Assimilation of SMOS soil moisture products into a hydrological model to improve streamflow simulations in the Oueme catchement, West Africa.

Abstract:

The Ouémé catchment is located in the central part of Benin in West Africa. Its climate, extremely dry in winter and heavy rains in summer, makes the water cycle a true challenge to model, especially for the water management in the agricultural areas. DHSVM (Distributed Hydrology Soil Vegetation Model) is a physically based and distributed hydrological model that solves the energy and water balance. This model simulates the soil moisture at each soil layer, the snow quantity, the evapotranspiration, the runoff and the streamflow. It was used with an hourly time step at a 1 km resolution. Model parameters have been calibrated using 2010 in situ streamflow and soil moisture measurements along with in situ precipitations. When using satellite precipitation observations, the streamflow simulations are no longer in agreement with in situ measurements. The goal of this work is to assimilate the SMOS soil moisture product (Level 3 product available at a 25 km resolution with a 3-day global coverage) into the hydrological model DHSVM using a Kalman filter for a better constraint on the water cycle model when using satellite precipitation observations. SMOS acts on the system by adding or removing some water when satellite observations are either under or overestimating the precipitations. Besides the choice of the assimilation method, the difference in spatial resolutions is also being taken care of by using an influence circle area. The results show an improvement of the simulated soil moisture (error decreased by 20% at the surface level and by 45% at 1 meter depth, while the correlation is much improved) and on the streamflow (Nash model efficiency increases from 0.2 to 0.6) proving that coarse observations from space can still provide useful information into a finer hydrological model. Furthermore, DHSVM also models the water table depth and a first attempt to estimate groundwater volume for the entire watershed is presented in this study.