Rainfall Runoff Relationships

Instructional Objectives
At the end of this lesson,
the student shall learn
1. How hydrograph varies with the catchment characteristics
2. How hydrograph varies with the rainfall characteristics
3. What is Unit Hydrograph,
its assumptions and limitations
4. Application of the
Unit Hydrograph to find the Direct Runoff Hydrograph
5. What is S – Curve and its applications
6. Derivation of the Unit
Hydrograph for gauged catchments
7. How to estimate Unit
Hydrograph for ungauged catchments
8. Conceptual and Physically based catchment rainfall – runoff models
2.3.0 Introduction
Lesson 2.2 it was explained
what a hydrograph is and
that it indicates the
response of water flow of a given catchm
ent to a rainfall input. It consists of
flow from different phases of runoff, like
the overland flow,
interflow and base
flow. Methods to separate base flow from the total stream flow hydrograph to
obtain the direct runoff hydrograph as we
ll as infiltration loss from the total
rainfall hyetograph to determine the effect
ive rainfall have been discussed. In
this lesson, a relationship between the direct runoff hydrograph of a
catchment observed at a location (the
catchment outlet) and the effective
rainfall over the catchment causing t
he runoff are proposed to be dealt with.
We start with discussi
ng how the various aspects of a catchment’s
characteristics affects t
he shape of the hydrograph.
2.3.1 Hydrograph and the catchment’s characteristics
The shape of the hydrograph depends
on the characteristics of the
catchment. The major factors are listed below.
2.3.1.1 Shape of the catchment
A catchment that is shaped in the form
of a pear, with the narrow end towards
the upstream and the broader end nearer
the catchment outlet (Figure 1a)
shall have a hydrograph that is fast rising and has a rather concentrated high
peak (Figure 1b).
Version 2 CE IIT, Kharagpur