Simulating Water Use and Yield for Full and Deficit Flood-Irrigated Cotton in Arizona, USA

Improved irrigation guidelines are needed to maximize crop water use efficiency. Combining field data with simulation models can provide information for better irrigation management. The objective of the present study was to evaluate the effects of two flood irrigation treatments on fiber yield (FY) and quality during the 2023 and 2024 growing seasons in Maricopa, Arizona, USA. Two irrigation treatments, denoted as F100% and F80%, were arranged in a randomized complete block design with three replicates. Then, AquaCrop was used to simulate cotton yield (YTot), water use (ETobs), and total soil water content (WCTot) for the two irrigation treatments. Six statistical metrics, including the coefficient of determination (R2), the normalized root-mean-square error (NRMSE), the mean absolute error (MAE), simulation error (Se), the index of agreement (Dindex), and the Nash–Sutcliffe efficiency coefficient (NSE), were employed to assess model performance. The results of the field trial demonstrated that reducing the irrigation rate to 80% of ETc negatively impacted cotton FY and ET water productivity (ETWP); the FY declined by 45.2% (ETWP = 0.097 kg·ha−1) in 2023 and by 38.1% (ETWP = 0.133 kg·ha−1) in 2024. Conversely, F100% produced a more uniform and stronger fiber than F80%, with the uniformity index (UI) and fiber strength (STR) measuring 81.7% and 29.5 g tex−1 in 2023 and 82.2% and 30.0 g tex−1 in 2024, indicating that UI and STR were well correlated with soil water during both growing seasons. AquaCrop showed an excellent performance in simulating cotton CC during the two growing seasons. The R2, NRMSE, Dindex, and NSE were between 0.97 and 0.99, 8.45% and 14.36%, 0.98 and 0.99, and 0.96 and 0.98, respectively. Moreover, the AquaCrop model accurately simulated YTot during these seasons, with R2, NRMSE, Dindex, and NSE for pooled yield data of 0.93, 8.05%, 0.95, and 0.78, respectively. The model consistently overestimated YTot, ETobs, and WCTot, but within an acceptable Se (Se < 15%) during both growing seasons, except for WCTot under the 80% treatment in 2023 (Se = 26.4%). Consequently, AquaCrop can be considered an effective tool for irrigation management and yield prediction in arid climates such as Arizona.