Deforestation can considerably affect transpiration dynamics and magnitudes at the catchment scale and thereby alter the partitioning between drainage and evaporative water fluxes released from terrestrial hydrological systems. However, it has so far remained problematic to directly link reductions in transpiration to changes in the physical properties of the system and to quantify these changes in system properties at the catchment scale. As a consequence, it is difficult to quantify the effect of deforestation on parameters of catchment-scale hydrological models. This in turn leads to substantial uncertainties in predictions of the hydrological response after deforestation but also to a poor understanding of how deforestation affects principal descriptors of catchment-scale transport, such as travel time distributions and young water fractions. The objectives of this study in the Wüstebach experimental catchment are therefore to provide a mechanistic explanation of why changes in the partitioning of water fluxes can be observed after deforestation and how this further affects the storage and release dynamics of water. More specifically, we test the hypotheses that (1) post-deforestation changes in water storage dynamics and partitioning of water fluxes are largely a direct consequence of a reduction of the catchment-scale effective vegetation-accessible water storage capacity in the unsaturated root zone (SU, max) after deforestation and that (2) the deforestation-induced reduction of SU, max affects the shape of travel time distributions and results in shifts towards higher fractions of young water in the stream. Simultaneously modelling streamflow and stable water isotope dynamics using meaningfully adjusted model parameters both for the pre- and post-deforestation periods, respectively, a hydrological model with an integrated tracer routine based on the concept of storage-age selection functions is used to track fluxes through the system and to estimate the effects of deforestation on catchment travel time distributions and young water fractions Fyw.

It was found that deforestation led to a significant increase in streamflow accompanied by corresponding reductions of evaporative fluxes. This is reflected by an increase in the runoff ratio from CR=0.55 to 0.68 in the post-deforestation period despite similar climatic conditions. This reduction of evaporative fluxes could be linked to a reduction of the catchment-scale water storage volume in the unsaturated soil (SU, max) that is within the reach of active roots and thus accessible for vegetation transpiration from ∼258 mm in the pre-deforestation period to ∼101 mm in the post-deforestation period. The hydrological model, reflecting the changes in the parameter SU, max, indicated that in the post-deforestation period stream water was characterized by slightly yet statistically not significantly higher mean fractions of young water (Fyw∼0.13) than in the pre-deforestation period (Fyw∼0.12). In spite of these limited effects on the overall Fyw, changes were found for wet periods, during which post-deforestation fractions of young water increased to values Fyw∼0.37 for individual storms. Deforestation also caused a significantly increased sensitivity of young water fractions to discharge under wet conditions from dFyw/dQ=0.25 to 0.36.

Overall, this study provides quantitative evidence that deforestation resulted in changes in vegetation-accessible storage volumes SU, max and that these changes are not only responsible for changes in the partitioning between drainage and evaporation and thus the fundamental hydrological response characteristics of the Wüstebach catchment, but also for changes in catchment-scale tracer circulation dynamics. In particular for wet conditions, deforestation caused higher proportions of younger water to reach the stream, implying faster routing of stable isotopes and plausibly also solutes through the sub-surface.@en

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 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. With detailed data on root depths and soil porosities, this volume can be estimated. While such data may be available for individual plants or for small forest stands at experimental sites, they remain problematic to acquire for scales larger than that, let alone for entire catchments. Zooming out to the macroscale and following evidence that plants efficiently develop root systems to meet the canopy water demand while minimizing sub-surface resource allocation provides a new perspective on the understanding of the storage capacity in the unsaturated root zone (Sumax). A series of experiments based on data from more than 400 catchments across a wide gradient of climates and landscapes strongly suggests that Sumax can be robustly estimated at the catchment scale based exclusively on long-term water balance data, independently of detailed information on root systems and soil porosities. It is essentially controlled by climate characteristics, such as precipitation seasonality or the aridity index and, in a feedback, by different types of vegetation. Based on further data from catchments that underwent well documented land use change (i.e. deforestation), experiments suggest that these storage capacities are not only significantly and predictably affected by deforestation, but also that Sumax exhibits distinct post-deforestation signatures in different environments, which can be directly linked to contrasting changes in post-deforestation hydrological response dynamics in these systems. More specifically, while in humid climates with little seasonality vegetation only needs to develop small additional storage volumes for continuous access to water, much larger root-accessible water volumes are needed in seasonal climates. Consequently, deforestation in the latter does affect the partitioning of water into drainage and evaporative fluxes, and thus the fundamental hydrological response dynamics, much stronger than in the first case. Due to a more significant reduction of the storage capacity, less water can be stored for eventual evaporation and plant transpiration, while more water is available for drainage. These changes in how catchments store and release water as a function of storage capacity in the unsaturated root zone, does not only affect stream flow and its signatures, such as runoff coefficients, but also catchment-scale transport dynamics: further experiments demonstrated that changes in the root zone storage capacity due to deforestation significantly alters transit 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. @en