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results are very sensitive to the lapse rates used for modifying temperature data. The use of higher (lower) lapse rates would reduce (increase) the effect of temperature changes on runoff. The GCM scenarios used in the model runs are also listed in Table 2. In all cases at least two GCM grid points Intersect the Colorado River Basin and, at the same time, Include vast areas outside of the basin. Figure 2 shows the approximate location of grid points and the modeled hydrologic sub-basins. The grid points represent spatially averaged data and, as such, misrepresent any particular point within the box. In selecting GCM grid-point data for use In hydrologic modeling, we chose not to modify the data in any way (i.e. through interpolation) because we found little justification for doing so. Each of the sub-basins (White, -Animas, and East Rivers) fell well within a specific GCM grid box, although not always the same grid box, depending upon the GCM. The Two-elevation model, on the other hand, was spread across two different grid boxes in each GCM. In the case of the GISS and GFDL models, there was little difference in the scenarios generated by the adjacent grid points, and thus only one point from each model was used. In the case of the UKMO model, however, the adjacent grid points yielded substantially different scenarios so that data from both points (labeled UKMO 1 and UKMO 2) were applied to the Two-elevation model. The available GeM data consist of mean monthly changes in temperature and precipitation developed from a historical baseline that encompasses years 1951 through 1980. These data were averaged to obtain mean annual changes in temperature and precipitation and then applied uniformly to the long-term historical data. As in the case of the hypothetical scenarios described previously, changes in temperature 18