658003b0-3eff-4cf2-93f7-bfb8cfd3cc32 2 4 5 decimal

&[DATE] &[TIME] - &[FILENAME]

Estimating Runoff - SCS Runoff curve number method (Not for CN less than 40)

-SCS runoff equations is: where: Q=runoff (in) P=rainfall (in) S=potential maximum retention after runoff begins (in) Ia=initial abstraction (in)

Q P I.a - (2^(P I.a - (S + / :

-Initial abstraction (Ia) is all losses before runoff begins. It includes water retained in surface depressions, water intercepted by vegetation, evaporation, and infiltration. Ia is highly variable but generally is correlated with soil and cover parameters. Through studies of many small agricultural watersheds, Ia was found to be approximately by the following empirical equation: Ia = 0.2S

I.a 0.2 S * :

-S is related to the soil and cover conditions of the watershed through the CN:

S 1000 CN / 10 - :

-Removing Ia as an independent parameter the runoff equation becomes:

Q P 0.2 S * - (2^(P 0.8 S * + (/ :

-Considering the Entire Site Characteristics (All C-type soils):

A 1.545 acre * :

P 2.40 in * :

-Predevelopment scenario:

CN.pre 92 :

S.pre 1000 CN.pre / (10 - :

S.pre 0.8696

-Predevelopment runoff Q in inches and volume in ac-ft

Q.1yrpre P 0.2 S.pre * (- (2^(P 0.8 S.pre * + (/ :

Q.1yrpre #

RV.1yrpre Q.1yrpre A * :

RV.1yrpre #

-Postdevelopment scenario:

CN.post 91 :

S.post 1000 CN.post / (10 - :

S.post 0.9890

-Postdevelopment runoff Q in inches and volume in ac-ft

Q.1yrpost P 0.2 S.post * (- (2^(P 0.8 S.post * + (/ :

Q.1yrpost #

RV.1yrpost Q.1yrpost A * :

RV.1yrpost #