Found inside – Page 92If the rainfall intensity is less than , then Time ( h ) the infiltration rate is equal to the rainFig . ... The amount of rainfall in excess of the index is called rainfall excess . In connection with runoff and flood studies it is ... runoff) rate becomes equal to the precipitation rate (see Fig. At that stage rainfall excess (i.e. 59c. Types of Runoff 3. Too much water means a waste of water which is so precious in arid countries. The losses are the difference between Precipitation and Precipitation in excess. Topographic characteristics include those features of watershed, which create effect on runoff. Under this condition the source of stream flow is only the ground water flow. This is the stage indicating after end of the rainfall. 66. Geography, Rainfall, Runoff, Meaning of Runoff. watershed. The capacity of any soil to absorb the rain water, falling con­tinuously at an excessive rate, goes on decreasing with time until infiltration is reached at the minimum rate i.e., at basic intake rate. ploughing, mulching, ridging, etc.). Found inside – Page 167At saturation the rate of rainfall excess (runoff) will equal the rate of rainfall and a proportional relationship may be developed, where, at any time S/S' = Q/P (8.14) Combining equations (8.13) and (8.14) gives (P-Q)/S' = Q/P (8.15) ... Prohibited Content 3. This concept assumes that there is too much runoff is build up at the beginning of storm and too little at the end of storm due to the fact, that the peak rate of surface retention and infiltration rate decrease with time throughout the storm period. 0000002148 00000 n Although the same amount of water (35 mm) has been supplied by both showers, the high intensity shower is less profitable to the crops. Imagine the same open container (Section 4.1.1) as used for the collection of rain water, but this time with a depth of 10 mm of water in it; leave the container in the field for 24 hours. 0000018076 00000 n 61. The depth of root penetration is primarily dependent on the type of crop, but also on the type of soil. Estimation of surface runoff volume from a large area having uniform infiltration characteristics, is done by subtracting the infiltration loss from the total rainfall. 2. The rainfall amount in excess of ɸ-index is referred as rainfall excess. This process is called transpiration of the plant. Each hectare (10 000 m2) receives 10 000 m2 x 0.015 m = 150 m3 of water. Q=Precipitation excess (runoff) [inches] This allows the ordinates of the UH to be dimensioned based on the precipitation excess (Q), as previously determined by the SCS runoff equation. Found inside – Page 130Center Mass of Rainstorm Basin Lag Peak Flow Rate In Type 2, soil infiltration capacity is low or rainfall ... The volume of flow under the storm hydrograph will be equal to the sum of the rainfall excess in all the blocks of time ... If during a specific time of 6-h, there happens 15 mm rainfall over the watershed of 250 ha, then total volume of rainfall is equal to 250 × 104 × 0.015 m3 = 3.75 × 104m3. :��Gp����y�:�5��{G O �M�p�X��P�\�r�G���7��M��-�w�4H.�6+�}VU�TD���w^�-Y�n~����1[{��U�ٻK? Found inside – Page B-775 SOIL - MOISTURE - ACCOUNTING COMPONENT 0 25 50 100 PERCENTAGE OF AREA WITH INFILTRATION CAPACITY EQUAL TO OR LESS THAN INDICATED VALUE FIGURE 4. - The relation which determines rainfall excess ( QR ) as a function of maximum ... For example, in case of a sloppy watershed, the time to reach the flow at outlet is less, because of greater runoff velocity, which results into formation of peak runoff very soon; and vice-versa. The north or south orientation of watershed affects the time of melting of collected snow. Since, runoff is through the channel, stream/or rivers etc., therefore, sometimes it is also called as channel flow. This loss varies greatly in the early part of rainfall, which subsequently gets decease with increase in rainfall duration (Fig. Infiltration indexes assume that infiltration rate is constant throughout the storm duration. All the rainwater is trapped in the basin and has time to infiltrate (Fig. A watershed having greater D.D. In fine textured soil especially, the soil aggregates break down rapidly into fine particles that seal the soil surface (see Fig. But on the other hand if the rain occurs in a close succession, as in the rainy season, then runoff yield gets sufficiently increased. 0000008849 00000 n 123 0 obj << /Linearized 1 /O 125 /H [ 1361 809 ] /L 1254910 /E 45931 /N 12 /T 1252331 >> endobj xref 123 45 0000000016 00000 n In addition, the sub-surface flow is also started and meets to the stream flow. Thus the Horton model for abstractions is … The fan shape watershed tends to produce higher peak rate of runoff very early than the fern shape, due to the fact that in former one all parts of the watershed contribute the runoff to the outlet simultaneously, comparatively in little period of time than the fern shape watershed. In this article we will discuss about:- 1. In this stage the rainfall causes overland flow, base flow and development of channel storage. 111: Rainfall Excess Overland Flow ROGER E SMITH1 AND DAVID C GOODRICH2 1Civil Engineering Department, Colorado State University, Fort Collins, CO, US 2Southwest Watershed Research Center, ARS USDA, Tucson, AZ, US One of the processes that can generate surface runoff is rainfall excess, which is a process controlled at the surface of the soil. An effective rainfall of 2-hour duration produced a flood hydrograph peak of 200 m 3 /s. It is equal to the total amount of rainfall minus all abstractions … The by a shower of higher intensity: (see Fig. Found inside – Page 91By definition , a rainfall excess that has a duration of 2 hours and a constant intensity of 1/2 inch per hour will produce a direct runoff hydrograph ( DRH ) that is equivalent to UH2 . Then , with the linearity assumptions of the unit ... Found inside – Page 56Rainfall minus (p-index is the rainfall excess. If the rainfall intensity (= the rainfall depth/time interval) is less than or equal to the Q-index, the infiltration rate is equal to the rainfall intensity and if the rainfall intensity ... Direct surface runoff or precipitation excess in inches uniformly distributed over a watershed can readily be calculated by picking values of DRH discharge at equal time increments through FIGURE7.6 (a) Typical infiltration curve. After 24 hours, 6 mm of water is left in the container. Under this condition W-index is known as Wmin – index. In coarse textured soil, water infiltrates quickly but a large part of it percolates below the rootzone. 64. Rainfall is characterized by its amount, intensity and distribution in time. The infiltration is then reduced, and surface runoff increases (see Section 4.1.4). :�%���pʃ� /��߲��j��d!�j���t�~0c.�X�����G�&. And if average infiltration rate of watershed is 1.25 mm/h, then loss of rain water will be 1.25/1000 × 6 × 250 × 104 m3 = 18750m3. Check that the total depth of direct runoff is equal to the total excess precipitation (watershed area = 7.03 mi 2). K = decay time constant. Direct runoff ordinates for a storm of given duration are assumed directly proportional to rainfall excess volumes.

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