Reduction of vegetation-accessible water storage capacity after deforestation affects catchment travel time distributions and increases young water fractions in a headwater catchment
Authors: Hrachowitz Markus , Stockinger Michael , Coenders-Gerrits Miriam , van der Ent Ruud , Bogena Heye , Lücke Andreas , Stumpp Christine 
Published: September 2021

Abstract. 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.