Average annual runoff is the amount of water running over the land surface during the year. Generally this number is given in units of millimeters per year, which can seem confusing at first. In reality, this number is the volume of water running over this gridcell divided by the area of the gridcell itself (mm3 per year / mm2 over 1 yr = mm/yr).
This dataset was compiled from a number of different point sources and then integrated upstream to determine the values between the different river gauging stations using a climate driven water balance model to fill in most of the uncertainties.
Fetke, Balazs M. et al (2000). Global Composite Runoff Fields Based on Observed River Discharge and Simulated Water Balances. Complex Systems Research Center, University of New Hampshire. UNH-GRDC Composite Runoff Fields v1.0. Available at this address/.
(All images are provided at a resolution of 1600x1200 pixels)
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