Modeling the dynamic root water uptake and its hydrological impact at the Reserva Jaru site in Amazonia

An empirical approach is proposed to represent the impact of dynamic root water uptake by plant roots in two land surface models (Community Land Model version 3 (CLM3) and Integrated Biosphere Simulator version 2 (IBIS2)). This approach is compared with a more physically based approach that simulates the hydraulic redistribution, using the Reserva Jaru site in Amazonia as an example. For each model and each approach, two different root profiles are experimented on, an exponential profile and an observed, deeper profile. In both CLM3 and IBIS2, including dynamic root uptake significantly improves the model simulation of latent heat fluxes, regardless of what root profile is used. In both models, the impact of hydraulic redistribution (as accounted for by the physically based approach) is comparable to that of a low degree of dynamic root water uptake (as accounted for by the empirical approach). The latent heat flux simulation in IBIS2 is closer to observation than is that in CLM3, with or without the impact of dynamic root water uptake. Despite the model improvement due to including dynamic root water uptake, significant biases in soil moisture simulations remain. Assimilating soil moisture observations into the land surface models produces a remarkably improved latent heat flux simulation. Parameterization in surface and subsurface runoff (which influences soil moisture) is suggested as a likely cause for the severe biases in the default model simulations, which highlights the critical importance of correctly simulating the fundamental hydrological processes in land surface models.