Water Losses
• Portion of water that is lost from a supply system
before reaching its intended users
• Are major concern for water utilities &
municipalities, as they lead to waste resources,
higher operation cost, & environmental impacts.
Factors that Create Water Losses
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Interception loss-due to surface vegetation
Evaporation from water surface
Evaporation from soil surface
Transpiration from plant leaves
Evapotranspiration for consumptive use from
irrigated/cropped land
Infiltration into the soil at the ground surface
Watershed leakage
Evaporation
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Water transfer into a gas/water vapor
Factors affecting evaporation:
✓ Air & Water Temperature
✓ Relative Humidity
✓ Wind Velocity
✓ Surface Area
✓ Barometric Pressure
✓ Water Salinity
✓ Vapor & Atmospheric Pressure
Rate of Evaporation
(a function of the differences in vapor pressure at
the water and in the atmosphere)
• Wind Speed - The higher the wind speed, the more
evaporation.
• Temperature - The higher the temperature, the
more evaporation.
• Humidity - The lower the humidity, the more
evaporation.
Dalton’s Law of Evaporation
John Dalton (1802)
“Rate of evaporation is proportional to the difference
between saturation vapor pressure (SVP) at water
temperature (𝑒𝑤 ) and actual pressure in the air (𝑒𝑎 ).”
𝐸 = 𝐾(𝑒𝑤 − 𝑒𝑎 )
If we consider the wind velocity, it becomes:
𝐸 = 𝐾(𝑒𝑤 − 𝑒𝑎 )(𝑎 + 𝑏𝑉)
E = rate of evaporation (mm/day)
K, a, & b = constant
𝒆𝒘 = saturated vapor pressure at the water temperature
𝒆𝒂 = vapor pressure of the air (about 2m above)
V = wind velocity
Method in Estimating Evaporation
➢ Water Balance Method
❖ used to understand and manage the
distribution and movement of water within a
specific area or system.
❖ relies on measuring the changes in storage of
water within a defined region (all inflows &
outflows), such as a watershed, basin, or a
smaller area like a catchment.
❖ 𝐼 = 𝑂 ± ∆𝑆
Where;
I = inflows O = outflows ∆𝑆 = change in storage
➢ ENERGY BUDGET Method
❖ A thermal budget for water body
❖ used to estimate evaporation by analyzing the
balance of energy at the earth’s surface,
particularly at the surface of water bodies
❖ 𝐻𝑒 = 𝐻𝑠 + 𝐻𝑤 − 𝐻𝑙 − 𝐻𝑐 − 𝐻𝑟 (calories
per square inch)
❖ 𝐸=
𝐻𝑒
𝐿
; L = latent heat of vaporization
➢ MASS TRANSFER Method
❖ An empirical approach to estimating
evaporation, based on the principle that
evaporation is driven by the movement of
water vapor from the evaporating surface to
the atmosphere.
❖ 𝐸 = 𝑏(𝑒𝑠 − 𝑒𝑎 )
Where;
E = evaporation & b= empirical coefficient
❖ Convection/Eddy Diffusion – transfer of heat
by mass movement of air
❖ Conduction – upward movement of heat w/in
the boundary layer
Instruments used in Measuring
Evaporation & Transpiration
(Using Evaporimeter Data)
➢ Us Class a EVAPORATION PAN
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Used by the US Weather Bureau.
made of unpainted galvanized iron sheet.
Monel metal is used where corrosion is a
problem.
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placed on a wooden platform of 15 cm height
above the ground to allow free circulation of
air below the pan.
measurements are made by measuring the
depth of water with a hook gauge in a stilling
well.
➢ ISI EVAPORATION PAN
• Used by Indians
• also known as modified Class A Pan.
• made of copper, tinned inside and painted white
outside.
• placed over a square wooden platform.
• The top of the pan is covered fully with a
hexagonal wire netting of galvanized iron to
protect the water in the pan from birds.
• evaporation is found to be less by about 14%
compared to that from unscreened pan.
➢ COLORADO SUNKEN PAN
• A square pan made up of unpainted galvanized
iron sheet and buried into the ground
• Advantage is that radiation and aerodynamic
characteristics are similar to those of lake.
• Transpiration Ratio – ratio of the weight of
water absorbed (thru root system)
𝑇𝑟𝑎𝑛𝑠𝑝𝑖𝑟𝑎𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑖𝑜 =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑊𝑎𝑡𝑒𝑟 𝑇𝑟𝑎𝑛𝑠𝑝𝑖𝑟𝑒𝑑
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝐷𝑟𝑦 𝑀𝑎𝑡𝑡𝑒𝑟 𝑃𝑟𝑜𝑑𝑢𝑐𝑒𝑑
Evapotranspiration
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loss of water from vegetated areas
Roles:
✓ Water Loss Regulation
✓ Influence in Climate & Weather Pattern
✓ Water Availability for Ecosystem
✓ Impact on Water Resource Management
✓ Groundwater Recharge
• Potential Evapotranspiration – evapotrans
from the short green vegetation when roots are
supplied with unlimited water covering soil
Method of Estimating Evaporation
o Tanks & Lysimeter Experiments
o Field Experimental Plots
o Installation of Sunken (Colorado) Tanks
o Equations by Lowry-Johnson, Penman, Thornthwaite,
Blaney-Criddle, etc.
o Evaporation Index Method by Hargreaves &
Christiansen
➢ BLANEY-CRIDDLE METHOD
• Used for consumptive use determinations
➢ US GEOLOGICAL SURVEY FLOATING PAN
• kept free on the water body whose evaporation
needed to be calculated.
• This square pan (900 mm side and 450 mm
depth) supported by drum floats.
Pan Coefficient
• the pan evaporation data have to be corrected to
obtain the actual evaporation from water
surfaces of lakes and reservoirs by multiplying
by a coefficient called pan coefficient and is
defined as:
𝑃𝑎𝑛 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 =
𝐿𝑎𝑘𝑒 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛
𝑃𝑎𝑛 𝐸𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛
• pan coefficients range from 0.67 – 0.82 with an
average of 0.7
Transpiration
• process by which the water vapor escapes
from the living plant leaves & enters the
atmosphere
• Methods:
➢ Phytometer – closed water tight tank w/
sufficient soil for plant growth
• Water consumed by transpiration (𝑊𝑡 ):
𝑊𝑡 = (𝑊1 + 𝑤) − 𝑊2
➢ EVAPORATION INDEX METHOD
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Indicate a high degree of correlation
between pan evaporation values &
consumptive use.
𝐸𝑡 = 𝑘(𝐸𝑝 ); k = coefficient; 𝑬𝒕 & 𝑬𝑷 = ratio
Factors Affecting Evapotranspiration
1. Climatological factors (sunshine hrs %, wind
speed, mean monthly temp, & humidity)
2. Crop factors – type and % of growing season
3. Soil Moisture level
Measures to Reduce Lake Evaporation
Deeper storage reservoirs &with less surface area
Growing tall trees like Casuarina on the
windward side of reservoirs as wind breakers
By spraying certain chemical that will form a film
on the surface of water. [e.g Acetyl Alcohol
(hexadecanol)]
By allowing flow of water, temperature is reduced
and evaporation is reduced
By removing the water loving weeds and plants
like Phreatophytes from the periphery of the
reservoir.
By straightening the stream-channels
By providing mechanical coverings like thin
polythene sheets to small agricultural ponds and
lakes.
By developing underground reservoirs
The reservoir is surrounded by huge trees and
forest
Soil Evaporation
➢ Evaporation from wet soil surface after rain
➢ Causes:
• Sunlight & Heat
• Air Movement & Temperature
• Soil Type & Moisture
➢ Evaporation Opportunity- Expressed as % of
evaporation from free water surface
𝐸𝑣𝑎𝑝. 𝑂𝑝𝑝 =
𝐴𝑐𝑡𝑢𝑎𝑙 𝑒𝑣𝑎𝑝. 𝑓𝑟𝑜𝑚 𝑡ℎ𝑒 𝑙𝑎𝑛𝑑 𝑎𝑡 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒
𝑥100
𝐸𝑣𝑎𝑝. 𝑓𝑟𝑜𝑚 𝑎𝑛 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑤𝑎𝑡𝑒𝑟 𝑠𝑢𝑟𝑓𝑎𝑐𝑒
Interception
➢ Process where precipitation is captured and held by
vegetation, such as trees, shrubs, and grasses, before
it reaches the ground
➢ Throughflow – not intercepted rain/drip down to the
ground
➢ Stemflow – water that reaches the ground via trunks
& stems of the vegetation
Interception Storage
➢ the capacity of vegetation (such as leaves, branches,
and stems) to capture and temporarily hold
precipitation before it reaches the ground.
Depression Storage
➢ water that is lost because it becomes trapped in the
numerous small depressions that are characteristic
of any natural surface
➢ water temporarily accumulates in a low point with no
possibility for escape as runoff
➢ assumes that all water has a chance to infiltrate/evaporate
Runoff Phenomenon
➢ runoff – all water that flows over the land surface;
occurs when the rate of precipitation exceeds the
rate of soil’s infiltration capacity; caused by
gravitational force
Types of Runoffs
Surface runoff – portion of rainfall which enters
the stream immediately after the rainfall.
Subsurface runoff - part of rainfall which first
leaches into the soil and moves laterally without
joining the water table, to the stream, rivers or
ocean; infiltrating water moves laterally in the
surface soil due to relatively impermeable
stratum in the subsoil
Base Flow - delayed flow defined as the part of
rainfall, which after falling on the ground the
surface, infiltrated into soil and meets to the
water table and flow the streams, ocean etc.
Factors Affecting Runoffs
✓
✓
✓
✓
✓
✓
✓
Precipitation characteristics
Shape & Size of the Catchment
Topography
Geological characteristics
Meteorological characteristics
Character of the catchment surface
Storage characteristics
Time of Concentration
➢ Time required for runoff to travel from the
hydraulically most distant point in the watershed
to the outlet
➢ Remotest Point – point from w/c runoff requires
the greatest amount of time to flow to the point
of analysis
➢ Hydraulic Path – path taken by the runoff from
the remotest point to the point of analysis
➢ Empirical Nomograph – use to determine the
time for shallow concentrated flow
➢ Stream flow – usually the last and the fastest flow
to occur along the hydraulic path; time can be
computed using Manning’s Equation:
1.49 2 1
𝑉=
𝑅3 𝑆 2
𝑛
➢ Hydraulic Radius can be computed by:
𝑊𝑒𝑡𝑡𝑒𝑑 𝐴𝑟𝑒𝑎
𝐴
𝐻𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑅𝑎𝑑𝑖𝑢𝑠 =
=
𝑊𝑒𝑡𝑡𝑒𝑑 𝑃𝑒𝑟𝑖𝑚𝑒𝑡𝑒𝑟 𝑃
Runoff Calculations
METHODS:
➢ RATIONAL METHOD – first & most enduring
method; introduce in England in 1889; used to
compute the peak runoff, Qp, following a rainfall
event. It makes no attempt to estimate runoff
before or after the peak but simply estimates the
one quantity of flow that is greatest; originally
given by:
𝑄𝑝 = 𝐴𝑖;
𝑄𝑝 = peak runoff, A = drainage Area, i = rainfall intensity
Drainage Patterns
Later became: 𝑄𝑝 = 𝑐𝐴𝑖;
𝑄𝑝 = peak runoff, A = drainage Area, i = rainfall intensity,
c = dimensionless runoff coefficient
➢ MODIFIED RATIONAL METHOD – Adopted in
19702, expands the original Rational Method to
yield a hydrograph for use in detention basin
design.
➢ NRCS METHOD - a procedure for computing a
synthetic runoff hydrograph based on empirically
determined factors developed by the Soil
Conservation Service (SCS). Now became
Natural Resources Conservation Service,
developed in 1950s.
• TR -55 contains charts and graphs that
allow the user to compute peak runoff and
runoff hydrographs for watersheds
located within the United States
DENDRITIC PATTERN – A tree-shaped
pattern, most common, also known as pinnate
drainage; land erodes in fairly uniform manner
so streams randomly branch and advance upslope
RECTANGULAR PATTERN – formed as
streams follow the faults; main stream bends at
right angles & the tributaries join at right angles;
found in regions that have undergone faulting
TRELLIS PATTERN – formation of stream
when land surface is folded/ is a broad, gently
sloping plane; the short subsequent streams meet
the main stream at right angles
Stream Orders
• A measure of the relative size of streams
• First-Order Stream – smallest to no tributaries
• !2th – Order Waterway – largest river in the world
(Amazon)
Streams
Drainage Network
• Are runoff flows over watersheds, collect in
streams, and flows downslope toward the ocean.
• Two mechanical functions:
1. Transport water from higher to lower
elevations
2. transport sediment - earthen materials
better known as rocks, sand, silt, & clay
• Classifications:
Perennial Stream – flows year-round & has
well-defined channel; flows in the stream at
least 90% of the time; usually have
continuous base flow from groundwater
Intermittent Stream – flows only during
wet-seasons & after heavy rains; has welldefined channel; water depends on the
position of the water table
Ephemeral Stream – often called gully, only
flows after rain/ for a short time; have no
base from groundwater; no defined channel
• Are also classified as young, mature, or old
Young Streams – flow rapidly & continually
cut their channels; sediment loads are
transported w/ no deposition
Mature Streams – sloped have been reduced
& no downcutting of channels; flows are
adequate to transport most of the incoming
sediment load
Old Streams – have gentle slopes & sluggish
flows; have broad floodplains & channels are
meander; sediment deposition often leads to
delta formation
• A stream system that contributes to the discharge
at a specified point in a higher order stream
Flood Prediction
• A
process of predicting the occurrence,
magnitude, timing, and duration of floods in a
specific area, often by analyzing various
hydrological, meteorological, and environmental
factors
• Requires several types of data:
✓ The amount of rainfall occurring on a realtime basis.
✓ The rate of change in river stage on a realtime basis
✓ Knowledge about the type of storm
producing the moisture, such as duration,
intensity and areal extent
✓ Knowledge about the characteristics of a
river's drainage basin, such as soil-moisture
conditions, ground temperature, snowpack,
topography,
vegetation
cover,
and
impermeable land area