Transpiration Across Elevations: Modeling the Impact of Climate Change on Plant Groundwater Use at Mountainous Headwaters

Abstract

As temperatures have increased with a changing climate, streamflow in the Upper Colorado River Basin (UCRB) has declined, posing a severe threat to public well-being. Much of this downstream surface water originates as groundwater in mountainous catchments. As conditions warm, plants’ demand for water increases, potentially reducing groundwater's contributions to surface water downstream. Yet this response to warming is complicated by transpiration’s dependence on energy and water limitation, which varies spatially and temporally in topographically complex catchments of the UCRB.

This study quantifies the effect of warming on summer transpiration in the East-Taylor Watershed (ETW), a representative catchment of the UCRB, under conditions of varying energy and water limitation. ParFlow-CLM is used to model hydrologic and land surface processes with meteorological forcing input from a wet year (WY2017) and dry year (WY2018). Baseline temperature forcings are then uniformly increased by 1.5oC to explore the effects of projected warming alongside different precipitation inputs, resulting in four total simulations. Analysis focuses on transpiration and recharge, studying the spatial variation of these fluxes with land cover type and elevation. Results indicate that warming has a more substantial effect on plant groundwater use in the ETW during an energy limited water year. Analysis of daily soil moisture change across each root zone layer further reveals an increased reliance on deeper root zone moisture with warming, which is strongest under hot and dry conditions.