Influence of Irrigation on Diurnal Mesoscale Circulations: Results

28 In order to understand the impact of irrigation on weather and climate, the 2018 Great Plains 29 Irrigation Experiment collected comprehensive observations straddling irrigated and non30 irrigated regions in southeast Nebraska. Using these observations, we examine how irrigation 31 affects diurnal terrain-generated slope circulations, specifically the slope wind. We find that 32 irrigation applied to upslope regions of gently sloping terrain reduces terrain-induced 33 baroclinicity and the associated pressure gradient force by up to two-thirds. This leads to the 34 reduction in the afternoon and evening upslope wind and is supported through comparisons to 35 the High-Resolution Rapid Refresh (HRRR) operational model, which does not explicitly 36 account for irrigation. Additionally, the presence of irrigation decreases daytime sensible heat 37 flux (Bowen ratio reduced 40% compared to non-irrigated regions), weakening turbulent 38 transport of momentum. Modifications to the terrain-forced circulation by irrigation has the 39 potential to affect moisture transport and thus cloud and precipitation formation over the Great 40 Plains. 41 42 Plain Language Summary 43 Agricultural irrigation alters input of heat and moisture from the land surface to the atmosphere, 44 which can affect weather and climate. Irrigation is expanding on all continents except Antarctica 45 and is thus a major pathway through which humans impact the environment. However, the 46 observations required to study the mechanisms though which irrigation affects weather and 47 climate are lacking. The Great Plains Irrigation Experiment (GRAINEX) was conducted to 48 collect such observations near the boundary between irrigated and non-irrigated regions in 49 southeast Nebraska. During the daytime, the slope of the Great Plains causes near-surface 50 upslope winds and downslope winds in the atmosphere above. At night this pattern reverses. This 51 wind system influences storm formation by forcing upward motion and transporting moisture. 52 Using the observations from GRAINEX and comparing to a weather model we find that 53 irrigation weakens this wind system, potentially affecting cloud and rain formation in this region. 54 55