Deep Infiltration Model to Quantify Water Use Efficiency of Center-Pivot Irrigated Alfalfa

Water shortages in arid regions present challenges in administering water and requires robust water accounting. In southeast Idaho, the Eastern Snake Plain Aquifer (ESPA) supports an important agricultural sector. Due to connectivity between surface and groundwater in the ESPA, quantifying aquifer recharge is also important. Historically, leaching from excess surface irrigation supported incidental recharge to the ESPA, but more efficient irrigation techniques reduced incidental recharge. This paper outlines a deep infiltration (DI) model developed to evaluate infiltration losses from different irrigation practices and soil types. Twelve scenarios were created to simulate an alfalfa growing season under varying climatic and soil conditions. Under some scenarios, modeled infiltration losses increased by 10%-20% coincident with increased application efficiency. The concept of consumptive use efficiency (CUE) is introduced to quantify the proportion of irrigation beneficially used by crops. The model results show that CUE decreased with increasing application efficiency and suggest CUE could be improved 8%-10% for well-drained loamy soils; clay loam soils showed little opportunity for improvement. The results indicate that more efficient irrigation application techniques may increase DI loss if irrigation schedules do not explicitly include soil water storage for the entire rooting zone. These results indicate that in conditions where losses from DI can be reduced, improving water use efficiency depends on precision irrigation scheduling linked to infiltration rates. This model provides a practical method by which infiltration losses from irrigated lands can be estimated. Considering site-specific infiltration would facilitate and prioritize investments meant to improve water use efficiency. Irrigation in arid regions usually entails some degree of inefficiency, partly due to water lost through deep percolation. Although some drainage loss occurs under most real-world conditions, it is difficult to measure actual loss. A lack of real information hampers precise estimates and increases uncertainty about actual conveyance and irrigation efficiency. On the other hand, soil infiltration models can utilize existing soil survey maps and guide prioritization and incentives to encourage best practices. This study demonstrates a straightforward application of a soil water drainage model combined with regional soil maps, as could be implemented by water managers. The model was calibrated using observations of crop water use, irrigation application, and soil water content from an irrigated alfalfa field. The calibrated model was then used to describe potential drainage rates from common soil types and weather conditions in southeastern Idaho. The model results indicate that efficient irrigation practices may be best suited to well drained soils, and fewer benefits were observed in poorly drained soils. Even a relatively simple application of this model highlights conditions that are best suited to particular efficiency strategies and help prioritize limited resources for developing best practices for water conservation.