Modeled reconfiguration of root water uptake amplifies warming-induced subsurface storage depletion in a high elevation catchment

Increased transpiration with warming is a growing concern in the Upper Colorado River Basin (UCRB), as plants’ use of subsurface storage diverts water that may otherwise contribute to streamflow. Prior in-situ studies suggest that plants continue transpiring during dry and warm periods by shifting water uptake to deeper soil layers; however, models used to simulate the effects of warming rarely capture these complex subsurface processes. Here, we use the integrated hydrologic model ParFlow-CLM to simulate the effects of 1.5 o C of warming under interannual precipitation variability in a representative headwater catchment of the UCRB. We model these meteorological scenarios using two different water stress formulations – one that assumes plants are limited by water stress in the topsoil, and another that allows increased reliance on deeper root zone storage when the shallow soil dries out. Accounting for shifts in water uptake depth led to higher modeled transpiration, especially during low-precipitation periods, and amplified increases in transpiration with warming relative to the simplified stress formulation. As a result, summer subsurface storage declines with warming were larger when considering plants' sustained use of deeper root zone storage upon surface drying. The demonstrated effects of water stress formulation on transpiration and subsurface storage have important implications for future modeling studies that seek to understand the vulnerability of UCRB hydrology to growing interannual precipitation variability, and anthropogenic warming.