Groundwater and Nitrogen Recharge Model for the On-Farm Flood Flow Capture Project in California
Delta Stewardship Council, Delta Science Program
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On-Farm Flood Flow Capture (OFFC) is a method to divert flood flow to agricultural lands for groundwater recharge and downstream flood damage reduction. An OFFC pilot project was conducted at the Terranova Ranch in western Fresno County (36°34'27.18”N, 120°5'39.69”W) in the Central Valley in California on vineyards, which can be ponded in winter without severely damaging the crop. The Terranova Ranch has access to water in the adjacent James Bypass, which retains overflow from the Kings River in wet years. The OFFC experiments were conducted using water in the James Bypass in 2011 to assess its feasibility and cost effectiveness. Groundwater recharge timing and nitrate leaching potential from OFFC likely affects the project benefits and feasibility. Therefore, this study assessed groundwater and nitrate recharge quantities and timings for OFFC using a HYDRUS 1-D model. In addition, the possibility of nitrate capturing soil pores was assessed through field infiltration tests. The modeled results indicated an equilibrium transport without the consideration of soil pore immobilizing affect can predict nitrate transport well. Therefore, an equilibrium transport model was developed for the 60 m vadose zone with multiple OFFC event and irrigation scenarios, using James Bypass’s flow and climate data from 1983 to 2002. The model was calibrated with water contents monitored during the OFFC experiments in 2011 for the top 1.8 m soil. The model showed a quick increase (within 1 year) in groundwater recharge at the depth of 60 m when annual infiltration exceeds approximately 3 m, which is equivalent to 45 days of ponding at the project site. Nitrate recharge patterns varied as a function of deep vadose zone flow parameters and water application scenarios, but resulted in at least 70% of initial soil nitrate leached to groundwater by the end of modeling period in all scenarios. The findings indicate that an aquifer is recharged more quickly by applying OFFC to a small field for a longer period than applying it to a large field for a shorter period. Also it is important to select OFFC sites with smaller initial nitrate concentrations since most of the soil nitrate likely leaches to groundwater.