Quantitative estimation of groundwater potentiality and rainfall infiltration in a typical crystalline environment

Transcription

1 Proc. Indian Acad. Sci. (Earth Planet. Sci.), Vol. 92, Number I, March 1983, pp Printed in India Quantitative estimation of groundwater potentiality and rainfall infiltration in a typical crystalline environment B H BRIZ-K1SHORE Centre of Exploration Geophysics, Osmania University, Hyderabad , India. MS received 29 October 1981: revised 30 December 1982 Abstract. A sophisticated method of measuring water levels is adopted to determine the groundwater potentialities in a typical weathered and fractured environment by storage method. The frequency of the rainfall has shown a direct bearing on recharging thegroundwater system compared to the intensity of the rainfall. The Thiessen polygon method is used to determine the weighted average of the rainfall over the basin area to estimate the ground water infiltration rates. The long term average infiltration to the medium is established as 15% of the total rainfall. Keywords. Crystalline environment; fluctuation; specific yield; recharge; correlation coefficient; Thiessen polygon; infiltration rate, 1. Introduction Replenishable character of groundwater systems always warrants systematic assessment for.proper lalanning and development. Techniques of assessment of this resource are broadly classified into direct and indirect methods (Niyogi 1971). In the direct methods, recharge is calculated by determining the specific yield of the formation and the water level fluctuation. Indirect methods involve comprehensive study of water balance by quantitatively assessing various inputs to and outputs from the basin. Application of these techniques to crystalline environment is more difficult because of differential weathering, fracturing and absence of primary porosity. Several investigators (Radhakrishna 1971; Singhal 1971; Venkatanarayana 1972) attempted to make such estimates in these terrains by the above methods. Their findings show wide variations because of the experimental nature of the investigations spread over limited number of years and the inherent complexity of the environment. Hence long duration studies over a period of five years are aimed at in the representative crystalline environment near Hyderabad, the capital city of Andhra Pradesh. The selected Shadnagar basin extends to 437 sq km weathered and fractured down to a depth of 60 m bgl. Groundwater and surface water divides coincide in the basin and the outflow is seen only through a portion of the northern boundary (figure 1). Hence all the other sides are considered as impermeable boundaries. Twenty one borewells drilled by the Central Ground Water Board (CGWB 1975) are uniformly spread over the basin area. For sophisticated monitoring of water levels at different depths in weathered and fractured granitic system, piezometric nests are installed in 14 out of the 21 borewells. Each of these nests consists of two or three piezometric tubes installed at different depths depending on the nature of weathering and fracturing as identified during drilling. To avoid interference from one zone to another, tubes are 63

2 64 B H Briz-Kishore,~ i,o <- I=o '<" ],-, s WJ~/.~ N,'.. ~ - ~ ~ "- / ] :, ~ c ~'600 "\ L,"U, ~&>r~.=;''~~ o.,?, l I r;.,--....,.~ ~ ~ -J -J /,<-, 7_3k--,,6 1"$ k.-,01, ~, ~ ~ ' " ~ i 7 ~-'~C-C,-~l~ ''~\ / i / " ~ " T ~ - - "'~'-_ "f"#"<is ~] ~.,('~]'.-IAiSAlliMI..,6. ~ <~ ~_\~..~ ~ %\ :., Y ',,.I." ~.~... ~o.~> u,~f~ 5 ~ 7..~0,~,~...~.~, " 1-4 _ ""--:-\~ ~ ~-.o.,.~ k./~ '~ ] ',~ rl~_,~ ~ ~ ~ / / "--" " ~ o ~ ~-'l"/~',.s.~---.,' ~ ~'..,,-.,,~ IlWkT,-~ '; '" / - - V ~ -',. O -.~ " o.,,,.,.,,, ~ S~ "~.', / io- To~ogfoph~ Coniour (~n mel~e$ obovem.s.l) ~.." ~'" ~.~ Bosin Bouhdory ~F ~4 n gouge ol 0 I 20' Figuce 1. Drainage and topography of the basin along with wells and rain gauge location. Table!. Rainfall data at different stations in basin ( ) Location of Rainfall (cm) Station raingauge Average station ( ) cm Chippalapalli ! Maisaram / 120. I Nandigaon "/ Palmakul Parandla Shahbad ' Shadnagar Welijorla Annual average rainfall in the basin Average rainfall in the basin for six years ( ) = cm. isolated by cement seals and water levels could be independently measured from each of these tubes. Eight rain gauge stations (figure 1) distributed over different parts of the basin and adjacent to it are utilized for daily rainfall measurements. The total annual rainfall data at each rain gauge station during 1973 to 1978 are Shown in table 1 along with the station averages. The rainfall in the basin is generally low to moderate

3 Groundwater potential in crystalline environment 65 with an average of 90 cm. The various studies for the estimation of recharges and groundwater infiltration ant described in this paper. 2. W-ter level mexsurements The locations of the 14 piezometric nests and the depths at which they are installed are shown in figure 2. All the nests are continuously monitored at monthly intervals from January 1974 and the data up to December 1978 are used in the present studies. During the measurements water levels in all the piezometric tubes installed in any piezometric nest are nearly the same (Briz-Kishore and Bhimasankaram 1981) and consistent with each other during pre-monsoon and post-monsoon periods. This behaviour has continued over the entire period of study of five years and reveals that water levels in shallowweathered mantle and deep-seated fractures are the same and the aquifer is continu, ous. Hence the average water levels in all the piezometric tubes for each of the piezometric nests are considered. The pre-monsoon (max bgl) and post-monsoon (min bgi) water level data are presented in table 2 along with the fluctuation and reference level of the well site for the years 1974 to Correlation study between elevation and fluctuation Table 2 shows that the annual fluctuations in water levels in different wells range from 0.15 to 8.27 m bgl throughout the basin area, with an average annual water level fluctuation ranging from 2.23 to 5.34 m bgl. These fluctuations also seem to be accentuated whenever the drought periods are succeeded by high rainfall periods i.e and ~../ \ I~I /",%ToIIoQu d o "~.-~-* ~... j \, ) 37 Am lerpet KambLomenotonda IQ /: i Kottur ~ li~! I Uppugudotcm4o ' / ) I S Pendyal i% 32 4%../... / Anlorom 45 \ eocjom~oh od I~,4j~ hondroyonaludo 40 "~., ~. ~,~ ~...~. Mohammadoltguda 32 Enmulanora "~ / "~" ~74 0 -,. 4% ShodnoQor [", "~. hatonpo h 43,, ~ ~" LEGEND... Welijorio )' \ Z% Speccf,c Yietd % ~,/"% f'" " ~../"-~" ""J 2~ Pi xomete# nest wilt. settin 9 (Deptl~ in metres. ) 17~-" 3~.,, '~t'o' I,,' ~" i ' ' 'ei '~' Figure 2. Location and depth of piezometric installations.

4 1978 Pre. Post Diff , i !! o ~, o~ I q3 Table 2. Mean annual water level fluctuations ( ). Depth to water level in m bgl Location 1974 Reference Level Pre. Post Diff Pre. Post Diff Pre. Post Diff Pre. Post Diff. Ameerpet Antaram Bodampahad. Chandrayanaguda Chatanpalli Enmulnara Kothur Kambamenatanda Mohammadaliguda Pendyal Shadnagar Tailaguda Uppaguda Tanda Welijorla i i O I ! i !i Average fluctuations of the basin:

5 Groundwater potential in crystalline environment 67 Another interesting aspect is that low fluctuations are associated with relatively low lying areas and higher fluctuations are observed over higher topographic areas. The statistical correlation coefficient is calculated between elevations and fluctuations for years 1974 to High values of correlation coefficients ranging from 0.67 to 0.82 are noticed between these two variables for different years of observation. This direct correlation can be more clearly visualised from figure 3 where the fluctuation is represented graphically with the varying elevations of different wells for Studies on intensity and frequency of rainfall To study the effect of rainfall on water level fluctuation the average rainfall is compared with average fluctuation in the basin and presented in table 3(a). It is seen that the average fluctuation in the basin does not directly correspond with the intensity of rainfall. To find the probable explanation for this behaviour, the events of rainfall (number of rainy days) recorded yearwise in three representative locations of the basin area, viz. Welijorla, Nandigaon and Palmakul are presented in table 3(b). The effect of rainfall over the groundwater levels is seen to depend on frequency apart from intensity. In order to present more vividly the influence of the intensity of rainfall and tts frequency, the data in tables 3(a) and 3(b) are classified into two groups having nearly equal average fluctuations. In group I, although greater intensity of rainfall is recorded in 1976, the average level of fluctuation has become lesser since the effect is largely countermanded by much smaller frequency of rainfall in this year. A strikingly similar observation could be made from group II comparing the data for the years 1975 and ]E Z Z " 8 "?~ 17 1.o LEGEND Well Number s~o s~o s;o 6;0 6~o 6~o 6~o 6;0 ELEVATION IN I,,1 (AMSL) Figure 3. Correlation of elevation and fluctuation of different wells.

6 68 B 1t llriz-kishore Table 3(a). Comparison of average rainfall and fluctuations in different years Item/Year Group! Group! Average rainfall in the basin (cm) Average fluctuation in the basin (M) , Table 3(b). Comparison of events of rainfall (number of rainy days) in different years Group I Group I! Welijorla (upland area) Nandigaon (mid slope area) Palmakul (low lying area) Point and net recharges of the basin area 3.! Estimation of point recharges Specific yield values determined (CGWB 1975) at five representative places over the basin area are used to estimate point recharges. The location of these wells along with specific yield values is given in figure 2. The point recharge at all the well locations is calculated for the years 1974 to 1978 as the product of mean annual fluctuations and the specific yield factor as inferred from the nearest well site. As an example, the recharge contour map is prepared for the normal rainfall year 1977 and is presented in figure 4. The point recharge ranges from 0.01 to 0.2 m/year over the entire area of the basin depending on the location and rainfall during the year. 3.2 Estimation of net recharges To estimate the net recharge of groundwater system in different years the specific yield factors obtained at different well sites in the basin area are analyzed. It is observed that the estimated value of specific yield is around 4% at most of the wells in the basin area but a lesser value at two well sites, Pendyal and Tallaguda. In fact the Pendyal well site whose specific yield is observed to be 1% is very close to Uppugudatanda having specific yield of 4%. Even Tallaguda well site whose specific yield also is! % is situated at a low topographic elevation with potential aquifer condition. In addition, at both the well sites good yields of around 5 lps are noticed with a thick-weathered zone of 15 m. Hence, these two specific yield values may be due to purely local conditions and may not be considered as the representative of the entire basin area. To verify the already arrived specific yield values, a digital model with a computer program (Briz-Kishore and Avadhanulu 198 I) is utilized. These values are obtained through a process of adjustment while simulating short duration pumping tests over a period of 100 minutes and by obtaining a close match between the observed and

7 Groundwater potential in crystalline environment 69 rd,o' ~ 1'~' 78"12o' r 12~' f".,.,/"~.~'-r "" ~",,\ Iq o' v/ ~ /. ~-.-.-o.oj./--:j ~ a~, -..6~-"-..- k 17" 7~ '<.. ~ ~ I/'--~ro.,s~.f f /'' '" '"... b " 17 0 T.o.o~-::;,7,. =;7." :':.7, ' Z:::7.7"''" ".....i ~ ~el,o' 7~'I,5' ~'12o ' ~ 125 ' Figure 4. Recharge contour map of normal rainfall year (1977) Table 4. Computed specific yield values at representative well sites Village Computed value of specific yield Ameerpet 0.04 Chandrayanaguda Enmulanara 0.05 Kottur Mohammadaliguda 0.04 Uppuguda Tanda 0.04 Yeerakunta 0.04 computed drawdowns. The specific yield values thus computed at seven representative wells presented in table 4 show that the values are mostly restricted to the range of 0.04 indicating that the overall specific yield value is around 4%. This estimated value conforms to the value reported by Ground Water Overexploitation Committee of Agricultural Refinance Development Corporation (ARDC 1979) after systematic field investigations over different parts of the granitic terrains. Hence considering the 4% specific yield, mean annual fluctuation and area of the basin, the net recharges are calculated for the years 1974 to 1978 and are presented in table 5. It is seen that the annual recharge of the basin ranges from to MCM during normal rainfall years. 4. Rainfall infiltration 4.1 Weighted averages of rainfall To determine the percentage of the rainfall recharging into the system, the total rainfall and the total recharge are considered. The total rainfall over the entire basin

8 70 B H Briz-Kishore area is estimated by Thiessen polygon method from the rainfalls observed at different rain gauge stations. This method gives an allowance for irregular spacing among the raingauge stations by giving weightage to the rainfall in proportion to the areas represented by the gauges. The computations to determine the total volume of rainfall at each Thiessen polygon for the year 1974 are presented in table 6. It can be seen from the table that the volume of rainfall at different stations varied from to 8624 sq km era. Similarly, the total volume of rainfall for subsequent years was also estimated and presented in table Groundwater infiltration rates Using the estimated groundwater recharge in table 5, the percentages of the rainfall infiltration into the basin are calculated and presented in table 7, which shows that the annual infiltration rates vary from 11.4 to 19.8% over a period of five years ( ) and the average infiltration coefficient of the basin comes to about 15%. Table 5. Estimates of groundwater recharge in different years Year Average water Point recharge Net recharge level fluctua- in water column ( MCM ) tions in the when specific basin (M) yield is 4% Average Recharge (5 years)= MCM Area of the basin = sq km Table 6. Volume of rainfall by Thiessen method at different stations Rain gauge Thiessen Observed Rainfall station polygon rainfall volume (sq km ) (cm) (sq km cm ) Chippalapalli Maisaram Nandigaon Palmakul Parandla I Shahbad Shadnagar Welijorla Total volume of rainfall over the entire basin area = MCM

9 Groundwater potential in crystalline environment 71 Table 7. Rainfall infiltration rates during the years 1974 to 1978 Year Total rainfall Ground water Ground water by Thiessen recharge by infiltration Polygon method storage method rates (MCM) (MCM) (%) Average infiltration rate = % 5. Conclusions A direct relation is observed between elevation and fluctuation with high positive values of correlation coefficient. The frequency of rainfall is shown to have a direct bearing on the amount of recharge compared to the intensity of rainfall itself. Representative specific yield values for the entire basin area are computed using a mathematical model. Weighted averages of rainfall at different stations are also determined by Thiessen polygon method which is designed to account for the irregular spacings of the rain gauge stations. The average rainfall infiltration using this method is established at 15% of the total rainfall over the entire basin area. Acknowledgements The author acknowledges Sri B K Baweja for providing certain basic data and facilities for these studies. The author also wishes to thank Prof. V L S Bhimasankaram and Dr K V Raghava Rao for useful discussions. References ARDC 1979 Groundwater Over Exploitation Committee Report, Bombay Briz-Kishore B H and Avadhanulu R V S S 1981 Groundwater Briz-Kishore B H and Bhimasankaram V L S 1981 Groundwater CGWB 1975 Canadian-assisted groundwater project, Hydrogeology Technical Report, December Niyogi B N 1971 Groundwater potential in hard rock areas. Seminar Volume, GSI, Bangalore 45 Radhakrishna B P 1971 Groundwater potential in hard rock areas, Seminar Volume, GSI, Bangalore 27 Singhal B B S 1971 Groundwater potential in hard rock areas, Seminar Volume, GSI, Bangalore 42 Venkatanarayana B 1972 Problems in Groundwater development, Procs. of the Seminar, Directorate of Groundwater, Madras 36