This chapter discusses key concepts in hydrology including: - The hydrologic cycle and its major processes like precipitation, infiltration, evapotranspiration, surface runoff, and groundwater flow. - The global distribution of water resources, with oceans holding over 96% of water as saline water and the small fraction of freshwater resources on land and in ice. - The water budget equation that accounts for inputs, outputs, and changes in storage of water in a given catchment area over time. - Major components of the hydrologic cycle and their roles in transporting water throughout the environment.

1. Chapter 2
The Hydrologic Cycle and
hydrologic processes
Prof. Dr. Ali El-Naqa
Hashemite University
June 2013

2. INTRODUCTION
 HYDROLOGY and HYDROGEOLOGY
 Scope of Hydrogeology
 Historical Developments in Hydrogeology
 Hydrologic Cycle
 groundwater component in hydrologic cycle,
 Hydrologic Equation
 HYDROLOGY and HYDROGEOLOGY
 HYDROLOGY:
 the study of water. Hydrology addresses the occurrence, distribution,
movement, and chemistry of ALL waters of the earth.
 HYDROGEOLOGY: includes the study of the interrelationship of
geologic materials and processes with water,
 origin
 Movement
 development and management

3. Hydrologic Cycle
 Saline water in oceans accounts for 97.2% of total water on earth.
 Land areas hold 2.8% of which ice caps and glaciers hold 76.4% (2.14%
of total water)
 Groundwater to a depth 4000 m: 0.61%
 Soil moisture .005%
 Fresh-water lakes .009%
 Rivers 0.0001%.
 >98% of available fresh water is groundwater.
 Hydrologic CYCLE has no beginning and no end
 Water evaporates from surface of the ocean, land, plants..
 Amount of evaporated water varies, greatest near the equator.
 Evaporated water is pure (salts are left behind).

4.  When atmospheric conditions are suitable, water vapor condenses and
forms droplets.
 These droplets may fall to the sea, or unto land (precipitation) or may
evaporate while still aloft
 Precipitation falling on land surface enters into a number of
different pathways of the hydrologic cycle:
 some temporarily stored on land surface as ice and snow or
water puddles (depression storage)
 some will drain across land to a stream channel (overland
flow).
 If surface soil is porous, some water will seep into the ground by
a process called infiltration (ultimate source of recharge to
groundwater).

5.  Below land surface soil pores contain both air and water: region
is called vadose zone or zone of aeration
 Water stored in vadose zone is called soil moisture
 Soil moisture is drawn into rootlets of growing plants
 Water is transpired from plants as vapor to the atmosphere
 Under certain conditions, water can flow laterally in the vadose
zone (interflow)
 Water vapor in vadose zone can also migrate to land surface,
then evaporates
 Excess soil moisture is pulled downward by gravity (gravity
drainage)
 At some depth, pores of rock are saturated with water marking
the top of the saturated zone.

6.  Top of saturated zone is called the water table.
 Water stored in the saturated zone is known as ground water
(groundwater)
 Groundwater moves through rock and soil layers until it discharges
as springs, or seeps into ponds, lakes, stream, rivers, ocean
 Groundwater contribution to a stream is called baseflow
 Total flow in a stream is runoff
 Water stored on the surface of the earth in ponds, lakes, rivers is
called surface water
 Precipitation intercepted by plant leaves can evaporate to
atmosphere

7. Groundwater component
in the hydrologic cycle
 Vadose zone = unsaturated zone
 Phreatic zone = saturated zone
 Intermediate zone separates phreatic zone
from soil water
 Water table marks bottom of capillary water
and beginning of saturated zone

8. Distribution of Water
in the Subsurface

10. Units are relative to annual P on land surface
100 = 119,000 km3/yr)

12. Hydrologic Equation
 Hydrologic cycle is a network of inflows and outflows,
expressed as
 Input - Output = Change in Storage (1)
 Eq. (1) is a conservation statement: ALL water is
accounted for, i.e., we can neither gain nor lose water.
 On a global scale
 atmosphere gains moisture from oceans and land areas E
 releases it back in the form of precipitation P.
 P is disposed of by evaporation to the atmosphere E,
 overland flow to the channel network of streams Qo,
 Infiltration through the soil F.
 Water in the soil is subject to transpiration T, outflow to the
channel network Qo, and recharge to the groundwater RN.

13.  The groundwater reservoir may receive water
Qi and release water Qo to the channel network
of streams and atmosphere.
 Streams receiving water from groundwater
aquifers by base flow are termed effluent or
gaining streams.
 Streams losing water to groundwater are called
influent or losing streams

14.  A basin scale hydrologic subsystem is connected to
the global scale through P, Ro , equation (1) may be
reformulated as
P - E - T -Ro = DS (2)
DS is the lumped change in all subsurface water. All
terms have the unit of discharge, or volume per unit
time.
 Equation (2) may be expanded or abbreviated
depending on what part of the cycle we are
interested in. for example, for groundwater
component, equation (2) may be written as
RN + Qi - T -Qo = DS (3)

15.  Over long periods of time, provided basin is in its
natural state and no groundwater pumping taking
place, RN and Qi are balanced by T and Qo, so
change in storage is zero. This gives:
RN + Qi = T + Q0 (4)
 => groundwater is hydrologically in a steady state.
 If pumping included, equation (4) becomes
RN + Qi - T -Qo - Qp = DS (5)
Qp= added withdrawal.

16.  As pumping is a new output from the system,
 water level will decline
 Stream will be converted to a totally effluent,
 transpiration will decline and approach zero.
 Potential recharge (which was formerly rejected due to a wt at or
near gl) will increase.
 Therefore, at some time after pumping starts, equation (5)
becomes:
RN + Qi - Qo - Qp = DS (6)

17.  A new steady state can be achieved if pumping
does not exceed RN and Qi.
 If pumping exceeds these values, water is
continually removed from storage and wl will
continue to fall over time. Here, the steady state
has been replaced by a transient or unsteady state.
 In addition to groundwater being depleted from
storage, surface flow has been lost from the
stream.

18. Example
groundwater changes in
response to pumping
Inflows ft3/
s
Outflows ft3/s
1. Precipitation 2475 2. E of P 1175
3. gw discharge to sea 725
4. Streamflow to sea 525
5. ET of gw 25
6. Spring flow 25

19. Example, contd.
 Write an equation to describe water balance.
SOLUTION:
Water balance equation:
Water input from precipitation – evapotranspiration of
precipitation – evapotranspiration of groundwater –
stream flow discharging to the sea – groundwater
discharging to the sea – spring flow = change in storage
P –ETp – ETgw –Qswo – Qgwo –Qso = ∆S

20. Example, contd
Is the system in steady state?
Substitute appropriate values in above
equation:
2475 – 1175 -25 -525 -25 = ∆S 0=

21. 1. Basic Hydrology Concept
 Water is vital for all living organisms on Earth.
 For centuries, people have been investigating where
water comes from and where it goes, why some of it is
salty and some is fresh, why sometimes there is not
enough and sometimes too much. All questions and
answers related to water have been grouped together
into a discipline.
 The name of the discipline is hydrology and is formed
by two Greek words: "hydro" and "logos" meaning
"water" and "science".
1.1. Introduction

22.  What is Hydrology?
 It is a science of water.
 It is the science that deals with the occurrence,
circulation and distribution of water of the earth and
earth’s atmosphere.
 A good understanding of the hydrologic processes is
important for the assessment of the water resources,
their management and conservation on global and
regional scales.

23. In general sense engineering hydrology
deals with
 Estimation of water resources
 The study of processes such as
precipitation, evapotranspiration, runoff
and their interaction
 The study of problems such as floods and
droughts and strategies to combat them

24. 1.2 Hydrologic Cycle
 Water exists on the earth in all its three states, viz.
liquid, solid, gaseous and in various degrees of
motion.

25. Hydrologic cycle….
 Water, irrespective of different states, involves
dynamic aspect in nature.
 The dynamic nature of water, the existence of
water in various state with different hydrological
process result in a very important natural
phenomenon called Hydrologic
cycle.

26. Hydrologic cycle….
 Evaporation of water from water bodies, such as oceans
and lakes, formation and movement of clouds, rain and
snowfall, stream flow and ground water movement are
some examples of the dynamic aspects of water.

27.  Evaporation from
water bodies
 Water vapour
moves upwards
 Cloud formation
 Condensation
 Precipitate
 Interception
 Transpiration
 Infiltration
 Runoff–streamflow
 Deep percolation
 Ground water flow
Hydrologic cycle….

28. Hydrologic cycle….
 The hydrologic cycle has importance influence in a variety
of fields agriculture, forestry, geography, economics,
sociology, and political scene.
 Engineering application of the knowledge are found in
the design and operation of the projects dealing with
water supply, hydropower, irrigation & drainage, flood
control, navigation, coastal work, various hydraulic
structure works, salinity control and recreational use of
water.

29. 1.3 Water Budget Equation
 The area of land draining in to a stream or a water
course at a given location is called catchment area /
drainage area / drainage basin / watershed.
 A catchment area is separated from its
neighbouring areas by a ridge called divide /
watershed.
Catchment area

30. 1.3 Water Budget Equation
 A watershed is a geographical unit in which the
hydrological cycle and its components can be
analysed. The equation is applied in the form of
water-balance equation to a geographical
region, in order to establish the basic
hydrologic characteristics of the region. Usually
a watershed is defined as the area that appears,
on the basis of topography, to contribute all the
water that passes through a given cross section
of a stream.
Catchment area….

31. Watershed and watershed divide
Watershed/
catchment
Watershed/
catchment

32.  If a permeable soil covers an impermeable substrate,
the topographical division of watershed will not always
correspond to the line that is effectively delimiting the
groundwater.
Catchment area….

33. Watershed characteristics

34. Water Budget Equation
 For a given catchment, in an interval of time ∆t, the
continuity equation for water in its various phases can
be given as:
Mass inflow – Mass outflow = change in mass storage
 If the density of the inflow, outflow and storage
volumes are the same:
Vi - Inflow volume in to the catchment, Vo - Outflow volume
from the catchment and ∆S - change in the water volume
i oV V S  D

35. Water Budget Equation…
 Therefore, the water budget of a catchment for a time
interval ∆t is written as:
P – R – G – E – T = ∆S
P = Precipitation, R = Surface runoff, G = net ground water flow out of the
catchment, E = Evaporation, T = Transpiration, and ∆S = change in storage
 The above equation is called the water budget equation for
a catchment
NOTE: All the terms in the equation have the dimension of
volume and these terms can be expressed as depth over
the catchment area.

36. Components of hydrologic cycle
Precipitation
Infiltration
Evapo transpiration
Inter flow
Groundwater flow
Base flow
Stream flow
(Runoff)

37. 1.3 World Water Budget
Total quantity of water in the world is
estimated as 1386 M km3
 1337.5 M km3 of water is contained in
oceans as saline water
 The rest 48.5 M km3 is land water
 13.8 M km3 is again saline
 34.7 M km3 is fresh water
 10.6 M km3 is both liquid and fresh
 24.1 M km3 is a frozen ice and glaciers in the polar
regions and mountain tops

38. Estimated World Water Quantitites
96%
1%
1%
2% Ocean-saline
Land - saline
Fresh - Liquid
Fresh - Frozen

39. Global annual water balance
SN Item Ocean Land
1 Area (km2) 361.3 148.8
2 Precipitation (km3/year)
(mm/year)
458,000
1270
119,000
800
3 Evaporation (km3/year)
(mm/year)
505,000
1400
72,000
484
4 Runoff to ocean
Rivers (km3/year)
Groundwater (km3/year)
44,700
2,200
Total Runoff (km3/year)
(mm/year)
47,000
316

40. Water Balance of Continents
Area (M km^2)
30.3
8.7 9.8
20.7
17.8
45
0
10
20
30
40
50
Africa Asia Australia Europe N.America S.America
Precipitation (mm/yr)
686 736 734 670726
1648
0
500
1000
1500
2000
Africa Asia Australia Europe N.America S.America

41. Water Balance …….
Precipitation (mm/yr)
686 736 734 670726
1648
0
500
1000
1500
2000
Africa Asia Australia Europe N.America S.America
Evaporation (mm/yr)
547 510
415 383
1065
433
0
200
400
600
800
1000
1200
Africa Asia Australia Europe N.America S.America
Total Runoff (mm/yr)
139
226
319
287293
583
0
100
200
300
400
500
600
700
Africa Asia Australia Europe N.America S.America
Drop of water …..
Matter…..

42. Water Balance of Oceans
107
12
75
167
780
240
1010
1210
1040
120
1380
1140
0
200
400
600
800
1000
1200
1400
1600
Atlantic Arctic Indian Pacific
Area M km^2
Precp (mm/yr)
Evap. (mm/yr)
Water flow in Ocean
200 230
70 60
350
-300
130
-60
-400
-200
0
200
400
Atlantic Arctic Indian Pacific
Continental Inflow (mm/yr)
water exch. with ocean(mm/yr)

43. 1.4 Application in Engineering
 Hydrology finds its greatest application in the
design and operation of water resources engineering
projects
 The capacity of storage structures such as reservoir
 The magnitude of flood flows to enable safe disposal
of the excess flow
 The minimum flow and quantity of flow available at
various seasons
 The interaction of the flood wave and hydraulic
structures, such as levees, reservoirs, barrages and
bridges

51. Chapter Headings
 The hydrologic cycle
 Precipitation
 Runoff
 Surface and
groundwater storage
 Evaporation
 Condensation
 Climate and weather
 Climate
 Monitoring climate
change
 Weather
 Weather modification
 Floods
 Drought

55. Groundwater Storage
Fetter, Applied Hydrology

56. Groundwater Storage
 Groundwater recharge
 Water added to groundwater usually through
percolation down through the soil to the water table
 Groundwater discharge
 Water lost from groundwater usually through springs,
streams, and rivers

57. Groundwater Storage
Fetter, Applied Hydrology

58. Introduction
 Precipitation is any form of solid or liquid water that
falls from the atmosphere to the earth’s surface.
Rain, drizzle, hail and snow are examples of
precipitation.
 Evapotranspiration is the process which returns
water to the atmosphere and thus completes the
hydrologic cycle. Evapotranspiration consists of two
parts, Evaporation and Transpiration.
 Evaporation is the loss of water molecules from soil
masses and water bodies. Transpiration is the loss
of water from plants in the form of vapour.

59. Precipitation types
 The can be categorized as.
 Frontal precipitation
 This is the precipitation that is caused by the expansion of air on
 ascent along or near a frontal surface.
 • Convective precipitation
 Precipitation caused by the upward movement of air which is
 warmer than its surroundings. This precipitation is generally
 showery nature with rapid changes of intensities.
 • Orographic precipitation
 Precipitation caused by the air masses which strike the mountain
 barriers and rise up, causing condensation and precipitation. The
 greatest amount of precipitation will fall on the windward side of the
 barrier and little amount of precipitation will fall on leave ward side.

60. Measurement of rainfall
 One can measure the rain falling at a place by placing a measuring
cylinder graduated in a length scale, commonly in mm. In this way,
we are not measuring the volume of water that is stored in the
cylinder, but the ‘depth’ of rainfall.
 The cylinder can be of any diameter, and we would expect the same
‘depth’ even for large diameter cylinders provided the rain that is
falling is uniformly distributed in space.
 In practice, rain is mostly measured with the standard non-
recording rain gauge the details of which are given in Bureau of
Indian Standards code IS 4989: 2002. The rainfall variation at a point
with time is measured with a recording rain-gauge, the details of
which may be found in IS 8389: 2003. Modern technology has
helped to develop Radars, which measures rainfall over an entire
region

61. Variation of rainfall
 Rainfall measurement is commonly used to estimate the amount of
water falling over the land surface, part of which infiltrates into the
soil and part of which flows down to a stream or river. For a scientific
study of the hydrologic cycle, a correlation is sought, between the
amount of water falling within a catchment, the portion of which that
adds to the ground water and the part that appears as streamflow.
Some of the water that has fallen would evaporate or be extracted
from the ground by plants.

62. Variation of rainfall
 In Figure 1, a catchment of a river is shown with four rain gauges, for
which an assumed recorded value of rainfall depth have been shown
in the table. It is on the basis of these discrete measurements of
rainfall that an estimation of the average amount of rainfall that has
probably fallen over a catchment has to be made. Three methods are
commonly used, which are discussed in the following section.

63. Average rainfall depth
 Average rainfall depth
 The time of rainfall record can vary and may typically range from 1 minute to
1 day for non – recording gauges, Recording gauges, on the other hand,
continuously record the rainfall and may do so from 1 day 1 week,
depending on the make of instrument. For any time duration, the average
depth of rainfall falling over a catchment can be found by the following three
methods.
 The Arithmetic Mean Method
 The Thiessen Polygon Method
 The Isohyetal Method
 Arithmetic Mean Method
 The simplest of all is the Arithmetic Mean Method, which taken an average
of all the rainfall depths as shown in Figure 2.

64. Average rainfall depth
 Average rainfall as the arithmetic mean of all the records of the four rain
 gauges, as show in below:
 The Theissen polygon method
 This method, first proposed by Thiessen
in 1911, considers the representative area
for each rain gauge. These could also be
thought of as the areas of influence of each
rain gauge, as shown in Figure 3.

65. Average rainfall depth

66.  These areas are found out using a method consisting of the following
three steps:
 1. Joining the rain gauge station locations by straight lines to form
 triangles
 2. Bisecting the edges of the triangles to form the so-called
“Thiessen polygons”
 3. Calculate the area enclosed around each rain gauge station
 bounded by the polygon edges (and the catchment boundary,
 wherever appropriate) to find the area of influence corresponding to
 the rain gauge.
 For the given example, the “weighted” average rainfall over the
catchment is determined as
Average rainfall depth

67. Average rainfall depth
 The Isohyetal method
 This is considered as one of the most accurate methods, but it is
dependent on the skill and experience of the analyst. The method
requires the plotting of isohyets as shown in the figure and
calculating the areas enclosed either between the isohyets or
between an isohyet and the catchment boundary.
 The areas may be measured with a planimeter if the catchment map
is drawn to a scale.

68. Average rainfall depth

69. Average rainfall depth
 For the problem shown in Figure 4, the following may be assumed to be the
 areas enclosed between two consecutive isohyets and are calculated as
 under:
 Area I = 40 km2
 Area II = 80 km2
 Area III = 70 km2
 Area IV = 50 km2
 Total catchment area = 240 km2
 The areas II and III fall between two isohyets each. Hence, these areas may
 be thought of as corresponding to the following rainfall depths:
 Area II : Corresponds to (10 + 15)/2 = 12.5 mm rainfall depth
 Area III : Corresponds to (5 + 10)/2 = 7.5 mm rainfall depth
 For Area I, we would expect rainfall to be more than 15mm but since there is
 no record, a rainfall depth of 15mm is accepted. Similarly, for Area IV, a
 rainfall depth of 5mm has to be taken. Hence, the average precipitation by the
isohyetal method is calculated to be

70. Average rainfall depth
 Please note the following terms used in this section:
 Isohyets: Lines drawn on a map passing through places having
equal amount of rainfall recorded during the same period at these
places (these lines are drawn after giving consideration to the
topography of the region).
 Planimeter: This is a drafting instrument used to measure the area
of a graphically represented planar region.

71. Class A evaporation pan
www.novalynx.com

72. Evaporation
 Evaporation – loss of liquid water from land and water
surfaces as it is converted to a gas (water vapor)
 Transpiration – liquid water moving from soil through
a plant and evaporating from the leaves
 Evapotranspiration (ET) – combination of evaporation
and transpiration