Controls on flood managed aquifer recharge through a heterogeneous vadose zone: hydrologic modeling at a site characterized with surface geophysics

Abstract. In water-stressed regions of the world, managed aquifer recharge (MAR), process intentionally recharging depleted aquifers, is an essential tool for combating groundwater depletion. Many groundwater-dependent regions, including Central Valley in California, USA, are underlain by thick unsaturated zones (ca. 10 to 40 m thick), nested within complex valley-fill deposits that can hinder or facilitate recharge. Within saturated zone, interconnected coarse-grained material (sands and gravel) act as preferential pathways, while fine-textured facies (silts clays) accommodate majority long-term increase storage. However, this relationship more vadose zone. Coarse capillary barriers restrict flow, contrasts matric potential draw water from flow paths into fine-grained, low-permeability zones. To determine impact unsaturated-zone stratigraphic heterogeneity on MAR effectiveness, we simulate at a almond orchard surveyed with towed transient electromagnetic system. First, identified three outcomes interest sites: infiltration rate surface, residence time root zone saturated-zone efficiency, which defined storage induced MAR. Next, developed geostatistical approach parameterizing 3D variably model using geophysical data. We use resulting workflow evaluate perform Monte Carlo simulations quantify their uncertainty function input parameters spatial uncertainty. Model results show rapid rates contiguous blocks fine-grained sediments >20 % likely remain longer than trees tolerate. Simulations also reveal capillary-driven draws unsaturated, sediments, limiting efficiency. Two years after inundation, retain average 37 across all simulations, where it inaccessible either plants pumping wells. Global sensitivity analyses demonstrate each outcome most sensitive describe fine facies, implying future work reduce should focus characterizing sediments.