Sensitivity of Simulated Mountain Block Hydrology to Subsurface Conceptualization

Mountain block systems are critical to water resources and have been heavily studied and modeled in recent decades. However, due to lack of field data, there is little consistency in how models represent the mountain block subsurface. While there is a large body of research on subsurface heterogeneity, few studies have evaluated the effect that common conceptual choices modelers make in mountainous systems have on simulated hydrology. Here we simulate the hydrology of a semi‐idealized headwater catchment using six common conceptual models of the mountain block subsurface. These scenarios include multiple representations of hydraulic conductivity decaying with depth, changes in soil depth with topography, and anisotropy. We evaluate flow paths, discharge, and water tables to quantify the impact of subsurface conceptualization on hydrologic behavior in three dimensions. Our results show that adding higher conductivity layers in the shallow subsurface concentrates flow paths near the surface and increases average saturated flow path velocities. Increasing heterogeneity by adding additional layers or introducing anisotropy increases the variance in the relationship between the age and length of saturated flow paths. Discharge behavior is most sensitive to heterogeneity in the shallow subsurface layers. Water tables are less sensitive to layering than they are to the overall conductivity in the domain. Anisotropy restricts flow path depths and controls discharge from storage but has little effect on governing runoff. Differences in the response of discharge, water table depth, and residence time distribution to subsurface representation highlight the need to consider model applications when determining the level of complexity that is needed. Plain Language Summary Computer models are a commonly used tool to understand and predict the behavior of mountainous watersheds. However, modeling groundwater is uniquely challenging in these systems because groundwater flow is strongly influenced by the geometry of the bedrock, yet observations of depth to bedrock and measurements of hydraulic properties with depth are infrequent. Modelers commonly rely on simplifying assumptions to represent the geologic layering in mountainous watersheds. However, the impact of these assumptions on simulated hydrology is rarely evaluated. Our study compares six commonly used representations of the subsurface to explore how basic choices in model construction influence the resulting hydr