download-manuals-ground water-manual-gw-volume8operationmanualdataprocessingpartiv

Government of India & Government of The Netherlands

DHV CONSULTANTS &DELFT HYDRAULICS withHALCROW, TAHAL, CES,ORG & JPS

VOLUME 8DATA PROCESSING AND ANALYSIS

OPERATION MANUAL – PART IV

GROUNDWATER RESOURCE ASSESSMENT

Operation Manual – Data Processing and Analysis (GW) Volume 8 – Part IV

Data Processing and Analysis March 2003 Page i

Table of Contents

1 GROUNDWATER RESOURCE ASSESSMENTCONCEPT 1-1

1.1 RECHARGE COMPONENTS 1-11.2 DISCHARGE COMPONENTS 1-21.3 INDEPENDENT AND DEPENDENT COMPONENTS 1-31.4 STABILISING INFLUENCE OF THE DEPENDENT COMPONENTS 1-3

2 OBJECTIVES 2-1

2.1 ESTIMATION OF GROUNDWATER RESOURCE 2-12.2 VALIDATION 2-12.3 PRIORITISATION OF THE COMPONENTS 2-12.4 ESTIMATION OF AN UNKNOWN COMPONENT 2-2

3 CHOICE OF AREAL UNITS 3-1

3.1 MINIMISING THE DEPENDENT RECHARGE/ DISCHARGE COMPONENTS 3-13.2 IMMUNITY FROM EXTERNAL INFLUENCE 3-13.3 EVALUATION OF POSSIBLE AREAL UNITS 3-1

4 CHOICE OF WATER BALANCE PERIOD 4-1

5 COMPONENTS INCLUDED 5-1

6 ORGANIZATION OF THE DATABASE 6-1

6.1 DESCRIPTION OF THE AREA AND THE RETRIEVAL ZONES 6-16.2 DESCRIPTION OF THE AREAL BOUNDARY 6-16.3 DESCRIPTION OF THE PERIOD AND OBJECTIVE OF THE WATER BALANCE 6-16.4 RETRIEVAL OF DATA FROM THE BASIC MODULE 6-16.5 REVIEWING AND UPGRADING THE DATABASE 6-26.6 DIRECT ENTRY OF DATA 6-2

7 PROCESSING POINT DATA TO SPATIAL DISTRIBUTIONS 7-1

7.1 GENERATION OF CONTOURS/ SPATIAL DATA 7-17.2 WATER LEVEL DATA 7-17.3 AQUIFER PARAMETERS 7-17.4 RAINFALL 7-27.5 TOPOGRAPHICAL LEVEL 7-2

8 ESTIMATION OF WATER BALANCE COMPONENTS 8-1

8.1 STORAGE FLUCTUATION 8-18.2 RAINFALL RECHARGE 8-18.3 LATERAL FLOWS ACROSS BOUNDARY 8-28.4 STREAM AQUIFER INTER-FLOW 8-38.5 VERTICAL INTER-AQUIFER FLOW 8-38.6 EVAPOTRANSPIRATION FROM WATERTABLE 8-48.7 GW DRAFT 8-48.8 CANAL SEEPAGE 8-58.9 RECHARGE DUE TO RETURN FLOW FROM APPLIED IRRIGATION 8-58.10 RECHARGE FROM TANKS, PONDS, CHECK DAMS AND NALA BUNDS 8-5

9 TIME SERIES ANALYSIS 9-1

10 INTERFACING WITH THE GEC-97 NORMS 10-1

10.1 SALIENT FEATURES OF THE NORMS 10-110.2 IMPLEMENTATION OF THE NORMS 10-210.3 SUMMARY REPORT 10-4

11 REFERENCES 11-1


Data Processing and Analysis March 2003 Page 1-1

1 GROUNDWATER RESOURCE ASSESSMENT CONCEPT

A groundwater (GW) balance study involves an application of the continuity equation to an aquifer(usually unconfined). The continuity equation in this context is a statement to the effect that thedifference between the net recharge volume (I) and net discharge volume (O) equals the change ofgroundwater storage (∆S). I, O and ∆S must be in respect of the same aquifer area and the timeperiod.

I - O = ∆S (1.1)

The continuity equation may be applied to a large area treating the entire study area as a single entity.The flow across the boundary is estimated by an application of Darcy’s law. Such a study is known aslumped water balance (LWB). Alternatively, the continuity equation may be applied at a micro level.Thus, the area of interest is divided into a finite number of cells and the continuity equation is appliedto each cell. The flow between the adjacent cells is estimated by Darcy’s law. This approach could betermed as distributed water balance, and is conducted through the application of specialized softwarefor groundwater modelling (such as the widely used MODFLOW).

The dedicated software comprises a module on groundwater balance and GIS tools. The scope of thededicated software is restricted to only the lumped water balance. Therefore, only the lumped waterbalance is addressed in the following text.

The main advantage of the lumped water balance approach is its simplicity. The data processingcomprises only simple arithmetic/graphical procedures devoid of any higher mathematics. In spite ofits simplicity, this approach leads to validation of various procedures/ parameters. It also yieldsconsistent estimates (that is, estimates satisfying the continuity equation) of various components ofthe recharge and discharge. These estimates in turn permit an estimation of the permissiblegroundwater development.

The LWB permits only an analysis of current and historical data and as such is not a good tool forprojection of the aquifer response. The area for a LWB study could be a basin (hydrologic andhydrogeologic), inter-basin or an administrative/ political unit. The time period could be a year, aseason or a smaller unit like a fortnight or month and is only constrained by the frequency in which thebasic data are monitored.

The LWB study essentially comprises estimation of various components of the recharge/ dischargeand the change of storage in a given area of the given aquifer in a given time period followed by anoverall cross check for the consistency.

1.1 RECHARGE COMPONENTS

The various components of the recharge are as follows. The components are classified as per thegoverning process. The governing processes are vertical flow through unsaturated and saturatedzones; lateral (that is, predominantly horizontal flow) through the saturated zone; and vertical inter-aquifer flow (see Figure 1.1).

Recharge as a consequence of vertical unsaturated flow

• Rainfall recharge

• Recharge from applied irrigation- either surface water or groundwater or both



Figure 1.1: Recharge as a consequence of vertical saturated flow

• Recharge from canal seepage over the deep water table (depth below the canal bed more than1.5 times the canal width at the free surface)

• Recharge from storage tanks, ponds, percolation tanks, check dams and nala bunds over deepwater table

Recharge as a consequence of lateral saturated flow

• Subsurface seepage from hydraulically connected drains

• Subsurface horizontal inflow across a non-hydraulic boundary

• Recharge from canal seepage to the shallow water table (depth below the canal bed less than 1.5times the canal width at the free surface)

• Recharge from storage tanks, ponds, percolation tanks, check dams and nala bunds over shallowwater table

Recharge as a consequence of vertical inter-aquifer flow

• Recharge from an underlying leaky confined aquifer

1.2 DISCHARGE COMPONENTS

The various components of the discharge are as follows. The components have been classified inaccordance with the governing process. The governing processes are the vertical discharge throughan external sink, saturated vertical and lateral flow, and inter-aquifer flow.

Vertical discharge through an external sink

• Groundwater pumpage

• Evapotranspiration from shallow water table (within the capillary fringe/ root zone)

Discharge as a consequence of lateral saturated flow

• Subsurface outflow to hydraulically connected drains (rivers, sea)

• Subsurface horizontal outflow across a non-hydraulic boundary

Subsurfaceoutflow to

drains

Groundwaterdraft

Recharge fromcanal seepage

Recharge fromponds or tanks

Recharge fromirrigation

Rainfall rechargeEvapotranspiration

Leakage tounderlying aquifer

Subsurfacegroundwater

outflow

Subsurfacegroundwater

inflow

Recharge fromunderlying aquifer

Change ingroundwaterstorage: ∆S



Discharge as a consequence of vertical inter-aquifer flow

• Leakage to an underlying leaky confined aquifer

1.3 INDEPENDENT AND DEPENDENT COMPONENTS

Recharge and discharge as a consequence of the lateral saturated flow and the inter-aquifer flow andevapotranspiration are dependent upon the position of the water table. These are termed as thedependent components. Other components of recharge and discharge are almost independent of theposition of the water table. These are termed as the independent components. This classification ofthe components has an important bearing upon the choice of an areal unit for groundwater resourceassessment by LWB; and shall be referred back to subsequently.

1.4 STABILISING INFLUENCE OF THE DEPENDENT COMPONENTS

A close examination of variation of the dependent components as a consequence of any temporaltrend of water table, reveals that the variation tends to stabilise the causative trend. Thus, as thewater table declines, the recharge components increase and the discharge components (includingevapotranspiration) decrease. This tends to stabilise the falling trend. Similarly, as the water tablerises, the recharge components decrease and the discharge components increase. This tends tostabilise the rising trend.



2 OBJECTIVES

Broadly speaking, a water balance study can serve one or more of the following objectives:

2.1 ESTIMATION OF GROUNDWATER RESOURCE

The LWB as enumerated above provides the validated historical estimates of different components ofrecharge and discharge and as such may be used to estimate the status of the resource in a certaintime span. Such estimation is an essential management tool for arriving at permissible groundwaterdraft and development stage.

2.2 VALIDATION

Unfortunately many components of recharge and discharge can not always be estimated rationallyeither because of the complexity of the process or due to an inadequate database. Thesecomponents are estimated empirically using some locally accepted (or some times even imported)norms/ practices.

Even if rational algorithms are available and adopted, there would still be some uncertainty in theestimates on two counts. Firstly, even an apparently rational equation may have some hiddenassumptions. Secondly, it would invariably comprise some parameters, which may not always be wellknown. Thus, there is some inherent uncertainty in the estimates of almost all the components of thewater balance equation. This calls for a validation criterion.

A water balance study can be taken up to validate various adopted algorithms (rational or empirical)/norms/ practices/ parameter values. This can be accomplished by checking if the independentlyestimated components of recharge and discharge; and the storage change satisfy the water balanceequation. For this purpose the water balance equation is rewritten as follows:

I - O - ∆S = ε.∆S (2.1)

Where: ε.∆S is the residue term in the water balance equation. This term can be viewed as thevolumetric imbalance between the net recharge (that is, recharge minus discharge) and the storagefluctuation. The term ε is the normalized imbalance that is, the imbalance expressed as a fraction ofthe storage fluctuation.

Ideally, the residue should be equal to zero. However, in practice it may at the best be a negligiblequantity. Thus, a small enough residue may provide the desired validation. However, there is always apossibility that the errors in the estimates of the individual components may have compensated eachother. Thus, the validation can never be absolute. Nevertheless, if the residues turn out to be smallenough consistently over a number of time periods, one may be reasonably confident about thevalidation. Thus, it is necessary to carry out a multiple period water balance study.

2.3 PRIORITISATION OF THE COMPONENTS

The above procedure apart from validating the various procedures/ parameters, also leads to anunderstanding of the relative significance of the various component(s) of the water balance. Estimateof each component is normalised by expressing it as a fraction of the storage fluctuation. Thesefractions are thus indices of the relative significance of the respective components. The indices canprovide a basis for prioritisation of the components. Larger effort should be put in to estimate the highpriority components and vice versa. Larger effort implies refining the algorithm and carrying out directmeasurement of variables or conducting experimental field work to improve upon the estimates of theconcerned parameters.



2.4 ESTIMATION OF AN UNKNOWN COMPONENT

It follows from the preceding discussion that many components of recharge occur as a consequenceof flow through the unsaturated zone that extends from ground surface to the watertable. Thesecomponents thus, need to be estimated by simulating the flow through the unsaturated zone. Thissimulation requires data on the soil and surface characteristics that are invariably not available on aregional scale. Further, the computational efforts may be prohibitively large. Thus, these rechargecomponents are usually estimated empirically. This however, may introduce considerable uncertaintyin the results of a water balance study. The uncertainty can be quite severe if a dominant/ crucialcomponent is estimated empirically.

This problem can be overcome by first identifying the most crucial component (say X) which is notamenable to a rational estimation. An example of X could be the rainfall recharge during a monsoonseason. Subsequently, this component can be estimated by substituting the estimates of all othercomponents and of the storage fluctuation into the water balance equation. The residual term (ε) isassumed to be zero. Thus, the estimate of X shall be reliable provided the estimates of all othercomponents and of storage fluctuation are more or less error free. This calls for a precedent validationof the procedures/ parameters adopted for estimating components other than X. This is possible onlyif there exist seasons/ time periods during which these components are significant but the componentX is insignificant or absent. If the estimates of these components during such seasons satisfy thewater balance equation with consistently insignificant residue, the procedures/ parameters standvalidated. Subsequently, these validated procedures/ parameters can be used to estimate thecomponent X, as outlined above.

Typically, this approach may be adopted for estimating the recharge from monsoon rainfall. Theapproach shall involve first dividing a hydrologic year into the monsoon and non-monsoon periods.The procedures/ parameters adopted for estimating components other than the monsoon rechargeare first validated by carrying out multiple water balance of the non-monsoon seasons included in thedatabase. Subsequently, the rainfall recharge is estimated by carrying out the water balance study ofthe monsoon seasons of the database. It is worthwhile to observe here that the average watertablefluctuation method of estimating the groundwater resource (R = ∆h.A.Sy) is essentially asimplified water balance of the monsoon period. In this method it is assumed that all dischargecomponents (e.g., pumpage, evapotranspiration, etc.) and the recharge components other than therainfall recharge (e.g., recharge from irrigation) are negligible as compared to the rainfall recharge.



3 CHOICE OF AREAL UNITS

The choice of an areal unit for assessment of groundwater resource by the LWB is governed by thefollowing two considerations:

3.1 MINIMISING THE DEPENDENT RECHARGE/ DISCHARGE COMPONENTS

Evapotranspiration, recharge and discharge as a consequence of the lateral saturated flow and theinter-aquifer flow are dependent upon the position of the water table. Thus, these components can notbe assigned historical estimates while planning for future groundwater development/ recharge. Thesecomponents (the dependent components) shall inevitably change as the planned groundwaterdevelopment/ recharge takes place and the water table declines/ rises accordingly. Other componentsof recharge and discharge are almost independent of the position of the water table. Thus, thehistorical estimates of these components (the independent components), after due normalisation mayhold for future planning.

3.2 IMMUNITY FROM EXTERNAL INFLUENCE

For the sake of uniqueness, the areal unit may preferably be so selected that the groundwaterresource is exclusively dependent upon the conditions there in and is independent of thepumping/recharge activity occurring outside the aquifer volume contained in the unit.

3.3 EVALUATION OF POSSIBLE AREAL UNITS

A hydrogeological basin

This is essentially an area bounded by impervious hydrogeological boundaries. Thus, the water tablein the area is totally immune to the pumping/recharge activities occurring outside the area. It is alsoimmune to the pumping activities (inside or outside the area) and boundary conditions in respect ofthe underlying confined aquifer. The resource of the unconfined aquifer in such a unit shall notdepend upon what is happening out side (laterally as well as vertically).

Thus, it shall not comprise any of the dependent components enumerated earlier (except for theevapotranspiration, which may not be activated unless the area is water logged). As such, this is anideal unit for resource assessment by the LWB.

However, such a unit shall no longer remain an ideal unit if the unconfined aquifer in the unit isunderlain by a leaky confined aquifer. In such a case the unconfined aquifer is affected by thepiezometric elevation of the underlying leaky confined aquifer and its position relative to the watertableelevation and hence by the pumping activities, and as such the resource can not be estimateduniquely by the LWB. However, if the vertical inter-aquifer flow is relatively small in magnitude, theseeffects may be negligible and as such, this may be a near ideal unit for resource assessment by theLWB.

Hydrological (surface drainage) basin

This is essentially an area bounded by a topographical divide. It is usually believed that atopographical divide is underlain be a water table divide but this may not always hold. Thus, theexternal influence and the dependent components could be significant. A hydrological unit thus, neednot necessarily be an ideal unit for assessing the groundwater resource. As explained earlier in thecontext of a hydrogeological unit, the uncertainty is further aggravated in case the unconfined aquiferis hydraulically connected to an underlying leaky confined aquifer through an aquitard.



In hard rock areas the topographical and hydrogeological divides may generally overlap each other.However, in alluvial areas and also in low relief hard rock terrain, an unconfined aquifer may cutacross a topographical water divide and the lateral flows in alluvial aquifers may be relatively far moresignificant due to high transmissivities.

Doab (Inter-basin)

This is an area bounded by two major perennial streams fully penetrating the unconfined aquifer.Such stream boundaries ensure that there are no flows across the boundary. Thus, the water tableinthe aquifer is immune to the pumping/ recharge activities out side the area. However, the componentof the stream-aquifer inter-flow is prevalent and is dependent upon the position of the watertablerelative to the stream gauge. Further, even a doab would invariably have a non-hydraulicboundary across which the external influence may occur. Thus, it is also not an entirely appropriateunit for the estimation of resource by the LWB.

An administrative unit

In such a unit, all the dependent components of recharge and discharge are activated. The lateralinflows/outflows across the boundary are dependent upon pumping/ recharge events outside the arealunit. As such, an administrative unit is the least desirable but most commonly used areal unit. Theonly advantage of such a unit (which should not be underestimated) is that it falls under a singleadministration, which may enforce uniform data acquisition and management, practices.



4 CHOICE OF WATER BALANCE PERIOD

Theoretically speaking, a water balance study could be carried out for any period. However, inpractice there are certain constraints, which are essentially derived from the intended objectives of thewater balance study.

For an optimal validation of the norms/ parameters etc., the period must permit the maximum possibleactivation of the concerned variables, and the components must be amenable to a reliable estimation.For example, if the objective is to estimate the specific yield by analysing the depletion of the watertable/ storage during the dry season (as recommended in the GEC-97 norms), the period mayincorporate the maximum possible decline from the view point of activation of the specific yield. Thisimplies that the period may span between the discrete times of the highest and the lowest water table.However, a split to monthly water balance may be considered to minimise the error in the draftestimate (draft may be negligible in the first month after the monsoon season).

For estimation of a specific component of the water balance, it is necessary to select a period duringwhich the component is just fully generated. Any period less than that shall lead to an underestimationof the component. A longer period shall attenuate the predominance of the component and henceshall lead to a less reliable estimate of the component. For example, if the water balance is carried outfor estimating the rainfall recharge by analysing the build up of the water table/storage during themonsoon season (as recommended in the GEC-97 norms), it shall be required to define a periodduring which the entire rainfall recharge just occurs. This period shall be necessarily different from theperiod of the monsoon rainfall because of the inevitable time lag. The period must span between thediscrete times of the lowest and the highest water table and not the start and end of the rainy season.

The frequency of manual monitoring of GW levels (usually two to four times a year under Indianpractice) is generally not sufficient to define the seasonal hydrograph and therefore may miss eitherpeak or low or both. The wide scale deployment of automatic water level recorders (DWLRs)implemented through the Hydrology Project, shall provide comprehensive and almost continuouswater table hydrographs. These hydrographs shall permit a correct identification of the optimal period.



5 COMPONENTS INCLUDED

The dedicated software package includes modules and tools that permits a comprehensive waterbalance computation, which can be applied for estimation of groundwater resource in general, and inaccordance with the GEC-97 norms in particular. The components of the water balance included inthe module are (see also Figure 5.2):

∆S - the change of groundwater storageRrf - recharge from rainfallRc - seepage from canalsRi - recharge from irrigationRt - recharge from storage tanks and pondsRwc - recharge from conservation structuresI - algebraic sum of lateral outflow (+) and inflow (-) across the boundaryDg - gross draftB- algebraic sum of subsurface outflow to (+) and inflow from (-) hydraulically connected streamsL - algebraic sum of vertical inter-aquifer outflow (+) and inflow (-)E - outflow from the aquifer due to evapotranspiration from the aquifer

The volumetric estimates of these components are substituted in the following equation:

(Rrf + Rc + Ri + Rt + Rwc) - (I + Dg + B + L + E) - ∆S = ε.∆S (5.1)

and ε is estimated.

Figure 5.2: Components of the water balance

Ri

Rrf

Rc Rt

E

L

B

Dg

I

∆S

Rwc



6 ORGANIZATION OF THE DATABASE

The database necessary for estimating the components included in the above equation is organisedin the following steps.

6.1 DESCRIPTION OF THE AREA AND THE RETRIEVAL ZONES

The assessor is prompted to input the digitised boundary of the water balance area (termedhenceforth as the unit area) and four reference circumscribing rectangles for the search of theobservation wells, rain gauge stations, river gauging points and the pumping test points. Thereference rectangles, oriented along the N-S and E-W directions, are defined in terms of thelongitudes and latitudes of the sides.

It may be appreciated that the data not only from the unit area, but also from beyond the boundary ofthe unit area could be relevant and useful for the study. The water level from an area extending up tothe hydraulic boundaries (that is, a dyke or a hydraulically connected river or a GW divide) should beretrieved. The aquifer parameter data from area of similar hydrogeology should be retrieved. Therainfall data from meteorologically similar area should be retrieved. The river stage data from theupstream and down stream reaches having similar slope should be retrieved. This extension of thesearch areas shall permit more reliable spatial distributions, such as contouring, zoning, Thiessen`spolygons, interpolation, extrapolation; and identifying/ reconfirming hydrological/ hydrogeologicalboundaries.

6.2 DESCRIPTION OF THE AREAL BOUNDARY

The assessor is then prompted to input the hydraulic description of the digitised boundary or its partswhich could be a hydraulically connected river boundary, an impervious boundary or a GW divide, ora non-hydraulic (that is, administrative) boundary. Further, the assessor is prompted to extend theriver alignment up to the corresponding reference rectangle.

6.3 DESCRIPTION OF THE PERIOD AND OBJECTIVE OF THE WATERBALANCE

The assessor is then prompted to input the period of the water balance and the objective of the study(as already stated, the objective could be either validation or estimation). In case the objective isestimation, the assessor is prompted to indicate the component to be estimated.

6.4 RETRIEVAL OF DATA FROM THE BASIC MODULE

Based upon the response of the assessor, the module retrieves the necessary basic data from theBasic module. The data comprise the following:

• Coordinates and ground levels (above MSL) of the observation wells falling within and on therelevant reference rectangle, and the corresponding available water levels at all the discretetimes falling in the period of the LWB. The water levels at the beginning and at the end of theperiod are mandatory. The water levels shall comprise water table elevations; and alsopiezometric elevations in case the vertical inter-aquifer flow is to be accounted for. Pre and postmonsoon water table elevations of the preceding ten years are also retrieved for the trendanalysis stipulated in the GEC-97 norms. However, if the database of the Basic module is notexhaustive enough to permit such retrieval, these data may be entered externally.

• Coordinates of the rain gauge points falling within the relevant reference rectangle and thecorresponding recorded rainfall depths within the stipulated period. However, if the database of



the Basic module is not exhaustive enough to permit such retrieval, these data may be enteredexternally.

• Processed thematic maps (topographical contours, forest cover, hydrogeological andgeomorphic units) in respect of the unit area. The forest cover may be defined in terms ofboundary of well-defined forest area. In case there are no well-defined forests in the unit area,the average forest/ canopy cover expressed as a fraction of the geographical area may beentered.

• In case a part (or whole) of the unit boundary comprises a river, the coordinates of the gaugestations (if any) falling within or on the relevant reference rectangle and the associated stagevalues at discrete times falling within the stipulated period. However, if the database of the Basicmodule is not exhaustive enough to permit such a retrieval, interpolated/ extrapolated river stageat a few points within the unit area along with the coordinates of the points may be enteredexternally.

• Coordinates of the pumping test points falling within and on the relevant reference rectangle andthe corresponding estimates of the relevant parameters. The relevant parameters aretransmissivity and specific yield and also the hydraulic resistance of the aquitard in case thevertical inter-aquifer flow is to be accounted for. However, if the database of the Basic module isnot exhaustive enough to permit such a retrieval, these data may be entered externally.

• Blocks falling within the unit area, total area and the effective area (that is, area falling in the unitarea) of each block, type wise number of groundwater production structures falling within theeffective area of each block. However, if the database of the Basic module is not exhaustiveenough to permit such a retrieval, these data may be entered externally

6.5 REVIEWING AND UPGRADING THE DATABASE

The retrieved/ entered database in respect of water table, piezometric head, rainfall, pumping testsand river gauge are displayed. The database comprises the locations and the corresponding datamagnitudes. These data locations symbolised appropriately shall be displayed superposed over theunit area and the reference rectangles. The data magnitudes shall be displayed at the respectivelocations.

The assessor may view the database. There upon using his professional judgement he may exerciseone of the following options:

• Accept the database as such without any modification.

• Delete any number of the retrieved data points (apparently inconsistent data, water level datafrom beyond a dyke or a hydraulically connected river, aquifer parameter data from a differenthydrogeological zone, etc.). The data corresponding to the deleted points shall not be used forthe subsequent calculations.

• Redefine the domain(s) of any one or more of the reference rectangles to enhance the database.

Thus, the assessor can iteratively modify his database till he is satisfied.

6.6 DIRECT ENTRY OF DATA

The assessor shall be prompted to enter the following data directly:

• Population density and fractional load on groundwater for domestic and industrial water supply,in the unit area (refer page 57 of the GEC-97 norms).

• Wetted areas, bed reduced levels and running days in respect of the lined and unlined canalsfalling in the unit area.

• The volumes of the canal water (released at the outlet) in the unit area during the stipulated timeperiod for paddy and non-paddy crops.



• The assessor will be prompted to state whether the stream is fully penetrating. If the answer isYes, the attenuation factor shall be taken as 1.0. If the answer is No, the assessor is prompted toeither enter the values of Wp, d and e (necessary for estimating the attenuation factor) or todirectly enter the value (less than one) of the attenuation factor as per his hydrogeologicaljudgement.

• The assessor shall be prompted to enter the capillary height and evaporation rate for estimatingthe direct evaporation loss from the shallow water table areas.

• The assessor shall be prompted to enter the root zone depth of the trees and theevapotranspiration rate for estimating the evapotranspiration from the forested areas.



7 PROCESSING POINT DATA TO SPATIAL DISTRIBUTIONS

The database so created shall be processed on a GIS platform in the following steps:

7.1 GENERATION OF CONTOURS/ SPATIAL DATA

The module permits generation of contours and the corresponding spatial data in Raster/ Vectormodes. As discussed subsequently, the assessor may use this capability for processing discrete pointdata of water level, aquifer parameters and rainfall. The processing involves the following steps:

• The available discrete point data are analyzed by an appropriate algorithm (such as Kriging,Least squares polynomial, Spline function) and a Raster data set is generated. The pixelspacings of the Raster data shall be chosen by the assessor.

• The assessor shall specify the desired contour interval. The Raster data are processed togenerate and store Vector data corresponding to the resultant contours.

• The assessor, using his professional judgement and guided by relevant maps etc. may edit someor all the contours manually.

• Vector data and hence the Raster data corresponding to the edited contours are generated andstored together with the data corresponding to the unedited contours (if any).

7.2 WATER LEVEL DATA

The retrieved water level and the river stage data are used for contouring. While drawing the watertable contours, the interpolated/ extrapolated stage values along the boundary, are treated as thewater table data. This ensures a compatibility along the boundary. The assessor is prompted toexercise his choice of the contouring algorithm.

The module returns the contours of the water table and also of the piezometric head (if relevant) at allthe discrete times. It also returns the contours of water table fluctuation (that is, water table elevationat the end of the period minus the water table elevation at the beginning of the season, or water tabledepth at the beginning of the season minus the water table depth at the end of the season).

The assessor may edit the contours any number of times. Raster data corresponding to the finallyaccepted contours are stored (refer Section 6.7.1).

7.3 AQUIFER PARAMETERS

The assessor has the following two options for obtaining the spatial distributions of transmissivity,specific yield, and also the hydraulic resistance, if relevant.

• The assessor may decide to use the contouring capability of the dedicated software. There upon,he is prompted to exercise his choice of the contouring algorithm. The module returns thecontours of the aquifer parameter. The assessor may edit the contours any number of times.Raster data corresponding to the finally accepted contours are stored (refer Section 6.7.1).

• Usually the data are available at a very few points only and as such, automatic contouring (thatis, the first choice) may not be applicable. The assessor may ask for a display of the unit areaand thematic maps, with the concerned database superposed over it. He may definehomogenous zones and define a value for each zone. In the absence of any hydrogeologicalsupport of such zones, the assessor may choose to define Thiessen’s polygons around the datapoints (see the following Section on Rainfall). Corresponding Raster data are generated andstored.



For obtaining the spatial distribution of the infiltration factor usually only the second option shall berelevant.

7.4 RAINFALL

The assessor has the following two options for estimating spatial distribution of rainfall over the unitarea during the stipulated period. The estimation has to be essentially based upon the retrieved dataof point rainfall depths, corrected for orographic effects.

• The assessor may decide to use the contouring capability of the dedicated software. There upon,he is prompted to exercise his choice of the contouring algorithm. The recommended algorithm isKriging. The module returns the contours (also known as isohyets) of the rainfall depth, withtopographical contours superposed over them. The assessor may edit the contours any numberof times. Raster data corresponding to the finally accepted contours are stored (refer Section6.7.1).

• The assessor may opt to estimate the average rainfall by Thiessen’s polygon approach. AThiessen’s polygon is drawn for each rain gauge by joining the right bisectors of the straight linesjoining the neighboring rain gauge points. Thus, all the points falling within any polygon are closerto the respective rain gauge station than to any other rain gauge point. Therefore, the rainfallwithin a Thiessen`s polygon is assumed to be equal to the rainfall recorded at the respective raingauge. Corresponding spatial data are generated and stored.

7.5 TOPOGRAPHICAL LEVEL

In case a processed topographical map is not available in the database, the topographical contoursshall be obtained on the basis of the ground level data if available for the sites of the observationwells/ piezometers. The assessor is prompted to exercise his choice of the contouring algorithm. Themodule returns the contours of the topographical level. Corresponding Raster data are generated andstored.



8 ESTIMATION OF WATER BALANCE COMPONENTS

Subsequent to the contouring and spatial distribution, the components of the water balanceenumerated above are estimated as follows:

8.1 STORAGE FLUCTUATION

Using the Raster data of the water tablelevations at the period’s beginning (hb) and at the end (he)and of the specific yield, the storage fluctuation shall be estimated by a GIS assisted integration of theproduct of the spatially distributed water table fluctuation and the spatially distributed specific yieldover the unit area (A). The integration is as follows:

dASy)hbhe(SA∫ −=∆ (8.1)

The integration involves pixel by pixel calculation of the storage fluctuation (pixel area multiplied bypixel water tableelevation multiplied pixel specific yield) and summation.

However if a uniform specific yield is assigned over the entire unit area or the area is divided into afew homogenous parts, the storage fluctuation is estimated as follows:

∑ ∫ −=∆i Ai

dAi)hbhe(SyiS (8.2)

Where, Ai is the ith homogenous part having an average specific yield Syi.

The assessor has an option of performing the calculations in a semi-automatic mode, if he so desires.In case this option is exercised, the storage fluctuation shall be computed in the following steps:

• Draw contours of water table fluctuation (as described in 6.7.1).

• Draw zones of equal specific yield (as described in 6.7.3).

• Superpose the zones of the specific yield over the fluctuation contours.

• Delineate the areas lying in between each pair of successive contours and the areal boundary.Divide each inter-contour area among the intersecting zones and measure the sub areas, soobtained. All the sub-areas of an inter-contour area are assigned an average watertablefluctuation equal to the arithmetic mean of the ratings of the enveloping contours. Estimatethe storage change in each sub-area by multiplying the sub-area by its respective specific yieldand the average water tablefluctuation. Estimate the total storage fluctuation by adding theestimated storage fluctuations in the sub-areas.

8.2 RAINFALL RECHARGE

Using the Raster data of the rainfall (r) and the infiltration factor (α), the volume of rainfall recharge(R) shall be estimated by GIS assisted computation of the following integration over the unit area (A):

∫ α=A

dArR (8.1)

The assessor has an option of performing the calculations in a semi-automatic mode, if he so desires.In case this option is exercised, the rainfall recharge shall be computed in the following steps:



• Draw contours/ Thiessen’s polygons of rainfall depth (as described in 6.7.4).

• Draw zones of assumed equal infiltration factor (as described in 6.7.3).

• Superpose the zones over the rainfall contours/ Thiessen’s polygons.

• In case rainfall contours have been drawn, delineate the areas lying in between each pair ofsuccessive contours and the areal boundary. Divide each inter-contour area among theintersecting zones and measure the sub areas, so obtained. All the subareas of an inter-contourarea are assigned an average rainfall equal to the arithmetic mean of the ratings of theenveloping contours. Estimate the rainfall recharge in each subarea by multiplying the subareaby it’s respective infiltration factor and the rainfall depth. Estimate the total rainfall recharge byadding the estimated rainfall recharge in the subareas.

• In case Thiessen’s polygons have been drawn, estimate the average infiltration factor over eachpolygon. Estimate the rainfall recharge by computing the recharge over each polygon bymultiplying the corresponding recorded rainfall depth and the polygon’s area with the averagedinfiltration factor.

8.3 LATERAL FLOWS ACROSS BOUNDARY

Using the following Raster data:

• Water tableelevations at the period’s beginning and end,

• Water tableelevations at other intermediate discrete times,

• Transmissivity,

and the Vector data of the non-hydraulic boundary, the volume (I) of subsurface lateral flows (inflowsand outflows) across the boundary shall be estimated in accordance with Darcy’s law, by integratingthe product of water table gradient normal to the boundary (i) and the transmissivity (T) over theboundary (C) and the period (t) of the LWB. Since the water balance equation, as written in Section6.5, treats the outflow as positive, an outward falling gradient is treated as positive and vice versa.The integration has been programmed in the module.

∫ ∫=t C

TidCdtI (8.4)

The estimation thus, shall essentially involve estimation of spatial gradients from the spatial watertable data and the boundary alignment, followed by numerical integration over the boundary and thewater balance period.

The assessor has an option of performing the calculations in a semi-automatic mode, if he so desires.In case this option is exercised, the lateral flow shall be computed in the following steps:

• Obtain contours of water table(above MSL) at the beginning, end and at other interveningdiscrete times, as per the data availability.

• Mark the boundary across which the flow is to be computed on each contour map.

• Divide the boundary length among segments of nearly uniform transmissivity.

• Estimate the average hydraulic gradient across each segment at each discrete time. Henceestimate the corresponding flow rates by multiplying the segment length with the correspondingtransmissivity and the estimated average hydraulic gradient.

• Estimate the flow rate across the boundary at each discrete time by adding algebraically thecorresponding computed flow rates across the segments.

• Estimate the flow volume in each time period (falling in between two successive discrete times)by multiplying the time duration with the average flow rate. The latter may be taken as thearithmetic mean of the flow rates at the starting and the ending discrete time.

• Estimate the total flow volume by adding the individual period-wise flow volumes.



8.4 STREAM AQUIFER INTER-FLOW

This component can be computed in the same way as the subsurface lateral flow in case of fullypenetrating streams. However, in case of the partially penetrating streams, the integral shall have tobe multiplied by an attenuation factor (less than 1.0) to account for the attenuation of the stream-aquifer inter-flows due to the partial penetration. The factor (f) can be computed as follows:

)ed)(e5.0Wp5()eWp5.0(Wp5f ++

+= (8.5)

Where Wp is the wetted parameter of the stream (defined as the length of contact between water andthe stream bed, normal to the flow direction), d is the water depth in the stream and e is the saturatedthickness of the aquifer below the stream.

8.5 VERTICAL INTER-AQUIFER FLOW


• Water table elevation at the period’s beginning and end

• Water table elevations at the intervening discrete times

• Piezometric elevation at the period’s beginning and end

• Piezometric elevation at the intervening discrete times

• Hydraulic resistance (C) of the intervening aquitard

the vertical inter-aquifer flow volume (L) across the unit area (A), in the water balance period (t) shallbe estimated by performing the following GIS assisted integration:

dtdALt A

ChH∫ ∫= − (8.6)

where H and h are respectively the space-time variant water table elevation and the piezometric head.

The assessor has an option of performing the calculations in a semi-automatic mode, if he so desires.In case this option is exercised, the vertical inter-aquifer flow shall be computed in the following steps:

• Estimate the spatially averaged water table(H) and piezometric elevation (h) at each discretetime.

• Estimate the spatially averaged hydraulic resistance (C).

• Estimate the rate of vertical inter-aquifer flow rate (Lr) at each discrete time, using the followingequation:

C

hHLr

−= (8.7)

• Estimate the flow volume in each time period (falling in between two successive discrete times)by multiplying the time duration with the average flow rate. The latter may be taken as thearithmetic mean of the flow rates at the starting and the ending discrete time.

• Estimate the total flow volume by adding the individual period-wise flow volume.



8.6 EVAPOTRANSPIRATION FROM WATERTABLE

This component (ETV) has two parts, that is, direct evaporation and evapotranspiration. Loss of waterdue to direct evaporation occurs from such unforested area where the depth to the water table is lessthan the capillary height. This loss can be estimated by integrating the time varying loss rate (volumeper unit time) over the period of the water balance. The loss rate at different discrete times is theproduct of the corresponding evaporating areas and the evaporation rates. Both, the evaporating areaas well as the evaporation rate varies with time. The dedicated software permits an estimation of theevaporating area (A1) at various discrete times for a given capillary height. The capillary height mayvary from few tens of centimeters (for sands) to about three meters (for clays). The evaporation rates(E) may be close enough to the potential evaporation rate which could be estimated from panevaporation rate.

Loss of water due to evapotranspiration occurs from such forested area where the depth to the watertableis less than the root zone depth. This loss can also be estimated by integrating the time varyingloss rate (volume per unit time) over the period of the water balance. The loss rate at different discretetimes is the product of the corresponding evapotranspiring areas and the evapotranspiration rates.Both, the evapotranspiring area as well as the evapotranspiration rate varies with time. The dedicatedsoftware permits an estimation of the evapotranspiring area (A2) at various discrete times for a givenroot zone depth. The root zone depth may be of the order of a few meters. Both theevapotranspiration rates (ET) and the root zone depth could be ascertained from the local forestauthorities.


• Water table depth at the period’s beginning and end,

• Water table depth at the intervening discrete times,

• Forested area,

the areas A1 and A2 at different discrete times can be estimated through GIS. The volume ofevapotranspiration (ETV) can be computed by the following numerical integration over the period (t) ofthe water balance. The integration has been programmed in the module.

∫ +=t

dt)ET.2AE.1A(ETV (8.8)

In case there are no well defined forests in the unit area, the evapotranspiration may be estimatedfrom the average forest/ canopy fraction, β (that is, average forest/canopy area per unit geographicalarea) and the area (A3) with water tabledepth ranging from the capillary height to the root zone depth.The area A3 at different discrete times can be estimated from the Raster data of the water tabledepth.The volume of evapotranspiration can be calculating by the following numerical integrationprogrammed in the module:

∫ +β+β−=t

dt]ET)3A1A(E.1A)1[(ETV (8.9)

8.7 GW DRAFT

Since direct draft figures are almost non-existent, this component is estimated from the retrieved dataof the block areas, and the block-wise number of groundwater production structures of different types(dug wells, various tube wells).



The number of structures (of different types) in the unit area shall be tentatively estimated assuming auniform distribution within each block. These numbers (block-wise and type wise) along with the basicdata shall be displayed on the console and the assessor shall be prompted to modify the numbers ifhe so desires. The modification could be on the basis of the assessor’s knowledge of the spatialdistribution of the structures with in each block.

Upon a reconciliation of the numbers the assessor is prompted to enter the fractions of the totalannual pumpage occurring in the stipulated time period. The groundwater draft shall be computedfrom these data and the average annual gross drafts incorporated in the GEC-97 norms (refer pages61 and 62 of the norms). The computed figure shall be displayed on the console. The assessor shallbe prompted to modify the figure, if he so desires (refer paragraph 3 on page 60 of the GEC-97norms).

8.8 CANAL SEEPAGE

This component is estimated using the retrieved canal data. The calculations shall be performed inaccordance with the criteria stipulated in the GEC-97 norms (refer Section 5.9.3, page 54 of thenorms).

However, the assessor may like to cross check and validate the estimated value by studying the data(if available) of recorded discharge at the upstream and downstream of the study area. The differencebetween the two discharge values is obviously the upper limit of the seepage. The true seepage hasto be less than that due evaporation and diversion of water. The assessor shall be prompted to entera new value, if he so desires.

8.9 RECHARGE DUE TO RETURN FLOW FROM APPLIED IRRIGATION

This component is estimated using the retrieved data of the canal irrigation and the Raster data of thetopographical levels and of the water tableelevations at the beginning and at the end of the stipulatedperiod. These data shall be used to estimate the spatially and temporally averaged depth towatertable. The calculations shall be performed in accordance with criteria stipulated in the GEC-97norms (refer Section 5.9.4, pages 54-55 of the norms).

8.10 RECHARGE FROM TANKS, PONDS, CHECK DAMS AND NALA BUNDS

These components shall be estimated using the data of average area spread (for storage tanks andponds) and of the gross storage (for percolation tanks, check dams and nala bunds). The assessorshall be prompted to enter these data. The calculations shall be performed in accordance with thecriteria stipulated in the GEC-97 norms (refer Sections 5.9.5 and 5.9.6 on page 55 of the norms).

However, the assessor may like to cross check and validate the estimated value by carrying out awater balance of the reservoir, considering among others, the storage fluctuation and evaporation.The latter could be estimated from the pan evaporation data

.



9 TIME SERIES ANALYSIS

The water balance study provides estimates of the recharge and the discharge components of thecontinuity equation. An excess of the net recharge over the net discharge should be reflected as arising trend in the time series of the water table; and vice versa. Thus, there must be a consistencyamong the water balance estimates and the trend in the time series. This requirement can be used foran indirect validation of the water balance estimates. As such, it is desirable to follow up the waterbalance study by a time series analysis of spatially averaged as well as point water table data from afew representative wells.

The analysis shall require GIS assisted spatial averaging followed by a check for first and secondorder stationarity. In case stationarity is not inferred, a regression analysis shall be carried out todetermine the nature of the trend, that is, rising or falling and it’s rate over the years.



10 INTERFACING WITH THE GEC-97 NORMS

Groundwater estimation committee (97) has stipulated detailed norms for estimating the groundwaterresource of unconfined aquifers. The norms essentially lie within the broad frame work of the lumpedwater balance approach described in the preceding paragraphs. Thus, the resource evaluationmodule of the dedicated software can be used to implement the procedures incorporated in thenorms.

10.1 SALIENT FEATURES OF THE NORMS

The salient features are as follows:

Areal and time units

The recommended areal unit for the hard rock areas is a hydrological basin. It is suggested that inhard rock areas the hydrological and hydrogeological boundaries may overlap and as such thegroundwater flow across the boundaries of a hydrological basin may be negligible. Recognizing that inalluvial areas an aquifer may cut across a topographical divide, the recommended unit for alluvialareas is an administrative block.

The recommended periods for the water balance are the monsoon and dry seasons. The dry seasonwater balance (recommended only for non-command areas) is aimed at estimating the specific yield.The monsoon season water balance is aimed at estimating the rainfall recharge.

Normalization of the monsoon rainfall recharge

The recharge occurring in a normal monsoon year is estimated by analysing the results of themonsoon season water balance for the current and the preceding years. The data of the monsoonrainfall and the corresponding computed rainfall recharge are subject to a regression analysis,assuming a linear relationship (with or without a constant) between the monsoon rainfall and thecorresponding recharge. The computed coefficient and the constant permit estimation of the rainfallrecharge corresponding to a pre computed value of the normal monsoon rainfall.

Categorization of areas for GW development

The GEC-97 norms suggest a categorization of the areas for groundwater development (that is, safe,semi critical, critical and over exploited) on the basis of the stage of the groundwater development.The stage is defined as the ratio of the existing groundwater draft to the net annual groundwateravailability. The net annual groundwater availability has been defined as the estimated resourceminus a certain allowance for the natural groundwater discharge. The latter has been defined as theexisting natural groundwater discharge during monsoon season plus a permitted natural groundwaterdischarge during dry season. The suggested figure for the permitted discharge is five to ten percent ofthe estimated resource.

However, keeping in view the inherent uncertainties in the estimates of the existing groundwater draftand the resource, the norms further suggest a confirmation of the categorization by looking at thelong-term trend of the water level in the unit. It is stipulated that a composite plot of the pre and postmonsoon water levels of at least ten preceding years should be presented along with the stagecalculations.



Components not included

The component of subsurface lateral inflow/ outflow has not been included in the suggestedapproach. However, it has been pointed out that this component may not always be negligibleespecially in the case of alluvial areas where the transmissivities are relatively large and choice of theassessment unit is based upon administrative consideration. Further, it is suggested that the assessormay like to estimate this component and include in the water balance equation appropriately.

Stream-aquifer inter-flow has been ignored in the LWB for the monsoon season. This componentcould be quite significant in high transmissivity areas. Evapotranspiration over water logged andforested areas have been ignored for both the seasons. This component could be quite significantduring monsoon period when the water table may be quite shallow. The inter-aquifer flow has also notbeen included.

10.2 IMPLEMENTATION OF THE NORMS

The water balance of the dry and monsoon seasons and the post water balance calculations,envisaged in the norms can be implemented through integrated use of the resource assessmentmodule and the GIS tools of the dedicated software.

As discussed in the preceding Section, the norms suggest exclusion of a few components of the waterbalance equation. These components have apparently been excluded to avoid the subjectivityinherent in their manual/ graphical estimation. On the other hand the dedicated software, on accountof a computerised/GIS approach, permits an easy and objective estimation of these components.Therefore, it is suggested that these components be included while implementing the norms throughthe dedicated software.

Steps of the implementation shall be as follows:

Dry season water balance

The estimates of the all the recharge and discharge components and of the storage fluctuation aresubstituted in the continuity equation. The imbalance, expressed as a fraction of the storagefluctuation (magnitude) is computed and conveyed to the assessor. In case the imbalance is smallenough, the assigned specific yield distribution stands corroborated. In addition, the estimates of therecharge components also stand corroborated. However, since the draft estimate is prone to possiblylarge errors, the validation may not be beyond doubt.

Monsoon season water balance

The principal end product is the rainfall recharge. This is estimated by substituting the estimates of therecharge components (other than the rainfall recharge), all the discharge components and the storagechange into the continuity equation.

Normalization of the rainfall recharge

The normalization of the recharge is aimed at estimating the monsoon rainfall recharge correspondingto the normal monsoon rainfall. Such a study can be taken up provided the estimates of monsoonrainfall recharge have been arrived at for the preceding years. The normalization is accomplished bycarrying out a regression analysis on the historical data of monsoon rainfall and the correspondingestimated monsoon rainfall recharge. The analysis provides a functional relation between themonsoon rainfall and the corresponding recharge.



The dedicated software permits a rigorous regression between the monsoon rainfall (r) and thecomputed monsoon recharge (R). The analysis apart from estimating the coefficients, is aimed atarriving at the optimal form of the functional relation between the monsoon rainfall and the monsoonrainfall recharge. For example, it can be checked whether the suggested constant in the functionalrelation is statistically significant or whether the assumed linearity holds (refer equations 8a and 8d onpages 42-43 of the norms). The performance of more elaborate nonlinear functional relations could beevaluated for possible adoption.

Functional relations included in the dedicated software are as follows. The assessor may design hisown functional relations as well.

R = Ar - B (R=0 if r < B/A) (10.1)

n)Br(AR −= (R=0 if r< B) (10.2)

The goodness of the finally adopted functional relation is quantified by computing the statistics of theregression including model efficiency (or the correlation coefficient).

The assessor is prompted to enter the normal monsoon rainfall. The normal monsoon rainfall isusually taken as the arithmetic mean of last fifty years’ data. There upon, the normal monsoon rainfallrecharge is estimated in accordance with the regressed functional relation.

Estimation of the total annual recharge/net annual GW availability

The total annual resource is estimated by adding the normalized monsoon recharge and the dryseason recharge. The dedicated software permits estimation of the net annual groundwateravailability as per the GEC-97 norms (refer Section 6.10.1.3); and also permits the assessor tosuggest his own figures for the permitted natural discharge during the monsoon and the non-monsoonperiods. The prevalent natural discharge during the two seasons, as estimated by the LWB, aredisplayed on the console to assist the assessor in taking a decision in this regard.

Estimation of the stage of GW development

The stage of the groundwater development is estimated in accordance with the GEC-97 norms (referequation 15 on page 57 of the norms).

Time series analysis

The time series analysis aims at identifying the trends (that is, rising or falling) in pre and postmonsoon water table elevations. The Raster data of the pre and post monsoon water table(elevationabove MSL or depth below the ground) of the preceding ten years are generated. These data areintegrated over the unit area to estimate the spatial averages.

The magnitude as well as the sign of the trend (if any) are identified by subjecting the time series ofthese average levels/ depths to a trend analysis. The inferred trends in the pre and post monsoonwater levels are accompanied by computerised plots of the averaged water levels. The levels/depthsfrom all or a few representative wells are also plotted.



10.3 SUMMARY REPORT

GEC-97 norms require a presentation of a summary of the entire resource evaluation exercise in astandard form given on pages 68 to 70 of the norms. The module permits a computerisedpresentation of such a summary.



11 REFERENCES

Ministry of Water Resources, Govt. of India, Ground Water Resource Estimation Committee-1997.New Delhi, 1997.

download-manuals-ground water-manual-gw-volume8operationmanualdataprocessingpartiv

Technology

recharge components

groundwater balance

water level data

dependent components

retrieval of data

basic data

aquifer area

processing point data