EFFECTS OF DIGITAL ELEVATION MODEL MAP SCALE AND DATA RESOLUTION ON ATOPOGRAPHY-BASED WATERSHED MODEL

The effects of digital elevation model (DEM) map scale and data resolu tion on watershed model predictions of hydrologic characteristics were determined for TOPMODEL, a topography-based watershed model. The effe cts of topography on watershed hydrology are represented in TOPMODEL a s the distribution of ln (a/tan B), where ln is the Napierian logarith m, a is the upslope area per unit contour length, and tan B is the gra vitational gradient. The minimum, maximum, mean, variance, and skew va lues of the ln (a/tan B) distribution were computed from 1:24,000-scal e (24K) DEMs at 30- and 90-m resolutions and from 1:250,000-scale (250 K) DEMs at 90-m resolution for 71 areas in Pennsylvania, New York, and New Jersey. An analysis of TOPMODEL showed that model predictions of the depth to the water table, the ratio of overland flow to total flow , peak flow, and variance and skew of predicted streamflow were affect ed by both the DEM map scale and data resolution. Further TOPMODEL ana lyses showed that the effects of DEM map scale and data resolution on model predictions were due to the sensitivity of the predictions to th e mean of the ln (a/tan B) distribution, which was affected by both DE M map scale and data resolution. DEM map scale affected the mean of th e ln (a/tan B) distribution through its influence on the mean of the l n (a) distribution, which characterizes land-surface shape, and the me an of ln (1/tan B) distribution, which characterizes land-surface slop e. DEM resolution, in contrast, affected the mean of the ln (a/tan B) distribution primarily by its influence on the mean of the ln (a) dist ribution.