The document discusses water sources and the water cycle. It describes the water cycle as the continuous movement of water on, through, and below the Earth's surface, changing between liquid, solid, and gas states. Water sources include groundwater found underground in aquifers and surface water such as rivers, lakes, and rainwater. Selection of a water source considers factors like the purity and available volume of raw water, the permanence and recharge rate of the source, and the elevation of the water relative to the area to be supplied.

1. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University//CE2023: Water & Water Supply
INTRODUCTION
THE WATER CYCLE:
The water cycle, also known as the hydrologic cycle or the H2O cycle, describes the continuous
movement of water on, above and below the surface of the Earth. The mass of water on Earth remains
fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water,
saline water and atmospheric water is variable depending on a wide range of climatic variables. The
water moves from one reservoir to another, such as from river to ocean, or from the ocean to the
atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, runoff,
and subsurface flow. In doing so, the water goes through different phases: liquid, solid (ice), and gas
(vapor).
The water cycle involves the exchange of energy, which leads to temperature changes. For instance,
when water evaporates, it takes up energy from its surroundings and cools the environment. When it
condenses, it releases energy and warms the environment. These heat exchanges influence climate.
The evaporative phase of the cycle purifies water which then replenishes the land with freshwater. The
flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the
geological features of the Earth, through processes including erosion and sedimentation. The water
cycle is also essential for the maintenance of most life and ecosystems on the planet.
THE HYDROLOGIC EQUATION:
Hydrologic equation is also known as the Continuity or Mass Balance Equation. The
Hydrologic Equation is simply stated as:
Inflow = outflow +/- storage
OR,
Inflow - Outflow = change in storage

2. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University//CE2023: Water & Water Supply
 Inflow = precipitation, surface inflow, loss from a body of surface water, subsurface inflow,
overland flow, groundwater discharge into a body of surface water.
 Outflow = evaporation, transpiration, surface outflow, soil evaporation, loss of water through a
stream bed, loss of water to a body of surface water, loss to vegetation, percolation to the water
table, pumpage, removal to a water supply.
 Change in storage = increase or decrease in subsurface storage, increase or decrease in surface
storage.
Hydrologic budget is simply an H2O mass balance:
For a watershed (topographic basin) water balance is,
Fetter has written a more specific equation to represent the hydrologic cycle from the
perspective of an aquifer:
P = R + T + E +/- U +/- Storage terms
Where: P = precipitation R = runoff
T = transpiration E = evaporation
U = underflow
SOURCES OF WATER
1. GROUNDWATER
(Underground water) aquifers, this is found in wells, springs, infiltration galleries, etc. There are
two sources of ground water:
(a) Rainfall which infiltrates, percolate or permeates into the ground through the pores or cracks in the
rock formation and finally reaches the underground water table.

3. Priodeep Chowdhury; Lecturer; Dept. of CEE; Uttara University//CE2023: Water & Water Supply
(b) The water from streams: Rivers, lakes and reservoirs which seeps or permeates through the soil to
the underground water table
Because of the seepage through pores of soil and rock, ground water rarely needs any treatment before
consumption, unless there is a highly contaminated area around the particular ground water or aquifer.
2. SURFACE WATER
This is exposed to contamination and so it must be treatment before use. It may contain both
organic and inorganic impurities, gases and micro organisms. It is generally used for drinking purposes
when the ground water supply is inadequate in quantity.
a) Rain water: This is generally collected from the roofs of buildings and stored in cisterns for domestic
consumption. It may also be collected in reservoirs. Rain water is soft and suitable for laundry purposes,
it also corrosive when in equilibrium with atmospheric gases. It is normally in very small communities
which as other source of water supply (deserts).
b) River water: This is rainfall that does not percolate into the ground, taken in by plants or evaporates.
It flows on the ground surface as rivers, streams and into the sea. Water may be drawn from a river
continuously throughout the year or it may be stored during times of flood and then supplied to the
community during drought. Because of pollution river water has to be properly treated.
c) Lakes and ponds: These are areas of water surrounded by lend. Deep lakes and ponds having
sufficiently large surface areas can be used for municipal water supply. Here the water may be of better
quality than the river water and can be used for domestic purposes without much treatment. During
floods and heavy rain, lakes and ponds fill up and discharge to the rivers and streams increases.
FACTORS FOR SOURCE SELECTION
a) Purity of the raw water: This is with respect to the possibility of treatment (cost)
b) Volume of water available to be enough to satisfy minimum requirement. Normally inflow should be
greater than outflow.
c) Permanency of the sources: Available in large quantity with a continuous recharge.
d) Elevation of the water level with respect to the area to be supplied: Cost of lifting the water to a
higher elevation will be higher than that from a lower elevation and may requires the use of pumps.
e) Availability of finance: to carry out the design, construction, running and maintenance. This is
related to the kind of structures.