Growing Season Precipitation Percolates to Groundwater Past Older Water in Storage Across a Temperate Agricultural Catchment

How water is stored within- and released from-the vadose zone controls groundwater recharge, plant water uptake, and the movement of dissolved solutes (nutrients, carbon, pollutants). The goal of this study was to determine the age of water recharging groundwater during the growing season in a temperate agricultural catchment. We measured soil moisture and bulk soil water isotopic compositions (δ¹⁸O) twice per month at three locations across a hillslope as well as groundwater and surface water δ¹⁸O near the catchment outlet from March through October. We then calibrated ecohydrological models to these data with two competing representations of vadose zone flow: two-pore domain flow (TPD) and well-mixed flow (WM). Measurements of moisture δ¹⁸O across the upper 40 cm of the soil profile and in surface and groundwater all supported selection of TPD over WM as the more likely representation of vertical water movement through the vadose zone. Calibration of the TPD model resulted in substantially different soil parameter estimates from that of the WM model. The TPD model indicated that growing season percolate to groundwater was composed of water 1–2 weeks old, whereas evapotranspiration (ET) was sourced from prior seasons. In contrast, the WM model suggested that both percolate and ET originated as precipitation from prior months. These results carry significant implications for conceptual and numerical modeling of the fate and transport of nutrients that are surface applied to agricultural fields. Our findings highlight a critical need for improved process representations of soil water transport in hydrological and ecohydrological models.