The Importance of Hydrological Cycle
Water has a basic role in the climate system through the hydrologic cycle. Hydrological
cycle is the interdependence and continuous movement of all phases of water (ie liquid,
solid, and gas form the basis of the hydrology cycle) It is also a key process upon which
other cycles operate.
water is indestructible so the total quantity of water in the hydrological cycle
cannot be diminished as it changes from water vapor to liquid or solid and back
The processes of streamflow, groundwater flow and evaporation ensure the never
ending transfer of water b/w land, ocean and atmosphere, followed by its return
as precipitation to the earth’s surface
The really mobile fresh water, which contributes frequently and actively to rainfall,
evaporation and streamflow, represents only about 0.3% of the global total, and is
distributed unevenly in both space and time
Water as a form of precipitation and water vapour, provides the following functions to the
earth: water vapour in the atmosphere condenses and forms clouds. This may give rise
to precipitation and helped the earth’ heat balance because of its high albedo
Water, because of its nature of high specific heat capacity, act as a cushion in
As a source of fresh water : be stored in the forms of pools, puddles and surface
water or flow over the surface into streams and lakes, from where it will move
either by evaporation into the atmosphere, or by seepage toward the groundwater,
or by further surface flow into the oceans
Agriculture/Plant use: the groundwater component will eventually be removed
either by upward capillary movement to the soil surface or to the root zone of the
vegetation covers, whereas it will be returned by evapotranspiration.
The high albedo of ice and snow help to reflect sunlight back to the universe. It is
stored as snow and ice before melting or sublimation occurs, possibly after the
lapse of many years or even centuries.
Water management for the planner in the future can bring different benefits and threat to
human life and livelihood as between the developed and the developing world
10% of the world’s pop is now affected by chronic water scarcity
there is an urgent need for a must clearer understanding of the physical, economic,
social, and political consequences of large-scale water resource development and of
major irrigation and flood defense schemes which are designed to reduce the impact of
drought and flood disasters
the need for sustainable development, and increasing recognition that water must be
used more efficiently
7.7 In the light of growing information about climate variability and climate change, discuss
the following concepts: river regimes; mean annual flow; the 100-year flood
River regimes are an expression of seasonal conditions averaged over many years.
are driven largely by climate and are a reflection of the balance between rainfall and
Many small river exhibit comparatively simple regimes having one period of high
water and one period of low water each year. Such regimes may result from the
spring/summer melting of snowpacks or glaciers followed by a period of near zero flow during the winter months when temperatures are low and icemelt is negligible
When rainfall is evenly distributed throughout the year, low flow coincides with the
peak of evaporation during the summer months and high runoff values occur during
the winter months when evaporation is small
In tropical areas evaporation tends to be high throughout the year, so that the rainfall
distribution is the main determinant of river regimes, with high runoff during the wet
Complex I Regimes
characterized by four distinct phases throughout the year: two low flow and two high
The high runoff period result from snowmelt, may occur in spring and then be
followed by a period of low runoff
Later in the year, a second period of high water levels and runoff may occur in the
summer as a result of convectional rainfall over a ‘continental’ area, or in the autumn
as a result of Mediterranean storms, or in the winter as a result of an excess of rainfall
over evaporation in an oceanic area
Complex II regimes
Large rivers may flow through several distinct relief and climatic regions and may
receive the waters of large tributaries which themselves flow over varied terrain. The
regimes of such rivers tend to change with distance downstream.
Normally have simple or complex I regimes in their headwater reaches but
downstream are gradually influenced by a variety of factors such as snow or glacier
melt, rainfall and evaporation regimes.
These different inflows may either reinforce the trends found in the headwater regime
or may cancel each other out.
Mean Annual Flow
average amount of water that flows down a particular river, per year, expressed either as a
depth (in millimetres) of water spread evenly across the entire drainage basin, or as a
volume (in cubic metres) of water flowing past a given point.
are driven largely by climate and are a reflection of the balance between rainfall and
100 year Flood
A one-hundred-year flood is calculated to be the level of flood water expected to be
equaled or exceeded every 100 years on average. The 100-year flood is more accurately
referred to as the 1% annual probability flood, since it is a flood that has a 1% chance of
being equaled or exceeded in any single year. 5.1 Discuss the role of groundwater in the hydrological cycle.
Most of the precipitation that reaches the ground surface is absorbed by the surface layers
of the soil
Groundwater sustains streamflow during period of dry weather and is a major source of
water in many arid regions. Due to its long residence time, areas that currently have an
arid climate where there is little opportunity for water to percolate deeply, may
nevertheless have significant groundwater reserves which are the result of recharge in
former pluvial periods
Almost all of it is composed of precipitated atmospheric moisture that has percolated
downwards through the zone of aeration
Only small quantities of groundwater are derived from connate water, which originated as
sea water trapped in some rocks at the time of their deposition
Groundwater is the earth’s largest accessible store of fresh water and excluding ice-sheets
and glaciers, has been estimated to account for 94 per cent of all fresh water.
The role of groundwater as a vast regulator in the hydrological cycle can be seen from the
large residence time, averaging about 300 years
5.2 What types of materials make good aquifers and aquitards?
Layers of rock or unconsolidated deposits that contain sufficient saturated material to yield
significant quantities of water are known as aquifers and less permeable formations that
transmit water more slowly than the adjacent aquifers are commonly known as aquitards.
Aquitards restrict the movement of ground water into and out of adjacent aquifers.
A silt bed would be an aquitard in a stratigraphic sequence of alternate sand and silt layers
but, if interlayered with less permeable clay beds, the silt would be an aquifer.
Most of the major aquifers are composed of sedimentary deposits formed from the
erosion contrast, igneous and metamorphic rocks, formed under conditions of high
temperatures and pressures, generally have few interconnected pore spaces and
consequently most have only low water-bearing capacities.
The lower limit of groundwater occurs at a depth where interstices are so few and so small
that further downward movement is virtually impossible. This groundwater boundary is
frequently formed by a stratum of very dense rock, such as clay, slate or granite, or by the
upper surface of the parent rock where the groundwater body occurs within a surface
deposit of weathered material.
Alternatively, the compression of strata with depth, which results from the increasing
weight of the overly rocks, means that a depth is eventually reached beyond which the
interstices have been so reduced in both size and number that further water movement is effectively prevented.
The number of interstices tends generally to decrease with depth, and below about
10km all rocks may be considered to be impermeable
The depth at which this occurs will depend on the nature of the water-bearing rock,
and would be shallower in a dense granite than in a deep porous sandstone
5.3 Explain, with the help of diagrams, what is meant by unconfined, confined and perched
The upper boundary of the zone of saturation, water table, varies according to whether the
groundwater is confined or unconfined.
An unconfined aquifer has no confining bed above it and is usually open to infiltration from
this boundary is normally known as the water table, which is defined as the level where
the porewater pressure is equal to atmosphe