Hydrological Processes in a Drainage Basin:
A. The Hydrological Cycle
http://www.aquatic.uoguelph.ca/general/general3.htm
1. What is the hydrological cycle?
Water of oceans, atmosphere, and lands move in a great series of continuous
interchanges of both geographic position and physical state known as the hydrological cycle.
When the rainfall that has not been intercepted by vegetation reaches the ground
surface, part of it fills small surface depression (surface storage), part percolates into the soil,
and may reach the stream at a later time as ground water flow. The remainder, if any, flows
over the surface as overland flow. Through the rivers the water reaches the sea to be
evaporated again. Each component of this equation is highly variable, and depends not only o
the intensity of the rainfall, but also on soil, vegetation and surface gradient. The amount of
water intercepted by vegetation depends on the type of plants and their stages of growth.
Rainfall reaching the soil surface has to fill the small depression on the surface before any
overland flow can occur, even on a totally impermeable surface. This depression storage does
not vary with the amount of rainfall but with the nature of the surface, especially with slope
gradient, vegetation cover, and land use practices. under natural conditions depression storage
absorbs about 2 to 5 mm of rainfall in anyone storm. Contour ploughing is particularly
effective in increasing depression storage by as much as ten time.
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2. Components of the hydrological cycle.
a. The Atmospheric Sector:
i. Gain of Moisture from the Biospheric and Lithospheric Sectors:
Evaporation: It refers to the change in state of water from liquid to vapour. It
occurs whenever energy is transported to an evaporating surface if the air is not
saturated. The energy is generally provided by the removal of heat from the
immediate surroundings causing an apparent heat loss (latent heat) and a consequent
drop in temperature.
Transpiration: It is the loss water from plant surfaces, chiefly leaves. It occurs
when the vapour pressure in the leaf cells is greater than the atmospheric vapour
pressure, and is vital as a life function in that it causes a rise of plant nutrients from
the soil and cools the leaves. It occurs mainly during the day when the stomata
through which transpiration takes place are open.
In practice, it is difficult to separate water evaporated from the soil,
intercepted moisture remaining on vegetation surfaces after precipitation and
subsequently evaporated, and transpiration. Thus these complex processes are usually
referred to as evapotranspiration.
ii. Return of Moisture to the Biospheric and Lithospheric Sectors:
Condensation refers to the change in state of water from vapour to liquid. It
occurs when there is a drop in air temperature. As a consequent there is a release of
latent heat back into the atmosphere.
Precipitation refers to the fall of water in forms of solid or liquid from the
atmosphere to the ground. It may take the form of rainfall, snowfall, hail storm or
sleet (a mixture of snow and rain)
b. The Biospheric/ Land Sector:
i. Flow of water:
Before rainfall bit the ground surface directly, it is caught firstly by the leaves
and branches as interception. Thick vegetation covering ground surface catches
precipitation and holds it for a short time. In light rainfall, all the water is intercepted
and evaporated back into the atmosphere.
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After satisfying interception storage, water begins to get through the canopy as
canopy drip and stem flow, bringing along dissolved and suspended materials to the
forest floor. Occasional gaps in the forest may allow direct throughfall to reach the
ground.
Water upon reaching the ground floor will follow a number of routes.
According to the nature of the litter layer and the soil, a portion will infiltrate the soil.
Litter and soil will take up moisture as retention storage. If the amount of throughfall
exceeds infiltration capacity, the excess water will move on the soil surface
downslope as overland flow which may take the form of sheet flow if a film of water
spreads on the slope and flows downward, or channel flow if the flow of water is
confined within a channel.
Water is returned from the surface to the atmosphere via the process of
evapotranspiration.
ii. Storage of water:
Other than the movement of water , there is also the retention and detention of
water in surface depressions and the soil layer. Interception storage, depression
storage and channel storage may hold up water from active cycling for varying length
of time.
c. The Lithospheric/ Soil Sector:
Infiltration refers to the movement of water into spaces existing among solid
particles of mineral matter and organic matter in soil, the rate being dependent on the
sizes and shapes of air spaces in the soil and the amount of water already filling the
spaces.
After entering the soil, water will move either vertically downwards as seepage/
throughflow which refers to the sub-surface movement of water deflected downslope.
Deep seepage may reach the groundwater reservoir and become groundwater
storage, that is the storage in the joints and interstices of the bedrock. Such process of
downward movement of water passing through the soil into the underlying permeable
rocks is called percolation.
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Water below the surface will also move in a lateral downslope direction, as
interflow in the unsaturated zone between soil moisture and groundwater table, or as base
flow/ groundwater flow within the groundwater body itself.
Water held in the soil may be evaporated back to the atmosphere by evaporation
while underground water may reach the surface through the process of capillary action.
3. Global water cycle.
http://geography.uoregon.edu/envchange/clim_animations/
We can start from oceans, which are the basic reservoir of free water. Evaporation
from the ocean surfaces totals about 109,000 cu. mi. per year. At the same time, evaporation
from soil, plants and water surfaces of the continents total about 15000 cu. mi.. Thus the total
evaporation term is 124,000.
Precipitation is unevenly divided between continents and oceans; 26000 cu. mi. is
received by land surfaces and 98000 cu. mi. by the ocean surfaces. Notice that the continents
receive about 11,000 cu. mi. more water as precipitation than they lose by evaporation. This
excess quantity flows over or under the ground surface to reach the sea, it is collectively
termed runoff.
Water can state the global water balance as follows:
"P=E+G+R"
where P = Precipitation
E = Evaporation
G = Net gain or loss of water in the system
R = Runoff
Since the quantities of water in storage in the atmosphere, on the lands, and in the
oceans will remain about constant from year to year, the equation then simplified to:
"P=E+R"
therefore : 26000 + 98000 = 15000 + 109000
Therefore, the hydrological cycle is an open system to which energy (in the form of
solar radiation) is added to provide a motive force. Water is evaporated from the ocean to
produce water vapour which in turn forms precipitation over the oceans or land masses.
Precipitation over the land masses ultimately return to the atmosphere by evaporation from
surface water or transpiration from vegetation or alternatively enter the oceans directly as
runoff (riverflow or iceflow).
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4. Soil water cycle.
a. Infiltration and Runoff:
Most soil surfaces in their undisturbed, natural states are capable of absorbing the
water from light or moderate rains, a process known as infiltration. Such soils have
natural passage ways between poorly fitting soils particles as well as larger openings,
such as earth cracks resulting from soil drying, boring of worms and animals, cavities left
from decay of plant roots, or openings made by heaving and collapse of soil as frost
crystals alternately grow and melt. A mat of decaying leaves and stems breaks the force
of falling drops and helps to keep these openings clear.
If rain falls too rapidly to be passed downward through these soil openings, the
excess amount flows as a surface water film or sheet down the direction of ground slope,
a runoff process termed overland flow.
b. Evaporation and Transpiration:
Between periods of rain, water held in the soil is gradually given up by a two-fold
drying process. First direct evaporation into the open air occurs at the soil surfaces and
progresses downward. Air also enters the soil freely and may actually be forced
alternately in and out of the soil by atmospheric pressure changes. Ordinarily only the
first 30 cm of soil is dried by evaporation in a single dry season, but in the prolonged
drought of deserts, drying will extend to depth of many feet.
Second, plants draw the soil water into their systems through vast networks of tiny
rootlets. This water, after being carried upward through the trunk and branches into the
leaves, is discharged in the form of water vapour through leaf pores into the atmosphere,
a process termed transpiration.
The combined moisture loss from direct evaporation and transpiration of plants is
termed evapotranspiration. The rate of evaporation slows down as soil moisture supply
becomes depleted during a dry season because plant employ various devices to reduce
transpiration. In general, the less moisture remaining, the slower is the loss through
evapotranspiration.
5. Relationship between water cycle and plants.
a. Moisture in the soil:
When infiltration occurs during heavy and prolonged rains (or when a snow cover
is melting), the water is drawn downward by gravity through the soil pores, wetting
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successively lower layers. Soon the soil openings are filled with water moving downward
except for some air entrapped in the form of bubbles. Then the percolation continues
downward into the bedrock.
When a soil has first been saturated by water, then allowed to drain under gravity
until no more water moves downward. The soil is said to be holding its field capacity of
water. This takes no more than two or three days for most soils. Most is drained out
within one day. Field capacity is measured in units of depth, usually inches or
centimetres.
Agricultural scientists also use a measure of soil moisture termed the wilting point.
This is the quantity of soil moisture below which plants are unable to extract further
moisture from the soil and the foliage will wilt. The wilting point also depends upon
particle size.
b. The soil Moisture Budget:
The annual cycle of soil water changes can be presented in quantitative terms through
the equation of soil water balance calculated for each month of the year, using values of a
particular year of the averages of many years.
"P=E+G+R"
where P = Precipitation
E = Actual evapotranspiration
G = Change in soil water storage in the zone available to plants
R = Water surplus, which eventually becomes runoff
Analysis of the annual cycle of soil water balance has many uses, it enables water
resources to be calculated for any region. Estimates for irrigation needs a quantitative
study of the total precipitation and the quantities of surplus water that can be drawn upon
for irrigation.
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B. Water Cycles in Selected Regions:
1. The water cycle in tropical rain forest landscapes:
a. The Atmospheric Sector:
Equatorial rain forest areas experience hot humid climate (AF - Koppen). Inputs
in the form of precipitation is high, more than 2000 mm per year. Rate of evaporation
through high is lower than it could be because cloud cover reduces energy input. Uplift of
warm, moist, unstable air caused by convergence of tropical air masses and aided by
convection and orographic uplift produces towering cumulus clouds when condensation
takes place due to rising air cooling and becoming saturated.
b. The Biosphere/ Surface Sector:
Much precipitation does not fall directly to the ground but is intercepted by dense
vegetation cover - some of the intercepted rain evaporates into the atmosphere but most
reaches the ground by direct throughfall and by running down trunks or dripping off
branches.
Vegetation acts as a large storage reservoir within the water cycle. Water is
absorbed by vegetation during photosynthesis and by uptake from the soil reservoir.
Large amounts of water vapour are returned to the atmosphere by transpiration.
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Water reaching the ground either infiltrates into soil or flows across surface as
runoff into streams and lakes. Much water is evaporated from these surfaces and returned
to the atmospheric sector.
c. The Lithospheric/ Soil Sector:
Water infiltrating into soil forms a large storage reservoir available to plant roots.
Moisture extracted from this reservoir by plant roots is transported up to the biosphere
sector where it may be retained in the form of plant tissue or returned to the atmospheric
sector by transpiration.
2. The water cycle in desert landscapes.
a. The Atmospheric Sector:
Desert areas experience hot, dry climate (BWh - Koppen). Inputs in form of
precipitation is low and sporadic throughout the year, average less than 250 mm. It is due
to the general subsidence and stable conditions. Evaporation rates are high because of
clear, sunny skies which gives high energy input and prevailing low atmospheric water
content.
b. The Biosphere/ Surface Sector:
Sparseness of vegetation cover means that interception of precipitation is very
small since most precipitation reaches the ground directly. Plants are xerophytic and are
adapted to retain moisture. Thus storage in plant tissue and transpiration are small
components in the total balance.
Most precipitation reaching the surface infiltrates into upper surface layers or
more commonly flows across surface as runoff. Rainfall is often intense though of short
duration. Thus it produces sheet flow with little surface storage in lakes. Moreover, water
is rapidly evaporated and returned to atmospheric sector due to the high energy input, hot
surface and low humidity.
c. The Lithospheric/ Soil Sector:
Infiltration and storage in subsurface layers are usually at considerable depth and
water is not generally available to vegetation. Most water uptaken from surface layer after
rain is stored in plant tissue and very little is returned to atmospheric sector evaporation.
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