Large-scale variations of the continental water storage – Global hydrological modelling and time-variable gravity data from GRACE

In view of the pivotal role that continental water storage, particularly soil moisture, plays in the global water, energy and biogeochemical cycles, temporal and spatial variations of the water storage have for a long time not been known with sufficient accuracy for large areas. This was mainly due to the lack of adequate large-scale monitoring systems. Traditional soil moisture or groundwater measurements give only local estimates of the water storage. 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 therefore 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. For the model-based analysis of storage variations, the WaterGAP Global Hydrological Model (WGHM) (Döll et al., 2003, J. Hydr., 270, 105-134; Hunger & Döll, 2008, HESS, 12, 841-861) is applied. 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