The role of rock moisture in supporting the evapotranspiration under different climatic conditions

Abstract:

The water balance of the surface soil layer can be broadly described by the following equation: P-Q-ET-f_d = ΔS (1) where P is the precipitation, Q is the runoff, ET is the evapotranspiration, ΔS is the variation of the water in the soil layer, and f_d is the vertical hydraulic redistribution. We expect f_d (positive) to be dominated by drainage during wet seasons, and f_d (negative) be dominated by water uptake from the roots in the underlying substrate to the shallow soil layer during dry seasons. Indeed, although the dry conditions the roots in the fractured rocks still uptake water from soil pockets, where “rock moisture” can be wet.

We investigate the role of the hydraulic redistribution on the seasonal dynamics of ET using micrometeorological (eddy-covariance based) data from 20 sites of FLUXNET and AMERIFLUX networks, with different climates conditions, land vegetation cover types and soil depths. We have also added data from the Orroli site (Sardinia, Italy), which is an experimental site of the FLUXMED European project under the Water Joint Programming Initiative program.

The key role of the roots in the deep substratum was highlighted considering the seasonal contributions of f_d to ET in relation to the seasonal index of dryness (PE/P, with PE the potential evaporation). f_d was estimated as the residual term of the soil water balance (equation 1), using measured P, ΔS and ET (assessed by eddy covariance method), and estimated Q (based on the Soil Conservation Service method).

In wet seasons, f_d/ET values were positive, and were highly variable with precipitation. The f_d/ET values were negative or nearly negative in dry seasons. The seasonal f_d/ET is strongly related to PE/P.

Water uptake by roots in the rocks sustained transpiration in the dry season, as climate becomes drier, increasing f_d role in the water balance. Surprisingly, f_d/ET was not variable with soil depth, meaning that if trees grow over a land, root uptake from the deep layer (that can be both soil and fractured rock) can be up to 80-90% of ET independently of the soil depth, because roots can uptake water also from fractured rocks.