Abstract
Abstract. Quantifying how vegetation mediates water partitioning at different spatial and temporal scales in complex, managed catchments is fundamental for long-term sustainable land and water management. Estimations from ecohydrological models conceptualising how vegetation regulates the interrelationships between evapotranspiration losses, catchment water storage dynamics, and recharge and runoff fluxes are needed to assess water availability for a range of ecosystem services and evaluate how these might change under increasing extreme events, such as droughts. Currently, the feedback mechanisms between water and mosaics of different vegetation and land cover are not well understood across spatial scales, and the effects of different scales on the skill of ecohydrological models needs to be clarified. We used the tracer-aided ecohydrological model EcH2O-iso in an intensively monitored 66 km2 mixed land use catchment in northeastern Germany to quantify water flux–storage–age interactions at four model grid resolutions (250, 500, 750, and 1000 m). This used a fusion of field (including precipitation, soil water, groundwater, and stream isotopes) and remote sensing data in the calibration. Multicriteria calibration across the catchment at each resolution revealed some differences in the estimation of fluxes, storages, and water ages. In general, model sensitivity decreased and uncertainty increased with coarser model resolutions. Larger grids were unable to replicate observed streamflow and distributed isotope dynamics in the way smaller pixels could. However, using isotope data in the calibration still helped constrain the estimation of fluxes, storage, and water ages at coarser resolutions. Despite using the same data and parameterisation for calibration at different grid resolutions, the modelled proportion of fluxes differed slightly at each resolution, with coarse models simulating higher evapotranspiration, lower relative transpiration, increased overland flow, and slower groundwater movement. Although the coarser resolutions also revealed higher uncertainty and lower overall model performance, the overall results were broadly similar. The study shows that tracers provide effective calibration constraints on larger resolution ecohydrological modelling and help us understand the influence of grid resolution on the simulation of vegetation–soil interactions. This is essential in interpreting associated uncertainty in estimating land use influence on large-scale “blue” (ground and surface water) and “green” (vegetation and evaporated water) fluxes, particularly for future environmental change.
Highlights
Climate projections indicate increases in temperatures and extreme drought frequency in many areas, with expected decreases in summer baseflow (Papadimitriou et al, 2016) and reduced summer soil water storage (Grillakis, 2019), which in turn limit evapotranspiration (Jung et al, 2010)
This study demonstrated the effectiveness of a physically based, tracer-aided ecohydrological model in consistently simulating blue and green water fluxes and their relationships to vegetation, soil cover, and water storage in a 66 km2 catchment across different spatial resolutions
Long-term water security is dependent on quantitative knowledge of regional water storages and fluxes and how these are anticipated to change under the anticipated increased frequency of extreme events, such as droughts (Falkenmark and Rockström, 2006)
Summary
Climate projections indicate increases in temperatures and extreme drought frequency in many areas, with expected decreases in summer baseflow (Papadimitriou et al, 2016) and reduced summer soil water storage (Grillakis, 2019), which in turn limit evapotranspiration (Jung et al, 2010). There are concerns that long-term partitioning of blue (groundwater and stream water) and green There is a limited evidence base to project the likely relative effects of climate and land use change on blue and green water fluxes (Orth and Destouni, 2018) and the associated predictive uncertainties (Mao et al, 2015). Ecohydrological modelling provides an approach to quantify blue and green water fluxes and associated storage dynamics and project future change. Ecohydrological models can bridge a gap between complex hydrological and ecological processes and capture their integrated effect in controlling water partitioning in the critical zone, i.e. the thin layer of the Earth encompassing the top of the vegetation canopy down to the bottom of the groundwater (Grant and Dietrich, 2017; Brewer et al, 2018). The complexities of soil–vegetation interactions mandate further clarification of scaling effects and the resolution boundaries of fluxes across ecohydrological interfaces in the critical zone (Krause et al, 2017; Vereecken et al, 2019)
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