ARCHIVED REPORTS_1995 - Laserfiche WebLink

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Sent By: Paul R. Scnroeder Oa Apr '94 where E Lt standardized dimensionless length, (L/500 ft) S standardized dimensionless slope, (S/0.04) This same equation is used to adjust user-specified AMC-II curve numbers for surface slope conditions by substituting the user value for CNII, in Equation 31. Adjustment of Curve Number for Frozen Soil When the HELP program predicts frozen conditions to exist, the value of CNIIt is increased, resulting in a higher calculated runoff. value of CN ( is increased, resulting in a higher calculated runoff. value of CNI is increased, resulting in a higher calculated runoff. Knisel et al. 1985) found that this type of curve number adjustment in the CREAMS model resulted in improved predictions of annual runoff for several test watersheds. If the CN I for unfrozen soil is less than or equal to 80, the CNII for frozen sO conditions is set at 95. When the unfrozen soil CNII is greater than 80, the CNII is reset to be 98 on days when the program has determined the soil to be frozen. This adjustment results in an increase in CNI and consequently a decrease in SQg and S' (Equations 16, 23, and 27). From Equations 16 and 18, it is apparent that as S' approaches zero, Q approaches P. In other words, as S' decreases, the calculated runoff becomes closer to being equal to the net rainfall which is most ARk often, when frozen soil conditions exist, predominantly snowmelt. This will result in a decrease in infiltration under frozen soil conditions, which has been observed in numerous studies. Summary of Daily Runoff Computation The HELP model determines daily runoff by the following procedure: 1) Given CNII, CNI and S,, are computed once using Equations 28 and 27. 2) S is computed daily using Equations 24 and 25. 3) The daily runoff resulting from the daily rainfall and snowmelt is computed using Equation 18. 2.3.3 Prediction of Frozen Soil Conditions In cold regions, the effects of frozen soil on runoff and infiltration rates are significant. Because of the necessary complexity and the particular data requirements of any approach to estimating soil temperatures, the inclusion of a theoretically-based frozen soil model in the HELP program is prohibitive for the purposes of the program. However, for some regions, it is desirable to have some method for predicting the occurrence of frozen soil and the resulting increase in runoff. 28