The magnitude and variability of lateral seepage in California rice fields

Abstract

Lateral seepage can be an important water loss pathway at the field level and a conduit for the discharge of pesticides and nutrients from rice fields. However, few studies have directly measured lateral seepage rates in flooded rice fields. This study sought to characterize the magnitude and variability of lateral seepage in California rice fields, and to explore the relationship between lateral seepage and soil properties or hydrologic conditions. Lateral seepage was measured during the growing season at 50 locations spread across six rice fields using a methodology that operates analogously to a double-ring infiltrometer. Lateral seepage rates varied over four orders of magnitude (0.05–33.11 cm2 h−1 or 0.0011–1.05 cm h−1), though even the highest rate measured was small compared to typical water inputs in California rice fields, and lateral seepage corresponded to only 1.0%–1.9% of water inputs at sites where irrigation inputs were measured. Lateral seepage was negatively correlated with levee width (p = 0.025) and the relative water height in the adjacent field, supply canal, or drainage ditch (p = 0.045), though this relationship explained little of the variation in lateral seepage rates (marginal r2 = 0.129). Contrary to expectations, not all supply canals or higher flooded fields were a source for lateral seepage into fields, as unsaturated zones present in most levees served as a sink for lateral seepage from both sides of the levee. Future research should examine the prevalence of preferential flow pathways in rice field levees and the potential for transport of nutrients or agrochemicals through these flow pathways.