With current advance in satellite measurements, the freshwater discharge of river basins, individual continent, and globe can be estimated. The data sets used in this study are the 1-degree daily rainfall measurements from the Global Precipitation Climatology Project (GPCP) and the land evaporation estimated from a combination of the results from the Surface Radiation Budget (SRB) and the Global Land Data Assimilation System (GLDAS). The monthly Bowen ratio in each grid from the GLDAS is applied to the satellite observed surface net radiation flux of SRB for estimating the evaporation.

The discharge is calculated as the residual of the freshwater balance of rainfall and evaporation. The result for entire global continents is 38874.6Km^3 per year based on the 1997-2005 data, which is close to the climatology results of Dai and Trenberth (2002) based on gauge station measurements. There are five other studies with results ranging between 37713 to 38843 km^3 per year.

The monthly, seasonal, or annual variations at global, continental, or basin scale are hard to be derived from the current data of the gauge station observations due to spatial and temporal inconsistency among the thousands of gauge data sets. Since satellite data cover global regions within close temporal range, the time series of monthly and seasonal variations of river discharges can be estimated when the rainfall and evaporation data are merged with the measurements of monthly mean land water storage anomaly from the Gravity Recovery and Climate Experiment (GRACE}, which started the measurements since late 2002. Based on the 2003-2005 data, we find that the variability in discharge at monthly scale is much large than the annual variability. They can reach as high as 120 mm per moth for Australia and south-America continents and more than 200 mm per month for Africa. The results also show a delay between the rainfall and discharges.