Managing Global Resources for a Secure Future

Rice is grown on approximately half a million acres in California and it is the fourth largest agricultural water user in the state. The evapotranspiration or consumptive water use has been relatively well characterized for California rice production, but less is known about non-consumptive water losses like lateral seepage through border levees and percolation below the root zone. Previous work in California has relied on a water balance where inputs, tailwater drainage, and evapotranspiration were measured, leaving seepage and percolation to be calculated as the difference between inputs and outputs. Here we attempt to close the water balance through direct and indirect estimates of lateral seepage and percolation. Direct measurements of percolation were made with percolation rings in seven California rice fields that were distributed throughout the rice-growing region and indirect measurements were made in three rice fields using the subsurface hydraulic head gradients, the saturated hydraulic conductivity, and Darcy’s Law. Direct measurements of lateral seepage were also made in all seven of these fields by measuring the water loss from three-sided metal frames inserted into the border levees. The results provide the most reliable estimate to date of lateral seepage and percolation in California rice fields while the complete water balance highlights the importance of preferential flow pathways to subsurface water losses.