Overview of water balance determinations for high latitude watersheds

For a variety of reasons such as low population, extreme environments and logistical costs, efforts to enhance our understanding of the hydrological cycle of this region of the world have been thwarted. Both the daily encounters between the Earth's human population and available water resources here and the effects on the hydrological cycle of climate changes induced by our life style warrant our attention. We can, however, gain some insight into high latitude hydrological processes by compiling all the scattered water balance data sets that exist for this region. Results from 39 research watersheds collected in 20 papers in this volume with three other papers that highlight some of the significant problem areas in producing quality water balance results, are reviewed in this paper. Collectively, water balance data for over 500 years are offered here; temporally, these data are dispersed over the period from 1958 to the present. Spatially these data encompass the high latitudes from 44°N to 80°N, with a significant void in central Russia. It is generally known that with increasing latitude there is a decrease in precipitation ([sim ]16 mm year-1 degree-1 latitude for continental areas), although this trend is somewhat clouded by coastal stations. While the precipitation is decreasing, the ratio of runoff to precipitation increases in a northward direction. This is mostly due to the snowmelt event, with substantial runoff being the dominant hydrological event each year, and when significant precipitation events occur in the summer there is generally limited surface or subsurface storage. Evapotranspiration parallels precipitation with a decreasing trend with higher latitudes ([sim ]12 mm year-1 degree-1 latitude). Obviously this reduction is primarily due to lack of energy for phase change; in a few cases it is due to the lack of moisture for evapotranspiration. Where changes in storage are important, the difficulty of quantifying this variable is a challenge in water balance computations at time scales of less than a year.