Garrett Rapp, Laura E. Condon, and Katherine H. Markovich1
Department of Hydrology and Atmospheric Sciences
The University of Arizona
Mountain-block (MB) 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 MB subsurface. Few studies have evaluated the effect of these conceptualizations on simulated hydrology, and there is a need to compare various representations of the MB subsurface. In this study, we simulate the hydrology of a semi-idealized headwater catchment using six common conceptual models of the MB subsurface. These scenarios include multiple representations of hydraulic conductivity (K) decaying with depth, changes in soil depth with topography, and anisotropic geology. We evaluate flowpaths, discharge, and water tables to quantify the impact of subsurface conceptualization on hydrologic behavior. Our results show that adding higher-K layers in the shallow subsurface concentrates flowpaths, increasing the average saturated flowpath velocities. Increasing heterogeneity by adding additional layers or introducing anisotropy increases the variance in the relationship between the age and length of saturated flowpaths. 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 K in the domain. Variable soil depth affects simulated hydrology less than adding constant-thickness layers. Anisotropy restricts flowpath depths and controls discharge from storage but has little effect on governing runoff. Overall, some hydrologic variables appear more sensitive to subsurface conceptualizations than others. Results from this analysis can be used to make more informed decisions when building models of MB systems.
1US Geological Survey, Albuquerque, New Mexico