Across Semiarid Ecosystems, Evapotranspiration Responds to Environmental Drivers Over Longer Timescales When Conditions are Dry

Abstract:

In semiarid regions, evapotranspiration (ET) represents a major component in the local and regional water cycle, but temporal relationships between environmental drivers (e.g., vapor pressure deficit [VPD], air temperature [T_air], shallow and deep soil moisture, and precipitation) and ET remains poorly constrained. We aim to compare the sensitivity and response timescales of ET to these drivers across semiarid ecosystems. We applied a cross-wavelet coherence (CWC) analysis to longterm data from 3 tree-dominated ecosystems (i.e., juniper savannah [US-Wjs], piñon-juniper woodland [US-Mpj], ponderosa pine forest [US-Vcp]) in the New Mexico Elevation Gradient (NMEG), a network of AmeriFlux sites that represents a gradient in elevation, temperature, precipitation, and ecosystem type characteristic of the southwestern U.S. CWC identifies timeframes over which time series with nonstationary periodicities are correlated. Across the full 7-year record available for all 3 sites, ET consistently responded to VPD and T_air at annual timescales but to soil moisture (shallow and deep) at shorter timescales (seasonal) at wetter sites and longer timescales (multi-year) at drier sites. Within and between water years, relationships between ET and drivers were highly variable. During dry years (e.g., 2011), ET at US-Wjs and US-Mpj were more strongly correlated with soil moisture at longer periods than during wet years (e.g., 2010; seasonal versus week to month). In contrast, ET at US-Vcp was more strongly correlated with soil moisture over shorter time periods in dry years than wet years (month versus seasonal). Across the 3 ecosystems ET response to VPD and T_air in wet years occurred at more varied time periods, suggesting that the ecosystems responded to changing conditions more quickly when moisture was relatively available. As the region becomes drier, quantifying the dynamic sensitivity of ET to drivers becomes increasingly important for projecting regional and local water balance.