Modeling the Atmospheric Response to Irrigation in the Great Plains. Part I: General Impacts on Precipitation and the Energy Budget

Abstract

Since World War II, the expansion of irrigation throughout the Great Plains has resulted in a significant decline in the water table of the Ogallala Aquifer, threatening its long-term sustainability. The addition of near-surface water for irrigation has previously been shown to impact the surface energy and water budgets by modifying the partitioning of latent and sensible heating. A strong increase in latent heating drives nearsurface cooling and an increase in humidity, which has opposing impacts on convective precipitation. In this study, the Weather Research and Forecasting Model (WRF) was modified to simulate the effects of irrigation on precipitation. Using a satellite-derived fractional irrigation dataset, grid cells were divided into irrigated and nonirrigated segments and the near-surface soil layer within irrigated segments was held at saturation. Nine April-October periods (three drought, three normal, and three pluvial) were simulated over the Great Plains. Averaging over all simulations, May-September precipitation increased by 4.97 mm (0.91%), with localized increases of up to 20%. The largest precipitation increases occurred during pluvial years (6.14 mm; 0.98%) and the smallest increases occurred during drought years (2.85 mm; 0.63%). Precipitation increased by 7.86 mm (1.61%) over irrigated areas from the enhancement of elevated nocturnal convection. Significant precipitation increases occurred over irrigated areas during normal and pluvial years, with decreases during drought years. This suggests that a soil moisture threshold likely exists whereby irrigation suppresses convection over irrigated areas when soil moisture is extremely low and enhances convection when antecedent soil moisture is relatively high.

The Effects of Great Plains Irrigation on the Surface Energy Balance, Regional Circulation, and Precipitation

“…Noah has been shown to overestimate latent heat flux somewhat in highly vegetated regions [43] and wet soil conditions [44], which potentially could lead to underestimating sensible heat flux and depleting soil moisture too rapidly. Despite this latent heat flux bias, Noah has nevertheless been used successfully in a variety of irrigation simulations ranging from a stand-alone configuration forced with atmospheric conditions [45] to fully-interactive mesoscale model simulations [20,21].…”

“…They also noted that approximately 10% of the water evaporated by irrigation was locally recycled. Recent comprehensive modeling studies confirm that irrigation is accompanied by substantial increases in evapotranspiration and smaller increases in precipitation [20,21].…”

Simulation of semi-arid biomass plantations and irrigation using the WRF-NOAH model – a comparison with observations from Israel

“…Schemes have been devised by others independently though and incorporated into WRF for impact studies (see Harding and Snyder, 2012;Sridhar, 2013). Sridhar for instance simulated two kinds of surface irrigation: flood and sprinkler systems.…”

Crop growth and irrigation interact to influence surface fluxes in a regional climate-cropland model (WRF3.3-CLM4crop)

“…Cropland management, such as irrigation, has been found to affect climate through changes in water and energy budg-ets (Adegoke et al 2003;Cook et al 2011;Harding and Snyder 2012b;Jin and Miller 2011;Ozdogan and Salvucci 2004;Sorooshian et al 2011). Irrigation's impacts on energy budgets are complex.…”