Rezaul Karim

Environmental Science and Health Management

Jessore Science and Technology University

Hydrology: Chapter 2 Hydrological Cycle

Introduction

Hydrologic cycle and its components and process;

System Concept,

Water Balance,

World's surface water: precipitation, evaporation and

runoff,

Metrological Parameters Affecting Hydrologic Cycle,

Hydrologic Equation

common units of measurement;

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Recommended references

Tim Davie (2002) Fundamentals of hydrology, Routledge

Fundamentals of Physical Geography, 2nd ed. Routledge

270 Madison Avenue, New York, NY 10016

Viessman, W., Jr., and G.L. Lewis, 2003. Introduction to

Hydrology, 5th Edition. Harper Collins College Publishers,

New York, NY.

Raghunath, H. M. (2006) Hydrology: principles, analysis and

design. 2nd ed. New age international (p) limited, publishers

4835/24, ansari road, daryaganj, new delhi - 110002

Chaw, David and Larry, Applied Hydrology

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Hydrologic cycle

Water, which is found everywhere on the earth, is one of

the most basic and commonly occurring substances.

It is the only substance on earth that exists naturally in

the three basic forms of matter, i.e., liquid, solid, and gas.

The quantity of water varies from place to place and from

time to time.

Although at any given moment the vast majority of the

earth's water is found in the world's oceans, there is a

constant interchange of water from the oceans to the

atmosphere to the land and back to the ocean.

This interchange is called the hydrologic cycle.

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Hydrologic cycle

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Hydrologic cycle

The path water takes as it

circulates from the land to

the sky and back again.

Water is recycled this way

so we do not run out.

Important process

Evaporation

Condensation

Precipitation

Runoff

Infiltrtion

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Evaporation and evapotranspiration Evaporation is when the sun heats

up water in rivers or lakes or the ocean and turns it into vapor or steam.

The water vapor or steam leaves the river, lake or ocean and goes into the air.

Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as Evapotranspiration.

Here water given off through the pores of plants and animals joins the atmosphere as a vapor.

Annual Evapotranspiration amounts approximately = 505,000 km of water

From the ocean = 434,000 km.

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Condensation and Precipitation

Water vapor in the air gets

cold and changes back into

liquid, forming clouds. This is

called CONDENSATION.

PRECIPITATION occurs

when so much water has

accumulated that the air

cannot hold it anymore.

The clouds get heavy and

water falls back to the earth

in the form of rain, hail, sleet

or snow.

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Runoff and Infiltration When the precipitation reaches the

ground, several things can happen to it. First, it might be re-evaporated. For instance, we have all seen the mist rising off hot roads after a summer shower.

If it isnt re-evaporated, much of the water will become RUNOFF that goes into streams, lakes and rivers as it flows back to the ocean.

Some of the precipitation will be absorbed into the ground. This is called INFILTRATION. Once in the ground, the water can join the earths ground water supply.

This is one of the worlds largest store house of water. The water could also be absorbed from the ground by the roots of plants.

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Reservoirs

The largest reservoir is the collection of oceans,

accounting for 97% of the Earths water.

The next largest quantity(2%) is stored in solid form in

ice caps and glaciers.

The small amount accounts for approximately(75%) of all

fresh water reserves on the planet.

The water contained within all living organisms

represents the smallest reservoir

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Hydrologic cycle

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Hydrology is the study of several

physical processes;

Atmospheric processes: cloud condensation, precipitation.

Surface processes: snow accumulation, overland flow, river

flow, lake storage.

Subsurface processes: infiltration, soil-water storage,

groundwater flow.

Interfacial processes: evaporation, transpiration, sediment

water exchange.

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A Note on Units

Rainfall volume is normally measured in inches or cm

Rainfall rate or intensity in inches/hr or cm/hr

Infiltration is measured in inches/hr or cm/hr

Evaporation is measured in inches or in/hr (cm/hr)

Stream flow is measured in cfs or m3/s

One acre-ft of volume is 43,560 ft3 of water

1 ac-inch/hr is approx. equal to 1.008 cfs

Ground water flows are measured as ft3/day or m3/day

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System Concept

A system is a set of connected parts that form a whole.

The hydrologic cycle may be treated as a system whose

components are precipitation, evaporation, runoff, and

other phases of the hydrologic cycle.

These components can be grouped into subsystems of

the overall cycle; to analyze the total system.

The simpler subsystems can be treated separately and

the results combined according to the interactions

between the subsystems.

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Block Diagram representation of global

hydrologic cycle

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By analogy, a hydrologic system is defined as a structure or volume in space, surrounded by a boundary, that accepts water and other inputs, operates on them internally, and produces them as outputs (Fig. 1.3).

The structure (for surface or subsurface flow) or volume in space ( for atmospheric moisture flow) is the totality of the flow paths through which the water may pass as throughput from the point it enters the system to the point it leaves.

The boundary is a continuous surface defined in three dimensions enclosing the volume or structure.

A working medium enters the system as input, interacts with the structure and other media, and leaves as output.

Physical, chemical, and biological processes operate on the working media within the system; the most common working media involved in hydrologic analysis are water, air, and heat energy

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Water Balance

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Global annual water balance

Ocean Land

Area (km2) 361,300,000 148,800,000

Precipitation (km3/yr) 458,000 119,000

(mm/yr) 1270 800

(in/yr) 50 31

Evaporation

(km3/yr) 505,000 12,000

(mm/yr) 1400 484

(in/yr) 55 19

Runoff to ocean

Rivers (km3/yr) 44,700

Groundwater (km3/yr) 2200

Total runoff (km3/yr) 47,000

(mm/yr) 316

(in/yr) 12

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Retention time / Residence time:

The residence time of a reservoir within the Hydrological

cycle is the average time a water molecule will spend in that

reservoir.

It is a measure of the average age of the water in that

reservoir, though some water will spend much less time than

average and some much more.

The residence time, Tr is the average duration for a water

molecule to pass through a subsystem of the hydrologic cycle.

It is calculated by dividing the volume of water S in storage by

the flow rate Q.

Tr = S/Q.

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Example: Estimate the residence time of

global atmospheric moisture.

The volume of atmospheric moisture is 12,900 km3.

The flow rate of moisture from the atmosphere as

precipitation is 458,000 + 119,000 = 577.000 km3/yr,

So the average residence time for moisture in the

atmosphere is Tr = I2,900 /577,00 = 0.022 yr = 8.2 days.

The very short residence time for moisture in the

atmosphere is one reason why weather cannot be

forecast accurately more than a few days ahead.

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Water shed

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A watershed is the area of land draining into a stream at a given location.

The watershed divide is a line dividing land whose drainage fellows toward the given stream from land whose drainage flows away from that stream.

Hydrologic Equation

The hydrologic equation is simply the statement of the

law of conservation of matter and is given by

I = O + S; where I = inflow; O = outflow; S = change in storage

This equation states that during a given period, the total

inflow into a given area must equal the total outflow from

the area plus the change is storage.

While solving this equation, the ground water is

considered as an integral part of the surface water and it

is the subsurface inflow and outflow that pose problems

in the water balance studies of a basin.

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World's surface water: precipitation,

evaporation and runoff

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Metrological Parameters Affecting

Hydrologic Cycle

Air Temperature

It is measured by Thermometers located 1.25 m above the ground and sheltered.

Maximum temperature is measured by mercury thermometer while minimum temperature is measured by alcohol thermometer.

Mean daily Temp = 1/2 [Max day Temp + Min day Temp]

Mean Monthly Temp = 1/2 [Max Month Temp + Min Month Temp]

Mean annual Temp = Average of the monthly means over the year

Normal (daily, month) Temp = average of (daily, month.) over 30 years

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Humidity

Air has the property of absorbing water vapor (moisture) and is measured by the Psychrometer.

At each level of absorption, there is a certain level of vapor pressure ev. For every temperature, there is a maximum value of vapor pressure called saturated vapor pressure at which air cannot absorb moisture any more. Equation (1) gives the values of es corresponding to each Temperature.

es = 611 exp (17.27 T / 237.3 + T),,,,,,,,,,,,,,,,,, (1);

where, es in Pa and T in degree Celsius.

Relative Humidty (Rh) defines the air's capacity of absorbing moisture and can be expressed as a percentage:

Rh = (ev / es) X 100 ,,,,,,,,,,,,,(2)

Where ev and es are measured in units of mbar [1 mbar = 100 Pa]

or of 1 mm Hg [1 mm Hg = 1.33 mbar]

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Dew point (Td)

Dew point is the temperature at which ev reaches es for the same conditions and water vapor starts to condense.

Dew point can be computed using equation (1) if T is replaced by Td and the normal vapor pressure is considered as saturated one.

Wind

Wind speed, W, and its direction are measured by the anemometer and wind vane respectively. Wind speed is measured by units of Knot or mph, where, 1 Knot =1.852 km/h and 1 mph = 1.61 km/h

The relationship between wind speed and elevation is expressed by the power law profile equation

W/W0 = (Z / Z0)k,,,,,,,,,,,,,,,,,,,,,,,,,,,, (3);

where, k is Von Karman coefficient and it depends on the nature of surface and its value varies between 0.1 and 0.6.

Solar radiation

Solar radiation is the source of energy on the earth and it is measured by units of Watt/m2 and KJ/m2.

It is also measured by the radiometer in micro-meter (10-6 m) and the important term is the net radiation (Rn) that is used in some methods of estimating evapotranspiration.

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In the hydrologic cycle:

T, transpiration;

E, evaporation;

P, precipitation;

R, surface runoff;

G, groundwater flow;

I, infiltration and

S, storage

Subscripts s and g are introduced to denote vectors originating above and below the earth's surface respectively. For example, Rg, signifies groundwater flow that is effluent to a surface stream.

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Major aspects of hydrology

The main jobs of a hydrologist are collection and analysis of data, and making predictions out of this analysis.

Collection of data

The hydrologic data comprises rainfall data, snowfall and snowmelt data, runoff data, topographic maps and groundwater data

Analysis of data

Analysis of data includes checking it for consistency and homogeneity as well as its various statistical parameters.

Prediction

Prediction means findings design values and maximum possible floods and drought. Various approaches for prediction of hydrologic values

Statistical approach

Physical approach and

Deterministic approach

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Hydrological data For the analysis and design of any hydrologic project adequate data and length of

records are necessary.

A hydrologist is often posed with lack of adequate data. The basic hydrological data required are:

(i) Climatological data

(ii) Hydro-meteorological data like temperature, wind velocity, humidity, etc.

(iii) Precipitation records

(iv) Stream-flow records

(v) Seasonal fluctuation of ground water table or piezometric heads

(vi) Evaporation data

(vii) Cropping pattern, crops and their consumptive use

(viii) Water quality data of surface streams and ground water

(ix) Geomorphologic studies of the basin, like area, shape and slope of the basin, mean and median elevation, mean temperature (as well as highest and lowest temperature recorded) and other physiographic characteristics of the basin; stream density and drainage density; tanks and reservoirs

(x) Hydrometeorological characteristics of basin:

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(x) Hydrometeorological characteristics of

basin:

(i) long term precipitation, space average over the basin using isohyets and Several other methods

(ii) Depth-area-duration (DAD) curves for critical storms (station equipped with Self-recording rain gauges).

(iii) Isohyetal mapsIsohyets may be drawn for long-term average, annual and Monthly precipitation for individual years and months

(iv) Cropping patterncrops and their seasons

(v) Daily, monthly and annual evaporation from water surfaces in the basin

(vi) Water balance studies of the basin

(vii) Chronic problems in the basin due to a flood-menacing river (like Tapti or Tapi in central India) or silt menacing river (like Tungabhadra in Karnataka)

(vii) Soil conservation and methods of flood control

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Scope of hydrology

The study of hydrology helps us to know

the maximum probable flood that may occur at a given site and its frequency; this is required for the safe design of drains and culverts, dams and reservoirs, channels and other flood control structures.

The water yield from a basinits occurrence, quantity and frequency, etc; this is necessary for the design of dams, municipal water supply, water power, river navigation, etc.

the ground water development for which a knowledge of the hydrogeology of the area, i.e., of the formation soil, recharge facilities like streams and reservoirs, rainfall pattern, climate, cropping pattern, etc. are required.

The maximum intensity of storm and its frequency for the design of a drainage project in the area.

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Application of hydrology to

environmental problems It is true that humans cannot exist without water; it is also true that water,

mismanaged, or during times of deficiency (droughts), or times of surplus (floods), can be life threatening.

Furthermore, there is no aspect of environmental concern that does not relate in some way to water. Land, air, and water are all interrelated as are water and all life forms.

Accordingly, the spectrum of issues requiring an understanding of hydrologic processes is almost unlimited.

As water becomes scarcer and as competition for its use expands, the need for improved water management will grow.

And to provide water for the world's expanding population, new industrial developments food production, recreational demands, and for the preservation and protection of natural systems and other purposes, it will become increasingly important for us to achieve a thorough understanding of the underlying hydrologic processes with which we must contend.

This is the challenge to hydrologists water resources engineers; planners, policymakers, lawyers, Economists, and others who must strive to see that future allocations of water are Sufficient to meet the needs of human and natural systems.

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