This is Section 2.4 of the Arctic Climate Impact Assessment Lead Author: Gordon McBean. Contributing Authors: Genrikh Alekseev, Deliang Chen, Eirik Førland, John Fyfe, Pavel Y. Groisman, Roger King, Humfrey Melling, Russell Vose, Paul H.Whitfield
The terrestrial water balance and hydrologic processes in the Arctic have received increasing attention, as it has been realized that changes in these processes will have implications for global climate.There are large uncertainties concerning the water balance of tundra owing to a combination of:
the sparse network of in situ measurements of precipitation and the virtual absence of measurements of evapotranspiration in the Arctic;
the difficulty of obtaining accurate measurements of solid precipitation in cold windy environments, even at manned weather stations;
the compounding effects of elevation on precipitation and evapotranspiration in topographically complex regions of the Arctic, where the distribution of observing stations is biased toward low elevations and coastal regions; and
slow progress in exploiting remote sensing techniques for measuring high-latitude precipitation and evapotranspiration.
Uncertainties concerning the present-day distributions of precipitation and evapotranspiration are sufficiently large that evaluations of recent variations and trends are problematic.The water budgets of arctic watersheds reflect the extreme environment. Summer precipitation plays a minor role in the water balance compared to winter snow, since in summer heavy rains cannot be absorbed by soils that are near saturation. In arctic watersheds, precipitation exceeds evapotranspiration, and snowmelt is the dominant hydrologic event despite its short duration. In the boreal forest, water balance dynamics are dominated by spring snowmelt; water is stored in wetlands, and evapotranspiration is also a major component in the water balance. Xu and Halldin suggested that the effects of climate variability and change on streamflow will depend on the ratio of annual runoff to annual precipitation, with the greatest sensitivity in watersheds with the lowest ratios.
Permanent storage of water on land (2.4.1)
The great ice caps and ice sheets of the world hold 75% of the global supply of freshwater; of these, the Greenland Ice Sheet contains 2.85 million km3 of freshwater.The northern portions of mid-latitude cyclones carry most of the water that reaches arctic ice caps, with the result that precipitation generally decreases from south to north. Runoff often exceeds precipitation when ice caps retreat.The behavior of glaciers depends upon climate (see section 6.5).
Temperature and precipitation variations influence the arctic ice caps; for example, temperature increases coupled with decreased precipitation move the equilibrium line (boundary between accumulation and ablation) higher, but with increased precipitation, the line moves lower. Small shifts in precipitation could offset or enhance the effect of increasing temperatures. Water is also stored in permanent snowfields and firn (compact, granular snow that is over one year old) fields, perched lakes (lakes that are raised above the local water table by permafrost), and as permafrost itself. Whitfield and Cannon implicated shifts between these types of storage as the source of increases in arctic streamflow during recent warmer periods.The IPCC stated: “Satellite data show that there are very likely to have been decreases of about 10% in the extent of snow cover since the late 1960s, and ground-based observations show that there is very likely to have been a reduction of about two weeks in the annual duration of lake and river ice cover in the mid- and high latitudes of the Northern Hemisphere, over the 20th century”.
Hydrology of freshwater in the Arctic (2.4.2)
The Arctic has four hydrologic periods: snowmelt; outflow breakup period (several days in length but accounting for 75% of total annual flow); a summer period with no ice cover and high evaporation; and a winter period where ice cover thicker than 2 m exists on lakes. Four types of arctic rivers show different sensitivity to climatic variations:
- Arctic–nival: continuous permafrost where deep infiltration is impeded by perennially frozen strata, base flow and winter flow are low, and snowmelt is the major hydrologic event.
- Subarctic–nival: dominated by spring snowmelt events, with peak water levels often the product of backwater from ice jams. Groundwater contributions are larger than those in arctic–nival systems. In some areas, complete winter freezing occurs.
- Proglacial: snowmelt produces a spring peak, but flows continue throughout the summer as areas at progressively higher elevations melt. Ice-dammed lakes are possible.
- Muskeg: large areas of low relief characterized by poor drainage. Runoff attenuation is high because of large water-holding capacity and flow resistance.
Fens (peatlands) are wetlands that depend upon annual snowmelt to restore their water table, and summer precipitation is the most important single factor in the water balance. Actual evapotranspiration is a linear function of rainfall. If summer rainfall decreases, there would be an increase in the severity and length of the water deficit.Water balance has a significant effect on the carbon budget and peat accumulation; under drier conditions, peatlands would lose biomass, and streamflows would decrease. Krasovskaia and Saelthun found that monthly flow regimes in Scandinavia have stable average patterns that are similar from year to year. They demonstrated that most rivers are very sensitive to temperature rises on the order of 1 to 3 ºC, and that nival (snow-dominated) rivers become less stable while pluvial (rain-dominated) rivers become more stable. Land storage of snow is important in the formation of the hydrograph in that the distributed nature of the snow across the land “converts” the daily melt into a single peak. Kuchment et al. (2000) modeled snowmelt and rainfall runoff generation for permafrost areas, taking into account the influence of the depth of thawed ground on water input, water storage, and redistribution.
Where they exist, perennial snow banks are the major source of runoff, and as little as 5% of watershed area occupied by such snow banks will enhance runoff compared to watersheds without them.The resulting stream discharge is termed “proglacial”, and stored water contributes about 50% of the annual runoff. During winter, when biological processes are dormant, the active layer freezes and thaws. Spring snowmelt guarantees water availability about the same time each year, at a time when rainfall is minimal but solar radiation is near its maximum. Summer hydrology varies from year to year and depends upon summer precipitation patterns and magnitudes. Surface organic soils, which remain saturated throughout the year (although the phase changes), are more important hydrologically than deeper mineral soils. During dry periods, runoff is minimal or ceases. During five years of observations at Imnavait Creek, Alaska, an average of 50 to 66% of the snowpack moisture became runoff, 20 to 34% evaporated, and 10 to 19% added to soil moisture storage. All biological activity takes place in the active layer above the permafrost. Hydraulic conductivity of the organic soils is 10 to 1000 times greater than silt. Unlike the organic layer, the mineral layer remains saturated and does not respond to precipitation events. Soil properties vary dramatically over short vertical distances.The snowmelt period is brief, lasting on the order of 10 days, and peak flow happens within 36 hours of the onset of flow.
Evapotranspiration is similar in magnitude to runoff as a principal mechanism of water loss from a watershed underlain by permafrost.Water balance studies indicate that cumulative potential evaporation is greater than cumulative summer precipitation.
Snowmelt on south-facing slopes occurred one month earlier than on north-facing slopes in subarctic watersheds. On south-facing slopes, the meltwater infiltrated and recharged the soil moisture but there was neither subsurface flow nor actual runoff. The north-facing slopes had infiltration barriers, thus meltwater was impounded in the organic layer and produced surface and subsurface flows. Permafrost slopes and organic horizons are the principal controls on streamflow generation in subarctic catchments. Seppälä (1997) showed that permafrost is confining but not impermeable. Quinton et al. found that in tundra, subsurface flow occurs predominantly through the saturated zone within the layer of peat that mantles hill slopes, and that water flow through peat is laminar.
Beltaos showed that temperature increases over the past 80 years have increased the frequency of mild winter days, which has augmented flows to the extent that they can affect breakup processes.There are several implications of this change, including increases in the frequency of mid-winter breakup events; increased flooding and ice-jam damages; delayed freeze-up dates; and advanced breakup dates. Prowse and Beltaos (2002) suggested that climate change may alter the frequency and severity of extreme ice jams, floods, and low flows. These climate-driven changes are projected to have secondary effects on fluvial geomorphology; river modifying processes; aquatic ecology; ice-induced flooding that supplies water and nutrients to wetlands; biological templates; dissolved oxygen depletion patterns; transportation and hydroelectric generation; and ice-jam damage.
The hydrology and the climate of the Arctic are intricately linked. Changes in temperature and precipitation directly and indirectly affect all forms of water on and in the landscape. If the storage and flux of surface water changes, a variety of feedback mechanisms will be affected, but the end result is difficult to project. Snow, ice, and rivers are considered further in Chapters 6 and 8.
Chapter 2: Arctic Climate - Past and Present
2.2 Arctic atmosphere
2.3 Marine Arctic
2.4 Terrestrial Water Balance
2.5 Influence of the Arctic on global climate
2.6 Arctic climate variability in the twentieth century
2.7 Arctic climate variability prior to 100 years BP
2.8 Summary and key findings of ACIA on Arctic Climate - Past and Present
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