RAINFALL-RUNOFF RELATIONSHIP

Surface runoff- runoff is generated when the rainfall rate exceeds the infiltration capacity and then overland flows occur in portions of

the watershed (sometimes run in small rivulets or channels before

reaching the larger streams).

Subsurface flow- some portion of water that infiltrates moves rapidly downslope through the unsaturated zone to stream channels.

Baseflow- water infiltrates the ground and percolates the saturated zone, then groundwater flow in the saturated zone discharges into

the stream.

Saturation excess runoff- sometimes runoff is generated when the water table reaches the ground surface as in the case of spring.

Streamflow Generation Mechanism

Gaining Stream- baseflow entering stream. Typical in humid

regions. Discharge increases as you go downstream even no

rainfall or tributaries.

Losing Stream- water table lower than the streams water level. Loss of water occurs as you go downstream. Rate of loss is a

function of the depth of water and hydraulic conductivity of

underlying alluvium.

During baseflow recession, a stream may be gaining but

become a losing stream during floods.

Groundwater pumping near a stream can drop the water table

locally and cause a section of stream to be losing, while it is

gaining up and downstream.

THE WATERSHED

Watershed

Area draining to a stream

Streamflow generated by rainfall then water entering surface channels

Affected by

Physical, vegetative, and climatic features

Geologic considerations

Stream Patterns

Dry periods

Flow sustained from groundwater (baseflow)

Components of Streamflow

The rational method is the simplest rainfall-runoff formula, which allows the prediction of peak flow.

Only gives the maximum value of the flood hydrograph and not the complete runoff hydrograph.

Applicable for estimating storm peak runoff for areas not more than 81 hectares (0.81 km2).

Assumes that the rate of runoff resulting from any rainfall intensity is maximum when the rainfall intensity lasts as long as the time of

concentration. This assumption limits the size of the watershed that

can be evaluated by the rational method.

Estimates the peak flow in a watershed as a function of the drainage area, runoff coefficient, and mean rainfall intensity for

duration equal to the time of concentration.

Rational Method

where,

Q = peak discharge in m3/s

c = runoff coefficient

i = rainfall intensity at time of concentration

A = drainage area

ciAQ

Runoff coefficient- represents the fraction of the rainfall converted to runoff.

where,

i = rainfall intensity

tc = time of concentration

a, b = regression coefficients

Rainfall intensity- is a function of geographic location and design exceedence frequency or return interval. The longer the return

interval the greater the precipitation intensity for a given storm

duration. The longer the length of storm, the lower the storm average

precipitation intensity.

The relation between these 3 components, storm duration, storm intensity and storm return interval is represented by a family of curves

called intensity duration frequency curves. Sometimes approximated

by the following functional form:

bta

ic

where,

tc = time of concentration (min)

L = length of longest flow path (m)

S = weighted average slope of the basin

Time of concentration- the time required for a parcel of runoff to travel from the most hydraulically distant part of a watershed to the

outlet. Time basis for the selection of rainfall intensity for application

of the rational method. Approximated by the formula:

385.0

77.00195.0

S

Ltc

Rainfall Intensity-Duration-Frequency Data for Surigao Del Norte

Based on 15 Years of Record

Computed Extreme Values (in mm) of Precipitation

yrs mins mins mins mins mins mins mins mins mins mins mins hrs hrs hrs hrs

2 16.2 24.4 31.9 37.8 47.5 57.8 64.4 73.3 80.5 86.2 93.7 101.5 133.5 165.6 196.1

5 22.9 34.2 44.7 51.8 64.1 77.9 86.8 99.6 11.2 120.7 132.6 144.5 193.0 242.1 280.5

10 27.3 40.7 53.1 61.1 75.1 91.1 101.6 116.9 131.5 143.6 158.5 173.1 232.4 292.7 336.5

15 29.8 44.4 57.8 66.3 81.4 98.6 109.9 126.7 143.0 156.6 173.0 189.2 254.7 321.2 368.0

25 32.9 49.0 63.7 72.8 89.1 107.9 120.3 138.9 157.2 172.6 191.1 209.1 282.3 356.6 407.1

50 37.0 55.1 71.6 81.4 99.4 120.4 134.2 155.1 176.2 194.0 215.3 235.8 319.2 404.0 459.6

100 41.1 61.1 79.4 90.0 109.7 132.7 148.0 171.3 195.2 215.3 239.3 262.4 355.9 451.1 511.6

Equivalent Average Intensity (in mm/hr) of Computed Extreme Values

yrs mins mins mins mins mins mins mins mins mins mins mins hrs hrs hrs hrs

2 194.4 146.4 127.6 113.4 95.0 77.1 64.4 55.0 48.3 43.1 37.5 33.8 22.3 13.8 8.2

5 274.8 205.2 178.8 155.4 128.2 103.9 86.8 74.7 6.7 60.4 53.0 48.2 32.2 20.2 11.7

10 327.6 244.2 212.4 183.3 150.2 121.5 101.6 87.7 78.9 71.8 63.4 57.7 38.7 24.4 14.0

15 357.6 266.4 231.2 198.9 162.8 131.5 109.9 95.0 85.8 78.3 69.2 63.1 42.5 26.8 15.3

25 394.8 294.0 254.8 218.4 178.2 143.9 120.3 104.2 94.3 86.3 76.4 69.7 47.1 29.7 17.0

50 444.0 330.6 286.4 244.2 198.8 160.5 134.2 116.3 105.7 97.0 86.1 78.6 53.2 33.7 19.2

100 493.2 366.6 317.6 270.0 219.4 176.9 148.0 128.5 117.1 107.7 95.7 87.5 59.3 37.6 21.3

3 6 12 2480 100 120 15020 30 45 60Return

Period5 10 15

3 6 12 2480 100 120 15020 30 45 60Return

Period5 10 15

Rainfall-Duration-Frequency Curves

Surigao, Surigao Del Norte

0.0

80.0

160.0

240.0

320.0

400.0

480.0

560.0

0 2 4 6 8 10 12 14 16 18 20 22 24

Time (hrs)

Rain

fall

(mm

)

100-Yr Return Pedriod

50-Yr Return Period

25-Yr Return Period