Evaporation Processes and Changes Over the Northern Regions

Evapotranspiration (ET) is a key component in global water and energy cycles. This chapter presents and discusses recent research advances about ET over northern regions and watersheds. ET in northern regions tends to increase with the decrease of latitude. The largest ET typically appears in the forest ecosystem, while the grasslands and shrublands have small ET. While the seasonal variations in ET are usually high, the interannual variability in annual ET is usually low over the Arctic regions. Sublimation from snow cover accounts for about 15–25% of winter precipitation. Many factors, such as soil moisture, vegetation type and productivity, and ecosystem features affect ET over northern regions. In addition, precipitation plays a key role in impacting ET. ET is more sensitive to precipitation in the early growing season than in the late growing season. Furthermore, changes in freeze–thaw processes due to warming also affect land surface conditions and the ET processes. During 1983–2005, ET increased significantly in the Arctic region with a rate of 3.8 mm decade-1 because of regional warming and vegetation greening. Such an increase in ET may exert significant impacts on the regional hydrology and water resources. Advanced models can simulate past ET change over the large northern watersheds. Remote sensing has provided new ET data and information that support climate and hydrology research and applications. There is a key question: Will Arctic landscapes become wetter or drier as climate changes? According to global models and data analyses, annual ET has increased over the northern regions. In the future, summer PE is projected to decrease much of Canada, increase over Alaska, decrease over the western and northern Eurasian subarctic, and increase over parts of northeastern Russia. Over most of these areas, the sign of the projected change is not robust across the models at the 95% confidence level. Many factors contribute to the uncertainty in the projected changes in Arctic surface wetness. There is certainly a need to better quantify and narrow the uncertainties in global models in the northern regions.