METHODOLOGY OF THE DATA ANALYSIS
The "Annual Renewable Freshwater Supply Per Capita" map was created using two primary data sources: the University of New Hampshire/Global Runoff Data Centre runoff and flow direction data, and gridded population data from the Center for International Earth Science Information Network at Columbia University. (CIESIN). Both data sets are in the form of global raster grids with a spatial resolution of 0.5 degrees. The runoff data initially comes in units of mm per year, and the population data values are people per grid cell.

The desired units of the final map are cubic meters of water per person per year (m3/person/year) so the initial step was to convert the runoff values to cubic meters per year. To accomplish this, the runoff values were divided by 1000 and then multiplied by the surface area of each grid cell. (The grid cell area values are included in the UNH/GRDC data set and merely have to be converted from square kilometers to square meters). The resulting data set represents runoff per grid cell in cubic meters per year.

ESRI’s Flow Accumulation tool was used to combine the UNH/DRGC flow network grid with the adjusted runoff values to calculate the available water entering each grid cell from upstream. The runoff values were added once again to the resulting grid to calculate the total available water in each grid cell.

The total available water was then divided by the population of each grid cell, given by CIESIN’s gridded population data, resulting in the final data values of annual renewable fresh water supply per capita (m3/person/year).

REFERENCES
United Nations Environment Programme (UNEP). Global Environmental Outlook 4: Environment for development. Progress Press Ltd.: Malta.

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