Towards constraining hydrologic models using satellite retrieved soil moisture

Abstract:

Hydrological models are usually calibrated against observed discharge at the catchment outlet and thus are conditioned by an integral catchment information. This procedure ensures the fulfillment of the catchment's water balance but can lead to high predictive uncertainties in model internal states, like soil moisture, or a lack in spatial representativeness of the model. However, some hydrologic applications, as e.g. soil drought monitoring and prediction, rely on this information.

Within this study we propose a framework in which the mesoscale Hydrologic Model (mHM) is calibrated with soil moisture data from various sources. The aim is to condition the model on soil moisture (SM), while preserving good performance in discharge estimation. We identify the most appropriate objective functions by conducting synthetic experiments. The best objective function is determined based on i) deviation of synthetic and simulated soil moisture, ii) nonparametric comparison of SM fields (e.g copulas), and iii) by euclidian distance of model parameters. Those objective functions performing best are used to calibrate mHM against satellite soil moisture products (e.g. ESA-CCI, SMOS) and local in situ observations. This procedure is tested in 3 distinct European basins ranging from snow domination to semi arid climatic conditions.

The results of the synthetic experiment indicate that objective functions focusing on the temporal dynamics of SM are preferable to objective functions aiming on spatial patterns or catchment averages. The best performance in the sense of parameter distance is achieved using temporal correlation or the sum of squared distances from soil moisture anomalies of observed and estimated soil moisture. By comparing the copulas of the different objective functions no significant differences between the methods is observed. Employing satellite data, in a consecutive step, the calibrated model is able to catch soil moisture dynamics but deteriorates the discharge signal.