chapter 1 hydrologic cycle.pdf

Gollis University Course : Hydrogeology Lecturer: Eng. M.M.Qawdhan Water Engineer/Hydrologist

Chapter One Elements of Hydrologic Cycle and hydrologic processes

INTRODUCTION  HYDROLOGY and 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

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).



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).

 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.

 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

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

Distribution of Water in the Subsurface

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

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.

Example groundwater changes in response to pumping Inflows

ft3/ Outflows s

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

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

Example, contd  Is the system in steady state?

Substitute appropriate values in above equation:

2475 – 1175 -25 -525 -25 = ∆S =0

1. Basic Hydrology Concept 1.1. Introduction  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".

 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.

 In general sense 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

1.2 Hydrologic Cycle  Water exists on the earth in all its three states, viz.

liquid, solid, gaseous and in various degrees of motion.

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

cycle.

Hydrologic

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.

Hydrologic cycle….  Evaporation from water bodies  Water vapour

moves upwards  Cloud formation  Condensation  Precipitate  Interception  Transpiration  Infiltration  Runoff–streamflow  Deep percolation  Ground water flow

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.

1.3 Water Budget Equation Catchment area  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.

1.3 Water Budget Equation Catchment area….  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.

Watershed and watershed divide

Watershed/ catchment

Watershed/ catchment

Catchment area….  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.

Watershed characteristics

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:

V  V

 S

Vi - Inflow volume in to the catchment, Vo - Outflow volume i o from the catchment and ∆S - change in the water volume

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 the catchment area.

over

Components of hydrologic cycle Evapo transpiration

Precipitation Stream flow (Runoff)

Inter flow

Infiltration Base flow Groundwater flow

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

Estimated World Water Quantitites 96%

2% 1%

1%

Ocean-saline Land - saline Fresh - Liquid Fresh - Frozen

Global annual water balance SN

1 2 3 4

Item Area (km2) Precipitation (km3/year) (mm/year) Evaporation (km3/year) (mm/year) Runoff to ocean

Ocean 361.3 458,000 1270 505,000 1400

Land 148.8 119,000 800 72,000 484

Rivers (km3/year) Groundwater (km3/year)

44,700 2,200

Total Runoff (km3/year) (mm/year)

47,000 316

Water Balance of Continents Area (M km^2) 50

45

40

30.3

30

20.7

20 10

8.7

9.8

Australia

Europe

17.8

0 Africa

Asia

N.Am erica

S.Am erica

Precipitation (mm/yr) 2000 1648 1500 1000

686

726

736

734

670

Africa

Asia

Australia

Europe

N.Am erica

500 0 S.Am erica

Water Balance ……. Precipitation (mm/yr) 2000 1648 1500 1000

686

726

736

734

670

Africa

Asia

Australia

Europe

N.Am erica

500 0 S.Am erica

Evaporation (mm/yr) 1200

1065

1000 800 600

547 433

400

510

415

383

Europe

N.Am erica

Drop of water ….. Matter…..

200 0 Africa

Asia

Australia

S.Am erica

Total Runoff (mm/yr) 700 583

600 500 400

293

300 200

319 226

287

139

100 0 Africa

Asia

Australia

Europe

N.Am erica

S.Am erica

Water Balance of Oceans 1600

Area M km^2

1380

1400

1210

1200

1040

1000

Precp (mm/yr)

1140

Evap. (mm/yr)

1010

780

800 600 400

240 120

107

200

167

75

12

0 Atlantic

Arctic

Indian

Pacific

Water flow in Ocean 350

400 230

200 200

130

70

60

0 -200 -400

Atlantic -60

Arctic

Continental Inflow (mm/yr) water exch. with ocean(mm/yr)

Indian

-300

Pacific

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

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

Groundwater Storage

Fetter, Applied Hydrology

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

Groundwater Storage

Fetter, Applied Hydrology

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.

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.

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 nonrecording 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

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.

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.

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.

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.

Average rainfall depth

Average rainfall depth  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  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.

Average rainfall depth

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

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.

Conti…..InshALLAh