Preface to HW04 - Preface to Homework Number 4 Rainfall...

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Based on Handbook of Hydrology, 1993, McGraw-Hill, David R. Maidment, editor 1 Preface to Homework Number 4 Rainfall Excess and Surface Runoff 1 When rainfall or snow melt begins, surface runoff does not begin immediately. Some water may be intercepted by vegetation, some may be stored by the many small depressions in the land and form puddles, some may be transpired by plants (little during or soon after rainfall, probably, because it is humid when raining), and the most important delay of surface runoff arises because of water infiltration into the surface layer of the soil. Rainfall excess, or direct surface runoff, is water in excess of these deductions from a rainfall “event”. (So much less depressing than the term “Ithacation”! By the time you graduate you will know what “Ithacation” really means.) Surface runoff can be very fast, such as that which forms small rivulets and drains quickly to streams, or somewhat slower, such as that which flows over the surface of the land but is retarded by vegetation and land surface irregularities. Surface runoff is not the total story, of course. Some rain soaks into the ground and then flows more or less laterally and reappears in streams, perhaps by forming small springs nearer the streams, or simply by seeping out of the land and into the stream bed. Even though this is not surface runoff, it contributes to stream flow and potential flooding, particularly after extended precipitation. Other rain percolates into the deep ground and reappears much later in the streams. This underground flow, in particular, is why we see many streams continue to flow all summer, even during quite dry spells (although, admittedly, at a much reduced rate). The water is rainwater from storms many weeks or months or years previous.
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Background: General Comments On Hydrographs : Consider a rainfall “event”. A sprinkle is unlikely to lead to any runoff. If the event is a modest storm, there may or may not be runoff depending on the amount of rain and the soil permeability (inherent ability of the soil to absorb water, which determines whether water soaks into the ground before it can flow overland). If a heavy storm occurs, only the most permeable (sandy) soil types will have no surface runoff. However, more than soil type is involved, which is very important. Land use directly affects runoff – much less runoff flows from a mature meadow or forest than from a cultivated field (or a road or parking lot!), even though the underlying soil types may be identical. A third important factor is the recent history of rain. A modest rain can lead to flooding if it follows several days of heavy storms – or no runoff at all if the previous week was dry. A fourth factor is, of course, whether the ground is frozen (in which case either a rainstorm, or melting snow, can lead to much more surface runoff than the same situation would yield when the ground is not frozen). A fifth factor involves the vegetation canopy; actively growing vegetation reduces runoff potential by capturing water and holding it, and slowing its travel over the surface of the ground.
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Preface to HW04 - Preface to Homework Number 4 Rainfall...

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