Runoff curve number

The runoff curve number is based on the area's hydrologic soil group, land use, treatment and hydrologic condition. References, such as from USDA The runoff curve number, CN, is then related :S = \frac{1000}{CN} - 10 CN has a range from 30 to 100; lower numbers indicate low runoff potential while larger numbers are for increasing runoff potential. The lower the curve number, the more permeable the soil is. As can be seen in the curve number equation, runoff cannot begin until the initial abstraction has been met. The curve number methodology is an event-based calculation, and should not be used for a single annual rainfall value, as this will incorrectly miss the effects of antecedent moisture and the necessity of an initial abstraction threshold. ==Selection==

The NRCS curve number is related to soil type, soil infiltration capability, land use, and the depth of the seasonal high water table. To account for different soils' ability to infiltrate, NRCS has divided soils into four hydrologic soil groups (HSGs). They are defined as follows. • HSG Group A (low runoff potential): Soils with high infiltration rates even when thoroughly wetted. These consist chiefly of deep, well-drained sands and gravels. These soils have a high rate of water transmission (final infiltration rate greater than per hour). • HSG Group B: Soils with moderate infiltration rates when thoroughly wetted. These consist chiefly of soils that are moderately deep to deep, moderately well drained to well drained with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission (final infiltration rate of per hour). • HSG Group C: Soils with slow infiltration rates when thoroughly wetted. These consist chiefly of soils with a layer that impedes downward movement of water or soils with moderately fine to fine textures. These soils have a slow rate of water transmission (final infiltration rate per hour). • HSG Group D (high runoff potential): Soils with very slow infiltration rates when thoroughly wetted. These consist chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious materials. These soils have a very slow rate of water transmission (final infiltration rate less than per hour). Selection of a hydrologic soil group should be done based on measured infiltration rates, soil survey (such as the NRCS Web Soil Survey), or judgement from a qualified soil science or geotechnical professional. The table below presents curve numbers for antecedent soil moisture condition II (average moisture condition). To alter the curve number based on moisture condition or other parameters, see Adjustments. ==Values==

Runoff is affected by the soil moisture before a precipitation event, the antecedent moisture condition (AMC). A curve number, as calculated above, may also be termed AMC II or CN_{II}, or average soil moisture. The other moisture conditions are dry, AMC I or CN_{I}, and moist, AMC III or CN_{III}. The curve number can be adjusted by factors to CN_{II}, where CN_{I} factors are less than 1 (reduce CN and potential runoff), while CN_{III} factor are greater than 1 (increase CN and potential runoff). The AMC factors can be looked up in the reference table below. Find the CN value for AMC II and multiply it by the adjustment factor based on the actual AMC to determine the adjusted curve number. Initial abstraction ratio adjustment The relationship I_a = 0.2S was derived from the study of many small, experimental watersheds . Since the history and documentation of this relationship are relatively obscure, more recent analysis used model fitting methods to determine the ratio of I_a to S with hundreds of rainfall-runoff data from numerous U.S. watersheds. In the model fitting done by Hawkins et al. (2002) found that the ratio of I_a to S varies from storm to storm and watershed to watershed and that the assumption of I_a/S=0.20 is usually high. More than 90 percent of I_a/S ratios were less than 0.2. Based on this study, use of I_a/S ratios of 0.05 rather than the commonly used value of 0.20 would seem more appropriate. Thus, the CN runoff equation becomes: :Q=\begin{cases} 0 & \text{for } P \leq 0.05S \\ \frac{(P-0.05S_{0.05})^2}{P+0.95S_{0.05}} & \text{for } P>0.05S \end{cases} In this equation, note that the values of S_{0.05} are not the same as the one used in estimating direct runoff with an I_a/S ratio of 0.20, because 5 percent of the storage is assumed to be the initial abstraction, not 20 percent. The relationship between S_{0.05} and S_{0.20} was obtained from model fitting results, giving the relationship: : S_{0.05}=1.33{S_{0.20}}^{1.15} The user, then, must do the following to use the adjusted 0.05 initial abstraction ratio: • Use the traditional tables of curve numbers to select the value appropriate for your watershed. • Calculate S_{0.20} using the traditional equation:S = \frac{1000}{CN} - 10 • Convert this S value to S_{0.05} using the relationship above. • Calculate the runoff depth using the CN runoff equation above (with 0.05 substituted for the initial abstraction ratio). ==See also==