Changing Water Availability

Venkat Lakshmi

Global Freshwater

The Science issue 25 AUGUST 2006"Freshwater Resources"Vol. 313 (#5790) Pages 1016-1145 has devoted an entire section to the description of freshwater issues, namely-

•Global water cycle and freshwater supply•Pollutants in the aquatic system•Waterborne diseases•Sustainability •Desalination

Table 3.1. Climate-related observed trends of various components of the global freshwater system. Observed climate-related trendsPrecipitation Increasing over land north of 30°N over the period 1901–2005.Decreasing over land between 10°S and 30°N after the 1970s Increasing intensity of precipitation CryosphereSnow cover Decreasing in most regions, especially in springGlaciers Decreasing almost everywhere Permafrost Thawing between 0.02 m/yr (Alaska) and 0.4 m/yr (Tibetan Plateau) Surface watersStreamflow Increasing in Eurasian Arctic, significant increases or decreases in some river basins Earlier spring peak flows and increased winter base flows in Northern America and EurasiaEvapotranspiration Increased actual evapotranspiration in some areas Lakes Warming, significant increases or decreases of some lake levels,

and reductionGroundwater No evidence for ubiquitous climate-related trend Floods and droughtsFloods No evidence for climate-related trend but flood damages are increasingDroughts Intensified droughts in some drier regions since the 1970sWater quality No evidence for climate-related trendErosion and sediment transport No evidence for climate-related trendIrrigation water demand No evidence for climate-related trend

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The human dependence on water has beenexemplified by various disturbances such asfailure of crops during periods of droughts,lack of quality freshwater during floods, human development and the reduction ofrecharge to groundwater.

The interaction between the biosphere andthe atmosphere is controlled in most partby the movement of water via the processof evaporation and transpiration. The changes in the biosphere results in changesin these fluxes which result in feedback to the local and regional hydrometeorology aswell as systematic changes in global climate.

Fig. 5 Change in the average annual potential recharge period (compared to the Baseline) in the 2050s High climate change scenario

The warmer future climate leads to an increase in the length of the growing season, so that the region’s soils return to field capacity later in the autumn and start drying out soonerin the spring. This leads to a reduction in the length of the recharge period as shown in Fig. 5.

East Anglia UK watershed

Climate change impacts on groundwater recharge-uncertainty, shortcomings, and the way forward? I. P. Holman Hydrogeology Journal (2006) 14: 637–647

• Simple direct estimations of climate-change-only impacts on groundwater (assuming current land use distributions) in areas where:– current groundwater resource management is sustainable, and there are significant unutilized resources;– there are few groundwater-sensitive wetlands or aquatic systems;– such estimations demonstrate only minor impacts on groundwater recharge and sustainable water resource management, which is supported by sensitivity analysis.• Partially integrated assessments in areas where:– current groundwater resource management is sustainable, but there is little unutilized resources;– agricultural systems are prone to significant change, either short term due to changes in subsidies or environmental legislation; or longer term due to climate-change-induced changes in crop suitability;– spatial development planning pressures suggest significant possible increases in urban development;– land use and groundwater recharge quality may be sensitive to future coastal defence policy of managed re-alignment and resultant inundation of coastal lowlands.• Fully integrated assessments in areas where:– current groundwater resource management is unsustainable;or– there are important groundwater-sensitive wetlands or aquatic systems and current groundwater resources management

Water stress indicates the intensity of pressure put on water resources and aquatic ecosystems by external drivers of change.

Generally speaking, the larger the volume of water withdrawn, used and dischargedback into a river, the more it is degraded and/or depleted, and the higher the waterstress.

The higher the water stress, the stronger the competition between society’s usersand between society and ecosystem requirements (Raskin et al., 1997; Alcamo et al.,2003a).

A level of severe water stress indicates a very intensive level of water use thatlikely causes the rapid degradation of water quality for downstream users (wherewastewater treatment is not common) and absolute shortages during droughts.

“Water stress” also includes the pressure on water resources caused by climate change, in thesense that climate change could lead to changes undesirable to society (e.g. reducedaverage water availability), or to aquatic ecosystems (e.g. unfavourable changes inriver flow regime).

Hydrological Sciences–Journal–des Sciences Hydrologiques, 52(2) April 2007Future long-term changes in global water resources driven by socio-economic and climatic changesJOSEPH ALCAMO, MARTINA FLÖRKE & MICHAEL MÄRKERCenter for Environmental Systems Research (USF), University of Kassel, Kurt-Wolters-Strasse 3, D-34109 Kassel, Germany

Fig. 1 Water stress in the 2050s for the A2 scenario based on withdrawals-to availability ratio. “Water withdrawals” are the total annual water withdrawals fromsurface or groundwater sources within a river basin for various anthropogenic uses(excluding the maintenance of aquatic or riparian ecosystems). “Water availability”corresponds to annual river discharge, that is, combined surface runoff andgroundwater recharge.

Changes in precipitation will raise or lower the average volume of river runoff.

Meanwhile, the expected increase in air temperature intensifies evapotranspiration nearlyeverywhere, and hence reduces runoff.

These two effects interact differently at different locations and produce the net increase or decrease in water availability shown in Fig. 7. Since evapotranspiration increases nearly everywhere, it tends to counteract the effect of increasing precipitation wherever it occurs.

Hence, the area of increasing water availability is somewhat smaller than the area of increasing precipitation. For example, under scenario A2 in the 2050s, 57% of the Earth’s land area has increasing annual precipitation (relative to the climate normal period) as compared to 51% having increasing annual water availability.

Fig. 7 Change in average annual water availability between climate normal period(1961–1990) and the 2050s under the A2 scenario.

While increasing water availability could have a positive influence on society by reducing river basin water stress, an increase in water availability in one season maynot be beneficial during that season, nor transferable to another season.

An increase in annual water availability may also be accompanied by a higher risk of extremely high and damaging runoff events. For the 2050s, under the A2 scenario, we estimate a significantly increasing risk of higher runoff events over 10.5% of total global river basin area (using an indicator described in the caption of Fig. 8).

Included are many humid regions, such as northern Europe, western India, northern China and Argentina (Fig. 8).

For the same scenario and period, 16.3% of the global area of river basinsmay be subject to more frequent low runoff events (Fig. 8), including such arid regionsas southern Europe, Turkey and the Middle East.

Fig. 8 Change in extreme runoff events. This figure depicts different combinations ofchanges in mean precipitation and the coefficient of variation of runoff. Computed bythe WaterGAP model (A2 emission scenario, for the 2050s, ECHAM climate model).Orange indicates a decline between 5 and 25% in annual precipitation and an increasein the coefficient of variation of runoff of between 5 and 25%. Red indicates a declineof more than 25% in annual precipitation and an increase in the coefficient ofvariation of runoff of more than 25%. Light blue indicates an increase between 5 and25% of annual precipitation and the coefficient of variation of runoff, and dark blue anincrease of more than 25%.

Fig. 1. Worldmapshowing the links between undernutrition and hydro-climatic preconditions. Prevalence of undernourished in developing countries is shown at the country level as the percentage of population 2001–2002. Hydro-climatic distribution of semiarid and dry-subhumid regions are shown in gray. These regions correspond to savanna and steppe agro-ecosystems, dominated by sedentary farming and subject to extreme rainfall variability and high occurrence of dry spells and droughts.

Figure 5. Percentage increases in annual and winter effective runoff for 2041–2070 and 2061–2090 scenarios.

Although the studies assessing the impacts of climate change on California hydrologyhave differed in their methodological approach, their results tend to agree in certaincritical areas. Results consistently show that increasing temperatures associated withclimate change will impact Californian hydrology by changing the seasonal streamflow pattern to an earlier (and shorter) spring snowmelt and an increase in winter runoff as a fraction of total annual runoff (see Figure). These impacts on hydrology vary by basin, with the key parameter being the basin elevation relative to the ‘freezing line’ during snow accumulation and melt periods and the prediction of temperature increases

The evolution of climate change impact studieson hydrology and water resources in CaliforniaS. Vicuna & J. A. DracupClimatic Change (2007) 82:327–350

Figure 3.8. Illustrative map of future climate change impacts on freshwater which are a threat to the sustainable development of the affected regions. Ensemble mean change of annual runoff, in percent, between present (1981 to 2000) and 2081 to 2100 for the SRES A1Bemissions scenario (after Nohara et al., 2006).

IPCCFreshwater Resources andtheir managementWG 2 Chapter 3

Global population increase in all continentsbetween 1950s and 2050 has put a bigstrain on water resources

Population “centers” includeChina, India, North America.

Impact of population

• The increase in global population has decreased the per capita water availability

• Consider for example the changes between 2000 and 2004

Philosophy

• As we can see there have been numerous climate change impact studies on local water resources – watershed level activities

• There has been statistical analysis of population impact on water availability

• What should this chapter convey? This chapter should integrate the micro and macroscale studies so that we can get the complete picture.

IntroductionStresses due to:

Climate changePopulation increase

Quantifying water resources: Lakes, Rivers, glaciers, groundwater, precipitation

Data and MethodsIPCC reports on climate change

MonitoringMeasurement of water resources: In-situ and satellite remote sensingModels for estimation of water resources

Future scenariosCase Studies

California; Great Lakes; Colorado River BasinsCzech Republic; RhineTaiwan; Tarim and Yellow River BasinSwazilandArctic

Transboundary disputesFloods and droughts and natural disasters

Key Uncertainties policy framework [cross with Chapter 9]