To close the water use budget in irrigated agricultural fields in flood plains with substantial riparian corridors, it is necessary to understand groundwater usage by dominant phreatophyte vegetation, particularly when the primary source of the water for irrigation comes from groundwater abstraction. We report here results of measurements of sap flow in phreatophyte vegetation in a riparian corridor, which is part of a watershed located along the coast in Santa Cruz County, California. The riparian corridor is within a study area of 75 to 140 meters wide in the lower portion of Scotts Creek watershed, which is bounded to the west by the Pacific Ocean. Canopy cover in the study area often approaches 100 percent, with dominant trees being red alder (Alnus rubra), arroyo willow (Salix lasiolepis), and pacific willow (Salix lasiandra var. lasiandra). Other trees include boxelder (Acer negundo), big leaf maple (Acer macrophyllum), and California bay laurel (Umbellularia californica). Common understory vegetation includes California blackberry (Rubus ursinus), stinging nettle (Urtica dioica subsp. gracilis), poison hemlock (Conium maculatum), Cape ivy (Delairea odorata), and Italian thistle (Carduus pycnocephalus). For the study reported here, only the two most dominant phreatophyte species, namely red alders and arroyo willows, were instrumented with sap flow sensors. In addition to diurnal fluctuations, sap flow data collected hitherto also shows expected seasonal variation with summer maxima and winter minima, with transition fall and spring periods. Sap flow measurements from the study area are projected across the entire riparian forest using sampled tree sapwood area and used to estimate forest evapotranspiration (ET). The ET is then used in a groundwater flow model to more accurately predict observed groundwater fluctuations and usage by riparian vegetation.