# Reduction of vegetation-accessible water storage capacity after deforestation affects catchment travel time distributions and increases young water fractions in a headwater catchment

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 (inline-formulaSU, max) after deforestation and that (2) the deforestation-induced reduction of inline-formulaSU, 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 inline-formulaFyw.

page4888It 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 inline-formulaCR=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 (inline-formulaSU, max) that is within the reach of active roots and thus accessible for vegetation transpiration from inline-formula∼258inline-formulamm in the pre-deforestation period to inline-formula∼101inline-formulamm in the post-deforestation period. The hydrological model, reflecting the changes in the parameter inline-formulaSU, max, indicated that in the post-deforestation period stream water was characterized by slightly yet statistically not significantly higher mean fractions of young water (inline-formulaFyw∼0.13) than in the pre-deforestation period (inline-formulaFyw∼0.12). In spite of these limited effects on the overall inline-formulaFyw, changes were found for wet periods, during which post-deforestation fractions of young water increased to values inline-formulaFyw∼0.37 for individual storms. Deforestation also caused a significantly increased sensitivity of young water fractions to discharge under wet conditions from inline-formula $M15inlinescrollmathml\mathrm{normal d}{F}_{\text{yw}}/\mathrm{normal d}Q=normal 0.25$ 78pt16ptsvg-formulamathimg962c3881c4b00221f12b9f821495832a hess-25-4887-2021-ie00001.svg78pt16pthess-25-4887-2021-ie00001.png to 0.36.

Overall, this study provides quantitative evidence that deforestation resulted in changes in vegetation-accessible storage volumes inline-formulaSU, 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.

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Hrachowitz, Markus / Stockinger, Michael / Coenders-Gerrits, Miriam / et al: Reduction of vegetation-accessible water storage capacity after deforestation affects catchment travel time distributions and increases young water fractions in a headwater catchment. 2021. Copernicus Publications.

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