Deforestation reduces the vegetation-accessible water storage in the unsaturated soil and affects catchment travel time distributions and young water fractions

Abstract

<jats:p>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 of 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 are therefore to quantify the effects of deforestation in the Wüstebach experimental catchment on the partitioning of water fluxes and to directly associate these changes to changes in parameters of a hydrological model with integrated tracer routine based on the concept of storage age selection functions. Simultaneously modelling stream flow and stable water isotope dynamics using meaningfully adjusted model parameters both for the pre- and post-deforestation periods, respectively, the model 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 of stream flow, accompanied by corresponding reductions of evaporative fluxes. This is reflected by an increase of 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 ~ 225 mm in the pre-deforestation period to ~ 90 mm in the post-deforestations 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 higher mean fractions of young water (Fyw ~ 0.13) than in the pre-deforestation period (Fyw ~ 0.11). In spite of these limited effects on the overall Fyw, considerable changes were found for wet periods, during which post-deforestation fractions of young water increased to values Fyw ~ 0.40 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 0.43. Overall, this study demonstrates that deforestation has not only the potential to affect the partitioning between drainage and evaporation as well as the vegetation-accessible storage volumes SU,max, and thus the fundamental hydrological response characteristics of catchments, but also 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 subsurface. </jats:p>

The importance of ecosystem adaptation on hydrological model predictions in response to climate change

“…Under continued global warming, precipitation and temperature extremes are expected to further increase and the hydrological cycle is likely to further accelerate (Allen et al, 2010;Kovats et al, 2014;Stephens et al, 2021). In addition, natural land cover change and anthropogenic activities of land-cover change and land-use management can substantially alter a catchment's water balance (Brown et al, 2005;Wagener, 2007;Fenicia et al, 2009;Jaramillo and Destouni, 2014;Nijzink et al, 2016a;Hrachowitz et al, 2020;Levia et al, 2020;Stephens et al, 2020Stephens et al, , 2021. Considering the unprecedented speed of change, Milly et al (2008) declared that stationarity is dead and no longer should serve as a default assumption in water management.…”

“…The root-zone storage capacity is, therefore, the key element regulating the partitioning of water fluxes in many terrestrial hydrological systems. In addition, not only natural changes to the environment, but also human interference with vegetation affect transpiration water demand and hence the root-zone storage capacity (Nijzink et al, 2016a;Hrachowitz et al, 2020).…”

“…Detailed observations of rooting-systems are very scarce in time and space and difficult to integrate to the catchment scale due to heterogeneity of landscapes (de Boer-Euser et al, 2016;Hrachowitz et al, 2020). Instead, the catchment-scale root-zone storage capacity is often estimated through calibration of a hydrological model.…”

“…CC BY 4.0 License. storage capacity can be robustly and directly estimated from annual water deficits using water-balance data (Gao et al, 2014;de Boer-Euser et al, 2016;Wang-Erlandsson et al, 2016;Nijzink et al, 2016a;Bouaziz et al, 2020;Hrachowitz et al, 2020).…”

Climate controlled root zone parameters show potential to improve water flux simulations by land surface models

“…In this method S r is derived from water storage deficit calculations in the root zone at catchment scale, assuming vegetation is able to keep memory of past deficit conditions to size roots in such a way to guarantee continuous access to water (hereinafter S r,MM ) (Gentine et al, 2012;Gao et al, 2014). Recent studies demonstrated that this method provides plausible catchmentscale estimates of S r (e.g., Gao et al, 2014;Nijzink et al, 2016;Wang-Erlandsson et al, 2016;Hrachowitz et al, 2020), that result in improvements in modelling catchment discharge compared to soil derived S r estimates (De Boer- Euser et al, 2016).…”