Diagnosing seasonal vegetation impacts on evapotranspiration and its partitioning at the catchment scale during SMEX04-NAME

A fundamental problem in ecohydrology is diagnosing impacts of vegetation dynamics on the catchment response. This study uses a distributed hydrologic model and remote sensing data to evaluate the effects of seasonal vegetation greening on the basin water balance and the partitioning of evapotranspiration ET into soil evaporation, transpiration, and evaporation of intercepted water. Using remotely sensed data, updates are made to model vegetation parameters related to radiation, interception, and transpiration as ecosystems respond to precipitation during the North American monsoon (NAM). Comparisons of simulations with static and seasonally varying vegetation parameters reveal lower ET but higher vegetation-mediated ET losses because of the greening. Sensitivity analyses indicate that vegetation fraction is the primary control on ET and its partitioning, while interception parameters play a secondary role. As a result, spatial patterns in ET partitioning in the catchment exhibit a strong signature of vegetation fraction, though fine (coarse)-scale influences of soil moisture (radiation) are also observed. Vegetation-mediated ET losses were significant in large fractions of the catchment and exhibited ecosystem-dependent seasonal evolutions. The numerical simulations presented here provide the first spatially explicit estimates of ET partitioning accounting for vegetation dynamics obtained from remotely sensed data at the catchment scale.