Antecedent Moisture Condition
Ponce (1989) reported for the case of low curve numbers or rainfall depths, the curve number method is very sensitive to the curve number and antecedent moisture conditions. NEH-4 (SCS, 1985) showed an appropriate Antecedent Moisture Condition (AMC) level based on a 5-day antecedent rainfall for dormant and growing seasons. However, due to concerns such as not accounting for regional differences or scale effects, it was removed from the NEH-4 release of 1993 (Ponce and Hawkins, 1996). During the CN development, the natural scatter of points around the median CN was interpreted as a measure of the natural variability of soil moisture and associated rainfall-runoff relationships. The average curve numbers are the median values used to represent the average response of a site for given soil, cover and surface conditions. The theoretical limits of CN values are 0 and 100. The enveloping CN values were defined using P-Q plots for each site. This provided practical limits of CN values including accounting for the observed variability of CN values. Hence, the observed variability of rainfall-runoff response was represented by the AMC parameter (Ponce and Hawkins, 1996). Table below shows the curve numbers developed by SCS for AMC I (dry conditions), AMC II (average conditions) and AMC III (wet conditions).
CN FOR AMC II |
CN FOR AMC I |
CN FOR AMCIII |
98 |
98 |
98 |
95 |
87 |
98 |
90 |
78 |
96 |
85 |
70 |
94 |
80 |
63 |
91 |
75 |
57 |
88 |
70 |
51 |
85 |
65 |
45 |
82 |
60 |
40 |
78 |
55 |
35 |
74 |
50 |
31 |
70 |
45 |
26 |
65 |
40 |
22 |
60 |
35 |
18 |
55 |
30 |
15 |
50 |