- 1). Plot the location of rain gauges on the base map with a pencil.
- 2). Connect adjacent points with dashed lines using a straight edge and pencil.
- 3). Construct perpendicular bisectors across the dashed boundary lines.
- 4). Connect the bisector lines to outline polygons belonging to each station or region.
- 5). Count squares on the graph paper to determine the size of each area. Areas of polygons are calculated and expressed as fractions of the total area.
- 6). Create a chart using the data. For example, list four columns labeled Station Precipitation, Area in Basin, Percent of Total Area and Adjusted Precipitation. Under each column, record given data. Under Adjusted Precipitation, multiply the data in column one by the data in column three for each data point.
- 7). Calculate the sum of column four, the Adjusted Precipitation. This sum represents the total precipitation over the entire area.
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