Spatio-temporal covariation of evapotranspiration, plant productivity, and groundwater dynamics at the global scale

Abstract:

Over the last few decades, various modeling studies, mostly at relatively coarse spatial resolution (50-100 km), have shown that the groundwater dynamics can control evapotranspiration (ET) at regional and global scales. It is, hence, necessary to evaluate the spatial and temporal heterogeneities of land surface water, energy and carbon fluxes, and the underlying processes and mechanisms behind them, based on observational constraints.

In this study we, therefore, analyze the relationships between observation-based recent global estimates of water table depth (WTD), evapotranspiration (ET), and vegetation productivity, at 5 minutes (~10 km) resolution, to evaluate the controls of WTD on ET and gross primary productivity (GPP). For each climatological season, the partial correlations with WTD were calculated after controlling for confounding factors such as precipitation, net radiation, topography, soil porosity, and leaf area index among others.

Even after accommodating for collinearity with confounding variables, WTD explains as much as 50% of the spatial variability of ET in 16-23% of the world in different seasons. Depending on the availability of moisture and energy, spatial variability of ET in the same region shows large dependence on WTD in dry period, while the controlling factors could explain all the variability in wet period. In some regions with the vegetation characterized by higher water use efficiency, GPP shows stronger dependence on WTD in all seasons, even though ET shows little or no dependence.