Changing Evapotranspiration over the Columbia River Basin in Response to Climate Change and Land Management

Evapotranspiration (ET) is a major factor that influences surface air temperature, soil moisture, and water resource availability. However, ET is one of the least understand terms in the water budget, particularly with respect to global change. Quantifying ET variability and long-term change in response to climate change and land management actions will improve not only our understanding of future water resource availability; it will also improve our knowledge of how ET may feed back to affect atmospheric processes, thus narrowing the uncertainty around projections of future climate. As a snow-dominated mountainous region, the Columbia River Basin (CRB) is facing substantial changes in snowpack, frozen soil, vegetation phenology, and their interactions under the warming condition. As a result, the spatial-temporal patterns of ET over this region will be transformed because of the variability in both atmospheric demand (i.e. potential ET) and water supply (soil moisture and surface water storage, including snowpack and canopy and litter intercepted water) over various climate zones and seasons (wet vs. dry and warm vs. cold). Climate change affects ET due to the combined effects of warming (reduced snowpack water storage, increasing ET, earlier leaf-on, shortening or lengthening plant growth season, etc.), the CO2 fertilization effect (increasing net primary productivity and leaf-level water and energy use efficiencies particularly for C3 crops), and extreme climate events. Cropland conservation management (rotation, tillage, irrigation, and fertilization) in this region also affects ET by changing the soil and surface hydrological properties and soil moisture. Here, we applied the coupled macro-scale hydrologic and crop growth model (VIC-CropSyst) to estimate how water use efficiency (crop yield per unit of ET and irrigated water) and the runoff/precipitation ratio (representing the total available blue water resources) is impacted under future climate and management scenarios. Preliminary simulation results indicate that there is a trend of increasing ET over the CRB during the 21th century because of increasing moisture demand, while most of the increases occurred over the cold/wet season which has increased moisture supply as well. Effective land management actions, including no-tillage and conservational tillage, earlier planting, and optimized irrigation can increase the water use efficiency, but the overall effects are location- and crop-specific. The decreasing ET in the warm season may also increase the risk of heat waves and hence further increase the water stress of vegetation.