General Circulation Models (GCMs) are currently used to project future climate. The output of the models is then used to evaluate the effect of a climatic change on resources such as agriculture, forestry, and water resources. The GCMs used in long-term climate studies vary widely in the geographic resolution of their predictions. The approximate matching of resource data to the geographic scale of GCMs is an important step in the evaluation of the effects of climatic change on resources. As gridcell size increases, however, the distribution of resources within cells becomes more heterogeneous, and it becomes more difficult to evaluate the regional effects of climatic change. We quantify the change in resource heterogeneity as a function of gridcell size. Four resource variables (wheat yield, percent forest cover, population density, and percent of land irrigated) are analyzed on the basis of county-averaged data, while assignment to major drainage basins is based on exact watershed boundaries. A major change in resource heterogeneity within gridcells occurs at a grid length of from 1.2° to 3
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