Changing Water and Nitrogen Use Efficiency over Agricultural Lands of the Inland Pacific Northwest During the 21th Century: Implications for Adaptation and Mitigation

Abstract:

As water is the primary resource limitation for cropping systems over the inland Pacific Northwest (PNW), water use efficiency impacts regional water availability, crop yields, and net carbon sequestration. Furthermore, nitrogen (N) use efficiency affects the cost of farming and the total N flux to the environment (including leaching to aquatic ecosystems and greenhouse gas emissions to the atmosphere). Climate change affects water and nitrogen use efficiencies due to the combined effects of warming (reducing snowpack water storage, increasing ET, earlier leaf-on, shortening or lengthening plant growth season, etc.), the CO₂ fertilization effects (increasing net primary productivity and leaf-level water and energy use efficiencies for C3 crops), and extreme climate events (drought and flood). Cropland conservation management (rotation, tillage, irrigation, and fertilization) is widely practiced in this region for maintaining high productivity of agricultural lands. To reduce vulnerability to weather extremes and long-term climate change, management regimes will likely need to be adapted for a changing environment. Here, we applied the coupled macro-scale hydrologic and crop growth model (VIC-CropSyst) to study how climate change in the 21st century will change water and nitrogen use efficiencies over the PNW. Simulation experiments with different combinations of management options and climate scenarios are used for attributing effects of climate factors and management options on long-term trends and fluctuations on water and nitrogen use efficiency. Preliminary simulation results indicate that there is a trend of decreasing water and nitrogen use efficiency over the inner PNW domain during the 21th century because of increasing ET, a seasonal shift in water availability, and the intensification of extreme climate events. Effective managements, including no-tillage and conservational tillage and optimized irrigation can eliminate the decrease or even increase water and nitrogen use efficiencies over certain areas but the overall effects are location- and crop-specific. The social and economic consequences of these adaptation and mitigation costs must be assessed before using this information to inform large-scale practices.