Watershed Subdivision

The method is also applicable to complex watersheds, in which runoff hydrographs for multiple subbasins are computed, then routed to a common point and combined to yield the total runoff hydrograph at that location.
TxDOT research on undeveloped watersheds (0-5822-01-2) has indicated that there is little justification for subdividing a watershed for the purpose of improving model accuracy. In general, subdivision had little or no impact on runoff volume for the following reasons:
  1. In general, subdivision of watersheds for modeling results in no more than modest improvements in prediction of peak discharge. Improvements generally are not observed with more than about five to seven subdivisions;
  2. Watershed subdivision multiplies the number of sub-process model parameters required to model watershed response and introduces the requirement to route flows through the watershed drainage network. Discrimination of parameters between sub-watersheds is difficult to justify from a technical perspective;
  3. The introduction of watershed subdivisions requires hydrologic (or hydraulic) routing for movement of sub-watershed discharges toward the watershed outlet. The routing sub-process model requires estimates of additional parameters that are subject to uncertainty;
  4. The dependence of computed hydrographs on internal routing became more apparent as the number of subdivisions increased; and
  5. Application of distributed modeling, as currently implemented in HEC-HMS, was difficult and time consuming. It is unclear what technical advantage is gained by application of this modeling approach in an uncalibrated mode, given the level of effort required to develop the models.
There are circumstances in which watershed subdivision is appropriate. If one of the sub-watersheds is distinctly different than the other components of the watershed, and if the drainage of that sub-watershed is a significant fraction of the whole (20-50%), then a subdivision might be appropriate. Specific examples of an appropriate application of watershed subdivision would be:
  • the presence of a reservoir on a tributary stream,
  • a significant difference in the level of urbanization of one component of a watershed, or
  • a substantial difference in physical characteristics (main channel slope, overland flow slope, loss characteristics, and so forth).
  • unique storm depths are appropriate for the different subbasin areas.
  • computed hydrographs are needed at more than one location.