Deforestation reduces the water holding capacity in the root-zone and increases the fraction of young water in streams

The water storage capacity in the unsaturated root zone of soils is the principal source of nonlinearity in the response of terrestrial hydrological systems. This storage capacity (SU,max) between field capacity and the permanent wilting point represents the water volume required by and accessible to vegetation to ensure continuous access to water to bridge dry periods. It can be robustly estimated at the catchment scale based exclusively on long-term water balance data, independent of detailed information on root systems and soil porosities. Based on data from catchments that underwent well documented land use change (i.e. deforestation), we here test how forest disturbance affects SU,max and the related dynamics of water storage and release. The post-deforestation signature of the hydrological response in the research catchments suggests that the reduced canopy water demand of deforested surfaces result in a substantial reduction of SU,max. As a consequence, not only evaporation, stream flow and their respective signatures, such as runoff coefficients, but also catchment-scale transport dynamics change: it is demonstrated that changes in the root zone storage capacity SU,max due to deforestation significantly alters catchment travel time distributions. In particular during storm events much higher proportions of young water reach the stream. This does not only have implications for the nutrient budget of a system but also changes the susceptibility of a system to pollution. Pollutant inputs will be more directly and with less attenuation routed to the stream, resulting in higher pollutant peak concentrations. This in turn illustrates the importance of vegetation for moderating peak pollutant concentrations in streams.