Rainfall-recharge correlation; a method for evaluating potential groundwater

Transcription

1 Improvements of Methods of Long Term Prediction of Variations in Groundwater Resources and Regimes Due to Human Activity (Proceedings of the Exeter Symposium, July 1982). IAHS Publ. no Rainfall-recharge correlation; a method for evaluating potential groundwater B.K. BHATTACHARJEE State Water Investigation Directorate, West Bengal, Calcutta, India. ABSTRACT The macro-level water balance and its correlation with rainfall can be used to calculate groundwater recharge to an unconfined aquifer. The hydrographs of 15 observation wells in the Hooghly district of West Bengal for the years 1978 to 1980 were analysed. A good correlation was found between seasonal rainfall and changes in water level. By evaluating the increment in groundwater reserve, and checking the specific yield of the aquifer by pumping tests, the potential groundwater for the area has been calculated. NOTATION P = total annual rainfall (mm) R = annual increment in ground water reserve (mm) r = radius of influence (m) S = specific yield, T = transmissivity of aquifer (m day ) t = duration of pumping (days) INTRODUCTION With increasing demand for water for both agricultural and industrial use, the need to develop groundwater resources to the maximum possible extent has gained importance. Identification of parameters governing groundwater resources, and assessment so as to predict groundwater potential from these parameters, is essential for planning and developing this resource. Estimation of groundwater potential in unconfined aquifers can be made from the net recoverable recharge potential of rainfall. The rate of accretion to the water table in India from rainfall has been assessed by several investigators (Chaturvedi, 1944; Bhattacharjee et al., 1953; and Gupta, 1953). The water table fluctuation and specific yield approach has been successfully employed in areas of unconfined aquifer to determine the recharge potential of rainfall. The report of the Groundwater Over-exploitation Committee published by A.R.D.C. (1979) as well as the paper of Saxena (1980) have compiled the different techniques available for assessing groundwater recharge in India. The main component of recharge is precipitation, though contributions are also received by seepage from large water bodies (rivers, streams etc), seepage from unlined irrigation canals, and return flow from irrigation. All these recharges contributed to the annual increment 161

2 162 B.K. Bhattacharjee in the groundwater reserve in the study area, which in turn is reflected in the rise of water level during the post-monsoon period. The annual increment in groundwater reserve, as reflected by the average summer and average winter levels, indicates the total amount of recharge available in the area from all sources. The potential groundwater is equivalent to this amount of recharge. To determine an average value of potential groundwater in an area, the recharge (as reflected by the annual incremental value for rise of water level) has been correlated with corresponding annual rainfall, and the relationship of recharge and rainfall has been established. From this relationship, expressed in the form of an equation, and from the normal rainfall (50 years average) the potential groundwater of the area has been evaluated using specific yield values determined from pump tests. LOCATION AND HYDR0GE0L0GY 2 The project area (Fig.1) covers about 2800 km, lying between latitudes 22 and 23 N, and longitudes 86 and 88 E. It has two major rivers as natural boundaries - the Hooghly to the east and the Damodar to the west; the northern limit is the boundary of the Howrah District, the Southern is the Burdwan District. FIG.l Location of piezometric tubes for recording water level fluctuations.

3 Rainfall-recharge correlation: groundwater resources 163 The subsurface geology of the project area is known from lithologies of about 225 tubewells of 219 mm diameter. These lithologs reveal that the soil to the west of the area is characterised by the presence of latérites. < However, from the eastern bank of the river Darakeswar the land is typically characterised by an alluvial terrain with a gradual slope from north to south. Subsurface lithologies down to a depth of 150 m from the surface consist mainly of clay, silt and sand of different grades varying from fine to coarse, occasionally mixed with fine gravel. The total thickness of the aquifer varies from 27 to 30 m. Groundwater occurs in a thick zone of saturation within the alluvium, existing under water-table conditions in the entire area except for a small portion to the west beyond the river Darakeswar. The project area was divided into 15 blocks, in only one of which (Goghat) does the groundwater occur under confined conditions. THEORETICAL CONSIDERATIONS The potential groundwater in an unconfined aquifer can be calculated by the water level fluctuation and specific yield'approach. The formula used is as follows: potential groundwater = (specific yield) x (annual incremental groundwater reserve) x (area) The rises in water level during and after the monsoon period were obtained from 15 piezometers installed in the project area for the years 1978, 1979 and The specific yield (S) was evaluated from pumping tests of 15 wells in the project area by using the formula of Ramsahoye and Lanz (1961): 4 T t S = ^-^ (2) r S may be seen to depend on the transmissivity of the aquifer (T), the radius of influence of the test well (r), and the duration of pumping (t). RAINFALL The annual rainfall data, both intensity and distribution, are recorded and maintained in 15 stations within the project area by the Agriculture Department of the Government of West Bengal. The total annual rainfalls for each of the above stations for the years 1978, 1979 and 1980 were collected from the records of these stations and are presented in Table 1. The 50-year average normal rainfall for 6 (star marked) of the 15 stations mentioned earlier are available from published data of the Indian Meterological Department, Government of India ( ). These data, together with computed normal rainfall data of the other 9 stations, are presented in the same table. The computation of normal rainfall for 9 stations from the available data of 6 stations from I.M.D. was done by the Thiessen network method.

4 164 B.K. Bhattacharjee TABLE 1 Annual rainfall in the project area for the years 1978, 1979 and 1980* Block where Total rainfall (mm) Average rainfall (mm) rainfall was recorded ( ) Arambahg Khanakul Pursurah Tarakeswar Dhaniakhali Jangipara Haripal Singur Polba-Dadpur Chanditolla Pandua Balagarh Chinsurah-Mogra Srirampur-Uttarpara * * * * * These figures were collected from published records of the Indian Meterological Department for the years RISE IN WATER LEVEL Fifteen piezometers as shown in Fig.l were installed in the project area by the State Water Investigation Directorate of the Government of West Bengal to record the fluctuation of groundwater level. These piezometers tap the same aquifer as the production wells in the neighbourhood and consist of a 5 cm diameter pipe with strainer placed at the corresponding aquifer. The top of the piezometer is provided with a screw cap, which can be opened at the time of reading. The water level is recorded every fortnight by lowering a chalked tape into the piezometer and noting the resultant water mark below piezometer top. The water level was recorded for the years 1978, 1979 and 1980, and it should be noted that the piezometers were installed within 50 m of the previously mentioned rain stations. From these fortnightly water level data, hydrographs were prepared and the difference between summer and winter average water levels was calculated as presented in Table 2. As mentioned previously, specific yield has been calculated using formula (2) and the values obtained for the different blocks are given in Table 3. CORRELATION OF RAINFALL AND RECHARGE The correlation between rainfall (P) and rise in water level (R) between summer and winter was examined by analysis of variance

5 Rainfall-recharge correlation: groundwater resources 165 TABLE 2 Difference between summer and winter average water tables (m) Block Arambagh Khanakul Pursurah Tarakeswar Dhanikakhali Jangipara Haripal Singur Polba-Dadpur Chanditolla Pandua Balagarh Chinsurah-Mogra Srirampur-Uttarpara following multiple polynomial regression analysis. This was done on a Burrows computer at the Regional Computer Centre, Jadavpur University, following a BASIS/package programme. The highest P-ratio is given in polynomial of degree 2. With 2 and 38 degrees of freedom (as there were 41 data) and F(Z) = 0.99, the Fe value obtained was 5.21 whereas the F-ratio obtained in 2nd degree polynomial was at the confidence level. The correlation coefficient has been calculated TABLE 3 Potential groundwater of the Project Area Block Area Av. R Spec. Potential (km 2 ) (m) yield m 3 x 10 6 Arambagh Khanakul Pursurah Tarakeswar Dhanikakhali Jangipara Haripal Singur Polba-Dadpur Chanditolla Pandua Balagarh Chinsurah-Mogra Srirampur-Uttarpara

6 166 B.K. Bhattacharjee and found to be Student's T-test demonstrates that all regression coefficients of polynomial 2 are significant at F(Z) = All these show good correlation between the parameters. The relationship can be expressed by the following equation: R = P - 8 P 2 (3) The resulting equation (3) is in quadratic form. For zero rise in water level the equation can be solved by two roots and these have been obtained from the solution of quadratic equations as 0.58 and 2.18 respectively. The two figures indicate the minimum and maximum amount of rainfall which will produce zero rise in water level. In other words it may be said that 0.58 m or 580 mm is the minimum rainfall below which no accretion to the water table takes place. It is the amount of rainfall just sufficient to saturate the overburden. Similarly 2.18 m or 2180 mm of rainfall will saturate the overburden as well as the aquifer itself and no more accretion to the water table can take place. The excess water will disappear in surface run off. When equation (3) is differentiated with respect to R and the resultant is equated to zero, the solution of the equation will give the maximum rise in water level and consequently the maximum recharge. In this present case the amount of annual rainfall which gives the maximum rise in the water level in the project area works out to be 1375 mm. CONCLUSIONS The net annual potential for development of the groundwater in the project area was calculated from formula (1). The annual increment in groundwater reserve was calculated from formula (3) taking into account the average normal rainfall of the area. The potential was calculated from this annual increment in reserve and the specific yield. The results for potential groundwater of the project area is given in Table 3. The annual potential groundwater in the project area has been estimated to be of the order of 2360 x 10^ m 3. The safe limit of extraction is of the order of 1570 x 10^ m keeping one third in reserve for drought situations. ACKNOWLEDGEMENTS The author is grateful to Dr B.N.Chanda, Professor of Fluid Mechanics and Hydraulic Engineering, Jadavpur University, and to Dr S.N.Banerjee of the Regional Computer Centre, Jadavpur University, for the kind-help they have given in the preparation of this paper. REFERENCES Ag. Ref. and Dev. Corpn. (A.R.D.C.) (1979), Report of the Groundwater Over-exploitation Committee, Bombay.

7 Rainfall-recharge correlation: groundwater resources 167 Bhattacharjee, A.P., Jindal, S.R. and Srivastava, B.P. (1953), Penetration of Rainwater to Groundwater table in Duab West of the Ganga River, U.P.I.R.I. Tech. Memo. No. 24. Chaturvedi, R.S. (1944), Investiation of Groundwater Resources in the Kali Nadia Nim Nadi Duab, U.P. Tech. Memo. No. 14. Gupta, B.L. (1968), Hydrogeological Studies in Muzaffarnagar and parts of Meerut Dist. U.P. (India), Ph. D. Thesis, Roorkee. Saxena, R.S. (1980), Assessment of Groundwater Recharge in irrigated areas of India - Present studies and scope of future research. Proc. Third Afro-Asian Regional Conf., New Delhi.

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