Estimating Seasonal Crop Water Consumption in Irrigated Lands Using Soil Moisture and Reference Evapotranspiration

The increase in population and the megadrought that the American west has been experiencing for the past two decades and for the foreseeable future are the cause of great stress on the region’s water resources. In an effort to mitigate the worst effects of water shortages many public and private agencies have been pursuing ways to reduce water use while maintaining the current status quo of living standards, industrial productivity, and agricultural yield. This project aims to study water use in irrigated lands dedicated to agriculture with the objective of quantifying crop water use, often referred to as evapotranspiration (ET), as this is among the most important metrics when it comes to increasing water use efficiency, i.e., increasing the amount of agricultural output per amount of water input. This study aims to develop a methodology that allows land managers to estimate evapotranspiration using familiar tools and data i.e., soil moisture sensors. The first study site in Vernal, Utah, was used to develop the proposed methodology because alongside the soil moisture data, accurate evapotranspiration measurements were available which allowed for the calibration and verification of the model. The second study site in Modena, Utah, in which accurate evapotranspiration measurements were also available, was used as a testing ground for the estimates provided by the proposed methodology allowing to assess whether local calibration was required or not. The third study site in West Weber, Utah, where two irrigation methods (drip and flood irrigation) were employed, was used as a practical example of how this methodology could be implemented in a real farm scenario and what insight could be gained from it. The proposed Soil Moisture based EvapoTranspiration (SMET) estimation model reliably provided accurate results (within ±7%) in the first two study sites where they could be compared with results from more sophisticated methods. In the third study site the SMET model offered previously hard to obtain insight on how the water use is influenced by the two irrigation techniques employed and on how water use can differ even within the same field. The encouraging results from this study prove that this user-friendly and low-cost crop water use estimation method can provide managers, producers, and policy makers with an accurate number, allowing for better-informed decisions regarding water conservation and maximizing the beneficial use of this limited resource.