Disentangling first-order controls of rock-water-plant interactions: a case study with the WITCH2O critical zone model

Water is the first order controlling factor of the weathering reactions. In the recent years, efforts have been made towards the building of model cascades able to simulate the water fluxes and the residence time of the water in the various compartments of the critical zone. Those hydrological constrains are then injected into numerical models simulating the water-rock interactions from the surface down to the impervious bedrock. In this contribution, we describe such a model cascade, where the water-rock interaction model WITCH is fed by the process-based ecohydrological model EcH2O-iso. This model cascade, WITCH2O, is designed for the modeling of water fluxes & stores, as well as the weathering reactions and transport of weathering products (including atmospheric CO2 consumption), from the vertical profile to the catchment scale, and from the submonthly to decadal time scales. We deployed WITCH2O in a small tropical headwater catchment in peninsular India where long-term observations of water and geochemical fluxes are available, allowing for model calibration and evaluation across the different processes simulated. Using various temporal averages of simulated water fluxes and stores, we notably analyse the influence of hydrological variability upon the dynamics of weathering rates and nutrient concentrations. Our results also highlight the control of root zone uptake and lateral water redistribution upon spatial variability of geochemical patterns.