Why am I getting a runoff value of 0 in the peak discharge computation?
Runoff (Q) is computed as a function of Rainfall (P) and curve number (CN). If you look at the runoff equation in the appendix of the Flash55 manual, you will see that the term on top (in brackets) is squared. This bracketed value is not allowed to go below 0, or else decreasing values of rainfall (P) can produce increasing runoff, which we know intuitively is not true. This condition occurs with very small values of P or CN. For a given value of P, you can determine the minimum value of CN which is required to produce a partial result which is greater than or equal to zero (ie, P=0.40 requires CN>=83.3), and which is therefore the minimum CN value to produce runoff. These values agree closely with SCS TR55 Figure 2-1, “Solution of runoff equation” and Table 2-1, “Runoff depth for selected CN’s and rainfall amounts”. • Why am I getting a much greater runoff value with Flash55 than with Flash20/TR-20? Check and see if the affected areas have CN values below 40. Using the TR-55 method, t
