2. Lecture Notes_Hydrologic Cycle_Water Balance_24.pdf

Hydrologic Cycle and Water Balance

1. Hydrologic Cycle
And Water Balance

2. 2

3. •Water cycle is in balance, and the amount of precipitation
falling will slowly soak into the ground and eventually reach
the rivers
•The hydrological cycle is a good example of a closed
system: the total amount of water is the same, with virtually
no water added to or lost from the cycle.
•Water just moves from one storage type to another.
•Water evaporating from the oceans is balanced by water
being returned through precipitation and surface runoff.
Hydrological Cycle

4. 4
•The Global hydrologic cycle is one of the most important natural cycles that
operates on Earth.
•It describes the circulation of water through the atmosphere, the land, and the
oceans.
•This cycle consists of set of storages (snow, soil moisture, and groundwater)
and fluxes (precipitation, evapotranspiration, and runoff) that link the storages
together.
•Precipitation is the main driver of the water cycle and, on average, 70% of
annual precipitation is lost due to evapotranspiration.
Hydrologic Cycle

5. 1Precipitation may be in the form of rain or snow.
2Vegetation may intercept some fraction of
precipitation. A large fraction of intercepted water is
commonly evaporated back to the atmosphere.
3
How it Works : Language and Terminology
Precipitation that penetrates the vegetation is referred to as
throughfall and may consist of both precipitation that does
not contact the vegetation, or that drops or drains off the
vegetation after being intercepted.
5

6. 1
2
How it Works : Language and Terminology
There is also flux of water to the atmosphere through
transpiration of the vegetation and evaporation from soil
and water bodies, Evapotranspiration
The surface water input available for the generation of runoff
consists of throughfall and snowmelt.
Surface water input may accumulate on the surface in
depression storage, or flow overland towards the
streams as overland flow, or infiltrate into the soil,
where it may flow laterally towards the stream contributing to
interflow.
3
6

7. 1
2
3
How it Works : Language and Terminology
4
Infiltrated water may also percolate through deeper soil and rock
layers into the groundwater. The water table is the surface
below which the soil and rock is saturated and at pressure greater
than atmospheric.
Water table serves as the boundary between the saturated zone
containing groundwater and unsaturated zone. Water added to the
groundwater is referred to as ground water recharge.
Immediately above the water table is a region of soil that is close
to saturation, due to water being held by capillary forces. This is
referred to as the capillary fringe.
Lateral drainage of the groundwater into streams is referred to as baseflow, because it sustains
streamflow during rainless periods.
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8. 1
2
3
How it Works : Language and Terminology
4
Subsurface water, either from interflow or from groundwater
may flow back across the land surface to add to overland flow.
This is referred to as return flow.
Overland flow and shallower interflow processes that transport
water to the stream within the time scale of approximately a day
or so are classified as runoff.
The terms quick flow and delayed flow are also used to
describe and distinguish between runoff and baseflow.
Runoff includes surface runoff (overland flow) and
subsurface runoff or subsurface stormflow (interflow).
8

9. Exercise
Label the Rainfall-Runoff processes
depicted in the figure
9

10. Solution
A
B
C
D
E
F
G
H
I
J
10

11. Water Balance
•A water balance can be established for any area of earth’s surface by calculating the total
precipitation input and the total of various outputs.
•The water balance approach allows an examination of the hydrological cycle for any period of
time.
•The purpose of the water balance is to describe the various ways in which the water supply is
expended.
•The water balance is a method by which we can account for the hydrological cycle of a specific
area, with emphasis on plants and soil moisture.
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12. 12
“The mass in an isolated system can neither be
created nor be destroyed but can be transformed
from one form to another”

13. Water Balance
•The water balance is defined by the general hydrologic cycle equation, which is basically a statement
of the law of conservation of mass as applied to the hydrologic cycle. In its simplest form, this
equation reads
•Inflow - Outflow = Change in Storage
Inflow = Outflow + Change in Storage

•Water balance equations can be assessed for any area and for any period of time.
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The hydrologic cycle for a natural system is characterized by a water mass balance
equation:
Precipitation=Runoff + Infiltration + Evapotranspiration + ΔStorage.
Water Balance
Inflow - Outflow = Change in Storage
Inflow = Outflow + Change in Storage

15. 15

16. What is the water balance/budget?
In order to gain better understanding of water resources in a drainage basin, we use a
simple equation called the water balance.
The balance between water inputs and outputs of a drainage basin, may be shown as
water budget graph-that is, the drainage basin operates as Water Budget Equation of a
catchment for a time interval ∆t is written as: Input – Output = Change in Storage
P – R – G- E- T = ∆S
Where:
P = Precipitation
R = Surface runoff
E = Evaporation
T= Transpiration
G = Net groundwater flow out of the catchment
∆S = Change in Storage .
16

18. •A water balance can be established for any area of earth’s
surface by calculating the total precipitation input and the total
of various outputs.
•The water balance approach allows an examination of the
hydrological cycle for any period of time.
•The purpose of the water balance is to describe the various ways
in which the water supply is expended.
Applications of Water Balance Equation

19. •A region with an area of 1750 Km
2
received 1250 mm of annual
precipitation. Calculate the total amount of rainfall occurred in the
year (in m
3
)

20. •A catchment with an area of 1750 Km
2
received 1250 mm of annual
precipitation. Calculate the total amount of rainfall occurred in the
year (in m
3
)
•Sol: Area of the catchment = 1750 Km
2
= 1750 X 10^6 m
2
•Precipitation received = 1250 mm = 1.25 m
•Total annual precipitation = 1.25 X 1750X10^6 = 2187.5 X 10^6 m
3

21. Example Problem:
Precipitation over an area = 157500 m
3
Runoff volume = 54000 m
3
Estimate the Losses occurred in the considered region ?
21

22. Example Problem:
Precipitation over an area = 157500 m
3
Runoff volume = 54000 m
3
Losses = 157500 – 54000 = 103500 m
3
22

23. In a given year, a catchment with an average area of 1750 km2 received 1250 mm of
precipitation. The average rate of flow measured in a river draining the catchment
was 25 m3/s. Estimate the Losses ?

24. In a given year, a catchment with an average area of 1750 km2 received 1250 mm of
precipitation. The average rate of flow measured in a river draining the catchment
was 25 m3/s. Estimate the total flow occurred in the year along with the runoff
coefficient.
Precipitation = 1.25 X 1750 X 10^6 = 2187.5 X 10^6 m3
Total rainfall = 2187.5 X 10^6 / (365 X 24 X 60 X 60) = 69.36 m3/s
Total rate of flow = 25 m3/s
Runoff coefficient = Actual flow / Total precipitation occurred (Runoff/Rainfall)
Runoff coefficient = 25 / 69.36 = 0.36 (36%)
Runoff of the catchment / Rainfall of the catchment = 36 %
That is 36% of rainfall is converting into runoff and remaining 64% is losses.

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A lake has an area of 15 km2. Observation of Hydrological variables during a certain year :
P = 700 mm/year
Average Inflow, Q_in = 1.4 m3/s
Average Outflow, Q_out = 1.6 m3/s
Assume that there is no net water exchange between the lakes and the groundwater. Determine the
evaporation during this year.

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Sum of Inflows to the lake – sum of Outflows from the lake = Change in the Volume
(P + Q_In) - (E+ Q_out) = ∆S
Assuming storage changes over one year as Zero , ∆S = 0
E = P + Q_in – Q_out
Q_in = 1.40 X 3600 X 24 X 365 / 15 X 10^6 = 2943.4 mm
Q_Out = 1.60 X 3600 X 24 X 365 / 15 X 10^6 = 3363.8 mm
E = 700 + 2943.4 +3363.8 = 279.6 mm