Rainfall-Runoff Relation

Surface runoff refers to the draining or flowing off of a precipitation from a catchment area through a surface channel. It represents the output from the catchment in a given time interval. It occurs when the excess precipitation moves over the land surface to reach smaller channels. Then flows from smaller channels combine to form bigger channels until it reaches the catchment outlet.

definition of hydrograph

Hydrograph is the graphical representation of the instantaneous rate of discharge of a stream plotted with respect to the time in a given point. It is a special graph that correlates the river's discharge over a period of time and rainfall event.
The hydrograph is divided into several components, which includes

Rising Climb
Falling or Recession Climb
Peak discharge
Lag time
Time to Peak
Discharge

types of hydrograph

The hydrograph is classified into different types which are as follows:

Annual Hydrograph
Monthly Hydrograph
Seasonal Hydrograph
Flood Hydrograph
Unit Hydrograph
Storm Hydrograph
Synder's Synthetic Unit Hydrograph

factors contributing to hydrograph shape

The factors that affect the hydrograph shape is broadly grouped into climatic factors and physiographic factors. Each of the group contains a set of factors as shown in the left figure.

hydrograph separation

It refers to the various processes used to decompose hydrographs. Surface flow hydrographs are obtained from total storm hydrograph by separating the quick response flow from the slow response runoff.
Only the baseflow needs to be deducted from the total storm hydrograph in order to obtain the surface flow hydrographs.
There are 3 graphical methods used in decomposing hydrographs:

Straight line method
Fixed base method
Variable slope method

unit hydrograph

Unit hydrograph is defined as a direct runoff hydrograph resulting from 1 cm of excess rainfall generated uniformly over the drainage area at a constant rate for an effective duration.
Shown in the figure are the assumptions, limitations and applications of using unit hydrograph.

estimation of discharge using unit hydrograph

There are four methods used for estimating the peak discharge. These are:

Physical conditions of the past floods
Flood discharge formula
Flood frequency studies
Unit Hydrograph

Shown in the right figure are the steps on how to prepare a unit hydrograph for estimating peak discharge from an isolated storm.

sample problems

rational formula

It is most widely used for analysis of runoff response from a small catchment.
This method take into account the following hydrological characteristics such as rainfall intensity, duration, frequency, catchment area, hydrologic abstraction, and runoff concentration.
The steps in using rational formula, and the equations for calculating the peak value and its corresponding parameters are shown in the figure.

sample problems

SCS CURVE METHOD

SCS Curve method is a recognized, predictable, and stable conceptual method used for estimation of direct runoff depth based on storm rainfall depth. It depends on the Curve Number (CN) parameter.
The Curve Number method is based on two phenomena, the so-called initial abstraction which refers to the initial accumulation of rainfall before the start of runoff and actual retention, which refers to the amount of excess that is lost due to infiltration.
Shown in the figure is the curve number equation derived from various equations.

sample problems

assessment tasks:

PROBLEM SET #5

Rainfall Runoff Relation_BARTOLOME.pdf

QUIZ #5

Rainfall-Runoff Relation_BARTOLOME (1).pdf

VIDEO PRESENTATION LINK: https://youtu.be/PXpTs2GgzVs

REFLECTION:

It is evident that a runoff is affected by various rain and soil characteristics such as rainfall intensity, soil infiltration rates, and storage capacity. In order to simulate runoff processes in a more precise manner, it would be better to use graphical representations estimating the rainfall discharge rate such as in hydrographs. This would ensure significant kinds of hydrologic data that are necessary for calculating runoff from a single rainstorm event. Thus, the video presentation has precisely explained the interrelationships between the intensity of rain, peak rate of runoff, and efficiency of surface storage, which are determined empirically using various equations such as rational formula and SCS-CN method.

Page updated

Report abuse