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 (D r ) of the root zone in making irrigation scheduling decisions. While growers might use soil water sensors, actual D r in fields may be only vaguely assessed. In this study, daily crop evapotranspiration (ET sat ) 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 D r (D r 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 ET sat 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 (I n ) inputs were based on gross irrigation amounts, modified for evaporation losses using weather data from the AZMET station in Buckeye. D r sat values were compared to the average observed root-zone D r (D r 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 I n plus effective precipitation, 776 mm (ALEXI/DisALEXI) to 1130 mm (SSEBop) for ET sat , and 816 mm of ET obtained by a seasonal SWB (ET swb ) based on the average change in D r obs (ΔD r obs ) from first to last measurements. Average D r obs fluctuated between 59 and 133 mm, and ΔD r obs was −25.0 ± 42.0 mm. Model variations in ET sat resulted in different estimates of D r 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 ET sat capably tracked D r 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 ET sat and ET swb and ΔD r sat and ΔD r obs were small, and MBE for D r 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 D r in arid climate alfalfa fields. • OpenET models were incorporated into a soil water balance to estimate daily soil water depletion (D r ) for alfalfa. • OpenET ensemble daily ET could be a reliable source for estimating D r in arid climate alfalfa fields. • Future arid-land studies on OpenET are recommended, especially for alfalfa.