Using Rainfall Analysis to Manage Freeboard and Increase Rainfall Capture for Multiple-Inlet Rice Irrigation in the Lower Mississippi River Valley

AbstractResearch was conducted to help rice (Oryza sativa L.) farmers increase rainfall capture to offset groundwater use in the Lower Mississippi River Valley (LMRV). The LMRV is where a majority of US rice is produced and irrigated using groundwater withdrawn from the declining Mississippi River Valley Alluvial aquifer (MRVAA). Increasing rainfall capture in rice fields should reduce the region’s reliance on groundwater. Kolmogorov–Smirnov (K-S) frequency analysis was used to determine 50%–99% quantiles for daily rainfall occurring at nine LMRV locations. The rainfall quantiles were used as freeboard settings in a generalized water-balance model that simulates multiple-inlet rice irrigation (MIRI) of a 16-ha field. Compared with the case when freeboard was not intentionally managed, average irrigation use was reduced by up to 60 mm or approximately 10%. Average rainfall capture (RC) estimates increased from approximately 60% to 80% when at least 28-mm freeboard was maintained throughout an 86-day flood. Rainfall capture decreased when rain occurred in intense, closely-spaced events. Improved RC and application efficiency (AE) were each responsible for about half of the total irrigation use reductions estimated by the model. By maintaining at least 28-mm freeboard and promptly halting irrigation when the target flood depth is achieved, rice producers can reduce groundwater withdrawals, energy use, and runoff.