Water Resources Engineering

Hydrology & Irrigation

Rational-method runoff, the unit hydrograph concept and the duty–delta relationship of irrigation.

PART 1

Topic Breakdown & Traps

The Engineering Principle

The rational method estimates peak runoff from rainfall intensity, catchment area and a runoff coefficient. A unit hydrograph is the direct-runoff response to

1 cm

of effective rain in unit time. In irrigation, duty, delta and base period are linked by a single relation.

The Core Formula Matrix

Rational method:

Q = \frac{C I A}{360}

(

A

in ha,

I

in mm/hr,

Q

in m³/s).

Duty–delta:

Δ = \frac{8.64 B}{D}

(m),

B

=base period (days),

D

=duty (ha/cumec).

Runoff depth: runoff

=

rainfall

-

losses.

The ‘IIT Traps’

⚠
Unit care in the rational method. The $360$ factor pairs hectares, mm/hr and m³/s.
⚠
Delta is a depth (m), duty is an area per unit discharge — keep them distinct.
⚠
Effective rainfall ≠ total rainfall for a unit hydrograph; subtract abstractions.

📚 Standard references

Engineering Hydrology — K. Subramanya
Irrigation Engineering and Hydraulic Structures — S.K. Garg

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · NAT

A storm produces

80 mm

of rainfall with abstractions of

30 mm

. The direct runoff depth is _____ mm.

Q2MEDIUM2 Marks · NAT

For a catchment of

20 ha

C = 0.6

and intensity

50 mm/hr

, the peak runoff (rational method) is _____ m³/s.

Q3HARD2 Marks · NAT

For a base period of

120 days

and a duty of

800 ha/cumec

, the delta is _____ m.