Improving Riparian Land Surface Flux Estimation Through Root Zone Groundwater Interaction in Semiarid Environments

Accurate estimation of turbulent land surface fluxes in land surface models (LSM) is important for sustainability studies of riparian areas, e.g. how they react to changes in groundwater availability either through extended droughts or due to climate change. Because atmospheric evaporative demand is very high in semiarid environments, neglecting important sources of root water uptake can cause errors in the estimation of these fluxes. In this study we apply a simple parameterization of root-zone groundwater interaction to the Noah LSM and compare coupled (to groundwater) and uncoupled model runs to in situ observations of latent heat fluxes and soil moisture. Observations used in this study were collected at a riparian grassland site located near the San Pedro River, in Arizona, USA. Our results show that when the Noah model is run without the presence of a water table (uncoupled run) we get significant under prediction of the latent heat fluxes, and by introducing a root-zone groundwater interaction (coupled run) we are able to better capture the observed turbulent flux dynamics. However, the coupled model still does not predict observed soil moisture dynamics accurately and in general overestimates soil moisture in the upper soil layers. Our findings suggest that a different root water uptake mechanism than currently implemented in Noah may be required to simultaneously reproduce both the latent heat and root zone water balance dynamics.