Land and Watershed Management - Practices of Irrigation and On-farm Water Management - page 218

The basic assumption of the SCS curve number method is that, for a single storm,
the ratio of actual soil retention after runoff begins to potential maximum retention
is equal to the ratio of direct runoff to available rainfall.
Although this is an accepted method for runoff estimation, studies have indicated
that it should be evaluated and adapted to regional agro-climatic condition. The
curve number tables should be used as guidelines and actual curve numbers and
their empirical relationships should be determined based on local and regional data.
Curve number method does not account for the antecedent moisture condition - less
runoff under dry conditions and more runoff under wet conditions.

6.5.3.3 Sample Workout Problems on Runoff Estimation

Example 6.1

A field at East-Central Illinois was seeded with pasture. The soil of the field is silt-
loam. The magnitude of a storm event was 70 mm. The potential maximum retention
after runoff begins was approximated at 25 mm. Estimate the storm runoff from the
field.

Solution

0.2 S ) 2

( P −

We know, runoff estimates by SCS method, Q =

P + 0.8 S

Here,
P = 70 mm

S 25 mm

Putting the values in above equation, Q

=

25) 2 (70 − 0.2 ×

= =46.9mm(Ans.)

70 + 0.8 × 25

Example 6.2

A rice field in North China Plain consists of 12 ha clay-loam soil having moderate
organic matter. The topography of the basin is flat, and each unit plot is surrounded
by 10 cm levee. The rainfall recorded at a nearby weather station for a particular
storm event was 120 mm. Estimate the runoff from the field.

Solution

0.2 S ) 2 ( P −

We know runoff Q =

P + 0.8 S

Here,
P 120 mm

For the prevailing field condition, Assuming S

=

= =

11 cm 110 mm

110) 2

(120 − 0.2 ×

Thus, Q =

110 =

46 mm

120 + 0.8 ×

Runoff volume, V = Area × runoff depth