Time-variable Gravity and Surface Mass Processes: Validation, Processing and First Application of New Satellite Gravity Data

The GRACE (Gravity Recovery and Climate Experiment) satellite mission, launched in March 2002, allows for the first time to quantify mass variations caused by changes in continental water storage. These satellite data overcome the lack of direct measurements of changes in water storage at the scale of large river basins or continents and may provide a constraint for validating and improving macro-scale hydrological models. Up to now, water storage is a variable which has rarely been analysed as an output of macro-scale models and model performance to simulate water storage change is not known.

The overall objective of the TIVAGAM project is the development of methods to derive mass anomalies from global gravity models of the GRACE mission for the application in modelling of dynamic earth system processes. Strategies and filter techniques for the detection and the extraction of regional single-process related mass variations from global gravity models are developed. The focus is on variations in continental water storage and the possible integration of GRACE data into a state-of-the-art hydrology model. The model used is the WaterGAP Global Hydrological Model (WGHM) (Döll et al., 2003, J. Hydr., 270, 105-134; Hunger & Döll, 2007, HESSD, 4, 4125-4173). It represents the terrestrial hydrological cycle with a spatial resolution of 0.5 degree.

Water storage compartments taken into account in the model are:

• soil moisture
• groundwater
• snow
• surface water in rivers, lakes, reservoirs and wetlands

The specific objectives of the TIVAGAM sub-project “Integration of Surface Mass Variations from Satellite Data into Hydrological Modeling” at Section 5.4 Engineering Hydrology are:

• to develop methods for implementing satellite-based data on continental water storage change into hydrological models for validation and calibration.
• to compute improved and consistent model-based time series of continental water storage, and to support validation and adequate preparation of GRACE data from the perspective of hydrological applications.

The work tasks include a sensitivity analysis of the global hydrological model WGHM to identify the most sensitive model parameters for storage change simulations and the development and application of a multi-objective calibration approach (see Figure) for WGHM taking into account observations of river discharge (from gauging stations) and storage change (from GRACE).