Hydrologic engineers are called upon to provide information for a variety of water resource studies:

  • Planning and designing new hydraulic-conveyance and water-control facilities.
  • Operating and/or evaluating existing hydraulic-conveyance and water-control facilities.
  • Preparing for and responding to floods and/or droughts.
  • Regulating floodplain activities.
  • Developing plans that use water to enhance environmental function.

In rare cases, the record of historical flow, stage or precipitation satisfies the information need. More commonly, watershed runoff must be predicted to provide the information. For example, a flood-damage reduction study may require an estimate of the increased volume of runoff for proposed changes to land use in a watershed. However, no record will be available to provide this information because the change has not yet taken place. Similarly, a forecast of reservoir inflow may be needed to determine releases if a tropical storm alters its course and moves over a watershed. Waiting to observe the flow is not acceptable. The alternative is to use a model to provide the information.
A model relates something unknown (the output) to something known (the input). In the case of the models that are included in the program, the known input is precipitation, temperature, and perhaps other meteorologic data. The unknown output is usually runoff. For applications other than watershed runoff estimation, the known input is upstream flow and the unknown output is downstream flow.