SECTION-7 Ground Water and Hydrogeology

GROUNDWATER POTENTIAL ESTIMATION - A COMPARATIVE ANALYSIS
S. Mohan1 and V. Ramani Bai2


ABSTRACT
INTRODUCTION
EARLIER WORKS:
STUDY AREA
ESTIMATION OF GROUNDWATER RECHARGE
RECHARGE BY FLUCTUATION IN MONSOON SEASON
RECHARGE BY YEARLY WATER LEVEL FLUCTUATION
RECHARGE BY TEN YEAR FLUCTUATION
RECHARGE BY AVERAGE OF TEN YEAR FLUCTUATION
RESULTS AND DISCUSSION
CONCLUSIONS
ACKNOWLEDGEMENT
REFERENCES
TABLE 1: RECHARGE BY FLUCTUATION IN MONSOON SEASON
TABLE 2: RECHARGE BY YEARLY WATER LEVEL FLUCTUATION
TABLE 3: RECHARGE BY WATER LEVEL DIFFERENCE OVER TEN YEARS
TABLE 4: RECHARGE BY AVERAGE W.L DIFFERENCE OVER TEN YEARS
TABLE 5: THE COMPARISON OF RESULTS
FIGURE-1: TAMIL NADU PROJECT LOCATION MAP
FIGURE-2: VALGAI RIVER BASIN - THIESSEN POLYGON MAP
FIGURE-3: DEPTH OF GWFLUCTUATION Vs TIME
FIGURE-4: COMPARISION OF GW RECHARGE METHODS
FIGURE-5: ZONE WISE RECHARGE POTENTIAL


ABSTRACT: first topic previous topic next topic last topic

With the increase in demand for water for competing uses, it is difficult to meet the entire demand from a single source and it is a challenge to plan and manage the different water resources. Among the two major water resources, surface and ground water, it is the ground water resource, which needs to be managed carefully, especially in drought prone areas. The hydro-geological features such as sub-soil structure, rock formation, lithology and location of water play a crucial role in determining the potential of water storage in groundwater reservoirs. To assess the groundwater potential, a suitable and accurate technique is required for a meaningful and objective analysis. A critical study is carried out on the different methods of estimating the groundwater potential and compared to arrive at the most suitable technique for practical utility. In this work, four methods of estimating groundwater recharge were studied viz., yearly water level fluctuation, ten year average water level fluctuation, fluctuation between the lowest and highest water levels over ten years and fluctuation in monsoon seasons. The results of this study help in accurate prediction of groundwater availability, which in turn may avoid groundwater over exploitation and help restore the aquatic eco-systems.

INTRODUCTION: first topic previous topic next topic last topic

Groundwater was the major source for irrigation prior to the introduction of canal irrigation system. Water contained in the voids of the geologic materials that comprise the crust of the earth is the groundwater. It exists at a pressure greater than or equal to atmospheric pressure. The experimental and mathematical methods required for analyses are distinctly different as it is exploited and used in human affairs in different ways. The important hydrogeological parameters such as porosity and hydraulic conductivity of the geologic stratum determine the performance of the aquifer. Added to this, an important factor is the length of data considered for predicting the groundwater potential. Thus, with adequate length of the database, the prediction of any derived model will reproduce the statistical properties. Otherwise, it is difficult to predict the regional groundwater flow conditions, subject to measurable hydrological, hydrogeological and meteorological variables in nature. As it is largely uncertain in nature, this cannot be left in isolation.

Hence, it prompts the importance of understanding and estimating the regional groundwater flow regime. This article presents the methods of estimation of groundwater recharge on a 250 km long reach of the Vaigai river in southern part of Tamil Nadu. The approach specially accounts for the different time periods for the estimation of recharge potential of the aquifer. A comparative evaluation is made on these methods.

EARLIER WORKS: first topic previous topic next topic last topic

Different methods were proposed and many authors reviewed several applications on this problem. Rushton and Rathod (1985) have determined the velocity components from information about the groundwater-head distribution, groundwater potential, confined and unconfined aquifers; time-variant behaviour of aquifer and hydraulic conductivity. Mathematical models have been developed by Serrano and Unny (1987) as an innovative approach to the solution of groundwater forecasting problems considering the uncertainty generated by the use of data subject to environmental fluctuations and measurement errors. They have described in detail the development, solution and validation of two mathematical models describing groundwater potential at the Twin lake aquifer. Sondhi et.al. (1989) have determined the available additional groundwater potential and its distribution in the project area; estimation of groundwater recharge from the water conveyance and distribution system and the annual water balance of the project; 'recharge distribution coefficients' are done using digital simulation models. Chiew and McMahon (1990) estimated groundwater recharge using surface watershed modeling approach for both irrigated and non-irrigated areas. In all the above cases, they have not seen which time period will give appropriate prediction over the recharge value of a basin area. Uma and Kehinde (1992) described the analysis of the baseflow characteristics of numerous small basins to estimate the groundwater in the basins. Boonstra and Bhutta (1996) have worked on determination of seasonal net recharge considering temporal and areal recharge variations, geometry of aquifer system, historical water table elevations, drainage design and waterlogged areas, and developed numerical models for monsoon estimates, water-balance, and return period. A similar attempt is made here for estimating the ground water recharge potential of a river basin.

STUDY AREA: first topic previous topic next topic last topic

The study area Vaigai (N 9o 15'-- 10o 20' and E 77o 10' - 79o 15') is located in south of Tamil Nadu in India (Fig. 1). The river basin is underlined by a weathered aquifer, phreatic to semi confined aquifer in the alluvium and valley fills in the crystalline rock formation and semi confined to confined aquifer conditions in the sedimentary formations.

The total area of Vaigai river basin is 7031 sq.km. The net area irrigated by canals and system tanks accounts for 1613.23 sq.km. and non-system tanks irrigate 466.5 sq.km. The depth of aquifer varies from 7 to 22 m. The water level fluctuation in this zone varies from 2 to 15 m.

The geology of the site consists of charnockites and khondalites of archaean age, the two main rock types encountered. Charnockites include acid charnockite and related migmatites with bands of basic granulite and magnetite, and quartzite. The khondalite group of rocks consists of crystalline limestone, calc-gneiss, hornblende and biotite gneiss and related migmatites with bands of quartzite. Sedimentary rocks (of upper Gondwana age) consist of micaceous sandstone, limestone alternating shale and grits and river and coastal alluvium. Laterites of sub recent age are in localised pockets. Monthly ground water level data in 99 control wells of the basin, for a period 1971-1992, are used in this study.

ESTIMATION OF GROUNDWATER RECHARGE: first topic previous topic next topic last topic

The commonly used method for estimating groundwater storage available annually is based on

Q = (Area) x (Depth of fluctuation in Groundwater Table) x (Specific Yield) (1)

In the above equation, the depth of fluctuation in groundwater table or the drop in groundwater table has to be calculated by taking the duration over which the fluctuation occurs.

The area in equation (1) is the area of influence of the well in the basin. This is obtained by Thiessen Polygon Method (Fig 2). By this method, the watershed area is subdivided into polygonal sub areas using the wells as nodes. The Thiessen polygon around each well permits the assignment of weights on the basis of the relative areas of the respective polygons. The sub areas are used as weights in estimating the average depth of fluctuation for each well.

The dynamic reserve has been computed by measuring the net change in storage of the phreatic aquifer. The storage of the groundwater in the aquifer is dependent upon the input components such as precipitation; seepage and return flow of irrigated water during various seasons.

The fluctuation of groundwater depth over the study period is analysed and plotted as shown in Fig. 3, to understand the behaviour of the aquifer. High variations in annual recharges to groundwater body are observed between monsoons. To avoid compilation in the estimation and management of these replenishable resources due to variation in the annual recharge, quantification of optimum dynamic part of the groundwater resource is adopted in the dynamic groundwater reserve estimation.

RECHARGE BY FLUCTUATION IN MONSOON SEASON first topic previous topic next topic last topic

The determination of groundwater recharge (in a minimum period of monsoon variations) in the basin was done first. The difference between the highest and lowest water levels is calculated during the monsoon period and the equation (1) is used to estimate the recharge in the basin. This is repeated for all five zones (in the basin) using 20 years of water level data (1972 to 1991). The estimated quantity of recharge is listed in Table 1.

RECHARGE BY YEARLY WATER LEVEL FLUCTUATION first topic previous topic next topic last topic

In this method, the fluctuation of groundwater level is taken as the difference between the highest level (of the second season) and the lowest (of the first season in a year) and the equation (1) is used for estimating the recharge values for 20 years from 1972 to 1991. Zonewise recharge quantity is calculated and is tabulated in Table 2.

RECHARGE BY TEN YEAR FLUCTUATION first topic previous topic next topic last topic

The study is further extended for a long span of water level fluctuations in the aquifer. Here, the depth of fluctuation is taken as difference between the highest level (of second season) and the lowest (of the first season) over ten years. Considering these variations groundwater recharge was estimated by equation (1). This is repeated with 20 years of data. Zonewise quantity of recharge is computed and listed in Table 3.

RECHARGE BY AVERAGE OF TEN YEAR FLUCTUATION first topic previous topic next topic last topic

The estimation is worked out for the average fluctuations over ten years in the basin. The difference between the highest level of second season and the lowest of the first season for every year is taken and the average over the ten years is calculated. This is done for all the five zones and the quantity of recharge is calculated. The estimated quantity of recharge is given in Table 4. Thus, the assessment of groundwater potential is done in different time scale. This study is further extended to see the variations spatially. The recharge results are plotted zone wise in Fig 5.

RESULTS AND DISCUSSION: first topic previous topic next topic last topic

The groundwater fluctuation over the entire study period was analysed. It shows a depletion and subsequent recharge of groundwater every year (Fig.3). Especially in 1976, it appears that recharge has occurred largely due to the high storage space available as a result of the lowering of water level in the preceding year. Same effect is seen between the years 1972 to 1974. Besides, the exploitation of groundwater is higher on the upstream side of the basin and decreased towards downstream side and still reduced at the tail end of the basin.

The estimated values of bouncing reserve of groundwater in the basin from the year 1972 to 1991 illustrate the changes in recharge potential, when the time scale for analysis is changed. The comparison plot of four different methods is shown in Fig. 4. and the net recharge values are shown in Table 5.

As per the Groundwater Estimation committee norms, the recharge of 993.07 Mm3 has been computed by the C.E (GW) during 1992. The results obtained by the four methods show that the three methods are close to the results of the assessment made by CE (GW) except the third method. The percentage of deviation is shown in Table 5. Among the four methods, the values obtained by third method of taking the extreme fluctuation over ten years seems to be undesirable. Hence, this method could not be recommended. In other words, the larger time scale in assessment proves to be incorrect. For example, when there is sudden drought in one year and is normal for the remaining nine years the average taken over ten years will give anomalous results which is not the case during that period. On the other hand, the results obtained by the remaining three methods are falling in line with the values of CE (GW), which was used for planning of water resources of Vaigai basin. Hence, they can be recommended for future studies. In particular, the result obtained by the second method is more on a conservative side. The result obtained by the fourth method is best suited for estimation of groundwater recharge.

CONCLUSIONS: first topic previous topic next topic last topic

A more adequate description of the groundwater potential should be given in a statistical sense since they imply the statistical properties of the aquifer. The conclusions derived out of this study will give the hydrologist a more realistic tool to see the high variability and uncertainty associated with the groundwater flow regime. The recommended methods will provide with a quantitative evaluation of the uncertainty inherent to the system and at the same time with the degree of reliability of the methods as a predicting tool at different points in space and time. In this work, the optimum average yearly recoverable groundwater reserve that can be exploited from the aquifer in the basin is found out using different time scale for the depth of fluctuation. Among the four methods, the recharge obtained by second method is more conservative. The results obtained by third method have not been recommended. The result obtained by the fourth method is best suited for estimation of groundwater recharge. Further it shows that the groundwater storage receives bulk of the recharge from monsoon precipitation in the basin.

ACKNOWLEDGEMENT: first topic previous topic next topic last topic

This work is part of a project on "Application of system models for planning and management of water allocation" funded by Institute for Water Studies. Public Works Department, Govt. of Tamil Nadu through M/s Krishnan Associates provided necessary data for this study.

REFERENCES: first topic previous topic next topic last topic

  • Boonstra, J. and Bhutta, M. N., 1996. Groundwater recharge in irrigated agriculture: The theory and practice of inverse modelling. Journal of Hydrology, 174(3-4): 357-374.
  • Chiew, F. H. S. and McMahon, T. A., 1990. Estimating groundwater recharge using a surface watershed modelling approach. Journal of Hydrology, 114(3-4): 285-304.
  • Rushton, K. R. and Rathod, K. S., 1985. Horizontal and vertical components of flow deduced from groundwater heads. Journal of Hydrology, 79(1): 261-278.
  • Serrano, S. E. and Unny, T. E., 1987. Predicting groundwater flow in a phreatic aquifer. Journal of Hydrology, 95(3-4): 241-268.
  • Sondhi, S. K., Rao, N. H., and Sarma, P. B. S., 1989. Assessment of groundwater potential for conjunctive water use in a large irrigation project in India. Journal of Hydrology, 107(1-4): 283-295.
  • Uma, K. O. and Kehinde, M. O., 1992. Quantitative assessment of the groundwater potential of small basins in parts of southeastern Nigeria. Hydrological Sciences Journal, 37(4): 359-374.
  • TABLE 1: RECHARGE BY FLUCTUATION IN MONSOON SEASON first topic previous topic next topic last topic

    Year

    1972

    1973

    1974

    1975

    1976

    1977

    1978

    1979

    1980

    1981

    Zone-1

    338.5

    334.5

    366.03

    242.8

    401.19

    616.6

    252.83

    431.5

    194.87

    217.19

    Zone-2

    195

    187.1

    137.15

    181.4

    159.11

    309.1

    140.95

    259

    169.36

    186.78

    Zone-3

    195

    189.8

    166.26

    206.3

    171.48

    310.1

    182.92

    278.5

    138.88

    300.25

    Zone-4

    196.8

    201.5

    180.93

    236

    179.91

    315.2

    233.61

    245.1

    195.55

    269.68

    Zone-5

    146.5

    155.4

    115.66

    111.8

    120.9

    290.3

    128.2

    172

    102.73

    190.75

    Annual

    1072

    1068

    966.03

    978.4

    1032.59

    1841

    938.51

    1386

    801.39

    1164.7

    Year

    1982

    1983

    1984

    1985

    1986

    1987

    1988

    1989

    1990

    1991

    Zone-1

    219

    159.3

    242.28

    157.2

    130.6

    275.8

    145.8

    246

    188

    191.1

    Zone-2

    119.5

    138.2

    90.5

    84.15

    125.5

    189.3

    112

    121

    157.4

    128.7

    Zone-3

    154.2

    196.1

    96.06

    167.5

    158.5

    210

    113.5

    194

    139.2

    244.8

    Zone-4

    115.3

    208.4

    147.1

    135.2

    141.4

    169.5

    102.4

    178

    139.7

    173.5

    Zone-5

    150.6

    90.79

    114.22

    97.9

    88.39

    190.1

    107.8

    104

    130.7

    95.99

    Annual

    758.5

    792.7

    690.16

    641.9

    644.3

    1035

    581.5

    843

    755

    834.1

    TABLE 2: RECHARGE BY YEARLY WATER LEVEL FLUCTUATION first topic previous topic next topic last topic

    Year

    1972

    1973

    1974

    1975

    1976

    1977

    1978

    1979

    1980

    1981

    Zone-1

    185.1

    330.8

    270.23

    184.8

    303.87

    168.4

    357.35

    231.1

    214.35

    270.28

    Zone-2

    156.63

    174.82

    221.33

    109.20

    197.87

    141.91

    179.70

    188.12

    160.79

    153.61

    Zone-3

    121.42

    172.49

    203.56

    205.14

    191.73

    127.24

    196.03

    179.36

    176.04

    225.22

    Zone-4

    154.32

    198.79

    231.81

    165.21

    214.46

    139.24

    229.66

    226.03

    233.65

    227.44

    Zone-5

    157.47

    162.93

    169.45

    87.84

    154.29

    152.70

    181.05

    153.12

    151.74

    151.58

    Annual

    758.6

    1023

    1095.8

    747.8

    1064.42

    725.1

    1143.79

    947.1

    1001.7

    1028.13

    Year

    1982

    1983

    1984

    1985

    1986

    1987

    1988

    1989

    1990

    1991

    Zone-1

    263.5

    234.1

    169.79

    212.7

    169.4

    181.2

    255.4

    206

    212.5

    165.6

    Zone-2

    182.06

    125.71

    138.56

    139.1

    126.8

    111.9

    127

    122

    170.3

    128.4

    Zone-3

    215.67

    207.41

    130.09

    202.2

    233.7

    280.3

    175.4

    141

    233.7

    205.6

    Zone-4

    222.13

    137.40

    168.14

    200.8

    154.2

    163.4

    182.4

    121

    151.6

    153.5

    Zone-5

    179.09

    172.42

    114.42

    155.5

    127.7

    112

    150.6

    138

    139.1

    113.7

    Annual

    1018

    861.1

    743.74

    910.2

    811.8

    848.8

    890.8

    729

    907.3

    766.9

    TABLE 3: RECHARGE BY WATER LEVEL DIFFERENCE OVER TEN YEARS first topic previous topic next topic last topic

    Year

    1972-81

    1973-82

    1974-83

    1975-84

    1976-85

    1977-86

    1978-87

    1979-88

    1980-89

    1981-90

    1982-91

    Zone-1

    543.7

    566

    582.78

    577

    584.792

    547.4

    616.973

    619.8

    668.52

    642.05

    692

    Zone-2

    522.4

    563.6

    560.7

    562.5

    574.38

    583.9

    606.23

    605.4

    619.14

    558

    578.1

    Zone-3

    397.8

    373

    398.66

    430

    432.55

    429.9

    494.45

    493.8

    484.91

    475.77

    503.4

    Zone-4

    687.8

    651.3

    663.02

    622

    621.68

    639.1

    652.52

    667.2

    669.61

    652.57

    652.6

    Zone-5

    475.8

    475.1

    473.88

    443.6

    455.55

    488.3

    569.8

    520.7

    549.59

    531.65

    533.4

    Annual

    2627

    2629

    2679.1

    2635

    2668.95

    2689

    2939.96

    2907

    2991.8

    2860

    2959

    TABLE 4: RECHARGE BY AVERAGE W.L DIFFERENCE OVER TEN YEARS first topic previous topic next topic last topic

    Year

    1972

    1973

    1974

    1975

    1976

    1977

    1978

    1979

    1980

    1981

    1982

    Zone-1

    264.4

    265.3

    261.8

    257.2

    302.9

    253.4

    276.73

    221.1

    219.65

    188.79

    198.3

    Zone-2

    168.40

    170.94

    171.08

    164.53

    170.72

    165.49

    168.90

    167.00

    159.47

    157.25

    166.82

    Zone-3

    134.17

    187.45

    206.38

    162.01

    178.47

    136.27

    206.01

    180.10

    195.79

    216.01

    215.67

    Zone-4

    201.58

    208.41

    209.59

    207.13

    213.10

    212.88

    227.58

    227.08

    227.57

    224.62

    221.99

    Zone-5

    152.22

    154.38

    153.43

    151.43

    160.51

    161.55

    163.32

    158.89

    160.81

    165.34

    179.09

    Annual

    920.75

    986.49

    1002.28

    942.31

    1025.70

    929.55

    1042.54

    954.21

    963.29

    952.01

    981.86

    TABLE 5: THE COMPARISON OF RESULTS first topic previous topic next topic last topic

    Sl. No.

    Methodology

    Qty of G.W in Mm3 in a year

    % Deviation

    from CE (GW)

    1.

    By water level fluctuation in monsoon season

    941.26

    5

    2.

    By yearly water level fluctuation

    901.03

    9

    3.

    By the lowest & highest water levels over 10 years

    2865.32

     

    4.

    By 10 years average water level fluctuation

    972.82

    3

    FIGURE-1: TAMIL NADU PROJECT LOCATION MAP first topic previous topic next topic last topic


    FIGURE-2: VALGAI RIVER BASIN - THIESSEN POLYGON MAP first topic previous topic next topic last topic


    FIGURE-3: DEPTH OF GWFLUCTUATION Vs TIME first topic previous topic next topic last topic


    FIGURE-4: COMPARISION OF GW RECHARGE METHODS first topic previous topic next topic last topic


    FIGURE-5: ZONE WISE RECHARGE POTENTIAL first topic previous topic next topic last topic


    ADDRESS: first topic previous topic

    1.) Professor,
    Environmental and Water Resources Engineering Division,
    Department of Civil Engineering,
    Indian Institute of Technology,
    Chennai 600 036,
    Tamilnadu,
    India.

    2.) Research Scholar,
    Environmental and Water Resources Engineering Division,
    Department of Civil Engineering,
    Indian Institute of Technology,
    Chennai 600 036,
    Tamilnadu,
    India.