Elucidating the Interactions Between Rice Cultivation and Groundwater in California.

Rice (Oryza sativa) is grown on over 150 million hectares worldwide and irrigated rice accounts for over 75% of all rice production. Rice is unique from other agronomic crops in that it is typically cultivated under flooded conditions, which create the potential for more complex interactions between irrigation water and groundwater. Aboveground water flows (inflow, outflow, and evapotranspiration) have been well quantified for California rice systems by recent research, but much less is known about subsurface water flows – namely, lateral seepage and deep percolation. This was illustrated by the recent California drought, with anecdotal evidence suggesting increased water input requirements per unit area, likely due to increases in either lateral seepage or deep percolation. In 2016 we measured inflow, outflow, evapotranspiration, lateral seepage, and deep percolation in three typical commercial rice fields in California’s Sacramento Valley to develop a water balance for these fields. Deep percolation and lateral seepage were both measured directly with infiltration rings or frames. Head gradients that indicate the direction of water flow within the saturated soil profile were also monitored. Preliminary results show that the groundwater connects to the floodwater at some sites, whereas at other sites an unsaturated zone persists between the floodwater and the groundwater for much or all of the growing season. Head gradients show downward flow at sites where the entire profile is saturated, whereas in sites where an unsaturated zone between the floodwater and the groundwater exists, there is flow toward this zone from the groundwater below and from the floodwater above. However, these flows are small given the magnitude of the head gradients observed and the low saturated hydraulic conductivity that is usually associated with heavy clay soils.