Using an integrated hydrology model to elucidate plant water use in a headwaters research catchment

Abstract

Mountain headwaters are vulnerable to change. Increases in annual average temperature, changes in seasonal precipitation and drought stress will continue to alter the dynamics of these delicate ecosystems. Despite its significance in the water budget, both the quantity and partitioning of evapotranspiration (ET) is poorly resolved. Restricted by discrete point observations, physical observations in mountain headwaters are challenging, limited by uncertainties and difficult to scale. Integrated, physically-based models are tools for dissecting the mechanisms driving evaporation and transpiration. Understanding mountain vegetation water use is an essential component for predicting the vegetative response to stress. This study is motivated by evidence of drought-induced tree mortality in some Sierran catchments, a high degree of uncertainty in mountain regolith groundwater storage, and the impacts of subsurface characterization on mountain ecohydrology (Holbrook et al. 2014; Jepsen et al. 2016). Using a physically-based integrated modeling approach, this study explores the role of lateral groundwater flow on the drought-tolerance of montane vegetation. Despite a 68.8% decrease in precipitation during the drought period, evapotranspiration only decreased by 26.5%. From the pre-drought period to the drought period the total change in groundwater storage decreased by 470%. Incorporating lateral groundwater flow increased transpiration partitioning (T/ET) from 44.9% to 59.0% in the pre-drought period and from 52.8% to 63.5% in the drought period. These results suggest that plant stress is mitigated under drought conditions because lateral flow of groundwater storage sustains transpiration.