months to thedrier and much hotter valleys below. Therefore, moun-tain snowpack and snowmelt is an important predictor ofsummer streamflow and constitutes the primary sourceof water for large populations (Singh and Bengtsson,2004), and the natural release of water from snowmeltin conjunction with constructed reservoirs is supplyinghuman and ecosystems demands during the season ofgreatest need. Economically, the seasonal snowpack ismore valuable than glacier ice (Krishna, 2005). Thus, for

* Correspondence to: Iris T. Stewart, Santa Clara University, Environ-mental Studies Institute, Santa Clara, CA, USA.E-mail: [email protected]

the regional mountain to dry valley natural water supplysystems, understanding the processes of snow accumu-lation, snowmelt, and streamflow is key to managingscarce water resources, and questions about the hydro-logic impacts of climate variability and change are ofparticularly great importance. For example, in the west-ern United States, a region with very high urban andagricultural water demands, 50–70% of the precipitationfalls as snow (Serreze et al., 2001), and spring/early sum-mer snowmelt runoff accounts for 50–80% of the totalannual runoff for snowmelt-dominated basins (Stewartet al., 2004).

Snowpack represents the cumulative effects of snowaccumulation and ablation during the preceding months.Both mountainous and low-elevation seasonal snow coverare playing an important role in the Earth’s hydroclimaticsystem owing to its low thermal conductivity, large spa-tial extent, seasonal amplitude, and latitudinal variation.Snow cover acts as a control on summer soil-water stor-age, and variations in snowpack are a key component inthe global heat budget. Fresh snow has a high albedo (e.g.Groisman et al., 1994), and reductions in snow cover areexpected to contribute to polar amplification of globalwarming (Serreze et al., 2000).

The timing, volume, and extent of mountain snow-pack, and the associated snowmelt runoff, are intrinsicallylinked to seasonal climate variability and change. Warmercold season temperatures reduce snow accumulation, as agreater fraction of the precipitation comes as rain (lower

Copyright 2008 John Wiley & Sons, Ltd.

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CHANGES IN SNOWPACK AND SNOWMELT RUNOFF 79

snow to total precipitation (S/P) ratio), while warmerspring temperatures hasten snowmelt, thereby shifting thetiming of runoff to earlier in the season and reducing theamount of summer and fall flows (e.g. Mote et al., 2005;Stewart et al., 2005). Variations in precipitation amountsdetermine the total runoff volume, while the seasonalityof precipitation affects the fraction stored as snow andtherefore the volume of the spring snowmelt. The influ-ence of precipitation is such that higher cold-season pre-cipitation amounts have been linked to increases in snow-pack accumulation and later snowmelt (greater amountof energy required) and streamflow (e.g. Hamlet et al.,2007; Moore et al., 2007). Both lower S/P ratios and ear-lier melt result in higher winter and spring runoff ratesand an increased risk of winter and spring floods.

As a consequence of warming, then, the naturalcapacity of mountain areas to store water until thespring and summer seasons is diminished. The effectmay be complicated though, by concurrent precipitationchanges. Cold-season precipitation increases and theresulting larger snowpack and later melting may maskthe effects of a modest amount of warming, such that theimpact of temperature shifts in mountain regions maynot be felt until either the warming is sufficiently largeto overcome the effects of an increased precipitation,or until precipitation increases are no longer occurring,at which point mountain snowpack accumulation andstreamflow timing may experience a step change.

Trends in mountain snowpack volumes and extent,S/P ratios, and streamflow timing therefore serve asdynamic indices for detecting and monitoring spatiallyand temporally integrated hydrologic changes in themountain cryosphere on local to global scales (Barry,1985; Krishna, 2005). In addition, mountain snowpackand snowmelt runoff connect the effects of climate vari-ability to water supplies, plant and animal communities(such as life cycles of pests, wildlife, and aquatic life),and recreational opportunities (e.g. CIRMOUNT Com-mittee, 2006). Earlier mountain snowmelt and snowmeltrunoff result in a longer summer drought season anddiminished summer and fall stream flow in adjacent dryregions. These types of changes in the annual hydrographmay exert stresses on ecosystems owing to lower flowsand warmer stream and lake temperatures.

While snowpack and snowmelt runoff have variedwith climate on many temporal and spatial scales, recentglobal surface temperature increases due to anthropogenicgreenhouse gas emissions are now well recognized alongwith their potential implications on the hydrologic sys-tem. Global temperatures have risen 0Ð74 °C over the1906–2005 period, and the warming trend in the lat-ter part of the 20th century is twice that over the entire100 years (IPCC, 2007). The observed warming is greaterfor land areas than oceans and greater at higher lat-itudes. Regions with seasonal snow cover are experi-encing enhanced warming, as is indicated by Globalclimate models (e.g. Gates et al., 1999) and by observa-tions from the higher latitudes and mountainous regions.One such region consists of the western United States,

where temperature changes have on average been 1–2 °Clarger than in other parts of the country (CIRMOUNTCommittee, 2006). Recent evidence suggests that therate and direction of human-induced climate change mayhave exceeded the natural range of variability sincethe 1980s (Barnett et al., 2008). Anthropogenic green-house gas emissions have also contributed significantlyto increases in precipitation in the Northern Hemisphere(NH) mid-latitudes. While precipitation changes fromregion to region have varied and therefore canceled eachother out for previous global-scale assessments, the recentstudy by Zhang et al. (2007) suggests a global latitudinalredistribution of precipitation from subtropical to higherlatitudes, which is corroborated by both simulated andobservational data. As mountainous regions with annualsnow cover are predominantly located at higher latitudes,they have generally experienced precipitation increases.Regions specifically noted for their significant precipita-tion increases over the past century include the easternportion of North America, northern Europe, and northernand Central Asia, where key snowmelt-dominated moun-tain ranges are situated (IPCC, 2007). Thus it is impor-tant to note that under current trends both temperatureand precipitation are increasing over the snow-coveredmountainous areas of the mid- to high latitudes. Thiscomplicates the snow cover response as discussed aboveand may lead to increasing snow water equivalent (SWE)but decreasing spring snow cover extent (SCE) trends.Temperature trends are projected to continue through the21st century, although the degree of projected warmingdepends on the model and scenario used. Precipitationchanges are less clear with a number of models project-ing general precipitation increases, while others projectprecipitation declines.

While the human-induced warming by now has pro-duced detectable and consistent hydroclimatic conse-quences on the global scale in many compartments ofthe cryosphere (e.g. Lemke et al., 2007), seasonal moun-tain snow cover remains less well understood. Snowcover in mountain regions appears to be more sensi-tive to climatic variation than nearby lowland regionsin the high latitudes. The reasons for this added sen-sitivity are complex. Geographic location plays a largerole, as mountains are an important element of the cli-mate system and owing to the orographic effect receivelarger amounts of precipitation than surrounding areas.Geographic location also renders the mountain snowpackvery responsive to atmospheric circulation patterns (e.g.the Pacific North American pattern (PNA) and Pacificdecadal oscillation (PDO) for the Rocky Mountains, theNorth Atlantic oscillation (NAO) for the European Alps,and the Indian monsoon for the Himalayas), which com-plicates the identification of a climate change signal, asmost data records span only a few decades. For moun-tains with seasonal snowpack, much of the precipitationreceived during the cold season remains relatively closeto freezing and therefore very sensitive to small tem-perature increases (Beniston, 1997). In addition, warmerwinter and spring surface temperatures in mountains are

Copyright 2008 John Wiley & Sons, Ltd. Hydrol. Process. 23, 78–94 (2009)DOI: 10.1002/hyp

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80 I. T. STEWART

amplified, as warmer temperatures cause the snow coverto recede and therefore greatly reduce the surface albedo(e.g. Giorgi et al., 1997; Fyfe and Flato, 1999). Therefore,the warming over mountainous regions with seasonalsnow cover is more pronounced as compared to otherland surface areas, due both to their location at higher lat-itudes where warming is largest and to the amplificationof the warming via the snow-albedo feedback. The goalof this paper, therefore, is to review and evaluate the evi-dence for changes in the seasonal mountain snowpack andthe associated spring snowmelt runoff. Region-to-regionvariations in the change of the volume, extent, and sea-sonality of snowpack, snow accumulation, as well as andsnowmelt runoff for key mountain regions, are examinedfrom recent prior studies.

CHANGES IN MOUNTAIN SNOWPACK ANDSTREAMFLOW TIMING

Measuring and evaluating changes in snowpack andsnowmelt runoff

The volume of snowpack is assessed through the mea-surement of snow extent, snow depth, and SWE, bothin situ at snow courses and snow pillows, and via remotesensing methods. Snow courses are designated lines alongwhich the snow is sampled at appropriate times to deter-mine snow depth and SWE for forecasting water supplies.Snow pillows convert the overlying weight of the snowon pillows filled with antifreeze solution into an elec-tronic signal for transmission. For practical applicationsthe depth of the snow pack is less important than SWE,which corresponds to the depth of the water after meltingthe snow, and is a function of snow density. Snowpackdensity varies with rainfall, settling, metamorphism, andtemperature. In the western United States, where manualsnow pack measurements were widespread by the late1940s, most sites reach their seasonal maximum SWE onor around April 1. It should be noted that in situ snowmeasurements are affected by microclimatic, topographic,and vegetative effects on snow accumulation and melt,which can cause significant spatial variability at the localscale. Thus, automated, fixed point, in situ snow mea-surements, which are the norm at many observation sitesand are used for hydroclimatological analyses and deci-sion making, consistently over- or under-represent thelandscape mean of the snow depth (e.g. Neumann et al.,2006). In addition, nonclimatic factors, such as incon-sistencies in the observational practices, and measure-ment inhomogeneities can produce statistically significantbiases (Kunkel et al., 2007).

Remotely sensed SCE data can be gleaned fromthe visible-wavelength satellite maps produced by theNational Environmental Satellite Data and InformationService (NESDIS; NOAA, 2007). Maps for the NH dateback to 1966 with a resolution of 190Ð5 km and 7 days.Since 1997, the resolution of the maps improved to 25 kmand a daily time step, and in 2004 to 4 km. The veryhigh resolution radiometer (VHRR) has been supplying

imagery with 1Ð0 km resolution since 1972, and theadvanced VHRR, 1Ð1 km resolution data since 1978(Robinson et al., 1993)). Passive microwave satellitedata can enhance snow cover measurements because ofthe ability to penetrate cloud cover and darkness andthe potential to provide estimates of SCE, snow depth,and SWE on large scales (e.g. Armstrong and Brodzik,2001; Derksen et al., 2005; Yang et al., 2007). Mappingof the Southern Hemisphere (SH) began only in 2000with the introduction of the moderate resolution imagingspectroradiometer (MODIS) satellite data (NOAA, 2007).The current work addresses issues such as how remotelysensed data can aid in snowpack assessments and howremotely sensed data can be integrated with on-the-ground networks.

While remotely sensed snow cover data have beenvaluable and are a widely used tool to derive trends forclimate-related studies, their application in mountainousterrain is problematic. Only the more recent, higher-resolution National Oceanic and Atmospheric Adminis-tration (NOAA) products attempt to capture the variabil-ity of snow cover in mountain regions; thus, SCE, snowdepth, and SWE estimates for mountainous terrain areavailable only over a comparatively short period. Eventhe newer products are limited by their coarse resolu-tion as compared to the heterogeneity of the landscape,and the move to the higher-resolution daily product in1999 has compromised the continuity of the data seriesfor the mountain regions. In addition, evidence suggeststhat the NOAA weekly data sets consistently overestimateSCE during the spring melt period at very high latitudes(Wang et al., 2005). The use of passive microwave satel-lite data is also limited in mountainous areas becauseof the coarse spatial resolution, mixed performance inareas with deep or discontinuous snow pack (e.g. overthe Tibetan Plateau; Armstrong and Brodzik, 2001) orthose that are densely forested, and the overestimation oflate season SWE compared to in situ measurements (e.g.Mote et al., 2003a).

Snowmelt-dominated streamflow timing studies con-sider trends in streamflow timing measures such as thedate of the centre of mass for flow (McCabe and Clark,2005; Stewart et al., 2005; Hodgkins and Dudley, 2006b),the start day of the snowmelt runoff (Stewart et al., 2005),or the day at which each percentile of the annual flowoccurs (Moore et al., 2007). Streamflow and streamflowtiming are derived from in situ gauge measurements ofdischarge, which can be viewed as a spatially and tem-porally integrated hydroclimatic index, encompassing thearea of the watershed above the gauge and the periodbetween the timing of precipitation and runoff. Stream-flow timing assessments are limited by the availability ofsuitable gauges and the length of the record. Most sig-nificant mountainous streams in arid regions have beenaltered by dams or diversions, and therefore the natu-ral response of streamflow and streamflow timing forthe mid-elevations, where impacts are expected to be thegreatest, are often difficult to assess. At higher elevations,streamflow monitoring sites are generally sparse. Daily

Copyright 2008 John Wiley & Sons, Ltd. Hydrol. Process. 23, 78–94 (2009)DOI: 10.1002/hyp

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CHANGES IN SNOWPACK AND SNOWMELT RUNOFF 81

streamflow records often exist for only a few decades,and at many sites the data are discontinuous.

Detecting statistically significant changes in mountainsnowpack and snowmelt-derived streamflow timing isa multifaceted problem because of the complexities ofthe response and of both temperature and precipitationchanges (see above), high variability on many timescales,and comparatively short records that often exist foronly a few decades, commonly yielding a very lowsignal-to-noise ratio (e.g. Wilby, 2006; Moore et al.,2007). Further complexities arise from the distributionof monitoring stations, the great spatial heterogeneity ofsnow cover, and the complex interplay of temperature andprecipitation as discussed above. The spatial coverageprovided by stations is generally limited to low-elevationregions of the NH mid-latitudes and to snow coursesin mountainous regions (Robinson et al., 1993). Thus,for the SH, high and low latitudes, and high altitudes,the observational record is extremely limited, as veryfew longer records or proxies for snow cover exist. Inaddition, a bias in station location towards areas close tourban centres has been identified. Longer records of dailysnowfall observations, dating back to the 1800s, are onlyavailable for a few countries (Finland, the former SovietUnion, Switzerland, and the United States). Since the1950s, widespread on-the-ground observations of dailysnowfall have been carried out in the mountains ofwestern North America and Europe, and since the 1960salso in Australia (Lemke et al., 2007).

Changes on hemispheric scales

Changes in SCE on the hemispheric to global scalereflect variations in both mountainous and low-elevationsnow cover and are briefly discussed here, as the hemi-spheric perspective can serve as a reference frame forregional changes. In addition, later season (spring) snowcover is progressively retreating to higher elevations andlatitudes, so that variations in the spring regional SCEcan also serve as an indicator for changes in mountainclimates.

The maximum SCE is in January with an averagesurface area of 45Ð8 million km2, and the minimumis in August with an average of 1Ð9 million km2 ofland covered (calculated from Global Snow Lab, 2008data). Because of its larger land area, the NH representsthe more important seasonal snow reservoir of the twohemispheres with most of its snow-covered lands locatednorth of 50°N (Figure 1). In mid-winter, approximatelyone-half of the NH land surface is covered by seasonalsnow. As a result, water supply for large regions of theNH is derived from snowmelt-dominated streamflow.

The findings of recent studies regarding SCE changesover the NH are summarized in Table I, which are basedon the analysis of the NOAA satellite and reconstructeddata. Table I and the annual average SCE shown inFigure 2(a) illustrate the general decrease in annual snow-pack that has taken place for most regions over thepast several decades. This reduction has been accelerated

since the 1980s. Most notably, substantial SCE decreaseshave occurred for the spring and summer months, espe-cially March and April (Table I and Figure 2(c)). Overthe 1922–2005 period, March and April NH SCE hasshown a statistically significant decline of 7Ð5 š 3Ð5%(updated from Brown 2000 in Lemke et al., 2007), andApril SWE losses have averaged 4Ð4%/decade (Lemkeet al., 2007). The reduction in SCE is strongly con-nected to warmer winter and spring temperatures, andhas resulted in significant timing shifts; in particular, thecentre of mass of SCE has changed from February toJanuary, and spring snow pack melt has advanced over2 weeks for the 1972–2000 period. In contrast to thespring declines, winter SCE, in general, has not changed(Figure 2(b)), or has even regionally increased, in spiteof warmer winter temperatures. This finding is consis-tent with the NH mid-latitudinal precipitation increasespredicted from climate change models for warmer globaltemperatures. Brown (2000) indicates that precipitationincreases on the order of 2% are sufficient to offset theeffects of warming in large-scale SCE to date. The find-ings of the satellite data analysis were corroborated by acomprehensive analysis of in situ snow observations forNorth America and Eurasia (Robinson and Heim, 2006).It should be noted, however, that the observed changesin snow cover may be understated, as they were calcu-lated from linear trends and comparatively short records,and most of the warming and reduction in snow coverhas occurred since the 1980s. In addition, the interpre-tation of the results for mountainous areas and highlatitude was limited by major data gaps. Simulationsof future NH snow cover and SWE suggest a progres-sively shorter snow accumulation season and continueddecreases in SWE for the mid-latitudes, while the coldestregions, such as Siberia and the northern parts of NorthAmerica, could see increases in snow depth and SWE(Hosaka et al., 2006). It should be noted that orographicprecipitation and in particular snowfall are among themost difficult variables to simulate in climate models,even at high spatial and temporal resolutions (Benistonet al., 2002). Thus the response of mountainous snowcover to increased warming may not be adequately rep-resented.

The primary areas of SH snow cover are New Zealand,southeastern Australia, the higher elevation areas in SouthAfrica, and South America. The few records or prox-ies available for the SH show either decreases or nochanges in SCE for the past 40 years. Very little isknown about the spatial and temporal variability of thesnow cover in the SH, as NOAA visible imagery doesnot cover the SH; microwave observations are availableonly since 1978 and the transition from Scanning Mul-tifrequency Microwave Radiometer (SMMR) to SpecialSensor Microwave/Imager (SSM/I) in 1987 has not beenresolved for climate quality studies. Dewey (1993) ini-tiated an SH snow cover archive for inter-hemisphericcomparisons and comparisons of climate variability andtrends.

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82 I. T. STEWART

0.0 0.2 0.4 0.6 0.8 1.0

180° W 90° W 0 90° E 180° E90° N

45° N

0

45° S

Figure 1. The importance of snowmelt-derived streamflow in the Northern Hemisphere as illustrated by the ratio of accumulated annual snowfall toannual runoff over the global land regions and given in %. The red lines indicate regions where streamflow is snowmelt dominated, and where thereis no adequate reservoir storage capacity to buffer shifts in the seasonal hydrograph. The black lines indicate additional areas where water availabilityis predominantly governed by upstream snowmelt. The inset shows regions of the globe that have complex topography using the criterion ofAdam et al. (2006). Reprinted from Nature, 438, Barnett TP, Adam JC, Lettenmaier DP, Potential impacts of a warming climate on water availability

in snow-dominated regions, 303-309, Copyright (2005), with permission from Nature Publishing Group.

Table I. Summary of Northern Hemisphere (NH) snow cover changes from recent studies

Region Findings Reference Period

NH high latitudes 10% decrease in NH SCE since 1972; largespring and summer deficits sincemid-1980s; winter increases and springdecrease in snow depth over Canada andRussia since early 1900s

Serreze et al. (2000) 1972–1998; 1946–1995;1891–1992;1915–1992

NH mid-latitudes Overall increase in NA SCE; increases inDecember–February snow cover;substantial decreases in NH March andApril snow cover associated withwarming; data gaps for mountainous areasand high latitudes

Brown (2000) 1915–1997

NH Reduction in SCE since 1980s, especially forspring; shift of greatest snow extent fromFebruary to January

Robinson (2003);Robinson et al. (2003)

1966–1993

NH Decline in spring snow cover Robinson and Heim(2006)

1967–2005

NH Abrupt 5% decline in annual SCE sincemid-1980s; declines in late winter, spring,summer snow cover, no snow coverdeclines for fall and winter

Robinson and Frei (2000) 1972–2000

NH Shift in the month of maximum SCE fromFebruary to January; significant decline inspring SCE; earlier snow disappearance of3–5 days/decade, lengthening of snow-freeperiod

Dye (2002) 1972–2000

Northern Eurasia Declining spring snow cover; spring melt hasshifted to 2 weeks earlier

Dye and Tucker (2003) 1982–1999

SCE, snow cover extent; NA, North America.

The changes in the extent and timing of the annualsnowpack and melt have impacted the volume and tim-ing of snowmelt-dominated streamflow on the continen-tal to global scale. Evidence suggests emerging patternsof streamflow changes as a result of temperature andprecipitation changes that include increases in runoff vol-ume for South and East Asia, and decreases in runoffvolume for southern and eastern Europe, western Russia,

much of North America, most of South America, andpossibly South and East Asia (Arnell, 2003; Falloon andBetts, 2006; Nohara et al., 2006). Broad-scale stream-flow timing changes are discernable for the marginalsnowmelt-dominated zones, where increasing tempera-tures mean that less winter precipitation falls as snowand peak timing shifts to earlier in the season (e.g.Nohara et al., 2006). Projected changes include an initial

Copyright 2008 John Wiley & Sons, Ltd. Hydrol. Process. 23, 78–94 (2009)DOI: 10.1002/hyp

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CHANGES IN SNOWPACK AND SNOWMELT RUNOFF 83

decrease in global river flow until about 2060 with asubsequent increase by 4–8% (Falloon and Betts, 2006).However, large regional variations are expected. There issome indication that the combined effects of temperatureand precipitation changes could increase (decrease) themonthly maximum (minimum) flows and thus increasethe frequency and severity of flooding events (Falloonand Betts, 2006).

Changes on regional scales

Changes in key mountain ranges receiving seasonalsnow are discussed below for regions where relevantstudies were available, notably the European Alps andEuropean mid-elevation mountain ranges, the TibetanPlateau and surrounding mountains, the Eurasian moun-tain ranges, and the North American mountains. Moun-tains with an annual snowpack are shown in Figure 1as those regions that are both identified as having alarge snowmelt component (shown in blue on the main

Figure 2. (a) Northern hemisphere average annual snow cover extent(1972–2007) for the water year October 1 to September 30. (b) Winter(DJF) average snow cover extent over the northern hemisphere (1967–2007). (c) Spring (MAM) average snow cover extent over the NorthernHemisphere (1967–2007). Data provided by the Global Snow Lab (2008)

Figure 1) and as topographically complex (shown in redin the inset of Figure 1).

The European Alps and mid-elevation European moun-tain ranges. Changes in the snow cover throughout theEuropean Alps and for some mid-elevation mountainranges are summarized in Table II from recent studies.In general, snow cover throughout the European Alpineregion has been decreasing throughout the 20th century,but especially since the 1980s and for the latter part ofthe century. While the snow cover decreases are mainlyconnected to broad rises in temperature, a fraction ofthe observed shifts can be attributed to decadal-scaleatmospheric circulation changes. Warming for the win-ter season has been particularly significant, with averagewinter (December–February) temperatures elevated by1Ð0–1Ð5 °C over the past several decades. Evidence sug-gests that milder winters have yielded higher annual pre-cipitation totals in the region. The snow cover responseto these climatic changes has been greatly dependent onelevation, with the greatest impact at lower and mid-elevations. The greatest variability and declines in snowextent, snow depth, and SWE have been identified for thelower elevations below 1300 m. The decrease in low- andmid-elevation snowpack is accompanied by a significantdecline in the amount of precipitation falling as snowand a significant decrease in the number of snow days(Beniston et al., 2003; Laternser and Schneebeli, 2003;Scherrer et al., 2004). At high elevations, snow covershave not declined under the warming experienced to date;instead, higher winter precipitation totals have resultedin an increase in snowpack (i.e. Beniston et al., 2003;Vincent et al., 2007).

Further hydroclimatological shifts for the Alpineregion are expected for the coming century under allIPCC climate change scenarios (Table II). While theextent of the projected changes varies with region andelevation, the direction of change is the same for allmodels and scenarios (Jasper et al., 2004; Zierl and Bug-mann, 2005; Horton et al., 2006). Major decreases inAlpine snowpack and an overall shorter duration of snowcover, resulting in earlier snowmelt runoff, are expectedto occur for the low and mid-elevations owing to anincreasing fraction of the precipitation falling as rainand earlier snowmelt. These projected changes are theexpected response to a 1Ð3–4Ð8 °C warming by 2100and may be accompanied by a rise in the snowline of400 m or more, as well as a reduction in the averagesnow-covered area by close to 25%—from 18 000 km2

currently to less than 14 000 km2 (Beniston et al., 2003).Concurrent with snow extent, snow volume could be sig-nificantly reduced later in the century, with all snowbelow 1500 m removed in all catchments and signifi-cant reductions of snow at medium altitudes (Laternserand Schneebeli, 2003; Zierl and Bugmann, 2005). Bycontrast, sustained winter precipitation at high elevationscould continue to compensate the effect of rising tem-peratures. Model simulations suggest that for elevationshigher than 2700 m, snow volume is currently increasing,

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Table II. Summary of observed and projected changes in snow cover and snowmelt-derived streamflow for the European Alps andEuropean mid-elevation mountain ranges

Observed

Region Findings References Study period

Alps Snow accumulation has not changed athigh-elevation sites

Vincent et al. (2007) 1905–2005

Alps Mean snow depth, duration of snow covershow increasing trends until the 1980s,then significant decrease; greatest effect onsnow extent, snow depth and SWE at mid-and low altitudes, where warmer Ts causea greater portion of precipitation falling asrain

Laternser and Schneebeli(2003)

1931–1999

Alps For milder winters increased P results ingreater snowpack above 1700–2000 m anddecreased snowpack at lower elevations, asmore P falls as rain

Beniston et al. (2002) 1932–1998

Alps Length of snow season and snow amountdecreased since mid-1980s due to warmerTs and persistent high pressure systemover region, greatest sensitivity at altitudesbelow 1500–2000 m

Beniston (1997) 1945–1994

Alps Decreases in snow days below 1300 mlinked to T increases, difference inresponse between Northern and SouthernAlps; Northern Alps unaffected by NAO

Scherrer et al. (2004) 1958–1999

Bulgarian Mountains Significant decreases in snow cover, snowdepth, and snow cover duration at somesites, no changes or increases at others, inspite of warmer Ts

Petrovka et al. (2004) 1931–2000

Bulgarian Mountains Three different elevation-based snow coverresponses, significant variability linked todecadal-scale atmospheric patterns, laterstart of snow accumulation formid-elevations (1000–1500 m); no earlierspring melt or accelerated response forlower elevations

Brown and Petrovka(2007)

1931–2000

Finnish Mountains P increases; greater snow depth; shorterduration of snow cover; SWE increased ineast and north; decreased in south and west

Hyvarinen, 2003 1911–2000

but could decline for all elevations after 2020 (Zierl andBugmann, 2005).

Changes in streamflow and streamflow timing areprojected to accompany the reductions in snowpack overthe next decades, with the lower and mid-elevationsparticularly impacted. Under a warming climate, theAlpine mountain range will lose some of its functionas seasonal water storage, and snowmelt-derived runoffwill arrive increasingly earlier with potentially dramaticimplications for a wide range of sectors. In addition, thefraction of the spring and summer runoff, the peak runoff,and the mean annual runoff are expected to decline.A reduction in annual streamflow anticipated for lowto mid-elevations is likely connected to decreases inannual precipitation, and to higher evaporation rates withwarmer temperatures (Horton et al., 2006). The projectedchanges in runoff timing could also affect future floodregimes. Under current climatic conditions, most majorfloods in the Alpine regions occur in the summer (Bader,2000). With warmer temperatures, a greater fraction ofthe winter precipitation will fall as rain and run off

immediately, such that less precipitation will be storedas snow or ice. A concurrent shift of the peak flow toearlier could result in an increase (decrease) of winter(summer) flood events.

Changes in other European mid-elevation ranges havebeen less clear. In the mountainous regions of Bul-garia, some sites showed significant decreases in snowcover, snow depth, and snow cover duration, while oth-ers exhibited no change or significant increases in spiteof warmer temperatures. Responsible for the variationsare elevation-based differences in response amid sig-nificant variability linked to decadal-scale atmosphericclimatic patterns. While the duration of snow cover forthe mid-elevation (1000–1500 m) zone has shortened,no evidence for earlier spring melting or an acceler-ated response of low-elevation sites was found. Sincethe 1970s, Finland has seen trends towards greater pre-cipitation, greater maximum snow depth, but a shorterduration of the snow cover (Hyvarinen, 2003). SWE hasincreased in eastern and northern Finland and decreasedin the south and west. Evidence suggests a continued

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CHANGES IN SNOWPACK AND SNOWMELT RUNOFF 85

Table II. (Continued )

Projected

Alps Warming of 4 °C; reductions of snow volumeof 90% below 1000 m, 50% at 2000 mand 35% at 3000 m; earlier melt up to130 days earlier for low- andmid-elevations

2071–2100 Beniston et al. (2003)

Alps Strongly decreased snowpack, shortenedduration of snow cover, advanced andreduced runoff peak; substantial reductionsin summer flows and soil–wateravailability; greatest impact at lowerelevations

2081–2000 Jasper et al. (2004)

Alps With increased atmospheric CO2 andwarming average maximum snow depth,and snow cover duration are reduced forelevations less than 3000 m

N/A Etchevers et al. (2002)

Alps Earlier start of the snowmelt period; decreasein mean annual runoff; greater impacts atlower altitudes

2070–2099 Horton et al. (2006)

Alps Warmer Ts cause runoff regime shift withgreater winter runoff, reduced summerrunoff, and shift of snowmelt-induced peakflows to earlier; mid-elevations affectedthe most, but significant changes also athigh and low altitudes; increased winter Pat high elevations could increase snowpackthrough 2020, more frequent winterflooding events; P changes are less certainand effects of P small compared to that ofT

1981–2100 Zierl and Bugmann(2005)

Finnish Mountains Decrease in snow cover, increase in P andsnow volume

Through 2100 Dankers and Christensen(2005)

SCE, snow cover extent; SWE, snow water equivalent; T, temperature; P, precipitation.

decline of the duration of the snow cover and an increasein precipitation and snow volume by the end of the cen-tury (Dankers and Christensen, 2005).

The Tibetan Plateau and surrounding mountains. TheTibetan Plateau with its drier western and monsoonaleastern parts lies at an average altitude of over 4000 m(Zhang et al., 2004) and is surrounded by the mountainranges of northwestern China to the north and theHimalayas to the south. The Tibetan Plateau receivesmost of its precipitation in the summer months. In spiteof winter and spring temperatures well below freezing,snow cover on the plateau is comparatively thin andrare, as winter precipitation is blocked by the surroundingmountains (Dahe et al., 2006). Streamflow is thus mainlyderived not from seasonal snow cover melt, but fromglacial melt water in the western part of Tibet, and frommonsoonal precipitation in the eastern part (Rees andCollins, 2006). The Tibetan Plateau has experienced aregional warming and increases in precipitation over thepast several decades, but particularly since the 1980s. Thetemperature increases have been as large as 3 °C/10 yearsbetween 1960 and 2001. In the absence of consistent andlong-term climatic networks, a substantial deglaciationof Himalayan glaciers since 1962 indicates a regionalwarming over the past decades. Recent findings regarding

the effect of these changes on snow cover and streamflowin the region are summarized in Table III.

While Himalayan ice core data point towards a long-term decrease of annual snow accumulation since 1840,more recent increases in snow pack have been docu-mented for the Tibetan Plateau, where an increase insnow depth and a small increase in SCE with a simul-taneous decrease in snow cover duration has been foundfor the last few decades. The increase in snow depth sincethe 1970s appears to be connected to a remarkable risein precipitation over Eurasia and western Asia, and isconsistent for a region that remains well below freezingduring the winter season under the warming observed todate. Thus the Tibetan Plateau and western China havenot experienced the continental decrease in snow cover inthe 1980s and 1990s that has been reported for the lowerand mid-elevation locations of western North Amer-ica and the European Alps. The observed decrease insnow cover duration and concurrent greater snow depthsfor the Tibetan Plateau suggest warmer spring tempera-tures. Similar changes in temperature, precipitation, andsnow cover have been observed for the mountainousareas of northwestern China. Streamflow in the regionhas generally increased, likely due to both an increasedcontribution of the fraction of annual snowmelt withgreater snow volumes, and greater glacial melting with

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Table III. Summary of observed and projected changes in snow cover and snowmelt-derived streamflow for the Tibetan Plateau andsurrounding mountain ranges

Observed

Region Findings Study period References

Himalayas and Tibetan Plateau Decrease of snow accumulation, as evidencedfrom ice core data, attributed to weakeningof trade winds over the Pacific Ocean

1801–2000 Zhao and Moore(2006)

Mountains of northwestern China T increases by 5% (1 °C); P decrease in1970s, increase in 1980s and 1990s by6–8 mm/10 years, step change around1986; increased annual streamflow

1955–2000 Chen et al. (2006)

Tianshan Mountains Upward trend of air T and stream flow,mainly winter streamflow, especially since1980s

1956–2004 Liu et al. (2006)

Tibetan Plateau and northwestern China Small increases in snow cover consistentwith P increases from remotely sensed dataand 106 meteorological stations; westernChina did not experience widespreaddecreases in snow cover during thewarming of the 1980s and 1990s

1951–1997 Dahe et al. (2006)

Tibetan Plateau Increase in regional warming, precipitation,and relative humidity since the 1980s; Tincreases of 3Ð1 °C/10 years for allseasons, but especially winter; P increasesof 0Ð96 mm/10 years

1960–2001 Xu et al. (2007),

Himalayas and Tibetan Plateau Himalayan glacier decreases by 21% suggestdecadal-scale warming in the region

1962–2001 Kulkarni et al.(2007)

Tibetan Plateau Significant increase in spring (March–April)snow depth at 17 stations after mid-1970s;decreasing snow cover duration; no rapidchange in snow cover extent; excessive Pincrease and land surface cooling

1962–1993 Zhang et al. (2004)

Tibetan Plateau Earlier snowmelt connected to warmer Ts inthe upper Yangtze and Yellow Riversoriginating from the Tibetan Plateau;increasing flow fractions in late winter andearly spring, decreasing in spring andsummer; larger annual flows at somestations due to local increases in P

1951–1998 Ye et al. (2005)

Tibetan Plateau Snow cover increased in the 1960s and1970s, some decrease in the 1990s in thesource regions of the Yangtze and YellowRivers

1960–1990 Yang et al. (2006)

Projected

Western Himalayas Melt in snowmelt-dominated basins(2000–4000 m) could be reduced by 18%under a warming of 2 °C; evaporationsignificantly increased; response ofsnowmelt-fed and glacially fed streams towarming is opposite to each other

N/A Singh andBengtsson(2005)

Himalayas Regional differences in streamflow responseto warming are dependent on moisturesupply; snowfall in eastern basins willreduce the rate of initial flow increasefrom warming and glacial melt, and delaythe timing of peak discharge

1990–2140 Rees and Collins(2006)

T, temperature; P, precipitation.

warmer temperatures. For the upper Yangtze and YellowRivers, which originate from the Tibetan Plateau, there issome evidence for earlier snowmelt runoff, as increasesin spring (January–April) and decreases in summer to

fall (June–December) runoff have been noted (Ye et al.,2005).

Under the intensified warming expected for the com-ing decades, Dankers and Christensen (2005) project a

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decline of the duration of the snow cover and a continuedincrease in precipitation and snow volume over Tibet bythe end of the century. The response of glacially fedand snowmelt-fed streams to these climatic changes willlikely be opposite to each other. Glacially fed streamsinitially experience increases in flow with warmer tem-peratures as the glacial ice is melting. Once the glacialice has disappeared, stream flow significantly decreases.Snow-fed streams, on the other hand, will immediatelyexperience decreased spring flows with warmer temper-atures, while annual flow totals might or might not beaffected. For the snowmelt-dominated basins (elevationsof 2000–4000 m) in the western Himalaya regions, find-ings suggest a reduction in the annual snowmelt runoffunder a warming of 2 °C, possibly due to a lesser fractionof the precipitation coming as snow and higher evapora-tion rates.

Northern Eurasian mountain ranges (former SovietUnion). Several recent efforts have focused on delineating

the hydroclimatic response of the large snowmelt-dominated rivers originating in the Eurasian mountainsto climatic variability and change, as summarized inTable IV. Not all findings are in agreement. Some of thedisagreement stems from the smaller number of avail-able studies and the placement of monitoring stationsin the region such that the impact of human activitiescan often not be distinguished from climatic variability(Yang et al., 2004a). While one study reports possiblecooling and a corresponding delay in snowmelt for theYenisei watershed, other evidences point towards a winterwarming along with summer, fall, and winter precipita-tion increases for the large Eurasian watersheds over thepast several decades. The precipitation increases have ledto increased snowpack and larger annual runoff totals.Variations in the annual discharge are also correlatedwith changes in the NAO and global mean surface airtemperatures. Recent work has described a connectionbetween high annual SWE in the Ob, Yenisei, and Lenawatersheds and high spring flow peaks, and has associ-ated changes in winter stream flow with a reduction in

Table IV. Summary of observed and projected changes in snow cover and snowmelt-derived streamflow for the Eurasian mountainranges

Observed

Region Findings Study period References

Yenisei Watershed Decreases in flow during earlysnowmelt time, delays in snowcover melt possibly due to a coolingtrend

1935–1999 Yang et al. (2004a)

Siberian Lena River Earlier snowmelt and shift to earlierpeak flow in response to spring airTs

1935–1999 Yang et al. (2002)

Ob, Yenisei, Lena River basins Snow depth has generally increased inOb and Lena Basin; snow depth lowin the 1960s; spring Ts controlspring discharge

1936–1995 Ye et al. (2004)

Ob Watershed Increased winter flows, decreasedsummer flows in upper watershed

1936–1990 Yang et al. (2004b)

Eurasian mountains Annual discharge from large Eurasianstreams has increased; changes arecorrelated with the NAO, and P andglobal T increases

1936–1999 Peterson et al., 2002

Siberia Winter warming in northern Siberia;winter precipitation increases; snowdepth increases over northerncentral Siberia; increases in winterdischarge; possible indication of aseasonal regime shift due to recentclimate warming over the Siberianregions

Past 40–50 years Yang and Ohata (2006)

Large Siberian watersheds, Ob,Yenisei, Lena Basins

Decreasing snow cover for spring andfall; clear correspondence ofstreamflow to changes in snowcover extent

1966–1999 Yang et al. (2003)

Ob, Yenisei and Lena Earlier streamflow, dischargecorresponds to change in snowcover mass; high flood peakassociated with a high maximumSWE; consistency between SWE,SCE, and T, but not with winter P

1988–2000 Yang et al. (2007)

SCE, snow cover extent; SWE, snow water equivalent; T, temperature; P, precipitation; NAO, North Atlantic Oscillation.

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permafrost area. There is some evidence that recent ear-lier spring runoffs are linked to higher flooding intensityand frequency in the region (Cunderlik and Burn, 2002).Dankers and Christensen (2005) project a decline of theduration of the snow cover and a continued increase inprecipitation and snow volume over the Eurasian moun-tain ranges by the end of the century.

North American mountain ranges. During the latterpart of the 20th century (1979–2005), annual tem-peratures have increased by up to 0Ð7 °C/decade overNorth America as a whole, winter temperatures upto 1Ð1 °C/decade (Trenberth et al., 2007). The largestamount of warming has taken place over the higher lati-tudes as well as the western/southwestern United States,where a greater rate of warming has been observed for allseasons (Trenberth et al., 2007). The observed tempera-ture and precipitation increases over western North Amer-ica exceed those attributable to Pacific climate indices.

The observed warming has been linked with changes insnowpack and streamflow timing across North America.The changes in SCE, snowpack, snowmelt, and stream-flow timing are characterized by temporal and region-to-region differences, but are most apparent for the latewinter and spring seasons, and for the western portion ofthe continent. Recent findings, summarized in Table V,show that on the continental scale, spring SCE and snowdepth have generally declined, whereas winter SCE hasincreased. While winter increases can be attributed toregional increases in winter precipitation, possible causesfor the observed declines include (i) decreasing snowfall,either because of a decrease in cold season precipitationor because a greater fraction of precipitation is fallingas rain (e.g. Groisman and Easterling, 1994; Akinremiet al., 2001; Zhang et al., 2000; Hodgkins and Dudley,2006a), (ii) a shorter snow accumulation period (Dye,2002), or (iii) an increase in thaw events during win-ter and early spring (Dyer and Mote, 2007). In fact, adecreasing fraction of precipitation has come as snow for75% of the gauges throughout the North American West;the greatest decreases have been in the coastal areas,while gauges in the Rocky Mountain range have seenincreases (Figure 3). All of these result in a shallowersnowpack and more rapid melting. The decreasing NorthAmerican SCE and snow cover depth, especially in thespring, is of critical importance because of the influenceof the timing and magnitude of spring snowmelt runoffto regional hydrologic systems, especially in the westernportion of the continent.

Large portions of western North America are charac-terized by mountainous terrain and a dry climate, wherewinter snow storage provides the most important reser-voir for winter precipitation, releasing water during thespring and early summer snowmelt. In spite of one ofthe world’s most extensive system of dams, the amountof precipitation stored as snow still exceeds that storedin man-made reservoirs for most river basins of Wash-ington, Oregon, and California, and snowmelt runoffoften constitutes 50–80% of the total annual runoff for

Figure 3. (a) Trends in winter mean daily minimum wet day air tem-peratures across the western contiguous United States for 1949–2004.The symbol size is proportional to trend amount. Circles indicate sig-nificant (p < 0Ð05) trends and squares indicate less significant trends.(b) Winter-mean wet day minimum temperatures (TMINw). (c) WY1949–2004 fractional change in winter snowfall water equivalent afterremoving the effects of trends in precipitation. A total of 75% of stationshave experienced snowfall reductions as a result of widespread warming.From Knowles et al. (2006), reproduced by permission of the American

Meteorological Society

snowmelt-dominated basins (Stewart et al., 2004). Thusit is of particular concern that declining SWE trendshave been observed throughout the West, except in verycold, high-elevation areas or areas with large precipitationincreases. The decreases in SWE have been statisticallysignificant, have been connected with temperature rises,and in some areas have taken place in spite of someprecipitation increases. Where warmer temperatures havebeen coupled with precipitation declines, the most dra-matic decreases have taken place (Mote et al., 2005).

Concurrent with the changes in snowpack, significantshifts in the timing of snowmelt-dominated streamflowhave taken place throughout the West (Table V). Theshifts include an earlier start of the snowmelt runoffand timing of the centre of mass (Figure 4) and areconnected to warmer late winter, spring, and early sum-mer temperatures, depending on geographic location and

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Table V. Summary of observed and projected changes in snow cover and snowmelt-derived streamflow for the North Americanmountain ranges

Observed

Region Findings Study period References

NA continent SCE and SWE increases forDecember–February, decreases forMarch–April since mid-1980s

1915–1997 Frei and Robinson(1999); Brown (2000);Serreze et al. (2000;updated from Robinsonet al., 1993)

NA continent SCE increases for November, December,April; decreases for January–March

1980–1995 Brown (2000)

NA continent Decreasing snow depth from late January tospring melt; greatest decreases in CentralCanada

1960–2000 Dyer and Mote (2006;2007)

Canada Earlier recessing of snow cover, SWEdecreases

1955–1977 Frei and Robinson(1999); Brown (2000)

1915–1997Maine Decline in snowpack depth, peak of depth

and SWE in the 1950s and 1960s1926–2004 Hodgkins and Dudley

(2006a)Maine, coastal Significant trends towards earlier streamflow

timing between 44° and 55°N latitude1906–1921 Dudley and Hodgkins

(2002)1929–2000

New England Half of the sites had significant decreases inS/P ratios; significant trends for Decemberand March only

1949–2000 Huntington et al. (2004)

West, also PNW General decline of SWE and snow pack,especially in spring, except for cold highelevations or where P increased

1916–2002;1950–2000

Mote, 2003b; Mote et al.,2005; Regonda et al.,2005; Earman andDettinger, 2006; Kalraet al., 2006

West Reduced and earlier peak snow pack; greatestSWE changes in coastal areas wherewinter T remain close to freezing (Oregon,California); more winter runoff, earlierspring peak flows by up to 45 days; noconsistent P trends

1916–2003 Hamlet et al., (2007)

West Reduction in the S/P ratio connected to Tincreases, largest for sites that remainclose to freezing in winter; lessgroundwater recharge

1949–2004 Earman and Dettinger(2006); Knowles et al.(2006)

West Earlier start of the snowmelt runoff; earliertiming of the centre of mass by1–4 weeks; increasing March flow;decreasing April–June flows

1948–2002 Cayan et al., 2001;McCabe and Clark,2005; Regonda et al.,2005; Stewart et al.,2005; Kalra et al.,2006; Hamlet et al.,2007

Columbia–Missouri head waters Trends towards earlier timing are small andstatistically insignificant

1951–2005 Moore et al. (2007)

Projected

NE US Under a higher and lower emissions scenario,expect 10–15 days advance of springsnowmelt

1960–2099 Huntington et al. (2006)

PNW Warmer Ts, increase in P except in summer,reduction in snowpack

Through 2040 Mote et al., 2003

West Earlier streamflow timing by 30–40 days 1995–2099 Stewart et al., 2004California Further declines in winter snowpack; earlier

streamflow timing; declines in summerlow flows

1900–2099;2011–2100

Miller et al., 2003;Dettinger et al. (2004)

SCE, snow cover extent; SWE, snow water equivalent; NA, North America; T, temperature; P, precipitation; N, North; NE, North East; US, UnitedStates; PNW, Pacific Northwest.

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elevation. Generally, the largest changes have taken placefor the northwestern United States and lower and mid-altitudes, while no or small trends have generally beenassociated with the highest elevations and those regionsthat have experienced precipitation increases. For exam-ple, in the northern Rocky Mountains, trends towardsearlier timing were found to be small and insignificantcompared to the natural variability. Although a smallportion of the observed trends can be attributed to thePDO and El Nino Southern oscillation (ENSO), theseindices do not explain the largest fraction and the continu-ity of the observed changes in snowmelt runoff (McCabeand Clark, 2005; Stewart et al., 2005). A multi-variatedetection and attribution analysis using a variety of cli-mate, regional, and hydrological models suggests that theobserved hydroclimatic changes can be explained well byanthropogenic greenhouse gas emissions alone (Barnettet al., 2008). The observed changes pose a challenge forwater resource management in the region.

Signals of earlier streamflow timing in eastern NorthAmerican mountain ranges are not as consistent as thosefound on the western side of the continent. S/P ratios inNew England generally decreased because of decreasingsnowfall and increasing rainfall, but the trends were

200° 220° 240° 260°

40°

60°

Trends in the Spring Pulse Onset(1948-2002)

(a)

> 20d earlier

15-20d earlier

10-15d earlier

5-10d earlier

< 5d

5-10d later

10-15d later

15-20d later

> 20d later

200° 220° 240° 260°

40°

60°

Trends in CT(1948-2002)

(b)

240°

40°

Figure 4. Trends in (a) spring pulse onset and (b) date of centre of massof annual flow (CT) for snowmelt- and (inset) non-snowmelt-dominatedgauges across western North America. Shading indicates magnitude ofthe trend expressed as the change (days) in timing over the 1948–2000period. Larger symbols indicate statistically significant trends at the90% confidence level. Note that spring pulse onset dates could not be

calculated for Canadian gauges. From Stewart et al. (2005)

statistically significant for March and December only. Bycontrast, river basins in Maine exhibit significant trendstowards earlier streamflow timing. These basins receivesubstantial winter snowpack, yet their cold-season meantemperature is warm enough to render them sensitive tothe warming experienced to date. The increased winterand spring flows due to warmer temperatures can increasethe risk of flooding owing to mid-winter ice jams andpossibly have ecological impacts (Hodgkins and Dudley,2006b).

The hydroclimatic trends observed under warmer tem-peratures over North America to date are projected tointensify with continued warming, as summarized inTable V. If mean winter minimum temperatures rise, theS/P fraction across the western United States will likelydecrease further. An immediate consequence of warmertemperatures and reduced snowfalls would be a regional-scale reduction of snowpack. Assuming that the responseto increased warming remains linear, the projections indi-cate that stream flow timing might shift by 30–40 daysby the end of the century. As warming has been quick-est for the highest latitudes (Knowles et al., 2006), theseareas might reach a critical threshold faster than would beprojected from average temperatures. Decreasing snow-fall and snow accumulation in combination with evenwarmer winter and spring temperatures are expected toadvance snowmelt-dominated streamflow much furtherthan what has been observed to date. Evidence suggeststhat California will be particularly affected and experi-ence large declines in late winter snowpack and accumu-lation, much earlier streamflow timing, and significantdeclines in summer low flows (Miller et al., 2003; Det-tinger et al., 2004; Stewart et al., 2004; Maurer, 2007;Barnett et al., 2008). A combination of reduced seasonalsnow accumulation and earlier melt of the snowpacksignificantly reduces the natural surface water storagecapacity for the arid southwest of the United States andincreases the risk for winter and spring floods (Knowleset al., 2006), implying a need for water resource man-agement adaptation.

SUMMARY AND CONCLUSIONS

The seasonal mountain snow pack and snowmelt runoff,an integral part of the Earth’s cryosphere, have expe-rienced changes over the past several decades that aregenerally coherent and consistent with the consequencesexpected from global warming. Although snowpack andstreamflow studies have not been undertaken for all of themountain ranges that receive annual snowpack, the avail-able data suggest that mountain areas are reacting to tem-perature and precipitation shifts worldwide. In particular,there is evidence that mountain snowpack is respondingwith particular sensitivity to global warming trends dueto the geographic location, topography, greater warmingat the higher latitudes, and the added warming that stemsfrom the snow-albedo feedback. Mountain regions alsoreceive larger amounts of precipitation, which can influ-ence the amount and timing of snowpack and melt in the

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same or opposite direction as warming. Thus observedimpacts on mountainous snow accumulation and meltfrom global change must be interpreted as a simultaneousresponse to both temperature and precipitation changein the context of the physical characteristics (e.g. geo-graphic location, latitude, elevation (distribution), vege-tation, atmospheric circulation) for a particular location.

The observed changes in mountain snowpack and meltinclude the following:

ž There has been a general decrease in annual snowpackand a decline of the maximum SCE in the NH.Snowmelt and snowmelt runoff have come earlier.This large-scale response is consistent with the changesobserved for the low- and mid-elevation regions inwestern North America and the European Alps havebeen consistent with this large-scale response.

ž The greatest mountain snowpack and melt responsesto warming have been observed for the areas thatremain close to freezing throughout the winter season,such as the lower and mid-elevation regions of thewestern North America and Alpine mountain ranges.Here, decreasing snow to total precipitation ratios, adecreasing snowpack, earlier spring snowmelt runoff,and longer summer low flows have been noted.

ž At elevations that remain well below freezing duringthe cold season, the warming observed to date has pro-duced little or no effect on snowpack accumulationand melt. These areas include the higher elevations inthe western North American mountains and the Euro-pean Alps, and the Tibetan Plateau with its surroundingmountains.

ž Mountain areas where precipitation increases concur-rent with the warming trends have been noted have seenvariations in response. For the Tibetan Plateau and theEurasian mountains, increases in the snow depth andextent, but some decreases in the snow cover dura-tion and evidence for earlier melt have emerged. Forhigh elevation regions in the European Alps and acrosswestern North America, any warming effects have beencompensated by precipitation increases, resulting inincreased snowpack.

ž Not all gauge sites and studies agree with the generalfindings. Several reasons could be responsible for thisdisagreement: The local temperature and or precipita-tion variations may diverge from larger-scale regionaland global trends; the data record may be short, discon-tinuous, or affected by measurement techniques, errors,and changes; or an area may be under the influenceof atmospheric circulation changes that dominate theresponse.

ž Region-to-region variations in the direction and themagnitude of the hydroclimatic signal have beenobserved. A comparison of the findings suggests thatthese variations can be explained by differences in therate and seasonality of warming, the precipitation typeand amount, elevation, and location.

The observed trends in snowpack and streamflow tim-ing are an indication of future changes, and model simula-tions suggest that it is likely that the trends will continueand accelerate. The combination of continued warmingand an increasing recreational use of mountain areasincrease the stresses on mountain ecosystems with annualsnowpack. The greatest changes are predicted for milderclimates, where warmer temperatures could shift a pre-cipitation regime from snowmelt dominated to rain dom-inated. Projections of changes in snowpack and stream-flow timing consistently forecast substantial snowpackreductions, a greater fraction of the precipitation fallingas snow, and an earlier spring snowmelt runoff under theincreasingly warmer temperatures expected by the end ofthe century. While the magnitude and timing of the pro-jected response vary with the location, model, and dataused, the direction of the projected change is the same forall studies. Precipitation regime shifts could have seriousconsequences in regions where natural supply is matchedor exceeded by ever-rising human demands.

Taken together, these results appear to describe areasonably coherent picture of short-term hydroclimaticchange; however, trend detection and attribution remaindifficult because of the length and quality of the availabledata, and the complex relationship between changes intemperature and precipitation and their combined effecton snowpack and snowmelt-dominated runoff. All dataextend over a very short duration compared to the highinter-annual variability, and many of the hydroclimaticdata sets are of uncertain quality, or provide very limitedspatial coverage (e.g. Rees and Collins, 2006).

Thus one of the primary questions that must be raisedis whether the trends observed to date represent a trueglobal impact on snowpack and snowmelt runoff, or if aregional redistribution is taking place through concurrentchanges in temperature and precipitation. While resultsfrom different regions show a range of responses based onthe location and physical characteristics of an area thereare widespread indications for a generally reduced snow-pack at mid-elevations and earlier snowmelt in responseto warmer winter and spring temperatures in the NH; witha concurrent precipitation redistribution either exacerbat-ing or masking the effects of warming. From physicalprinciples, increasingly warmer temperatures will impactmountain snow accumulation and melt. While the impactmay be temporarily subdued by precipitation increases,a sufficiently large warming will eventually overcomethe precipitation effect. In addition, future changes inprecipitation as predicted from model simulations arefar less certain than temperature increases and may betemporary. If the effects of increased precipitation wereremoved, then temperature effects are likely to be ampli-fied. This author then argues that although the currentobservations may differ between regions and elevations,a global climate change impact on mountain snow coverand snowmelt runoff can be identified.

The societal and ecological impacts of the observedand projected hydroclimatic changes described here andthe development of strategies to cope with the wide

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ranges of expected climate impacts remain pressingmatters. They are especially crucial for dry regionswith large populations such as Central Asia, or thewestern United States, where the recent drought hashighlighted the vulnerability of the region to watersupply problems, wildfires, disease outbreaks, speciesmortality, and increases human mortality due to airpollution with warming (CIRMOUNT Committee, 2006;Jacobson, 2008). Therefore, statistical and modellingtechniques separating out the natural variability fromlong-term anthropogenic climate change on all spatialand temporal scales continue to be a research topic ofprimary importance. In addition, the regional sensitiv-ity of the mountain snowpack and melt runoff systemto warming and the past and future occurrence of pre-cipitation regime shifts are important areas of researchwhich have not systematically been explored for manykey mountain ranges worldwide, often because of a lackof data. There is a need to investigate these regions.Even in the developed parts of the world, the sup-port for continued and added instrumentation has oftenbeen lacking and many monitoring stations have recentlybeen discontinued. Thus streamflow monitoring sites thatare not impacted by water supply structures, and couldshow the unrestricted hydroclimatic response of a basin,are becoming increasingly rare. The most fundamen-tal resources necessary for continued regional-to-globalscale hydroclimatic research, then, are long-term and con-sistent snowpack and streamflow monitoring networksof unimpeded flow that are situated at critical eleva-tions and regions. In monitoring, assessing, and address-ing hydroclimatic changes, there is a need for nation-ally and internationally integrated research, cooperation,and exchange. Research findings must be communicatedacross disciplines and outside the scientific communityto provide an effective basis for land-use planning andnatural resource policy and management. A good basiscould be to develop and grant access to an internationalhydroclimatic database for regions and basins that havebeen free from human construction. This database couldprovide support for research and decision making in thescience and policy-making arenas.

ACKNOWLEDGEMENTS

The author would like to thank Tim Barnett and his co-authors for providing Figure 1, and Noah Knowles forproviding Figure 2 for this paper. This manuscript hasgreatly benefited from the helpful comments by PhilipMote and another anonymous reviewer. Support from theClare Booth Luce Foundation for the author is gratefullyacknowledged.

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