Upscaling water storage – flux – age interactions in larger catchments using tracer aided ecohydrological models

Abstract:

Quantifying how vegetation affects interactions between the fluxes, storage, and age of water at different spatial and temporal scales in complex, managed catchments is fundamental for long-term sustainable water and land management. Estimations from ecohydrological models conceptualizing how vegetation regulates the inter-relationships between catchment storage dynamics, evapotranspiration losses, and recharge/runoff fluxes are needed to assess water availability for a range of ecosystem services and how this might change under increasingly frequent extreme events, such as droughts. Currently, the feedback mechanisms between water and mosaics of different vegetation/land cover are not well understood across spatial scales, and the effects of scale on the skill of ecohydrological models need 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 scales (250, 500, 750, and 1000m) using data fusion of nested field data (including precipitation, soil water, groundwater, and stream isotopes) and remote sensed data. This approach also integrates water and tracer data at different temporal scales, from daily to bi-weekly. Multicriteria calibration at each scale showed the importance of using field measurements in larger-scale calibration. Calibration also revealed significant differences in the estimation of fluxes, storages, and water ages. Larger grid-scales simulated higher evapotranspiration, lower relative transpiration, increased overland flow, and slower groundwater movement. The larger grid-scales revealed higher uncertainty, lower overall model performance, and were unable to replicate the same observed streamflow and isotope dynamics at multiple locations in the way smaller pixels could. The results show that tracers provide effective calibration constraints on larger scale ecohydrological modelling and help elucidate the influence of grid-scale on the simulation of vegetation-soil interactions. This is essential in interpreting associated uncertainty in the large-scale “blue” (ground and surface water) and “green” (vegetation water) fluxes, particularly during more hydroclimatic extremes.