A Dynamic View of Plant Available Water

Abstract:

The capacity of the system to store plant available water is a key determinant of how incoming precipitation is partitioned between deep percolation and evapotranspiration. Wherever there is a significant dry season, the success of crops and the distribution of native vegetation in upland environments is strongly linked to this storage capacity because it modulates plant water stress and delivery of water to streams and groundwater. This has long been recognized and is part of most ecohydrology models. Unfortunately, storage capacity is usually viewed as a texture-based static soil property. From an ecohydrological viewpoint it is also dependent on overlying vegetation and the underlying geology or parent material. The dependence on vegetation is most obviously expressed in the rooting depth, which changes dramatically with different cropping systems, which often leave the soil bare, but also changes with vegetation type. The nature of underlying material is also critical. Traditional approaches generally do not consider regolith as “soil” in storage calculations or truncate the soil at an arbitrary depth. Plant roots do not respect those boundaries and may receive substantial amounts of water from below the soil. We illustrate these concepts with field data and model simulations from a gradient of precipitation regimes ranging from low to high annual precipitation and a variety of cover/geology combinations. These added considerations greatly alter vegetation/soil relationships both temporally and spatially. We believe that this more functional approach to quantifying plant available water storage will improve our understanding and interpretation of interactions between vegetation and hydrology.