estimation of natural ground water recharge final

8/2/2019 Estimation of Natural Ground Water Recharge Final

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ESTIMATION OF NATURALGROUND WATER RECHARGE


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CONTENTS

INTRODUCTION

TYPES OF ESTIMATION METHODS

RECHARGE ESTIMATION TECHNIQUES

(a)SOIL WATER BALANCE METHOD

(b)ZERO FLUX PLANE METHOD

(c)ONE DIMENSIONAL SOIL WATER FLOW MODEL

(d)GROUND WATER LEVEL FLUCTUATION METHOD(e)GROUND WATER BALANCE METHOD


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INTRODUCTION

Ground water recharge may be explained as theprocess where by the amount of water present in orflowing through the interstices of the sub-soilincreases by natural or artificial means.

Groundwater recharge is an important process forsustainable groundwater management, since thevolume-rate abstracted from an aquifer in the longterm should be less than or equal to the volume-rate that is recharged.

Rainfall is the principal source for replenishment ofmoisture in the soil water system and recharge ofground water. Other sources include recharge fromrivers, streams, irrigation water etc


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TYPES OF ESTIMATION METHODS

Physical methods use the principles of soil physics toestimate recharge. The directphysical methods arethose that attempt to actually measure the volume ofwater passing below the root zone. Indirectphysicalmethods rely on the measurement or estimation of soil

physical parameters, which along with soil physicalprinciples.

Chemical methods utilize the presence of relativelyinert water-soluble substances, such as an isotopictracer or chloride.

Numerical Models

Recharge can be estimated using numerical methods,using such codes as HELP, UNSAT-H, SHAW, WEAP,and MIKE SHE.


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RECHARGE ESTIMATION TECHNIQUES

The following methods are commonly in use forestimating natural ground water recharge:

i) Soil water balance method

ii) Zero flux plane methodiii) One-dimensional soil water flow modeliv) Ground water level fluctuation methodv) Ground water balance method

vi) Inverse modeling techniquevii) Hybrid water fluctuation methodviii) Isotope and solute profile techniques


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SOIL WATER BALANCE METHOD:

In hydrology, a water balance equation can be usedto describe the flow of water inand outof a system.

The method is essentially a bookkeepingprocedure, which estimates the balance betweenthe inflow and outflow of water.

Here, the volume of water required to saturate thesoil is expressed as an equivalent depth of waterand is called soil water deficit.


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EQUATION

The soil water balance can be represented by:

Ri = P - Ea+ W R0

Where,

Ri - recharge

P - precipitation

Ea - actual evapotranspiration.

W- change in soil water storage.

R0 - run off


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ZERO FLUX PLANE METHOD

The zero flux plane method relies on the location ofa plane of zero hydraulic gradient in the soil profile.

Recharge over a time interval is obtained bysummation of the changes in water contents belowthe plant.

The position of the zero flux plane is usuallydetermined by tensiometers.


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EQUATION

Darcy's law gives the flux q, defined as the volumeof water per unit time passing through the unit areaat any depth:


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ZERO FLUX PLANE METHODCONTD.

Thus, knowing the unsaturated hydraulicconductivity and potential gradient the flux may bedetermined.

The hydraulic conductivity estimation presentsmore problems such as

(i) K may vary by a factor of 103 or so over thenormal moisture content of a typical soil and

(ii) K differs depending on the place even in partlyhomogenous soil.


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ONEDIMENSIONALSOILWATERFLOWMODEL

For recharge to occur, water has to move throughthe unsaturated zone until it reaches the watertable.

The equations commonly used is Van Genuchtenfunction.

where n and m are characteristic constants whichhave to be determined for every soil type and m =1-1/n


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Equation in terms of reduced water content


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GROUNDWATERLEVELFLUCTUATIONMETHOD

This is an indirect method of deducing the rechargefrom the fluctuation of the water table.

The rise in the water table during the rainy seasonis used to estimate the recharge, provided thatthere is a distinct rainy season with the remainderof the year being relatively dry.

the rise in the water table is primarily due to therainfall recharge.


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the rise in water table is >Ds, then rainfall recharge,Ri is given by:


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GROUNDWATERBALANCEMETHOD


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GROUNDWATERBALANCEEQUATION


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GROUNDWATERBALANCEEQUATIONCONTD.

The boundaries of an area usually studied do notrepresent streamlines i.e., they are notperpendicular to the equipotential line. Hence, thelateral inflow and outflow must be accounted for the

balance equation.

All elements of the water balance equation arecomputed using independent methods whereverpossible.

The discrepancy of water balance is given as aresidual term of the water balance equation andincludes the error in the determination of thecomponents.


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CONCLUSION

While estimating natural ground water recharge, itis essential to have a good idea of the differentrecharge mechanisms and their importance in thestudy area.

Choice of methods should also be guided by theobjectives of the study, available data and thepossibilities to get supplementary data.

Economy, too is an important factor.

It is desirable to apply more than one methodbased on independent input data.


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estimation of natural ground water recharge final

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