Benefits of representing floodplains in a Land Surface Model: Pantanal simulated with ORCHIDEE CMIP6 version

Tropical floodplains have a significant impact on the regional water cycle and on land-atmosphere interaction but are not always considered in Land Surface Models (LSMs) or in Earth system models. This study evaluates the importance of representing tropical floodplains in an LSM to provide realistic river discharges, evapotranspiration fluxes and other variables crucial for land’s interaction with the ocean and atmosphere. Off-line simulations of the world’s largest tropical wetland, the Pantanal are conducted with ORCHIDEE, the LSM of IPSL’s regional and global Earth system model. We analyse the period 1961–2000, which includes both dry and wet decades. Atmospheric uncertainty is considered through the utilization of three forcing data sets, each one in two versions: the original dataset and a regionally bias adjusted version. The activation of the floodplain module leads to a systematic improvement of intra-annual variability and extremes of river discharge. Temporal evolution and spatial distribution of flooded area are coherent with satellite estimations, although the model, due to the coarse resolution of the topography, underestimates the extent of the area. Considering floodplains in ORCHIDEE enhance the evapotranspiration since it permits the water from the upstream region to evaporate in the plains. This have strong consequences on the water balance and on the spatial pattern of surface fluxes. The simulations allow us to perform a model-guided residual estimation of evapotranspiration through a water balance obtaining an annual evapotranspiration over Pantanal of 1220 mm while precipitation is estimated to be 1250 mm with an uncertainty of 180 mm.