recharge process and aquifer models of a small...

Hydrological Sciences—Journal—des Sciences Hydrologiques, 44(5) October 1999 gg]

Recharge process and aquifer models of a small watershed

A. SRINIVAS, B. VENKATESWARA RAO Water Resources Department, School of Environment, Water Resources and Remote Sensing, Jawaharlal Nehru Technological University, Hyderabad 500 018, India

V. V. S. GURUNADHA RAO National Geophysical Research Institute, Hyderabad 500 007, India e-mail: [email protected]

Abstract Kanchanapally watershed covering an area of about 11 km2 in Nalgonda district, Andhra Pradesh, India is located in granitic terrain. Groundwater recharge has been estimated from a water balance model using hydrometeorological data from 1978-1994. The monthly recharge estimates obtained from the water balance model formed input for the groundwater flow model during transient model testing. The groundwater flow model has been prepared to simulate steady state groundwater conditions of 1977 using the nested squares finite difference method. The transient groundwater flow model has been tested during 1977-1994 using the estimated recharge values. The present study helped verify the usefulness of monthly recharge estimates for accounting dynamic variations in recharge as reflected in water level fluctuations in hydrographs.

Recharge et modélisation de la nappe d'un petit bassin versant Résumé Le bassin versant de Kanchanapally (District de Nalgonde, Andhra Pradesh, Inde), d'une surface d'environ 11 knf, est situé sur des terrains granitiques. La recharge des eaux souterraines a été estimée à partir d'un modèle de bilan hydrologique utilisant des données hydrométéorologiques de la période 1978-1994. Les estimations de la recharge mensuelle issues du bilan hydrologique ont été utilisées comme entrées d'un modèle d'écoulement souterrain pour des essais en régime transitoire sur la période 1977-1994. Le modèle d'écoulement souterrain, utilisant des mailles emboîtées et une résolution en différences finies, avait été initialise en régime permanent sur la situation de 1977. La présente étude nous a aidé à vérifier l'utilité de l'estimation de la recharge au pas de temps mensuel pour rendre compte de sa variation dont on retrouve la trace dans les fluctuations des niveaux de l'eau.

INTRODUCTION

Kanchanapally watershed, spread over 11 km2, is located about 16 km from Nalgonda town in Nalgonda district, Andhra Pradesh, India. The watershed is a closed one with a stream outlet in the south and it was selected as a critical watershed by the Groundwater Department of Andhra Pradesh for regular monitoring (Fig. 1). About 150 open wells and bore wells are used for irrigation and domestic purposes and seven dug wells were selected for daily groundwater level measurements. Rainfall and open pan evaporation (Class A) were measured daily at Kanchanapally in the central part of the area and the rainfall and pan evaporation data are available since 1978. The surface runoff was estimated from a Soil Conservation Service (SCS) runoff curve number sub-model which was based on soil, land use, vegetation and hydrological conditions.

Open for discussion until I April 2000

mailto:[email protected]

682 A. Srinivas et al.

Fig. 1 Grid map of meshes with stream channels and watershed boundary.

The pan evaporation data were transformed into actual transpiration and actual soil evaporation components in the water balance model through pan coefficient, crop coefficient and water holding capacity of the soil zone during the crop growing season. Narasimha Reddy et al. (1994) used a water balance approach coupled with groundwater flow modelling for estimation of long-term recharge in Dulapally watershed in granites. The mathematical procedure followed for estimation of recharge from rainfall and pan evaporation data is discussed in detail by Narasimha Reddy et al. (1994).

Hydrogeological conditions

The topography is undulating and drainage is sub-dendritic and controlled partly by fracture pattern. Generally reddish brown sandy and loamy soils with an average

Recharge process and aquifer models of a small watershed 683

Table 1 Exploratory bore well particulars (after GWD, 1987).

1 2 3

Village

Kanchanapally I Kanchanapally II Marriguda I

Depth drilled (mb.g.l)

45 40 49

Water struck (mb.g.l) 20.2 17.5 21.0

Yield (1 s"') 0.62 0.33 1.38

SWL (mb.g.l) 12.87 13.80 18.00

b.g.l.: below ground level. SWL: static water level.

thickness of about one metre occur in the area. The watershed is underlain with peninsular gneissic complex. The country rocks are extensively weathered and fractured covered by alluvium under the command area of the tank. A random inventory of wells indicates that the weathered zone extends up to a maximum depth of 15 m. The drainage courses in the central part receive groundwater effluence during the rainy season. Fourteen aquifer tests were conducted around the watershed and well parameters such as depth, yield, etc. are presented in Table 1.

A fractured zone normally occurs along certain lineaments corresponding to surface drainage pattern. Generally fractures extend up to 40 m below the ground surface. The wells tapping the fracture zone have yielded up to 850 m3 day"1

(GWD, 1987). Secondary clay minerals fill joints which considerably reduce permeability of the weathered zone. Groundwater occurs in unconfmed to semi-confined conditions in the fracture zone and similar conditions were reported elsewhere in granitic terrain (Karanth, 1987; Narasirnha Reddy et al., 1994). Groundwater moves away from the topographic highs. The depth to water level varies between 4 and 10 m below ground level (b.g.l.). The groundwater divide in the watershed closely corresponds with the surface topographic divide and the groundwater flow path closely follows the drainage pattern, evident through the presence of more productive wells in the fracture zone along the drainage pattern (Narasirnha Reddy & Pradeep Raj, 1997).

Watershed characteristics

Red chalka soils are interspersed with black cotton soils in the area. The soil zone thickness varies from 0.3 to 0.8 m. The texture of the soil is coarse and vegetation is sparse. Rainfed crops like maize, sorghum and castor beans, are widely grown during the southwest monsoon season (June-September). Paddy is grown in small pockets under the command area of the tank. A weighted average runoff curve number has been computed for the area, considering the hydrological condition, soil group and crop practices (Table 2). In sandy loams, evaporation could occur from up to 10 cm depth of soil (Champbell & Diaz, 1988). The transpiration from plants occurs from the root zone, which depends on the vegetation type and its root growth. The average soil zone thickness in the watershed is about 45 cm and a two-layer model for the soil zone was considered, with the first layer thickness being 30 cm and the rest below it forming the second layer in the water balance model.


Table 2 Weighted average runoff curve numbers (CN) for Kanchanapally watershed (Srinivas, 1995).

Land use and cover

Area covered by barren land and habitation Area under irrigation and under cultivation (row crops) Area covered by tanks

Area (%)

27.5

65.0

7.5

Treatment/ practice

Straight row

Hydrological conditions

Poor

Good

Poor

Hydrological soil (CN value) I II 68

60

38

84

78

58

group C*

III

93

90

77 * Hydrological soil group C: soils having slow infiltration rates when thoroughly wetted and consisting chiefly of soils with a layer that impedes downward movement of water or soils with moderately fine to fine texture. The soils have a slow rate of water transmission (Campbell & Diaz, 1988).

Groundwater recharge is given by:

Recharge = P + Ir^ {Int + Rof{ + ETact + WQ (1)

where P = rainfall; Ir = irrigation (in this case assumed zero); Int = interception loss; i?0ff

= surface runoff; ETacX = actual évapotranspiration; and JVC = change in soil moisture storage.

A five days antecedent rainfall was chosen to decide the antecedent moisture condition (AMC) of I, II or III—undersaturated, saturated and excess saturated, respectively—for assessing the soil moisture status in the water balance computation. Based on the AMC status, a relevant runoff curve number will be selected for calculation of surface runoff from a particular rainfall event. The soil moisture status in the area is under AMC I from January to June. The moisture status may change from AMC I to AMC II or AMC III depending on the rainfall pattern. The runoff is computed using the following equation:

^off = (P - 0.2S)2/(P + 0.86) (2)

where S is potential maximum surface retention in mm and is given by:

S = 25 400/CN-254 (3)

where CN is the runoff curve number, a parameter dependent on soil type, land use and antecedent moisture conditions.

Evapotranspiration

Actual évapotranspiration was calculated using Food and Agriculture Organization (FAO) procedures (Doorenboos & Pruitt, 1984). Reference crop évapotranspiration (ETo) can be obtained from

ETQ = KpEpm (4)

where KP is the pan coefficient, and Epm is pan evaporation (mm day"1) measured in a Class A pan.

Values of KP depend on humidity, wind conditions and pan environment. The maximum évapotranspiration ETmm (mm day"1) depends on stage of crop development under a given climate and is given by:


ETmm = KcETo (5)

where Kc is the crop coefficient. Doorenboos & Kassam (1979) have proposed a relationship between soil moisture

availability and évapotranspiration (Fig. 2) based on the results of field and laboratory experiments for determining actual évapotranspiration:

£Tact = £Tmax when WC>{\- M)A WHC (6)

£r a c t = ETm WCI{ 1 - M)A WHC when WC<{\-M)A WHC (7)

where AWHC is the available water holding capacity of the soil zone (difference between field capacity and wilting point); WC is available moisture; and M is the moisture depletion factor.

The values of M for different crops and rates of potential évapotranspiration are listed by Doorenboos & Kassam (1979). The component of groundwater recharge could be obtained ultimately from the water balance computation of the soil zone. Based on the crop growth, évapotranspiration takes place as mentioned above. An interception loss of 0.2 mm per rainfall event has been assumed. If the rainfall for a day is less than 0.2 mm, the interception loss is equal to precipitation. The soils in the area fall under Hydrological Soil Group C (see Table 2) and, based on the land-use pattern, a weighted curve number 78 has been worked out under AMC II conditions. Depending on the antecedent moisture status on a particular rainfall day, an appropriate curve number is considered for runoff computation. The pan coefficient of 0.8 for light and medium humidity condition has been used. The crop coefficient has been determined based on land under various crops and crop growing periods. The actual rate of water uptake by a crop from the soil zone in relation to its maximum évapotranspiration demand is determined by the available water in the soil zone. An average effective root zone depth of 100 cm, having an available moisture holding capacity of 80 mm, has been considered for the entire watershed. The soil water depletion factor based on the soil moisture holding characteristics has been assumed as 0.5.

The annual groundwater recharge estimates from the water balance model are based on daily rainfall and pan evaporation data measured at Kanchanapally during 1977-1993. The annual water balance components indicate that average groundwater recharge in the watershed is 86.7 mm year"1 (Table 3). The estimates of monthly groundwater recharge and surface runoff from the water balance model are shown in

10

ETAct

ETMOX

Wilting (l-M) AWHC Field Point Capacity

Fig. 2 Relationship between soil moisture availability and évapotranspiration.

686 A. Srinivas et al

Table 3 Annual water balance components (in mrn) of soil zone.

Year

1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993

Rainfall

572.8 1069.9 690.4 524.9

1030.3 583.4 832.0 697.6 803.1 665.6 981.8 882.1 942.5 734.1 728.1 463.2 713.8

Interception loss

18.7 18.7 18.0 13.0 14.0 14.0 17.7 11.0 18.9 16.8 13.8 18.0 14.3 17.0 13.9 12.5 13.0

Runoff

31.9 180.3 90.5 40.5

189.1 36.0 98.5 80.2

163.1 82.6

184.7 138.0 228.6

69.8 58.1 44.9

117.5

Evapotranspiration

447.0 658.3 508.8 459.9 654.0 534.4 648.9 546.1 542.9 477.4 536.0 611.0 596.6 600.6 574.6 411.5 524.3

Recharge

41.2 232.9

68.5 23.6

161.9 3.7

55.7 62.9 99.4 51.3

258.5 141.4 91.3 58.5 53.3

8.7 59.8

Soil moisture

35.0 -23.3

4.6 -12.1

11.3 -4.7 11.2 -2.6

-21.2 37.5

-11.2 -26.3

11.7 -11.8

28.2 -14.4

-0.8

Tables 4 and 5 respectively. The average surface runoff in the watershed would be about 108 mm year"1 for an average rainfall of 759.6 mm year"1 (Srinivas, 1995).

Southwest monsoons, and sometimes northeast monsoons, during October-November, bring copious rains and may contribute to recharge. The groundwater recharge process is not continuous and is variable during different months depending on the rainfall pattern as is evident from Table 4. Groundwater recharge in the watershed takes place during a 3-4 month period while there could be no recharge in many years. Recharge is confined to July and August only in a few years. The aquifer

Table 4 Monthly estimates of groundwater recharge (in mm).

Year

1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993

June

25.3

28.7

12.8 21.8

12.9

July

65.6

1.2

3.7

29.3 47.2

27.9 23.4 42.2

26.5

August

60.7

13.4 41.3

25.5 23.4 22.7 46.0 67.7

5.9

September

77.1 34.8

61.4

15.0

39.4 39.6

5.0 8.6 9.4

October

4.2 5.9

59.2

40.6 7.8

3.2 10.8 9.3 6.3

31.9

November

41.2

0.2

28.6 181.4


Table 5 Monthly estimates of groundwater recharge (in mm).

Year

1977

1978 1979 1980

1981 1982

1983 1984

1985

1986 1987

1988

1989 1990 1991

1992 1993

June

3.6 8.2

27.7

1.2 9.9 0.1 0.5 54.4

0.2 9.6

1.5 15.1 24.1

73.6

July

18.8

11.6 7.7 10.4

18.1

8.8 2.2

38.8 95.9

2.5 18.5 60.7

130.9

3.3 42.1

19.8

August

6.0 74.2

2.4 41.6 8.6 7.1 32.9

5.2 32.4

56.3

26.8 1.1 4.6 1.9 0.6 19.8

September

87.1

46.3

73.7

42.8

7.7

46.4

81.1 16.4

28.8 2.2 9.0

October

4.7 0.3 3.1

54.5 8.7

46.3

7.9 7.6

7.8 4.0 14.0 12.0

15.0

response is dependent on the recharge process as is evident from the water level fluctuations in the hydrographs. Unless one understands and incorporates the dynamic variation of recharge in time while giving input to the groundwater flow model in monthly time steps, it is difficult to account for fluctuations in the water table registered in the observation wells. This variability of recharge has been appropriately taken care of by taking into account the recharge values estimated using the water balance model during the entire 16-year period of simulation.

Groundwater flow model

The watershed has been discretized into 321 finite difference nested square meshes of variable mesh spacing of 250 m and 125 m. There are two ephemeral streams which drain the area, which join the surface water tank at Kanchanapally in the south (Fig. 1). The streams mostly receive groundwater effluence from the aquifer system during monsoon season only. The computer program NEWS AM (Ledoux & Tille, 1981) was used to solve the governing partial differential equation of groundwater flow. The linear system of equations were solved iteratively by point successive over relaxation method (De Marsily et al., 1978). Pumping tests were carried out at two dug wells by the Groundwater Department of Andhra Pradesh to estimate aquifer parameters, namely transmissivity and storativity. Tranmissivity values ranged from 207 to

Table 6 Aquifer parameters estimated from pumping tests.

Well no. Transmissivity Storativity value (m2 day"1)

I II

207 308 0.01


308 m2 day"1 and the storativity value is 0.01 (Table 6). The transmis si vity values were assigned to meshes in two blocks: a northern block with 207 m2 da / 1 and a southern block with 308 m2 day4. The model boundaries were realized by terminating the meshes with a no flow boundary by zero transmissivity value except near the tank boundary. The outflow took place from the downstream side of the tank. The water level configuration during June 1977 was assumed to be in equilibrium and was considered as the initial water level for the flow model. The elevations of the bed of the stream channel were simulated as the known head boundary.

Input and output stresses

The recharge due to rainfall forms the main input to the groundwater regime. Seepage from the tank contributes some additional input to the aquifer. Groundwater withdrawal from open wells and bore wells forms the main output and also some groundwater effluence occurs to the stream channel during the monsoon season. Subsurface outflow from the aquifer to the stream occurs whenever the water table elevation becomes higher than the stream bed elevation (which was simulated as known head boundary in the model). The groundwater withdrawal was estimated based on detailed well inventory, average running hours of pumps and cropping pattern. The unit draft in a year works out as 0.7 ha m. The groundwater withdrawal for irrigation takes place during October-March for rabi crop and during May for raising paddy nurseries. However, drinking water pumping continues throughout the year. The draft values have been assigned to each mesh based on the density of

\ _~ 1

I

' u - k

i T 1 ! 1

I i

1 1

1 ' 1

1

|

1

1

1 1

1 i " ~-

i 1 2 1 1 ' 1

I I I 1

1 1 '

1 ! 1

1 2

1 1

1 |

1 1 1

1 1

1 —

UNIT DRAFT- 0-7 hom/annu

g]No. wells

1

I

2 1

- 1

1 1

J . 1 1 I

1 3

1 1

2 1

1 2

1 3 3

- 3 m i t

i

i

3

2

1

1

2

2

1 2 1

1

1 -

2 -

2 -

1

! 1

1

7

1 1

1

j j 1

1 l

1 1

1

1

2

1

1

1

1

1

1

Fig. 3 Spatial distribution of draft.


0

0

-1

2

3

1

H

2 t2_ 0

• 1 2 • 1 •1

i

2

2

3

0

-2

0

0

? 0 0 '1

- 2

- 1

1

0

0

-1

ojo 0

1-1 0 4 2 0

0 0 3

6 -1 -4

- 2

1-

~2

- 2

- 1

-1

-1

-, -1

0 2 2 0 -2 -2

-1 2 0 1 -1 -1

- 3

0

1

0

3

3

- 4

- 2

- 2

- 1

0

- 1

2

1 0 2 2 0-2 1 1

0

ojo

0 c 2 C 2 0 4 4

2

2

1

- 1

- 2

- 3

- 3

-3

2

3

3

3

1

1

* 0 o 0 ' 1 0 2 2 0 -0 --1 -2 Z

3

- 4

- 3

-3

-2

0 0 0 0

1

2 3 4 4 3 3 2 4 1 3 3 3

5

3

3

2

0 0 1 0 1 C 3 :

3 ? 2

(TJ IN METERS

< 2

3

2

1 ? 3 2 4 4 4 4

2

5

2

- 2

- 1 -0 C

1

3

3

3

1

? 3 3 3

4

3

2

1

» •1

-< 0 0

-1 -1 0 0

0

4

2

4

2

2

0

- 1

2

1

2

•1 -2 1 0

•1 -1 •1 0

0

4

3

3

2

0

,0 -1 2 2

-1 •2 ? •1

- 1

0 •1 -1 0

•2

•1

-( 0

- I

3

1

1

0 0

0 0 0 2

0 0 ? 2 1 ? 0 1

1 2

•3 •z

•3 •3

•2 H 2 1

1

0

0

- 1

4

1

2

1 1 •1 -1 Tl •2 -1 0

0 1 •1 4 .1 3 0 0

- 2

0

-1

2

0

-1

-1

.1

- 1

Fig. 4 Difference map of calibrated and observed water levels (m).

wells falling in a particular mesh (Fig. 3). The average annual recharge estimated from the water balance model has been uniformly assigned as input at all meshes except at those falling on the stream nodes. The stream nodes receive the groundwater effluence during most part of the year. The nodes covering tanks receive more input to the aquifer system through seepage of surface water in addition to normal recharge. It was assumed that, generally, surface water will be available in the tank from July to December and accordingly a higher value of 46 cm was distributed appropriately at these nodes during the six-month period as additional input. Minor changes in pumping were made during calibration of the steady state model to match computed water levels with observed ones (Fig. 4). The input stresses under transient conditions include time variant monthly recharge due to rainfall (ref. Table 4) and seepage from tanks. As 90% of the cultivated area is irrigated by pumping groundwater, it was assumed that return flow from the irrigated water would be negligible. The net annual input and output stresses are arrived at by summing the variable monthly recharge during each year of transient model simulation (Table 7).

RESULTS AND DISCUSSION

Hydrographs at six observation wells were considered for transient calibration of the model. Comparison of hydrographs at observation wells 14 and 39 indicates a close match during the post-monsoon period up to 1982 and a deviation of 2 m after that date (Figs 5 and 6). However, the computed and observed water level fluctuations in the


Table 7 Annual input and output stresses for transient condition in million cubic metres (m3 x 106).

Year Baseflow to streams and outflow

1977-78 1978-79 1979-80 1980-81 1981-82 1982-83 1983-84 1984-85 1985-86 1986-87 1987-88 1988-89 1989-90 1990-91 1991-92 1992-93

Input: Recharge due to rainfall and seepage from tanks

2.71 0.82 0.34 1.88 0.15 0.65 0.75 1.10 1.39 3.00 1.60 1.10 0.70 0.70 0.10 0.10

Output Draft

0.36 0.49 0.67 0.51 0.66 0.58 0.64 0.61 0.67 0.58 0.72 0.76 0.81 0.77 0,81 0.74

1.67 0.66 0.26 0.71 0.28 0.12 0.21 0.41 0.54 1.38 1.22 0.64 0.32 0.12 -0.33 -0.16

hydrographs indicate similar trends at six other wells. There was a decline of 4-5 m during the last 15 years at some wells (Srinivas, 1995). Matching of well hydrographs at wells 14 and 39 indicates that the water table decline is about 2.5 m in the central part. The highest water table was observed during 1978 and 1988, due to well distributed recharge processes taking place during four months of monsoon in those

Computed

Observed

1978 June

Fig. 5 Comparison of well hydrographs at observation well no. 14.


o o g I I 1 I 1 1 1 1 1 1 1 1 1 f

,7; I I I I I i i I I I i I I I I N I 9 7 8 80 82 84 86 88 90 92 93

June Ye o r s •

Fig. 6 Comparison of well hydrographs at observation well no. 39.

years. Also, a rare event of about 181 mm recharge which occurred during November 1987 was followed by well distributed temporal recharge during 1988, resulting in the highest water table in the watershed during the 1988 post-monsoon period. Even though the highest rainfall occurred during 1981 with a recharge of about 162 mm, it could not raise the regional water table because of the preceding year's low recharge (14 mm only). Thus monthly estimates of recharge from the recharge process model were found very useful to account for the temporal variations in input stress to the aquifer model during transient model calibration.

CONCLUSION

The groundwater flow model of the aquifer system in Kanchanapally watershed was calibrated from June 1977 to May 1993 under transient conditions using monthly recharge estimates of the water balance model. The matching of hydrographs at six observation wells was found satisfactory. The average groundwater recharge in the watershed is 86.7 mm year"1 for an average annual rainfall of 759.6 mm. The output of the recharge process model was found very useful to simulate dynamic variations of recharge in a groundwater flow model while giving input at monthly intervals. The importance of temporal variation in recharge controlling the fluctuations in water levels has been seen in all hydrographs and simulated accordingly by giving temporal variations in recharge. The estimated recharge from the recharge process model has helped while giving input to the groundwater flow model in monthly time steps. The importance of coupling the approach of a recharge process model output to the groundwater model input in monthly time steps has helped account the temporal


variations in recharge while matching the computed hydrographs with observed hydrographs in the watershed.

Acknowledgement The first author is grateful to the Director, NGRI for according permission to work at NGRI for completion of his MTech dissertation work and to the Director, Groundwater Department for allowing collection of the field data. The authors are grateful to Dr C. P. Gupta for encouragement and discussions during the study and to two anonymous reviewers for improving the manuscript.

REFERENCES

Champbell, G. S. & Diaz, R. (1988) Simplified soil water models to predict crop transpiration. In: Drought Research Priorities for the Dry Land Tropics (ed. by F. R. Bedinger & C. Johansen), 15-26.ICRISAT, Patancheru, India.

Doorenboos, J. & Kassam, A. H. (1979) Yield response to water. FAO Irrigation and Drainage paper 33, FAO, Rome, Italy.

Doorenboos, J. & Pruitt, W. D. (1984) Guidelines for predicting crop-water requirements. FAO Irrigation and Drainage paper 24 (revised) FAO, Rome, Italy.

De Marsily, G., Ledoux, E., Levassor, A., Poitrinal, D. & Salem, A. (1978) Modelling of large multilayered aquifer systems: theory and applications. J. Hydro!. 36, 1-34.

GWD (Groundwater Department) (1974) Pilot project report on Kanchanapally critical basin, Nalgonda district, AP. Groundwater Department, Andhra Pradesh, India.

GWD (Groundwater Department) (1987) Bore well directory, Nalgonda district, AP. Unpublished report, Groundwater Department, Andhra Pradesh, India.

Karanth, K. R. (1987) Groundwater Assessment, Development and Management. Tata McGraw Hill, London. Ledoux, E. & Tille, B. (1981) Programme NEWSAM: principle et notice d'emploi. Report CIG/LHM/RD/81/40, Ecole

des Mines de Paris, France Narasimha Reddy, T., Prakasm, P., Subralimanyam, G. V., Nageswarara Rao, T., Prakash Gowd, P. V., Harish, K.,

Gurunadha Rao, V. V. S. & Gupta, C. P. (1994) Groundwater recharge process model in a granitic terrain—a long term analysis. J. Geol. Soc. India 44(6), 645-662.

Narasimha Reddy, T. & Pradeep Raj (1997) Groundwater conditions in Nalgonda district. J. Geol. Soc. India 48(3), 230-239.

Srinivas, A. (1995) Aquifer modelling based on finite difference method—Kanchanapally watershed (AP). MTech dissertation, Water Resources Dept, Jawaharlal Nehru Technological University, Hyderabad, India.

Srinivas, A., Venkateswara Rao, B. & Gurunadha Rao, V. V. S. (1996) Groundwater flow model in a granitic watershed. In: Abstracts 30th Int. Geol. Congress (Beijing, China, 4-14 August 1996), vol. 3, 300.

Received 5 January 1998; accepted 28 January 1999

recharge process and aquifer models of a small...

Documents