Evaluation and comparison of OpenET models for estimating soil water depletion of irrigated alfalfa in Arizona

Growers in arid regions often estimate soil water depletion (Dr) of the root zone in making irrigation scheduling decisions. While growers might use soil water sensors, actual Dr in fields may be only vaguely assessed. In this study, daily crop evapotranspiration (ETsat) data from the six OpenET satellite-based models (ALEXI/DisALEXI, eeMETRIC, geeSEBAL, PT-JPL, SIMS, SSEBop) and the OpenET ensemble were incorporated into a soil water balance (SWB) to estimate Dr (Dr sat) for alfalfa (Medicago sativa L.) within a 46-ha, center-pivot field in Buckeye, Arizona. A daily SWB for each model input the average ETsat data acquired at four locations for both 30-m and 90-m pixel sizes, assuming an alfalfa root-zone of 1.8-m. Net irrigation (In) inputs were based on gross irrigation amounts, modified for evaporation losses using weather data from the AZMET station in Buckeye. Dr sat values were compared to the average observed root-zone Dr (Dr obs), as determined at the four locations by neutron moisture meter measurements on 21 dates between 05/23/2023 and 12/11/2023. Cumulative values were 841 mm for In plus effective precipitation, 776 mm (ALEXI/DisALEXI) to 1130 mm (SSEBop) for ETsat, and 816 mm of ET obtained by a seasonal SWB (ETswb) based on the average change in Dr obs (ΔDr obs) from first to last measurements. Average Dr obs fluctuated between 59 and 133 mm, and ΔDr obs was −25.0 ± 42.0 mm. Model variations in ETsat resulted in different estimates of Dr sat and agreements, where mean bias error (MBE) ranged from ≈ -35 % (PT-JPL) to 195 % (SSEBop). Despite notable uncertainties in the SWB parameters and observed data, the SWB based on both the ensemble and ALEXI/DisALEXI ETsat capably tracked Dr obs for most of the observations, excluding those made from late-July to mid-August. However, the best overall agreement was with the ensemble data, where differences between cumulative ETsat and ETswb and ΔDr sat and ΔDr obs were small, and MBE for Dr sat was less than 9 % and 15 % at 30-m and 90-m pixels, respectively. Findings suggest that using the single value OpenET ensemble daily ET could be a reliable source for estimating the Dr in arid climate alfalfa fields.