Hydrographs

By definition, a hydrograph is a plot of the variation of discharge with respect to time (it can also be the variation of stage or other water property with respect to time). Discharge is the volume of water flowing past a location per unit time (usually in cubic feet per second (cfs)).

Hydrograph Example

lewis Lake Divide SNOTL vs. Snake River at Flagg Ranch

Real-time hydrographs for data points in your HSA are available on Hydromet and Hydroview. These hydrographs are an easy and quick way to assess the hydrologic conditions in your HSA: whether rivers are rising, falling or remaining steady; whether flows are high, low, or near base flow; if the rivers have crested, or are near crest, etc..

A Unit Hydrograph is by definition, a hydrograph having a volume of 1 inch of runoff which is associated with a precipitation event of specified duration and areal pattern (uniform over the basin). The unit hydrograph is a theoretical hydrograph which is intended to describe how a river at a particular point will react to one inch of runoff, and can in turn be used to derive how the river will react to any amount of runoff. Based on current condition, the RFC's hydrologic models compute the amount of runoff expected at a point on a river. The calculated runoff amount is then applied to the unit hydrograph at that particular point, to produce a forecast hydrograph which is specific to that location and event.

It is not the intent of this section to describe in detail the theories of hydrographs and unit hydrographs. Please look up the following references to learn more about the techniques and theories of hydrographs.