Irrigation’s effects on summer precipitation and temperature over the CONUS in 1979-2018

Abstract:

Many agricultural regions across the Continental United States (CONUS) have undergone intensive large-scale irrigation to improve agricultural production since the 1950s. In 2010, for instance, irrigated agriculture over the High Plains and the California Central Valley accounted for about 20% of the entire U.S. produced crop value, and consumed about 97% of the groundwater pumped from the High Plains Aquifer (HPA) and 80% of the water supply of California, respectively. Besides imposing groundwater depletion risk over many heavily pumped areas, this irrigation has also affected regional climate over similar time scales—especially during the growing seasons. While empirical evidence suggests irrigation modifies land-atmosphere interactions enough to detectably alter climate, there has been limited evidence from numerical models. To help fill this gap, we implemented a novel irrigation scheme within WRF (v4.0.3) to study irrigation’s effects on summer climate over the CONUS in the past 40 years (1979-2018). The irrigation scheme embedded within the WRF-Noah-Mosaic module dynamically responds to soil moisture deficits, including realistic limits on individual irrigation event application amounts and frequencies. For each year, paired May-August irrigation and no-irrigation simulations are conducted with identical model configurations and physics parameterizations, to identify the irrigation signal over the CONUS in June, July, and August. Irrigated area in the model varies annually to represent real-world changes using best-available data: MODIS-derived observations every five years since 2002 across the entire CONUS; the biannual irrigation map for California since 1984, and an annual irrigation map for HPA since 1984. The study emphasizes investigations of irrigation’s interactions with regional climate under abnormally wet and dry conditions on extreme events. Results show that in general, irrigation increases summer precipitation and reduces 2-m air temperature across the majority of the CONUS, which reflects irrigation’s dominant cooling mechanism during summer seasons, and matches the observational evidence of enhanced precipitation downwind of heavily irrigated agricultural regions for the past century. Irrigation’s effects on precipitation’s frequency over the heavily irrigated HPA and its downwind Northeast and Southeast regions vary dramatically, with distinct patterns revealed during abnormally wet and dry climate backgrounds. Contrasting responses are seen especially for extreme events (above 90th percentile for both precipitation and temperature), indicating variable thermodynamic processes interacting with different dominant synoptic atmospheric circulations. In general, the irrigation induced temperature change is likely a result of both irrigation and increased precipitation’s cooling effect, while effects on precipitation reflect a more complex dynamic and nonlinear interactions at both local and regional scales.