<P> 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 . </P> <P> However, 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 . </P> <P> A unit hydrograph (UH) is the hypothetical unit response of a watershed (in terms of runoff volume and timing) to a unit input of rainfall . It can be defined as the direct runoff hydrograph (DRH) resulting from one unit (e.g., one cm or one inch) of effective rainfall occurring uniformly over that watershed at a uniform rate over a unit period of time . As a UH is applicable only to the direct runoff component of a hydrograph (i.e., surface runoff), a separate determination of the baseflow component is required . </P> <P> A UH is specific to a particular watershed, and specific to a particular length of time corresponding to the duration of the effective rainfall . That is, the UH is specified as being the 1 - hour, 6 - hour, or 24 - hour UH, or any other length of time up to the time of concentration of direct runoff at the watershed outlet . Thus, for a given watershed, there can be many unit hydrographs, each one corresponding to a different duration of effective rainfall . </P>

The word unit in the unit hydrograph denotes
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