Subsurface storage capacity influences climate–evapotranspiration interactions in three western United States catchments

C. L. Tague,E. S. Garcia

doi:10.5194/hess-19-4845-2015

Abstract

Abstract. In the winter-wet, summer-dry forests of the western United States, total annual evapotranspiration (ET) varies with precipitation and temperature. Geologically mediated drainage and storage properties, however, may strongly influence these relationships between climate and ET. We use a physically based process model to evaluate how plant accessible water storage capacity (AWC) and rates of drainage influence model estimates of ET–climate relationships for three snow-dominated, mountainous catchments with differing precipitation regimes. Model estimates show that total annual precipitation is a primary control on inter-annual variation in ET across all catchments and that the timing of recharge is a second-order control. Low AWC, however, increases the sensitivity of annual ET to these climate drivers by 3 to 5 times in our two study basins with drier summers. ET–climate relationships in our Colorado basin receiving summer precipitation are more stable across subsurface drainage and storage characteristics. Climate driver–ET relationships are most sensitive to subsurface storage (AWC) and drainage parameters related to lateral redistribution in the relatively dry Sierra site that receives little summer precipitation. Our results demonstrate that uncertainty in geophysically mediated storage and drainage properties can strongly influence model estimates of watershed-scale ET responses to climate variation and climate change. This sensitivity to uncertainty in geophysical properties is particularly true for sites receiving little summer precipitation. A parallel interpretation of this parameter sensitivity is that spatial variation in storage and drainage properties are likely to lead to substantial within-watershed plot-scale differences in forest water use and drought stress.

Highlights

In high-elevation forested ecosystems in the western US, the majority of precipitation falls during the winter; there is often a disconnect between seasonal water availability and growing seasonal water demand
Watersheds (CO-ROC and California Sierras (CA-SIER), correlations and p values reported in Table 3) where the years of highest annual P are correlated with the years of greatest annual ET
We find that the threshold value of accessible water storage capacity (AWC) varies across basins and is substantially higher in Colorado Rocky Mountains (CO-ROC) (265 mm) as compared to CA-SIER (195 mm) and Oregon Cascades (OR-CAS) (190 mm) (Fig. 5)

Summary

Introduction

In high-elevation forested ecosystems in the western US, the majority of precipitation falls during the winter; there is often a disconnect between seasonal water availability and growing seasonal water demand Forests in these regions are frequently water-limited, even when annual precipitation totals are high (Boisvenue and Running, 2006; Hanson and Weltzin, 2000). This disconnect between water inputs and energy demands highlights the importance of storage of winter recharge by both snowpack and by soils. Field and modeling studies have shown that the years with greater snowpack accumulation can be a strong predictor of vegetation water use and productivity for sites in the California Sierra (Tague and Peng, 2013; Trujillo et al, 2012)

Methods

Results

Discussion

Conclusion