The accuracy of drainage-area delineations for small areas depends on accuracy and scale of the elevation data used in StreamStats for the delineations. The elevation data that StreamStats uses for most areas are from the USGS 3D Elevation Program dataset, which is a seamless national elevation dataset that is refreshed regularly as new data are made available. When implementing StreamStats, we take a snapshot of the elevation data that is available at the time for the area to be implemented, and the area may include data of varying ages and accuracy. As a result, it is difficult to say without detailed investigation what the accuracy of the elevation is at a particular location.
Generally, StreamStats is able to delineate basins with reasonable accuracy down to around 0.05 square miles (32 acres) in terrain with moderate relief. Delineations for flat areas will have lower accuracy. Although, StreamStats can delineate such small basins with reasonable accuracy, the errors associated with estimates of streamflow statistics for such small area often is unknown, and may be very large. This is because the USGS operates very few streamgages in such small basins that can be used in regression analyses to develop equations for estimating the streamflow statistics. In outputs for user-selected sites, StreamStats indicates when the drainage area, or any other basin characteristic used as an explanatory variable in the regression equations, is outside of the ranges of the values computed for the streamgages that were used to develop the equations. When that happens, the errors associated with the estimated streamflow statistics are unknown.