Chapter 1
INTRODUCTION
1.1. What is Hydrology?
A science that deals with the waters of earth
their occurrence, distribution, and circulation of earths
waters as well as their chemical and physical
properties and their interaction with the environment
and living things.
1.2. Worlds Water Resources
The Worlds total water resources are
estimated at 1.36 10
8
ha-m.
Distribution of nearly 35 million km fresh
water resources of the world (after UNESCO 2003)
1.3. Scope of Hydrology
The study of hydrology helps us to know
1. The maximum probable flood that may
occur at a given site and its frequency; this is
required for the safe design of drains and
culverts, dams and reservoirs, channels and
other flood control structures.
2. The water yield from a basinits
occurrence, quantity and frequency, etc; this
is necessary for the design of dams, municipal
water supply, water power, river navigation,
etc.
3. The ground water development for which a
knowledge of the hydrogeology of the area
(i.e. the formation soil, recharge facilities like
streams and reservoirs, rainfall pattern,
climate, cropping pattern, etc.) are required.
4. The maximum intensity of storm and its
frequency for the design of a drainage project
in the area.
1.4. Hydrological Data
For the analysis and design of any hydrologic
project adequate data and length of records are
necessary. The basic hydrological data required are:
1. Climatological data
2. Hydrometeorological data like temperature,
wind velocity, humidity, etc.
3. Precipitation records
4. Stream-flow records
5. Seasonal fluctuation of ground water table or
piezometric heads
6. Evaporation data
7. Cropping pattern, crops and their consumptive
use
8. Water quality data of surface streams and
ground water
9. Geomorphologic studies of the basin, like
area, shape and slope of the basin, mean and
median elevation, mean temperature (as well
as highest and lowest temperature recorded)
and other physiographic characteristics of the
basin; stream density and drainage density;
tanks and reservoirs
10. Hydrometeorological characteristics of basin:
a.a.r., long term precipitation, space
average over the basin using isohyets and
several other methods (Rainbird, 1968)
Depth-area-duration (DAD) curves for
critical storms (station equipped with self-
recording raingauges).
Isohyetal mapsIsohyets may be drawn
for long-term average, annual and monthly
precipitation for individual years and
months
Cropping patterncrops and their
seasons
Daily, monthly and annual evaporation
from water surfaces in the basin
Water balance studies of the basin
Chronic problems in the basin due to a
flood-menacing river
Soil conservation and methods of flood
control
1.5. Hydrologic Cycle and Hydrologic Budget
Equation
The hydrologic equation states that for a given
time interval, difference of inflow to and outflow from a
system is equal to change of storage of the system.
Mathematically,
I - O = S/t
Where,
I = Rate of volume inflow (volume/time)
O = Rate of volume outflow (volume/time)
S/t = Rate of change of storage in time
The above equation is a storage equation which
only approximates some hydrologic processes.
The water budget of a catchment for a time
interval t is written as:
P R G E T = S
P = Precipitation
R = Surface runoff,
G = net ground water flow out of the catchment
E = Evaporation
T = Transpiration
S = change in storage
Sample Problem 1:
Flow of River Chenab at Marala Barrage varied
linearly from 34 m
3
/sec to 283 m
3
/sec in 10 hours during
a flood. The flow variation at Khanki Barrage,
downstream of Marala was observed to be from 28 to
255 m
3
/sec during the above mentioned time. Assuming
no lateral flow in or out of the reach, find out the rate of
change of storage of the river reach between Marala and
Khanki. What is total change in storage of the reach in
this period?
Problem Sets
1. A lagoon has a surface area of 350.5 x 10
6
m
2
. The
average annual rainfall and evaporation are
obtained as 1850.4 mm and 1142.7 mm,
respectively. If the increase in storage is 247.8 x 10
6
m
3
/yr, obtain the net annual inflow into the lagoon.
What are the hydrologic components in the net
inflow?
2. A city is supplied by water from a 1250-ha
catchment area. The average water consumption of
the community is 50,000 m
3
/day. The annual
precipitation in the region is 412 cm. a river with an
average annual flow of 0.35 m3/s originates in and
flows out of the catchment area. If the net annual
groundwater outflow from the area is equivalent to
16-cm depth of the water, what is the
evapotranspiration loss in m
3
/yr, which, if exceeded,
would cause shortage of the water supply to the
community? Assume that the storage of water in the
area at the beginning and at the end of the year are
equal.
3. Determine the volume of water lost through
evapotraspiration during a year from the surface of 1
1500-ha lake located in a region where the annual
rainfall is 135 cm. the increase in the depth of the
lake over the year is 10 cm. Neglect the effect of
groundwater flow.
