Geohydrologic data and a ground-water flow model were used to calculate a water budget and evaluate the contribution of regional groundwater flow to on-farm drainflow in a part of the western San Joaquin Valley, California. Regional ground-water flow is affected by the distribution of unconsolidated coarse- and fine-grained sediment. Predominantly coarse-grained sediment in the upslope areas results in a water table greater than 3 meters below land surface, but the low-lying areas are underlain by predominantly fine-grained sediments and have a water table within 3 meters of land surface. The vertical component of flow is downward in the upslope areas, but can be upward at some locations in the low-lying areas.
Results of model simulations indicate that about 18.5x106 cubic meters per year of drainflow originates as recharge within the fields that overlay the drainage systems (89 percent), and 2.3x106 cubic meters per year of drainflow is lateral-flowing ground water and upward-moving deep percolation originating as recharge within fields upslope of the drainage systems (11 percent). The drainage systems that intercept this upslope recharge overlay predominantly coarse-grained sediment associated with old stream channels. This ground water can move upward from depths greater than 29 meters below land surface and distances as great as 3.6 kilometers, requiring from 10 to more than 90 years to reach the drainage systems.
