WRM Lesson 3 - Hydrological Processes.pdf

Lesson 3: Hydrologic Processes
K.D.W.Nandalal
Professor
University of Peradeniya, Sri Lanka
kdwnandalal5@gmail.com

2
Hydrologic Processes
Hydrology: Study of the hydrologic cycle; occurrence,
distribution, movement, physical and chemical
properties of waters of the earth and their
environmental relationships.
Study and practice of hydrology: aids in
explaining and quantifying the occurrence of
water on, under and over the earth's surface.
Hydrology

3
Earth’s water resources are in a state of continuous
circulation linking the atmosphere, land and the oceans.
This endless process, which is powered by the forces of
nature is called the Hydrologic Cycle.
It is the basic framework upon which the science of
hydrology is built.
There are many processes that constitute the hydrologic
cycle.
Hydrologic Cycle

4
Hydrologic Cycle
Ref: Applied Hydrology by Ven Te Chow, David R. Maidment, Larry W. Mays

5
Block diagram representation of
the global hydrologic system
Hydrologic Cycle

6
Estimated world water quantities
Item
Area
(106 km2)
Volume (km3)
Percent of
total water
Percent of
fresh
water
Ocean 361.3 1338000000 96.5
Groundwater
Fresh 134.8 10530000 0.76 30.1
Saline 134.8 12870000 0.93
Soil Moisture 82.0 16500 0.0012 0.05
Polar ice 16.0 24023500 1.7 68.6
Other ice and snow 0.3 340600 0.025 1.0
Lakes
Fresh 1.2 91000 0.007 0.26
Saline 0.8 85400 0.006
Marshes 2.7 11470 0.0008 0.03
Rivers 148.8 2120 0.0002 0.006
Biological water 510.0 1120 0.0001 0.003
Atmospheric water 510.0 12900 0.001 0.04
Total water 510.0 1385984610 100
Fresh water 148.8 35029210 2.5 100

7
Largest sphere represents all of Earth's
water 1,386,000,000 km3
Fresh water (groundwater, lakes, swamp water,
and rivers) 10,633,450 km3
“Tiny" bubble represents fresh water in all the
lakes and rivers 93,113 km3
Available water in the planet

8
Global annual water balance
Ocean Land
Area (km2) 361300000 148800000
Precipitation (km3/yr) 458000 119000
(mm/yr) 1270 800
Evaporation (km3/yr) 505000 72000
(mm/yr) 1400 484
Runoff to ocean
Rivers (km3/yr) - 44700
Groundwater (km3/yr) - 2200
Total runoff (km3/yr) - 47000
(mm/yr) - 316

9
• Air is forced to rise
• Rising air cools because the ideal gas law says that the
temperature falls when the air pressure decreases.
• The air cools at the dry lapse rate until it reaches its
dewpoint.
• Once the air reaches its dewpoint, the relative humidity
reaches 100%, and clouds form.
• As the air continues to rise, the air cools at the wet lapse
rate, causing precipitation to form because the colder air
can not hold the excess moisture.
Precipitation

10
• The condensing water generates heat, causing the air to
warm slightly, so that the wet air lapse rate is less than the
dry rate.
• The excess heat generated by the condensing water
causes the air to rise faster (because warmer air rises
through colder air).
Precipitation

11
Three primary steps for formation of precipitation
•creation of saturation conditions in the atmosphere
(typically some type of lifting of air)
•phase change from vapor to liquid (cloud formation)
•growth of droplets to precipitable size (able to
overcome upward velocity)
Precipitation

12
Front: the boundary between two adjacent air masses of
different temperature and moisture content.
Warm Front: Warm air advancing on colder air
Cold front: Cold air advancing on warm air
Lifting Mechanisms

13
Convective Precipitation

14
Orographic Precipitation

15
Measured as a depth of water on a flat level surface if all the
precipitation remained where it had fallen
Units: inch, mm
Non Recording Gauge
Funnel
Measuring tube
Overflow can Measuring
stick
Measurement of Precipitation

16
US Standard Weather Bureau Gauge,
Diameter, Φ = 8 in (20.32 cm)
Area of the tube is 0.1 that of the funnel
Graduated Scale: To measure precipitation
Excess: Overtops the inner tube and collects in the
overflow can
Rainfall is measured by an observer at regular intervals
Measurement of Precipitation

17
Tipping Bucket Gauge
Consists of a pair of buckets pivoted
under a funnel
When one bucket receives 0,25 mm (0.01 in) of precipitation
it tips, discharging its content into the bucket, bringing the
other bucket under the funnel
Recording mechanism: Time of occurrence of each tip
measures rainfall intensity for short periods
Recording
Gauges

18
Weighting Type Gauge
Has a bucket supported by a spring or lever balance
Movement of the bucket is transmitted to a pen, which
traces a record of the increasing weight of the bucket
and its contents on a clock driven chart
Record: Accumulation of precipitation
Slope of mass diagram: Rate of rainfall

19
Floating type Gauge
Has a pen actuated by a float on the water surface in the bucket
Automatic siphoning arrangement empties the bucket when it is
full

20
Radar
Radar records a signal reflected of particles (raindrops) at a
high frequency. This signal is converted to cumulative
rainfall. This data can provide information on location and
movement of storms, data in ungauged regions.
However, there are difficulties or error in the measurements
due to various types of particulate (hail, snow, high
intensities)

21
A well distributed network of rain gauge stations within the
catchment is essential
To obtain reliable results, rain gauges should be evenly
and uniformly distributed
Total number of rain gauges installed within a given
catchment area should neither be too many as to be costly
nor should be too less as to give unreliable results
Design of Rain Gauge Networks

22
Areal density of rain gauges may vary considerably from
region to region in any country
Density of gauges may also depend upon the basic purpose
behind their installation
Guidelines: World Meteorological Organization
•(flat) temperate, mediterranean and tropical; 230 – 350
mi2 per station
•(mountainous) temperate, mediteranean and tropical; 40
– 100 mi2 per station
•arid polar zones 600 to 4000 mi2 per station
Design of Rain Gauge Networks

23
Cumulative Mass Curve
Typically the output from a recording gauge is in the form of
cumulative rainfall over time
0
20
40
60
80
100
120
140
160
180
1 3 5 7 9 11 13 15 17 19 21 23
15 minutes periods
Cumulative
volume
(mm)
At any given time
during a storm, the
intensity is the
slope of the
cumulative rainfall
curve at that point
in time
Data Representation

24
Rainfall Hyetograph
The slope of the cumulative mass curve at any point
represents the rate of rainfall (depth/time).
Typically we represent the rainfall distribution in discrete
time intervals using the cumulative mass curve to
calculate the depth over each time interval.
Next the depth or intensity is plotted versus time to
generate a hyetograph characterizing the time
distribution of a storm or selected time period.
The hyetograph is the most common input to hydrologic
models (surface hydrology models).

25
0
5
10
15
20
25
30
35
1 3 5 7 9 11 13 15 17 19 21 23
15 minutes periods
Average
depth
(mm/15
minutes)
It is expressed in depth units per unit time, usually as mm per
hour (mm/h) (or in/hr)
Rainfall Hyetograph

26
This is usually done for one or two years, months or days for a
long term record. Three methods commonly used and are based
on long term records of surrounding gauges (minimum of three)
Simple average of surrounding gauges
must be equal distance from point of estimate should have fairly
equal rainfall at known gauges
Precipitation measuring stations sometimes fail in providing a
continuous record of precipitation
Eg. Due to malfunction of instruments, failure to record data or
miss a visit to site
Missing Data Points
(also rainfall at ungauged locations)

27
Normal Ratio Method (missing data)
when surrounding gauges differ by greater than 10% in
measured rainfall, stations are weighted based on normal
annual precipitation ratio’s








+
+
+
=
k
P
k
N
x
N
P
N
x
N
P
N
x
N
k
x
P ....
2
2
1
1
1
where: Nx = normal long-term annual precipitation at location x
Pk = event precipitation at gauge k
Nk = normal long-term annual precipitation at gauge k
Px = estimated event precipitation at location x

28
Reciprocal Distance Method (missing data &
ungauged location)
Surrounding gauges are weighted based on reciprocals of the
sum of squares of distance away from the point of interest.
Sometimes called inverse weighted distance method
X and y are distances from the
unknown point (missing data
location) where precipitation is being
estimated
2
2 y
x
i
d +
=
∑
=
∑
=
=
=
n
i i
W
n
i i
W
i
P
x
P
i
d
i
W
1
1
2
1

29
Hydrologic abstractions are the losses of precipitation, which
prevent precipitation from becoming surface runoff
Eg.
Out of the 4300x106 m3 of rainfall receiving on the Walawe
Ganga catchment in Sri Lanka
only 945x106 m3 is discharged to the sea
85x106 m3 of water is used for drinking and industrial
purposes
Balance of 3270x106 m3 is evapotranspirated and
infiltrated
Hydrologic Abstractions

30
Average disposition of precipitation in the Walawe Ganga
catchment in Sri Lanka
Hydrologic Abstractions

31
The process by which water is transferred from land
and water masses of the earth to the atmosphere
There is a continuous exchange of water molecules
between an evaporating surface and its overlying
atmosphere
Evaporation is the net rate of vapour transfer
Evaporation

32
It is a function of
• solar radiation
• difference in vapour pressure between water surface
and the overlying air
• temperature
• wind
• atmospheric pressure
• the quality of evaporating water
• size of the water body
Rate is highest from open water
Ground surface: Rate depends on the type of soil and
saturation with water
Evaporation

33
Storm periods: vapour pressure gradients are reduced
and evaporation is usually not significant
Evaporation varies from day to day
Daily evaporation in Sri Lanka varies from 1 to 10
mm/day but on average it is taken as 4 mm/day
Evaporation is determined based on
•water budget
•energy budget
•empirical formulae
•pan-evaporation data
Evaporation

34
Evaporation Pans
Most widely used method
A standard evaporation pan
The standard weather bureau Class A pan: unpainted, galvanized iron
4-ft diameter (122 cm), 10 in (25.4 cm) deep, circular container,
mounted 12 in above the ground on a wooden frame
Usually filled to a depth of 20 cm and refilled when the depth has
fallen to less than 18 cm
Water surface is measured with a hook gauge

35
Pan evaporation estimates lake evaporation
Lake evaporation (EL) is usually calculated by multiplying the
pan evaporation by a factor called pan coefficient (PC)
EL = PC EP
Pan coefficient on an annual basis varies between 0.65 and
0.82
Evaporation rates vary with the time of the year
Greatest is during periods of intense sunlight and least is
during cold cloud-covered days
Evaporation Pans

36
Evaporation Reduction
Evaporation losses can be greatly significant at any location
Consequently, the concept of evaporation reduction is
receiving widespread attention
Evaporation losses from soils can be controlled by employing
various types of mulches or by chemical alteration

37
They may be reduced from open waters by;
a) storing water in covered reservoirs
b) making increased use of underground storage
c) controlling aquatic growths
d) building storage reservoirs with minimal surface
area
e) through the use of chemicals
Evaporation Reduction

38
Transpiration
Process by which water is drawn from the soil by plant roots and
transferred to the leaves, from which it evaporates
Controlled by
• solar radiation
• temperature
• wind velocity
• vapour pressure gradient
• characteristics of plants
• plant density
• soil moisture content

39
For small plant areas: determined by a closed
container in which humidity changes are measured
Soil is sealed to prevent evaporation from soil
These experiments are performed on site or by use of a
phytometer (a container with a particular plant rooted in
it). Soil surface is sealed so that the only escape of
moisture is by transpiration. Transpiration can be
determined by weighting the planted container at
desired intervals of time
Estimation of Transpiration

40
Precise determination is difficult
Extrapolations to other areas can be misleading
Water budgets are valuable, but again requires
estimates of other variables and thus the transpiration
estimates are only as accurate as the measurement of
the other variables
Estimation of Transpiration

41
Transpiration Reduction
Methods of control include
•use of chemicals to inhibit water consumption
(chemicals are applied in the root zone)
•harvesting of plants
•improved irrigation practices
•actual removal or destruction of certain vegetative
types a day when in leaf

42
Evapotranspiration
Sum total of water returned to the atmosphere from
surface and ground (soil) water and vegetation
Evaporation + Transpiration
Difficult to separate the effect of evaporation and
transpiration over land areas
Mostly, only total evaporation from an area (combined
evaporation plus transpiration: consumptive use) is of
practical interest to a hydrologist

43
- The maximum value of evapotranspiration
It is relatively easy to calculate potential evapotranspiration
for a saturated surface (open water), using local
meteorological parameters such as humidity, temperature
and wind speed
Actual evapotranspiration is always less than potential
evapotranspiration
Estimation of Evapotranspiration
Lysimeters
Penman’s Equation
Empirical formulae
Potential evapotranspiration

44
Evapotranspiration from satellite data
When a surface evaporates, it looses energy and cools itself.
This cooling can be observed from space.
Satellites can map the infrared heat radiated from Earth, thus
enabling to distinguish the cool surfaces from the warm
surfaces.
Very dry and desert-like surfaces show easily as they get
hotter than their surroundings.
From this qualitative reasoning, the scientific objective is to
determine quantitatively the amount of evapotranspiration that
occurs at given locations.

45
In practice, it consists in
entering various types of
satellite observations
(not just infrared) into
mathematical models of
the atmosphere.
The models, of various
complexities, are run in
algorithmic form on
computers.
Evapotranspiration from satellite data

46
Interception process + Depression storage
This represents the quantity of storage that must be satisfied
before overland runoff begins
Interception
Segment of gross precipitation input, which wets and adheres
to above ground objects until it is returned to the atmosphere
through evaporation
Initial Loss

47
It is part of a subcycle of the hydrological cycle
Precipitation striking vegetation may be retained on leaves or
grass, flow down the stems of plants and become stem flow,
or fall off the leaves to become part of the throughfall
Interception is a function of
a) the storm character
b) the species (type), age and density of prevailing plants
and trees
c) the season of the year
Typical interception loss curve
Interception

48
Most interception loss develops during the initial storm period and
the rate of interception rapidly approach zero thereafter
Determined by comparing precipitation in gauges beneath the
vegetation with that recorded nearby under the open sky.
Intercepted precipitation is dissipated as stemflow (measured by
catch devices around tree trunk) down the trunk of the trees and
evaporation from the leaf surface
Interception

49
Depression Storage
Precipitation that reaches the ground may infiltrate, flow over the
surface or become trapped in numerous small depressions from
which the only escape is evaporation or infiltration
The nature of depression as well as their size is largely a
function of the original land form and local land use practices
All depression must be full before overland flow supply begins

50
Infiltration
The movement of water through the ground surface into the
soil and on downwards
Infiltration rate and quantity are a function of
soil type
soil moisture
soil permeability
ground cover
drainage conditions
depth of water table
intensity and volume of precipitation

51
Infiltrated water percolates downwards by gravity until it
reaches the zone of saturation
Rate of downward movement is controlled by the
transmission characteristics of the underlying soil profile
Due to the infinite combinations of soil and other factors
existing in nature no general relationship exists to quantify
infiltration
Infiltration

52
Infiltrometer studies
a) Rainfall simulators
b) Flooding infiltrometers
Hydrograph analysis
Measuring Infiltration

53
Calculation of Infiltration
Calculations vary in sophistication from the application of
average rates to the use of conceptually sound differential
equations
Horton (1930s) studied infiltration process and suggested the
following relationship for determining infiltration
Where,
f = infiltration rate as a function of time, (depth/time)
fc = final or ultimate (equilibrium) infiltration rate
fo = initial infiltration rate
k = a constant representing the rate of decrease in
infiltration capacity
e
)
f
-
f
(
+
f
=
f kt
-
c
o
c

54
Infiltration Rate

55
Temporal and Spatial Variability of Infiltration Capacity
Infiltration rate generally varies both in space and time
Spatial variations are due to differences in soil type and
vegetation
Accommodate this type of variation by subdividing the total
region into compartments having approximately uniform
soil and vegetal cover
The infiltration capacity at a given location in a watershed
varies with time as shown before

56
Streamflow
Streamflow = surface runoff + underground flow
Throughflow Baseflow
flow through the soil in
the zone of aeration
(above the water table)
groundwater
River discharge (streamflow): Volume of water that
flows past a point in a certain time

57
The measurement, or gauging, of river discharge is
important for assessment of water resources, design of
water supply schemes, flood-control projects, hydroelectric
projects etc.
Streamflow Measurements
Measurement techniques can be broadly classified into
two categories as,
a) direct determination
(Area-velocity methods, Dilution techniques)
b) indirect determination
(Hydraulic structures [eg., weirs, flumes] )

58
Flow measuring methods can also be classified as shown
below too
Stage
(water surface
elevation)
Discharge Discharge
(structural)
Visual observation
Float
Pressure Sensor
Electrical
Resistance
Current meter
Dilution
Float
Indirect via
Manning's equation
Direct volume
collection
Weirs
Flumes
Orifices
Streamflow is the general term used to represent volumes
or rates of flow.

59
Continuous measurement of stream discharge is very difficult
(very time-consuming and costly)
Hence, a two-step procedure is followed
First, the discharge is related to the elevation of the water
surface (stage)
Next, the stage of the stream is observed routinely in a
relatively inexpensive manner and the discharge is
estimated by using the previously determined stage-
discharge relationship

60
Measurement of Stage
Stage: water surface elevation recorded relative to some
horizontal datum elevation, frequently MSL
Record of stage is called "stage hydrograph"
A stage recorder can be as simple as a ruler along a
bridge or other structure. It can be read periodically

61
Stage data
Stage data is presented as a plot of stage against chronological
time (stage hydrograph)
Besides its use in the determination of stream discharge, stage
data itself is of importance in flood warning and flood protection
works

62
Stream Discharge
Velocity-area method:
Based on continuity equation V̂
A
=
Q
Where,
Q = volume rate of discharge (m3/s)
A = cross sectional area (m2)
V = mean velocity in the cross section (m/s)
Discharge is determined by measuring cross sectional area
and the velocity

63
Cross-sectional area is determined from measurements of
depth of water taken at known intervals across the river
Velocity is measured by floats, by a pitot tube (restricted to
pipes or to experimental channels) or by using a current meter
Current meter is mostly used.
Velocity-area Method

64
Calculation of Mean Velocities
Type Depth in
vertical
Observation point Mean velocity
Single point 1 - 2 ft 0.6D from surface v = v0.6
Two point 2 - 10 ft 0.2D & 0.8D v=1/2[v0.2+v0.8)]
Three point 10 - 20 ft 0.2D, 0.6D & 0.8D v=1/4[v0.2+2v0.6+v0.8)]
Five point Over 20 ft S, 0.2D, 0.6D, 0.8D & B v=1/10[vs+3v0.2+2v0.6+3v0.8)+vB
]
VS is measured 1 ft below surface, and VB is measured 1 ft above stream bottom
Cross section of a river - current meter measurements
Discharge in subsection
Qsub = Asub x Vsub

65
a) Price current meter
This consists of six conical cups rotating about a vertical axis
Electrical device is used to count the revolutions
Meter is suspended
from a cable

66
b) Propeller Current Meter
Rotating element is a propeller turning about a horizontal axis
The relation between revolutions per second N of the meter cups and
water velocity v in m/s is given by an equation of the form
v = a + bN Where, a is the starting velocity or velocity
required to overcome mechanical friction

67
Stage - Discharge Relations
Stage-discharge relation (or rating curve) is constructed by
plotting measured discharge against stage (water surface
elevation) at the time of measurement
0
2
4
6
8
0 1 2 3 4 5 6 7
Discharge (m3
/s)
Stage
(m)

68
Assumption: if the stage at that point is the same, the
discharge will be the same
River discharge can be estimated from measurement of stage
alone
Constructed by plotting successive measurements of the
discharge and gauge heights on a graph
Rating curve must be checked periodically to ensure that the
relationship between the discharge and gauge height has
remained constant: scouring of the stream bed or deposition of
sediment in the stream can cause the rating curve to change
so that the same recorded gauge height produces a different
discharge.
Stage - Discharge Relations

69
Floats
A floating object on the surface of a stream is timed: provides
surface velocity
t
S
Vs =
Where,
S = distance
traveled in time
t

70
Though primitive still finds applications in special
circumstances, such as:
(i) a small stream in flood
(ii) small stream with a rapidly changing water surface
(iii) preliminary or exploratory surveys
Any floating object can be used
Normally specially made leakproof and easily identifiable
floats are used
Mean velocity is obtained by multiplying the observed
surface velocity by a reduction coefficient
Floats

71
Surface floats are affected by surface winds
To get the average velocity in the vertical, floats in which part
of the body is under water (rod floats) are used
The method is restricted to straight rivers having almost
uniform cross section throughout
The method is not good if the depth of water exceeds 1.5 m
or more
Floats

72
Weirs and Flumes
Employed where a high degree of precision or reliability of
the discharge measurements are required
Confined (or limited) to streams and fairly small rivers
For wide rivers this is extremely expensive
Stream discharge is made to behave according to certain
well known hydraulics laws

73
Weirs and Flumes

74
Dilution Method
Used where the conventional velocity-area method can
not be used
(eg. boulder - strewn stream)
Discharge is determined over a length of the stream
rather than at a single point
A known quantity of some exotic substance (a tracer) is
introduced. Samples are withdrawn at a downstream
point

75
The tracer may be a chemical
tracer or a radioactive tracer
If a tracer solution is injected into a stream at a constant rate
and samples are taken downstream at a point where
turbulence has achieved complete mixing, the steady flow
rate Q in the stream is given by
)
C
-
C
C
-
C
(
Q
=
Q
0
2
2
1
t
Where,
Qt = Steady dozing rate
C0 = Concentration of the tracer in the undosed flow
C1 = Concentration of the tracer in the dose
C2 = Concentration of the tracer in the dosed flow
Dilution Method

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