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This cookie is set by GDPR Cookie Consent plugin. The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional". The cookie is used to store the user consent for the cookies in the category "Analytics". These cookies ensure basic functionalities and security features of the website, anonymously. Necessary cookies are absolutely essential for the website to function properly. 3.8, 3.9, 3.10) have been developed for various geographic locations of India.įrom rainfall-retention relationships, following equation was developed by Wasiullah and Ram Babu (1970) for estimating runoff volume: For determining I’, therefore, charts for 6 hours rainfall for 10,25 and 50 years frequency (Figs. soil conservation service staff have also suggested 6 hours as the minimum storm period for flood-water-retarding structures. The arbitrary curve number N for different soil conditions can be obtained from the relation: Q = (I’ – 0.1S) 2/(I’ + 0.9S) …(3.13)Įquation (3.12) is applicable to all soil regions of India except the black soil areas for which Eq. On gaged watersheds S can be obtained directly from the plot of actual rainfall (I’) and runoff (Q) data as shown in Figs. For N = 100, S = 0 and, therefore, I’ = Q.Ĭurve numbers can be obtained from Table 3.10a. Where, N = an arbitrary curve number ranging from 0 to 100. For convenience in evaluating antecedent moisture, soil conditions,’ land use and conservation practices, the U.S. (3.10) the initial abstraction l a has been considered to be 0.2 S. With the curve number, runoff can be predicted for a given watershed rainfall using the relations (SCS, USD A): Knowing these watershed conditions, hydrologic soil-cover complexes and antecedent moisture, through the use of appropriate tables and graphs, one can determine the appropriate curve number. Where, I a is the initial abstractions over the catchment before runoff begins, expressed in equivalent depth of water, and S is the potential maximum reten­tion or infiltration (difference between rainfall and runoff). (b) Black soils region AMC I – I a = 0.3 S (a) Black soils region AMC II and III – I a = 0.15 The value of I a is assigned from the general condition of the region. The initial abstraction consists of intercep­tion losses, surface storage, and water which infiltrates into the soil prior to runoff. Initial abstraction of rainfall, before runoff begins, is also an important fac­tor determining the actual runoff. These conditions are defined both for dormant and growing seasons (Table 3.11).Ĭurve numbers for the three AMC conditions are inter-convertible using suitable correction factor (Table 3.12). It is defined as the summation of the 5 day precipitation before the runoff- producing storm, and is called antecedent moisture condition (AMC) I, II, III. These characteristics of the watershed are quantified in terms of runoff curve numbers as shown in Table 3.10a, b.Īntecedent moisture is also an important factor in determining the runoff. The land use and treatments on an area are further divided into three hydrologic conditions viz., poor, fair and good. Treatment classes consist of straight row cropping, contour­ing and terracing.