CIV E 634 - SURFACE WATER HYDROLOGY
FALL 2020
HOMEWORK 3:   RUNOFF CURVE NUMBER

Calculate the runoff curve number for watershed 68013 (WS-13), in Reynolds, Idaho, which has the rainfall-runoff (P-Q) data shown below (inches) (ordered series of annual maxima).

  • Plot the data in the P-Q plane.

  • For each P-Q pair, use Eq. 17 in the runoff curve number paper to calculate the potential retention S.

  • For each S, use Eq. 9 in the runoff curve number paper to calculate a CN.

  • The calibrated curve number is the average of the set of CN's, rounded to the nearest integer.

  • Use the calibrated CN obtained in step d and Eq. 10 in the runoff curve number paper to plot a curve in the P-Q plane. Note the constraint to Eq. 10.

  • Compare the curve plotted in step e with the raw P-Q data plotted in step a.

  • To what do you attribute the apparent spread in the data?


P(1) = 0.07
P(2) = 0.08
P(3) = 0.15
P(4) = 0.16
P(5) = 0.21
P(6) = 0.22
P(7) = 0.25
P(8) = 0.30
P(9) = 0.36
P(10) = 0.39
P(11) = 0.52
P(12) = 0.57
P(13) = 0.69
P(14) = 0.76
P(15) = 0.84
P(16) = 0.92
P(17) = 0.98
P(18) = 1.72
Q(1) = 0.0471
Q(2) = 0.0121
Q(3) = 0.0051
Q(4) = 0.0047
Q(5) = 0.0714
Q(6) = 0.0117
Q(7) = 0.0165
Q(8) = 0.0180
Q(9) = 0.0074
Q(10) = 0.3660
Q(11) = 0.0758
Q(12) = 0.1060
Q(13) = 0.6200
Q(14) = 0.0761
Q(15) = 0.1162
Q(16) = 0.3275
Q(17) = 0.1390
Q(18) = 0.1614