Impacts of climate change and irrigation management strategies on soil moisture, evapotranspiration, irrigation water availability, and crop productivity

Soil moisture and evapotranspiration (ET) are key components of the terrestrial water and energy cycles. These variables are tightly coupled and potentially affected by climate change and shifting agricultural practices. Variations in the magnitude of ET and soil moisture change the partitioning among components of water and energy cycles and impact water availability and agricultural productivity. This investigation aims at better understanding the impacts of climate change and irrigation management (i.e. different strategies for applying deficit irrigation, different types of irrigation systems, and different irrigation frequencies) on soil moisture, ET, irrigation water availability, and agricultural productivity in the Yakima River Basin (YRB) in central Washington State. This basin is an important component of the state's agricultural economy. Warming will exacerbate current water shortages in this snowmelt-dominated basin as streamflow seasonality shifts away from the summer season of peak irrigation demand and into the winter season, and as irrigation demands increase. Our research approach involves tightly coupling two process-based models: a spatially-explicit macroscale hydrologic model, the Variable Infiltration Capacity (VIC) model, and a cropping system model, CropSyst. The VIC model simulates all the hydrological phenomena including soil moisture, while CropSyst simulates transpiration from vegetation and crop growth and phenology. ET includes 4 components: crop transpiration (Tc), Soil water evaporation (Es), evaporation of canopy-intercepted water (Ec), and evaporation from droplets during irrigation (Ed). The analysis suggested an important change in the YRB water and energy balance as a result of climate change, and showed a high sensitivity of soil moisture and ET to irrigation management, with implications to water use policies to adapt to increasing water shortages in the region.