A semi-distributed ecohydrological modelling framework for catchment hydrology and river chemistry

Climate and land use changes can significantly impact ecosystem water quantity as well as quality. To understand the corresponding physical processes within a catchment, an ecohydrologicalbiogeochemical model that can simulate water, nutrients and vegetation dynamics simultaneously is therefore required. The recently developed spatially distributed model T&C-BG-2D has enabled the simulation of coupled vegetation, hydrological, and soil biogeochemical dynamics within catchments. However, its potential in exploring the impacts of different scenarios and interventions on ecosystems can be limited by computational costs due to grid representations. In this work, we present a semi-distributed abstraction framework for T&C-BG-2D to simulate hourly river discharge and chemistry (C, N, P, K, Ca, Si, Mg) at the (sub)catchment outlet with minimal computational costs. Leveraging recent available remote sensing and reanalysis datasets, an algorithm was developed to enable a novel calibration procedure for all key model parameters (e.g., soil properties, land cover, and vegetation traits) within the catchment across representative hydrological response units. The newly developed semi-distributed version T&C-BG-SD was benchmarked against the fully distributed T&C-BG-2D model in the Hafren (Wales, UK) and the Erlenbach (Swiss pre-Alps) catchments. To further evaluate the suitability of the framework in representing different catchment characteristics, the performance of the model was then examined across multiple catchments in the US and UK spanning diverse climatic conditions and land covers.