2. Hydrologic Cycle and
Concepts
2.1
Description of Hydrologic Cycle
The demand for water is continuous. In order to appropriate the available
water, one should be familiar with elements that dictate the movements of
water. For simplicity, where does the water come from? When is water in
motion, and is it in liquid or vapour form? Are there factors that intercept,
trap and remove water by some means? Is the water stored during
movement? How much is withdrawn by users? Since the hydrologic cycle
describes the process of water movement it is essential to thoroughly
understand the concepts of hydrology which provides a basis for the
underlying principles of a water budget. This section describes the
hydrologic cycle and its components, as one would observe in the
watershed.
The hydrologic cycle describes the processes of motion, loss and recharge
of the earth’s water. The processes can be visualized as shown in Figure
2.1. The cycle may be divided into the following principle components or
phases:
a)
b)
c)
d)
e)
f)
g)
h)
Precipitation
Interception
Evaporation / Evapotranspiration
Infiltration
Percolation
Runoff (Surface and Groundwater)
Storage (Surface and Groundwater)
Water Use and Demand
At any point in each of these phases, water may be in the state of
transportation, a state of temporary or long-term storage, or in a state of
change between solid, liquid and gaseous flow.
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Water Budget Analysis on a Watershed Basis
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Figure 2.1:
Hydrologic Cycle - Visual Definition of Main
Hydrologic Components (after EPA, 1998)
Any exposed surface may be considered as a unit area on which the
hydrologic cycle operates. For example:
•
an isolated tree;
•
the roof of a building;
•
the drainage basin of a river system or any of its tributaries;
•
a lake;
•
an ocean;
•
a country;
•
a continent; and
•
the earth as a whole.
A water balance is based upon the above noted factors and, is expressed as
a mathematical equation showing the relationship among the various
hydrologic components. The equation is as follows:
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Water Budget Analysis on a Watershed Basis
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P - E -T - I nt - Ds + Qin + Gin - Q out - Gout ± ∆S = 0
where:
P
E
T
Int
Ds
Qin
Qout
Gin
Gout
=
=
=
=
=
=
=
=
=
Precipitation;
Evaporation;
Transpiration;
Interception;
Detention Storage;
Surface Inflow;
Surface Outflow;
Groundwater Inflow;
Groundwater Outflow; and
∆S
=
Changes in Storage.
While precipitation generates the main input to the cycle, evaporation and
transpiration constitute the main losses of water from the groundwater and
surface water systems to the atmosphere. A schematic representation of
the hydrologic cycle is shown on Figure 2.21 .
From a global perspective, the world’s supply of water is stored mainly in
oceans (97.2%), while 2.15% is frozen water (ice caps) and 0.65% is fresh
water on land. Of this fresh water component, only 2.5% makes up
quantities in air, soil, lakes and rivers, while the remainder is groundwater,
most of which is unavailable for human use (Vander Leeden et al, 1990).
In a watershed, about one half to one third of precipitation enters the earth,
while some of the available water remains at the surface. The quantity of
precipitation that is available for runoff is a function of the topography, the
soil infiltration and the holding capacity of the surface. The mass balance
expresses the relationship of rainfall excess or runoff, R, to be equal to the
product of the precipitation, P, times the runoff coefficient, C, (R = CP),
where the storage is ∆S = (1-C) P. This runoff may be routed to streams,
sewers, detention ponds or reservoirs. Typical uses of the stored water are
recreation, dilution capacity, water supply, irrigation, hydroelectric power,
flood control, low flow augmentation, navigation, and natural resources
enhancement.
1
A complete list of definitions and terms is provided in the List of Symbols, Abbreviations and
Glossary (Appendix A).
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Water Budget Analysis on a Watershed Basis
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Legend *
ATMOSPHERIC
ET
P
ET
P
ET
P
E
VEGETATION
INT
LAND SURFACE
R
SR
P
ET
IF
SR
INT
=
=
=
=
=
I
=
RIVERS
PER =
I
IF MEDIA
SOIL
Figure 2.3: Canada Water Uses
IF
WETLANDS
PER
AQUIFER
Gw
STREAMS
BF
BF
=
Gw
=
LAKES
Precipitation
Evapotranspiration
Interflow
Surface Runoff
Interception by
Vegetation
Infiltration from
Land Surface to
Soil Media
Percolation from
Soil Media to
Groundwater
(recharge)
Base Flowprovided
by Groundwater to
Rivers and Streams
Groundwater
Underflow
OCEAN
Figure 2.2: A System Representation of the Hydrologic Cycle*
In Canada, major water uses are broken down as follows. Thermal and
hydroelectric power generation use about 60%, manufacturing 20%,
agriculture 9% and municipal about 11% (Environment Canada, 1992,
Freshwater Series A-6) as shown in Figure 2.3.
Figure 2.3: Canada – Water Uses
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Water Budget Analysis on a Watershed Basis
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