224 Northern Research Basins Water Balance (Proceedings of a workshop held at Victoria, Canada, March 2004). IAHS Publ. 290, 2004

Hydrological cycle on the North Slope of Alaska

DOUGLAS L. KANE 1, ROBERT E. GIECK1, DANIELLE C. KITOVER1, LARRY D. HINZMAN 1, JAMES P. MCNAMARA 2 & DAQING YANG1

1 Water & Environmental Research Center, University of Alaska Fairbanks, 306 Tanana Drive, DuckeringRoom 437, Fairbanks, Alaska 99775-5860, USA ffdlk@.uaf.edu

2 Department of Geosciences, Boise State University, Boise, Idaho 83725, USA

Abstract Sufficient hydrological data has been collected, since 1985 for Imnavait Creek and 1996 for Upper Kuparuk River, to perform water balance calculations on the North Slope of Alaska. Permafrost is continuous under these basins, and sub-freezing temperatures dominate through the annual cycle with the snow-covered season lasting 8-9 months. These two small headwater basins receive about one-third of their annual precipitation as snow and two-thirds as summer rain (occasionally mixed with snow). The snowmelt period is a significant hydrological event each year and often produces the highest annual runoff; the floods of record are rainfall or rain/snow mixed summer events. The highest runoff ratios occur in Imnavait Creek during snowmelt and in Upper Kuparuk River during the summer. The Upper Kuparuk River, besides being 60 times larger, has higher terrain, steeper slopes, extensive bedrock outcrops and a few lakes. Water fluxes leaving Imnavait Creek catchment are about equally split between évapotranspiration and runoff; for the Upper Kuparuk watershed, it is estimated that 64% left as runoff and 36% as évapotranspiration. The quality of the data in these basins is quite good, except during extreme events. K e y w o r d s A l a s k a ; g lac ie r s ; h igh la t i tudes ; hydro log ica l cyc le ; pe rmaf ros t ; p rec ip i t a t ion ; s n o w ; w a t e r b a l a n c e

INTRODUCTION

Since 1985 we have been collecting hydrological data on the North Slope of Alaska, first for a single, small watershed and eventually for a cluster of four watersheds (Kane et al, 2000). We have sufficient data for most years to perform water balance calculations for the two headwater catchments during snowmelt and summer periods. The North Slope of Alaska is an essentially treeless region underlain by continuous permafrost and exceeding 200 000 km" in area; for comparison purposes, it is greater in area than the 36 smallest states in the United States and about equal in size to the states of Nebraska or Minnesota. Prior to successful oil exploration and development of this region in the early 1970s, hydrological measurements in this region were rare and research was non-existent. Even today only six streams are gauged, and a handful of precipitation gauges exist, mostly for research purposes. Early measurements significantly underestimated annual precipitation, particularly the cumulative impact of trace amounts and the under-catch of precipitation (particularly snowfall) during windy events. True precipitation is two to three times greater than what was reported by the National Weather Service and other federal agencies (Yang et al, 1998).

Hydrological cycle on the North Slope of Alaska 225

Although interest in the water resources of the North Slope increased during oil development, no long-term strategy was developed for understanding the complete hydrological cycle. Stream gauges were installed, but no complementary precipitation gauges were installed in gauged watersheds to develop cause and effect relationships. However, in the 1980s new interest in the hydrology of this region developed on two fronts, the potential for global change and the impact of development on the arctic ecosystem. Environmental issues such as dust from roads, oil spills, atmospheric emissions, altered drainages, and the cumulative effect of all of these and more impacts were questioned. Almost simultaneously it became apparent that the Arctic was in a period of change, potentially related to climate change. It was first recognized that the permafrost of this region was warming by Lachenbruch & Marshall (1986). The climate of this region is closely tied with the energy and mass fluxes of the hydrological cycle, and the arctic climate is highly coupled with the climate of the mid-low latitudes and neighbouring regions (Walsh et al, 2000).

In response to the above issues, we have established a hydrological research programme on the North Slope of Alaska with the aim of developing a comprehensive understanding of its hydrology cycle. In 1985 we began studying a small 2.2 km 2

watershed, Imnavait Creek, in the northern foothills of the Brooks Range. Because there were no previous hydrological measurements of any kind in this area, we had to guess at the likely range of discharges that a flume (to be installed on Imnavait Creek) should be capable of measuring (as it turned out, it did not measure low flows very well, and the maximum flood exceeded the capacity by a factor of more than three). Gradually, we increased the area of study, until by 1993 we were studying the entire Kuparuk watershed (8140 km 2, Fig. 1). An additional watershed was added in 1999; the Putuligayuk River is a low-gradient watershed that drains the coastal plain. The core of this study is to quantify all of the major hydrological water fluxes at each of the four watershed scales under study; this paper will concentrate on the two headwater catchments that have the most complete data sets.

LOCATION AND SETTING

The two watersheds presented in this paper, Imnavait Creek and the Upper Kuparuk River (Figs 1 and 2), are centred on 68°38TSf, 149°24'W. Both are north-draining in parallel valleys. Imnavait Creek has a catchment of 2.2 km -, a stream length of 1.5 km, basin length of 2.0 km, and elevation range from 844 to 960 m. The Upper Kuparuk watershed has a drainage area of 142 km 2, stream length of 25 km, basin length of 16 km and elevation range of 698-1464 m. Imnavait Creek eventually flows into the main stem of the Kuparuk River, 12 km north of the stream gauging station. Imnavait Creek has been monitored since 1985 and the Upper Kuparuk since 1993, both continuously.

The entire area is underlain by continuous permafrost in the range of 250 to 300 m in thickness (Osterkamp & Payne, 1981). Typical thaw depths ofthe active layer are 50 cm, with a range of 25 to 100 cm. Pleistocene glaciation and subsequent erosion have shaped rolling hills with a frequency of 1 to 2 km and vertical relief of 25 to 75 m. The predominant soils are Histic Pergelic Cryaquepts, where about 10 cm of live and dead moss mantle and another 10 cm of decaying organic material are mixed with the underlying mineral soil. These organic soils then cover glacial till. The upper levels

226 Douglas L. Kane et al.

Toolik Lake Field Station

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A Gauging Site © Climatological Site o Logistical Support Base

F ig . 1 Location map of research watersheds with instrumented sites.

Hypsom etric Curve 16 0 0 -i

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U p p e r K u p a r u k S i t e s

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Imnava i t C reek S i t e s

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Fig . 2 Hypsometric graph of Imnavait Creek catchment and Upper Kuparuk River watershed with elevation indicators of climatological sites.

Hydrological cycle on the North Slope of Alaska 227

of the permafrost are ice-rich, and this results in a supra-permafrost water table forming in the active layer. During wet periods following snowmelt and rain, the supra-permafrost water table can be in the organic soils; during drier periods it is in the mineral soils. Wetness generally increases from the ridges to the valleys, although standing water is often observed on flat to gently sloping hilltops.

Mainly water tolerant plants growing in this cold environment are found in the watersheds (Walker et al, 1989). Tussock sedges are primarily found on the hillslopes and mosses in the valley bottom. Exposed areas like ridges have minimal vegetation and limited organic soils. Shrubs are common in water tracks and along valley drain­age channels. At lower elevations, but farther north than these headwater catchments, vegetation up to 10 m high can be found in riparian areas.

Subfreezing temperatures prevail for much of the year for these watersheds (Olsson et al, 2002). Mid-winter monthly temperatures (December, January and February) average around -20°C, with record lows below -40°C. Summer (June, July and August) monthly temperatures average between 6-10°C, with hourly record highs exceeding 20°C. Monthly mean wind speeds close to 4 m s"1 are typical throughout the year, with hourly maximums greater than 20 m s"1. The dominant wind direction is from the south and southeast.

FIELD MEASUREMENTS

For the Imnavait Creek catchment, there is one major meteorological station on the east ridge above the stream gauging site. Profiles of wind speed, humidity and air temperature are measured on a 10 m tower. Wind direction, incoming and outgoing short and long wave radiation, net radiation, summer precipitation, and barometric pressure are measured at one elevation (1.5 m). Radiation measurements are made prior to snowmelt through freeze-up. Rime ice during the winter months at these unmanned sites severely compromises the radiation data. There is one additional summer precipitation gauge at the southern boundary of the Imnavait Creek watershed. All our rainfall measuring instruments are equipped with wind screens to provide improved estimates of total rainfall, even during high wind events. An evaporation pan has also been operating at this site during the summer months following snowmelt. A Wyoming Snow gauge records snow accumulation throughout the winter, and rainfall during the warm season.

A calibrated flume was installed on Imnavait Creek to measure flow in 1985. We have a good stage-discharge relationship for this hydraulic structure. In the spring, it is small enough so that we can clear all the ice and snow out of it and for some distance upstream and downstream. Slush flows in this drainage are common at the onset of spring runoff. Large volumes of meltwater accumulate within the snowpack in the valley bottom; this water rapidly drains out of the basin shortly after the slush flow makes a channel through the snowpack. Flows greater than 1 m s"3 exceed the capacity of the flume, and estimates of flow are made using stage data and indirect flow estimates using Manning's equation. All of these measurements have been made continuously since 1985 in the same manner. Flows during the winter months are near zero or below the level that we can measure. The formation of small amounts of aufeis each winter in/along the channel indicates that there is meagre flow.

228 Douglas L. Kane et al.

Winter precipitation is a challenge to measure. There is a Wyoming precipitation gauge at this site (operated by USDA, Natural Resources Conservation Service (NRCS)) that undercatches winter snowfall by about 5-10% (Yang et al, 2000). In addition, we make extensive measurements along an east-west transect of snow depth (every 1 m) and snow water equivalent (five SWE measurements every 100 m) across the watershed in late winter to ascertain the basin-wide average. High resolution maps of snowpack distribution developed from measurements made throughout the basin over five seasons have demonstrated that the single transect provides a good estimate of the basin-wide average snowpack water equivalent (Hinzman et al, 1996). The rational for this approach is that the measured SWE is the water that potentially will produce the snowmelt freshet. In reality, our measurement of SWE equals the winter precipitation minus any sublimation that occurs over the winter months. The winter precipitation that we report represents the snow on the ground at the end of winter, not the total precipitation that occurred during the winter (Yang et al, 2000). The snow depth in these watersheds is very heterogeneous due to winter winds; exposed areas (ridges, windward slopes) have little or no snow while protected areas (depressions, valley bottoms, leeward slopes) have enhanced snow depth.

We have used a different approach for the larger Upper Kuparuk watershed. Within the basin, we have one major meteorological site with a 10 m tower instrumented similar to the one at Imnavait Creek (located near the gauging site). Located throughout the basin we have five additional sites where we measure summer precipitation in combination with air temperature and wind speed at one elevation.

For the measurement of streamflow, we have installed a stilling well; the stream is too large to feasibly install any type of hydraulic structure. We gauge the stream regu­larly and have developed a reliable stage-discharge relationship. During the largest flow recorded in 10 years on this stream, the original stilling well was washed away. A new stilling well was placed downstream from the old one; we are presently develop­ing a new stage-discharge relationship. Because of ice/snow in the channel during break-up, we try to gauge this stream twice per day (at low flow in the morning and high flow late in the evening) and record stage continuously.

Our approach to estimating the average SWE throughout the watershed is different from that for Imnavait Creek. In this case, we have 16 stations distributed throughout the watershed, where we make 50 depth measurements and five SWE measurements at each site.

RESEARCH CHALLENGES

The remoteness of these watersheds, lack of power, extreme climate, lack of previous studies and wildlife interference are all challenges to collecting good quality hydrolog­ical data. It takes about 8 h by land or 8 h by a combination of air/land to reach these headwater catchments. When an important short-term event is occurring in the basin, it is very difficult for us to get there quickly; usually summer floods are over before we can get on site. All the instrumentation is battery-operated and recharged by solar panels. This limits the type of instrumentation and communication equipment we can install. The lack of solar radiation during the coldest part of the winter places severe

Hydrological cycle on the North Slope of Alaska 229

limitations on our continuous measurements and real-time telemetry. Low tempera­tures, high winds, rime ice, ice-choked streams etc., require that the quality of collected data be scrutinized. The lack of data from previous studies means that we must estimate the range of data for various variables when installing equipment. A flume operating on Imnavait Creek functioned well from 1985 to 2002, when its maximum capacity was exceeded by a factor of between 3 and 4. Wildlife, particularly bears, have damaged several of our remote meteorological sites. Those sites further from the road access are most prone to damage by wildlife.

Initially we retrieved data at field sites when we visited them; in some cases, for remote sites accessible only by helicopter, this was done twice per year. Presently we have a system that transmits data hourly (under ideal conditions) by radio transmitters to a ground station that can interface with a communication satellite, or to an internet system (Johnson et al, 2002). In addition to transmitting environmental data, we can monitor the batteries used for the data collection and transmission. Now we only visit sites when we detect a problem in the data being transmitted. It is possible to communicate both directions; for example, it is possible to change the program that collects data at a field site from the university campus or anywhere within radio range.

WATER BALANCE CALCULATIONS

The determination of water balance for watersheds can be approached in many ways, depending upon watershed characteristics and important hydrological processes. One form of a general water balance equation is:

(Psnow + Prain) - (Qsnow + Qram) - ET - AS = X\ (1)

where: P s m w = water equivalent (SWE) of snow on the ground at end of winter; Prai„ =

precipitation during summer, occasionally snow; Q s n o w = runoff from snow-melt; Q r a i n

= runoff during remainder of summer after snowmelt; E T = combined évapotranspira­tion during snowmelt and summer period; A S = change in storage such as surface, active layer and sub-permafrost groundwater; q = error in water balance.

In our case we make assumptions that allow us to simplify the above equation:

(Psnow Prain) (Qsnow Qrain) ET (2)

Measurements of input of precipitation (both snow and rain) minus outputs of runoff (again both snow and rain) give an estimate of combined summer evaporation (E) and transpiration CT) or évapotranspiration (ET). The main assumption here is that there are no changes in reservoirs such as surface and subsurface storage. Surface storage is limited in this headwater basin because of moderate slopes and lack of water bodies. Subsurface storage is limited to the shallow active layer; it is assumed that the continuous permafrost precludes any interaction with deeper groundwater. There is no evidence during the winter months of deep subsurface discharge. Basically we are determining our annual water balance from freeze-up in one year to the same time the next year. Kane et al. (1989) reported that the mineral soils remain close to saturation throughout the year; obviously in the winter months these soils are frozen. Therefore there is very little storage change in the mineral soils. The surface organic soils (~20 cm) show the greatest variation in soil moisture content (Hinzman et al, 1991).

230 Douglas L. Kane et al.

The upper 10 cm or so of the organic layer have a low bulk density and a high porosity incapable of holding water for very long due to gravitational gradients, except in relatively flat areas, which are less than 5% of watershed area. High moisture content only occurs in the surface organic soils for short periods of time immediately following snowmelt and rainfall. In the lower, decomposed organic soils, from 10 to 20 cm depth, soil moisture can range from very dry to saturated.

Imnavait Creek catchment

For the Imnavait Creek watershed from 1985 to 2002, we have the highest expecta­tions for accurate closure of the water balance (Table 1) because of the relatively small size of basin (consequently increased density of measurements). The watershed is small enough so that we can make many SWE and snow depth measurements in the watershed to get an average basin-wide estimate of SWE. There are no large snow­drifts in this watershed; by making comparisons between various estimates, we feel that our estimates of SWE are within ±10%. For 18 years the SWE ranged from 69 to 181 mm, averaging 116 mm year"1. For summer precipitation, we use tipping bucket gauges with shields and -179 mm orifice openings. Summer precipitation ranged from 163 to 342 mm and averaged 239 mm. Monthly precipitation over the summer increases through August and then starts to decline in September when freeze-up begins.

Runoff measurements for Imnavait Creek are generally very good. During snowmelt runoff, the flow increases rapidly, following slush flows that cut a channel through the snowpack in the valley bottom. Ice flows for the most part do not exist in this small headwater basin. Snowmelt runoff ranged from 39 to 144 mm and averaged 78 mm. Summer runoff ranged from 28 to 188 mm and averaged 95 mm. Measure­ments with the flume are probably with ±1 to 2%, except during extreme flows.

Pan evaporation has been monitored continuously over the summer with a standard American Class A pan (25.4 cm deep, 120.65 cm in diameter). Results for ET from equation (2) are compared with the measured pan evaporation in Table 1 ; the ratio of ET by water balance to pan evaporation averaged 0.55 for a range of 0.34 to 0.79 (Kane et al, 1990). ET was also estimated by the Priestley-Taylor method (Priestley & Taylor, 1972) for Imnavait Creek and the error term (n) was determined by using equation (1) and assuming that there was no change in storage in the active layer over the calculation period. The maximum and minimum error using this approach was 69 and -50 mm, respectively; the average error was 1 mm and the standard deviation was 32 mm.

Upper Kuparuk River watershed

In addition to being over 60 times larger, the Upper Kuparuk watershed has much greater relief (Fig. 2), wider range of slopes, and a channel width greater than 20 m, as compared to sub-metre for Imnavait Creek. In both watersheds, water tracks are responsible for the movement of excess water off the slopes during snowmelt and major rain events (McNamara et al, 1999). The accuracy of various hydrological

Table 1 Water balance data for Imnavait Creek watershed, North Slope of Alaska, spring and annual water balances, 1985-2003.

water Eq. precip. precip. runoff runoff runoff evapotrans evapotrans q evap. PSNOW PRAIN PTOTAL RTOTAL PSUMMER Evap (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm)

1985 102 251 353 66 * * * * * * 0.65 * * * 0.41 * 1986 109 163 272 57 62 119 153 171 -18 310 0.52 0.38 0.44 0.48 0.67 0.49 1987 108 272 380 71 179 250 130 167 -37 320 0.66 0.66 0.66 0.28 0.40 0.41 1988 78 252 330 39 72 111 219 201 18 332 0.50 0.29 0.34 0.35 0.31 0.66 1989 155 257 412 94 78 172 240 171 69 420 0.61 0.30 0.42 0.55 0.60 0.57 1990 106 163 269 64 28 92 177 147 30 394 0.60 0.17 0.34 0.70 0.65 0.45 1991 82 249 331 56 43 99 232 208 24 377 0.68 0.17 0.30 0.57 0.33 0.62 1992 181 241 422 144 63 207 215 154 61 328 0.80 0,26 0.49 0.70 0.75 0.66 1993 125 208 333 100 125 225 108 109 -1 321 0.80 0.60 0.68 0.44 0.60 0.34 1994 80 271 351 52 98 150 201 211 -10 345 0.65 0.36 0.43 0.35 0.30 0.58 1995 142 209 351 98 139 237 114 150 -36 290 0.69 0.67 0.68 0.41 0.68 0.39 1996 133 188 321 106 60 166 155 160 - 5 280 0.80 0,32 0.52 0,64 0.71 0.55 1997 142 255 366 114 46 160 206 194 12 307 0.80 0.18 0.44 0.71 0.56 0.67 1998 96 238 334 62 119 181 153 203 -50 288 0.65 0.50 0.54 0.34 0.40 0,53 1999 69 342 411 43 143 186 225 231 - 6 286 0.62 0.42 0.45 0.23 0.20 0.79 2000 112 232 344 90 105 195 IM9 168 -19 335 0.80 0.45 0.57 0.46 0.48 0,44 2001 142 204 346 95 67 162 184 179 5 285 0.67 0.33 0.47 0.59 0,70 0.65 2002 126 303 429 55 188 243 186 216 -30 285 0.44 0.62 0.57 0.23 0.42 0.65 2003 185 284 469 108 186 294 175 172 3 327 0.58 0.65 0.63 0.37 0.65 0.54 Max 85 342 469 144 188 294 240 231 69 420 0.80 0.67 0.68 0.71 0.75 0.79 Min 69 163 269 39 28 92 108 109 -50 280 0,44 0.17 0.30 0.23 0.20 0.34 Std 33 45 51 28 51 56 40 30 32 40 0.11 0.17 0.12 0.16 0.17 0.12 Dev. Av. 120 241 359 80 100 181 179 178 1 324 0.66 0.41 0.50 0.47 0.52 0.55 *Data not collected; ** Water balance; ***Priestley Taylor. RSNOW, winter snowpack runoff; PSNOW, snowpack water equivalent; RRAIN, summer runoff; PRAIN, summer precipitation; RTOTAL, total annual runoff; PTOTAL, total annual precipitation.

Year Snowpack Summer Total Snowmelt Summer Total WB** PT*** Error Pan RSNOW/ RRAIN/ RTOTAL/ RSNOW/ PWINTER/ ET/Pan

232 Douglas L. Kane et al.

fluxes (Table 2) is lower than those measured for Imnavait Creek, for a variety of reasons discussed below. Although we have been studying this basin since 1993, we did not have enough data on spatial distributed SWE and summer precipitation to complete an accurate water balance for the first three years (1993-1995).

As mentioned earlier, since 1996 there have been six summer precipitation gauges in this watershed, plus two in adjacent Imnavait Creek (about one gauge per 18 km 2). The precipitation pattern during a given storm is similar between sites; however, there is a great deal of variation between storms. For example, during the July 1999 storm the Imnavait Creek gauge recorded the lowest precipitation of all the gauges, while in the August 2002 storm it recorded the greatest when compared to the others. Measurements with these gauges give a good picture of the watershed rainfall distribution in summer. For 7 years, the summer precipitation averaged 262 mm and ranged from 205 to 324 mm. The basin-wide average of SWE for the Upper Kuparuk watershed was 112 mm at winter's end, ranging from 54 to 150 mm.

Every year snowmelt produces a significant flood in these watersheds. It is hypothesized that although most annual floods will be snowmelt floods, the floods of record for watersheds in this size range will be produced by rainfall (Kane et al, 2003). For Imnavait watershed, snowmelt floods were the dominant floods 15 years out of 17. For the Upper Kuparuk watershed, with 10 years of data, five of the annual peak flood events were rainfall-generated and five were snowmelt-generated. For both watersheds, the maximum rainfall-generated flood exceeded the maximum snowmelt flood by a factor of greater than 3. Snowmelt runoff ranged from 27 to 89 mm and averaged 57 mm over 7 years. Summer runoff ranged from 129 to 274 mm and during the same period averaged 179 mm.

MEASUREMENT ERRORS

Although our goal is to collect the best data possible, certain factors complicate this objective. Sufficient research funding to install the best equipment that can be used does not exist; within our budget, we have taken the path that we believe will provide us with the best data. When installing the flume on Imnavait Creek, the lack of prior runoff data hindered our selection of an appropriately sized flume. It was too small for high flows (on about three occasions) and too wide for low flows during dry periods in the summer. A supplemental stilling well was installed directly in the stream above the flume to augment low flow estimates. During snowmelt, flows have slightly exceeded the maximum capacity of the flume, and during one summer runoff event (August 2002) it was seriously exceeded. This requires that we estimate the peak flow by the indirect method of measuring slopes and cross-sections from high water marks and estimating a Manning's n value. This method has been shown to produce errors as great as 50% (House & Pearthree, 1995). During break-up, slush flows in Imnavait Creek and floating ice in the Upper Kuparuk River compromise the quality of the runoff data. Flow is measured in the Upper Kuparuk River by holding a current meter in the channel from a boat held in place by a tag line. During peak flows this is quite challenging, particularly with large ice rafts coming down the channel. When slush flows occur they often inundate the flume and redirect flow around the flume.

Table 2 Water balance data for Upper Kuparuk River basin, North Slope of Alaska, spring and annual water balances 1996-2002.

Year Snowpack water eq. (mm)

Summer precip (mm)

Total precip (mm)

Snowmelt summer (mm)

Runoff runoff (mm)

Total runoff (mm)

Evapo­trans (mm)

RSNOW PSNOW

RRAN PRAIN

RTOTAL PTOTAL

RSNOW RTOTAL

PWINTER PSU-R

1996 147* 242 389 71 175 246 143 0.48 0.72 0.63 0.29 0.61 1997 150 310 460 89 215 304 156 0.59 0.69 0.66 0.29 0.48 1998 81 236 317 35 129 164 153 0.43 0.55 0.52 0.21 0.34 1999 54 324 378 27 176 203 175 0.50 0.54 0.54 0.13 0.17 2000 148 205 353 72 139 211 142 0.49 0.68 0.60 0.34 0.72 2001 117 222 339 68 148 216 123 0.58 0.67 0.64 0.31 0.53 2002 105 294 399 38 274 312 87 0.36 0.93 0.78 0.12 0.36 Max 150 324 460 89 274 312 175 0.59 0.93 0.78 0.34 0.72 Min 54 205 317 27 129 164 87 0. 0.54 0.52 0.12 0.17 Average 115 262 376 57 179 237 140 0.49 0.68 0.62 0.24 0.46 Stnd. 37 47 47 23 51 54 28 0.08 0.13 0.09 0.09 0.19 Dev. *Snowpack adjusted to include rain on snow.

234 Douglas L. Kane et al.

Accurate precipitation measurements are also challenging in this environment. Our gauges, although shielded, are prone to under-catchment and the tipping buckets poorly record the trace events. Although spatially distributed (Fig. 1), we do not have any precipitation gauges in the highest 30% of basin for the Upper Kuparuk basin. Our raingauges work well in the summer when rain occurs; however, it can snow on any day in the summer, and occasionally it snows a substantial amount. In August 2002, we had 40 cm of snowfall in a 24-h period. Smaller amounts of snowfall are collected in the gauge orifice and then measured when it melts; however, in August 2002, a large snowfall overwhelmed the gauges and most fell off onto the ground.

Measuring snowfall in the winter in this treeless and windy environment is problematic. The redistribution of snowpack by wind during the winter months is a perplexing problem in that it results in a very heterogeneous snowpack. In our case, most of the snow is re-deposited in areas (water tracks, valley bottoms) where runoff is enhanced. To get an estimate of snow on the ground at the end of winter from gauge snowfall measurements, we would also need an estimate of sublimation. Our approach has been to measure the snow on the ground at the end of winter and not attempt to quantify sublimation. This approach also has its problems in years when additional precipitation falls after the measurements have been made, but before ablation is completed. Although March, April and May are the driest months, there have been years when substantial precipitation in the form of either rain or snow has fallen. We try to estimate this from daily snow surveys made at a few sites. During this period of time, travel around the basin is difficult.

We would like to make some measurements throughout the year that we cannot make at this time, such as incoming and outgoing radiation fluxes. Our sites are unmanned during the winter months, so both freezing precipitation and rime ice are capable of debilitating these instruments.

DISCUSSION

For a variety of reasons, these two high latitude regions have fairly high runoff ratios during snowmelt. These ratios for Imnavait Creek catchment averaged 0.66 and the Upper Kuparuk watershed averaged 0.49. First, snowmelt runoff occurs over a relatively short period of time when soils are nearly saturated. Additionally, there are limited surface and subsurface storage reservoirs for meltwater. The Upper Kuparuk watershed has a few small lakes in the headwaters that could attenuate runoff through surface storage; Imnavait catchment has no lakes. In contrast, on the low-gradient coastal plain to the north, numerous lakes, ponds and wetlands afford considerable storage (Bowling et al, 2003). Second, the active layer is ice-rich and completely frozen, and only the surficial organic soils tender any storage possibilities. During the winter months, desiccation of the near-surface organic layer enhances potential storage; it was found that after dry summer periods (Kane et al, 1989) and during snowmelt (Kane et al, 1991) only 15 mm of water from rainfall was needed to initiate an increase in runoff. During years with heavier snowpacks (like 1992), the runoff ratio has been as high as 0.8. If the snowmelt period is prolonged due to cold spells, both the volume of runoff and peak flows are reduced due to delayed melt and losses back to the atmosphere.

Hydrological cycle on the North Slope of Alaska 235

Runoff ratios in the summer are also high compared to more temperate watersheds (Lilly et al, 1998; McNamara et al, 1998). Over the period of record, they have averaged 0.4 for Imnavait watershed and 0.7 for the Upper Kuparuk watershed. When contrasted with snowmelt runoff, it can be seen that these two basins reverse roles, with Imnavait Creek having high runoff during snowmelt and the Upper Kuparuk having high runoff during the summer. There are no proven hypotheses as to why this occurs, but the reversal is generally consistent from year to year. The Upper Kuparuk has a much greater elevation range, steeper slopes, and considerably less vegetation in higher elevations with less well-developed soils, and more extensive bedrock outcrops. Also, long after the peak snowmelt runoff event, there is still considerable snow in the basin in deep snowdrifts and at higher elevations. In Imnavait catchment, the elevation range is less than 100 m and the snowpack essentially melts all at once, with deeper snowdrifts melting last. High summer runoff ratios are more likely in both watersheds when a few intense storms occur vs numerous less intense storms.

Typically one-third of the annual precipitation is winter SWE and the remainder comes during the summer months. For Imnavait watershed, almost one-half (47%) of the annual runoff comes from snowmelt, and for the Upper Kuparuk basin it is one-quarter (24%). The long-term averages of measured precipitation estimates from gauges and snow surveys are fairly similar for both sites over the period of record: 116 mm vs 112 mm SWE, 239 mm vs 262 mm summer precipitation, and 353 mm vs 376 mm total precipitation for Imnavait Creek and Upper Kuparuk watersheds, respectively.

Using the ET estimates from equation (2), 50% of the precipitation falling on Imnavait Creek leaves through the evaporation and transpiration processes. For the Upper Kuparuk watershed, this same statistic is 36%o. It is possible that the much steeper slopes without vegetation in the headwaters of the Kuparuk watershed produce less ET and consequently greater runoff.

Acknowledgements This work was funded by the National Science Foundation grants OPP-9214927, OPP-9318535, OPP-9814984, OPP-0229705 and OPP-0229938. Some of the early data collection was funded by the US Department of Energy. Any assertions expressed here are those of the authors and not necessarily those of the funding agency. We would like to thank all of the students and research associates that have helped to collect data over the years.

REFERENCES

Bowling, L. C , Kane , D. L., Gieck, R. E., Hinzman, L. D. & Lettenmaier , D. P. (2003) The role of surface storage in a low-gradient Arct ic watershed. Water Resour. Res. 39(4) , 1087.

Hinzman, L. D., Kane, D. L., Gieck, R. E. & Everett, K. R. (1991) Hydrologie and thermal properties o f t h e active layer in the Alaskan Arct ic . Cold Regions Sci. Technol. 19, 9 5 - 1 1 0 .

Hinzman, L. D., Kane, D. L., Benson, C. S. & Everett , K. R. (1989) Hydrology of Imnavait Creek: an Arct ic watershed. Holarctic Ecol. 12, 2 6 2 - 2 6 9 .

Hinzman, L. D., Kane, D. L., Benson, C. S. & Everett , K. R. (1996) Thermal and hydrologie processes in the Imnavait creek watershed. In: Landscape Function: Implications for Ecosystem Response to Disturbance. A Case Study in Arctic Tundra (ed. by J. Reynolds & J. Tenhunen) 131—154. Springer-Verlag, Ecologie Studies Series.

House , P . K. & Pearthree, P. A. (1995) A géomorphologie and hydrologie evaluation of an extraordinary flood discharge est imate: Bronco Creek, Arizona. Water Resour. Res. 3 1 , 3 0 5 9 - 3 0 7 3 .

236 Douglas L. Kane et al.

Johnson, B. C , Irving, K., Hinzman, L. D. & Kane, D. L. (2002) Integration of remote weather stations with advanced telemetry options and remote image acquisit ion. In: 2002 ISSW (Proc. 2002 International Snow Science Workshop , Penticton, British Columbia) , A12-AX6.

Kane, D. L., Gieck, R. E. & Hinzman, L. D. (1990) Evapotranspirat ion from a small Alaskan Arctic watershed. Nordic Hydrol. 2 1 , 2 5 3 - 2 7 2 .

Kane, D. L., Hinzman, L. D., Benson, C. S. & Liston, G. E. (1991) Snow hydrology of a headwater Arct ic Basin: 1. physical measurements and process studies. Water Resour. Res. 27 ( 6 ) , 1099-1109.

Kane, D. L., Hinzman, L. D., McNamara , J. P., Zhang, Z. & Benson, C. S. (2000) An overview of a nested watershed

study in Arct ic Alaska. Nordic Hydrol. 31 (4 /5 ) , 2 4 5 - 2 6 6 .

Kane, D. L., McNamara , J. P. , Yang, D., Olsson, P. Q. & Gieck, R. E. (2003) A n extreme rainfall/runoff event in Arct ic

A l a s k a . / . Hydromet. 4 (6) , 1220-1228 .

Lachenbruch, A. H. & Marshal l , B . V. (1986) Changing cl imate: geothermal evidence from permafrost in the Alaskan Arct ic . Science 2 3 4 , 6 8 9 - 6 9 6 .

Lilly, E. K., Kane, D. L., Gieck, R. E. & Hinzman, L. D. (1998) Annual water balance for three nested watersheds on the north slope of Alaska. In: Permafrost (Proc. seventh Int. Conf., Yellowknife, Canada) , 669-67'4.

Lynch, A. H., Slater, A. G. & Serreze, M . C. (2001) The Alaskan Arctic frontal zone: forcing by orography, coastal contrast , and the boreal forest. J. Climate 14 (23), 4 3 5 1 - 4 3 6 2 .

McNamara , J. P., Kane, D. L. & Hinzman, L. D. (1998) An analysis of s treamflow hydrology in the Kuparuk River Basin,

Arct ic Alaska: a nested watershed approach. J. Hydrol. 2 0 6 , 3 9 - 5 7 .

McNamara , J. P., Kane, D. L. & Hinzman, L. D. (1999) A n analysis of an Arctic channel network using a digital elevation

model . Geomorphology 29 , 3 3 9 - 3 5 3 .

Olsson, P . Q., Hinzman, L. D., Sturm, M. , Liston, G. E. & Kane, D. L. (2002) . Surface cl imate and snow-weather relationships of the Kuparuk Basin on Alaska ' s Arct ic Slope. US Army Corps of Engineers, Technical Report ERDC/CRRELTR-02-W, 42pp.

Osterkamp, T. E. & Payne, M . W. (1981) Estimates of permafrost thickness from well logs in northern Alaska. Cold

Regions Sci. Technol. 5, 13 -27 .

Priestley, C. H. B . & Taylor, R. J. (1972) On the assessment of surface heat flux and evaporat ion using large-scale

parameters . Mon. Wealh. Rev. 100 , 8 1 - 9 2 .

Walker , D. A., Binnian, E., Evans, B . M. , Lederer, N . D., Norstrand, E. & Weber , P. J. (1989) Terrain, vegetation and

landscape evolution of the R 4 D research site, Brooks Range Foothil ls , Alaska. Holarctic Ecol. 12 , 2 3 8 - 2 6 1 .

Walsh, J. E., Lewis , E. L., Jones, E. P., Lemke, P., Prowse , T. D. & Wadhams , P. (eds) (2000) Global a tmospheric circulation patterns and relationships to Arct ic freshwater fluxes. N A T O Science Series. Partnership Sub-series, 2 , Environ. Security 7 0 , 2 1 - 4 3 . Kluwer Academic Publishers, Dordrecht , The Nether lands .

Yang, D. , Goodison, B . E., Benson, C. S. & Ishida, S. (1998) Adjustment of daily precipitat ion at 10 climate stations in Alaska: application of W M O Intercomparison Results . Water Resour. Res. 34 ( 2 ) , 2 4 1 - 2 5 6 .

Yang, D. , Kane, D. L., Hinzman, L. D., Goodison, B. E., Metcalfe, J. R., Louie, P . Y. T., Leavesley, G. H., Emerson, D. G. & Hanson, C. L. (2000) An evaluation of the Wyoming gauge system for snowfall measurement . Water Resour. Res. 36 (9 ) , 2 6 6 5 - 2 6 7 7 .