Effects of digital elevation model map scale and data resolution on a topography‐based watershed model

The effects of digital elevation model (DEM) map scale and data resolution on watershed model predictions of hydrologic characteristics were determined for TOPMODEL, a topography‐based watershed model. The effects of topography on watershed hydrology are represented in TOPMODEL as the distribution of ln ( a /tan B ), where ln is the Napierian logarithm, a is the upslope area per unit contour length, and tan B is the gravitational gradient. The minimum, maximum, mean, variance, and skew values of the ln ( a /tan B ) distribution were computed from 1:24,000‐scale (24K) DEMs at 30‐ and 90‐m resolutions and from 1:250,000‐scale (250K) 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 affected by both the DEM map scale and data resolution. Further TOPMODEL analyses showed that the effects of DEM map scale and data resolution on model predictions were due to the sensitivity of the predictions to the mean of the ln ( a /tan B ) distribution, which was affected by both DEM map scale and data resolution. DEM map scale affected the mean of the ln ( a /tan B ) distribution through its influence on the mean of the ln ( a ) distribution, which characterizes land‐surface shape, and the mean of ln (1/tan B ) distribution, which characterizes land‐surface slope. 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 ) distribution.