Hydrograph - Baseflow Separation

Baseflow Separation

A stream hydrograph is commonly conceptualized to include a baseflow component and a direct flow component. The former represents the relatively steady contribution to stream discharge from groundwater return flow, while the latter represents the additional streamflow contributed by surface runoff,

The separation of baseflow from direct runoff in a hydrograph is often of interest to hydrologists, planners, and engineers, as it aids in determining the influence of different hydrologic processes on discharge from the subject catchment. Because the timing, magnitude, and duration of groundwater return flow differs so greatly from that of direct runoff, separating and understanding the influence of these distinct processes is key to analyzing and simulating the likely hydrologic effects of various land use, water use, weather, and climate conditions and changes.

That said, the process of separating “baseflow” from “direct runoff” is an inexact science. In part this is because these two concepts are not, themselves, entirely distinct and unrelated. Return flow from groundwater increases along with overland flow from saturated or impermeable areas during and after a storm event; moreover, a particular water molecule can easily move through both pathways en route to the watershed outlet. Therefore separation of a purely “baseflow component” in a hydrograph is a somewhat arbitrary exercise. Nevertheless, various graphical and empirical techniques have been developed to perform these hydrograph separations. The separation of base flow from direct runoff can be an important first step in developing rainfall-runoff models for a watershed of interest -- for example, in developing and applying unit hydrographs as described below.