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<br /> <br />characteristics of the basin. It is variations from the normal areal pattern of <br />precipitation that cause the differences in unit hydrographs. <br /> <br />By definition. a unit hydrographs contains 1 unit of runoff from rainfall <br />and snowmelt occurring in a specified unit time. If the amount of runoff remains <br />constant (1 unit) and the duration in increased. the time base of the unit hydrograph <br />will be lengthened and the peak will be lowered. A separate unit hydrograph is <br />theoretically necessary for each possible duration of rainfall and snowmelt. Actually. <br />a few unit hydrographs for short durations will serve most requirements. and a unit <br />hydrograph for a short duration of rainfall and snowmelt can be used to develop unit <br />hydrographs for storms of longer duration. <br /> <br />Time-intensity patterns of rainfall can have a significant effect on unit <br />hydrographs and the effect is directly related to basin size. On large basins. changes <br />in storm intensity must last for several hours to cause distinguishable effects on the <br />hydrograph. On the other hand, clearly defined peaks in the hydro graphs may be <br />caused by short bursts of rainfall lasting only a few minutes in very small basins. For <br />large basins. valley storage tends to eliminate the effects of short-time intensities and <br />only major changes in the time-intensity pattern can usually be lessened by selecting <br />the computation interval to be used in developing a unit hydrograph short enough that <br />the changes are not great from one computation interval to the next. <br /> <br />Up to this point. it has been assumed that the physical characteristics of <br />a basin have remained relatively constant. But it is known that changes in the <br />physical characteristics can and do occur from natural and manmade causes. A <br />typical example is the drastic changes that can occur in the shape of hydrographs and <br />unit hydrographs developed over a period of time for basins that are being urbanized. <br />In this case, because of the reduction of natural valley storage, the unit hydrographs <br />will tend to have higher peaks and shorter times of concentration. Under natural <br />conditions, changes in physical characteristics can occur due to seasonal and long- <br />term changes in vegetation .or to other causes, such as fires. It should be noted that <br />these variations also affect loss rates. For example. the increase in impervious area <br />caused by urbanization will not only reduce the natural valley storage of a basin, but <br />will also lower the average loss rates, since some rainfall will contribute directly to <br />streamflow with no loss. <br /> <br />There is no prescribed set of rules for solving this problem, but the <br />engineer should be aware that many factors cause variations in unit hydrographs. <br /> <br />Since the ordinates of a unit hydrograph represent the distribution by time <br />and magnitude of 1 unit of runoff from a drainage basin, the unit hydrograph may be <br />applied to rainfall and snowmelt excess of any magnitude to determine the resulting <br />hydrograph, provided the duration of excess coincides with the unit duration of the <br /> <br />7-25 <br /> <br /> <br />