Trends and Sustainability of Groundwater in Highly Stressed Aquifers (Proc. of Symposium JS.2 at 57 the Joint IAHS & IAH Convention, Hyderabad, India, September 2009). IAHS Publ. 329, 2009. Characterization of water level response to rainfall in Narava Micro Watershed, Andhra Pradesh, India P. RAJENDRA PRASAD, N. V. B. S. S. PRASAD, N. L. K. REDDY, K. V. RAMAKRISHNA & D. NOOKA RAJU Department of Geophysics, Andhra University, Visakhapatnam-530 003, India rpatury@yahoo.com; ahivisakha@hotmail.com Abstract Groundwater response to rainfall and its intensity has been investigated, using groundwater fluctuations from five bore wells located in Narava basin. The Narava micro basin, extending over an area of 105 km 2 is covered with khondalitic suit of rocks and gneisses of Archaean age. Hourly time series data on rainfall and groundwater levels are correlated to derive the characteristic response of one over the other in time and space domains. The lag time responses are studied with specific reference to local hydrogeological conditions and regional recharge characteristics. A comparison between the well hydrograph and the associated hyetograph clearly reflects different response and lag times. The lag time, ranging from 5 to 10 days between the occurrence of rainfall events and the corresponding response in groundwater levels, is observed to have a close relationship with local hydrogeological conditions. The well hydrograph was correlated with the hyetograph at the same locations to quantify the response between rainfall and recharge. The lag time in recharge characteristics with reference to rainfall events are observed to be influenced by local hydrogeological conditions, as well as its proximity to recharge areas. A thick weathered kaolin zone is responsible for a large lag time due to its low permeability characteristics. However, further intensity of rainfall appears to have a direct relationship with raised water levels and, does not seem to have a one to one relationship with recharge response time. In general, it is observed that 10 mm of rainfall is a threshold value for triggering the groundwater recharge. Low rainfall of less than 10 mm is observed to contribute either to soil moisture or quick runoff. Sharp temperature variations in the groundwater, of around 0.05 to 0.1 C, are associated with characteristic time lags and are prominently connected to recharge phenomena. The characteristic variation in groundwater temperature vis-à-vis atmospheric temperature and the temperature of recharge water indicates that the change in temperature of groundwater is due to recharge water rather than the decreased atmospheric temperature. An attempt is made to establish a quantitative relationship between recharge, rainfall, temperature, pressure and recharge lag time characteristics in different geological terrains in a closed micro watershed. Key words water level fluctuations; response lag time; Narava micro watershed; hydrograph; hyetograph INTRODUCTION Water level fluctuation measurement in observation wells is an important aspect of groundwater studies. Water level fluctuations are mostly influenced by hydrological, hydrometeorological and hydrogeological phenomenon such as groundwater recharge, evapotranspiration and phreatophytic consumption, artificial recharge, groundwater pumpage, and return flows from irrigation. In many cases there may be more than one phenomena/process operating simultaneously. Under undisturbed natural conditions well hydrographs do not show any change in tendency with time because the recharge balances with the discharge. Aquifer response to recharge or discharge is reflected in water level fluctuations measured at different time periods. At any specific point the change in water level below ground surface depends not only on rates of pumping and recharge, but also on the intrinsic characteristics of the geological formations. Long and steady low rainfall on a loamy saturated soil with a highly permeable geological section and deep water table condition can result in a significant rise in the water table. Whereas, an intense rainfall event of shorter duration on a dry clayey soil with a shallow water table may not raise the water table a considerable amount. The rate of recharge and discharge can be ascertained from the hydrograph behaviour. The varied behaviour of hydrographs within the Narava micro watershed in India, reflects different geological formations. Copyright 2009 IAHS Press
58 P. Rajendra Prasad et al. LOCATION AND DRAINAGE Narava micro watershed covers an area of about of 105 km 2 and lies between 17 42 30 17 47'30 N latitude and 83 04 30 83 12 30 E longitude and is shown in Fig. 1. The area is covered in Survey of India toposheets 65O/1, O/2 and O/3. The Narava Gedda, an ephemeral river network, drains initially into the SW NE direction in the upper reaches and later turns to NW SE direction. The river network eventually merges into the Mehadhrigedda reservoir. This reservoir is one of the important drinking water storage reservoirs catering to the water needs of Visakhapatnam city. The Narava basin has attained semi maturity with a 4th order drainage network dominated by dendritic to sub-dendritic patterns. The drainage patterns of Narava are structurally controlled by lineaments. Fig. 1 Map of the study area showing drainage pattern. Rainfall The study area receives rainfall mainly during the southwest and northeast monsoons from June to November. To study the influence of rainfall on water level fluctuations, a regular collection of rainfall data is made at 10 representative locations in the Narava micro watershed (Fig. 2). The annual rainfall during the years 2007 and 2008 in the study area is observed to be 1197.7 mm and 715.5 mm, respectively, while the number of rainy days for the same years is 88 and 94, respectively.
Characterization of water level response to rainfall in Narava Micro Watershed, Andhra Pradesh, India 59 Fig. 2 Location map of rainfall observation stations. Geology The study area forms a part of the Eastern Ghat tectonic complex which is a major physiographic province and a principal Precambrian metamorphic unit of peninsular India. The prominent geological formation in the area belongs to Archean and Quaternary periods. The Archaean system includes mainly khondalites, Leptynites, Charnockites, Pegmatites and quartz veins mostly occurring as intrusive bodies. The Quaternary system includes laterites and surficial deposits.the geological sequence of the area is as follows: System Age Stratigraphic unit Quarternary Recent Surficial deposits (piedmont zones, colluvium red sediments and alluvium Sub recent Laterite /lateritic gravel Archaean Precambrian Quartz viens Pegmatites Granites Charnokites Leptynites khondalite Suit of rock METHODOLOGY A total of 96 observation wells were located in the Narava micro watershed for monitoring the groundwater levels in the unconfined aquifers of the region on a monthly basis. In addition, five automatic water level recorders were installed in the bore wells and continuous digital data on water levels were recorded at hourly intervals using level troll equipment. The water level fluctuations are plotted as hydrographs using the Winsitu 5.0 package. During the same period the continuous record of rainfall is also collected and hyetographs were prepared. The hydrographs and hyetographs are compared for correlation and interpretation. The lag times were calculated between the occurrence of rainfall events and the corresponding response in groundwater levels.
60 P. Rajendra Prasad et al. RESULTS AND DISCUSSIONS The well hydrographs of the study area directly reflect the aquifer response which is prominently due to recharge through rainfall. The rising limb of the hydrograph reflects the quick recharge character of the aquifer as well as the characteristic hydrogeological conditions and regional recharge characteristics (Chachadi & Choudri, 2004). The sharp rising limb of hydrographs indicates the existence of a permeable rock/soil formation in the upper unsaturated zone. Fig. 3 and aquifer response to rainfall in Granite Gneisses.
Characterization of water level response to rainfall in Narava Micro Watershed, Andhra Pradesh, India 61 The hydrographs and the hyetographs of the study area clearly indicate that the water table and the corresponding rainfall hyetograph follow each other (Figs 3 and 4). It is further observed that the water table rises in two different stages. The rise is steep in the first stage during the early rains of SW monsoon (June) and gentle thereafter up to the end of the NE monsoon (August November). However, occasional rains occurring in the months of January, February and March are found to have no appreciable effect on the water table. Fig. 4 and aquifer response to rainfall in khondalite formation.
62 P. Rajendra Prasad et al. It is also observed that the water level represents no or very little response to the rainfall of less than 10 mm, while the water level is observed to rise after a rainfall of 10 mm or more. In general, it is observed that 10 mm of rainfall is a threshold value for triggering the groundwater recharge. The minor deviations in fluctuations in the hydrograph are attributed to the local pumping. Figure 4 and clearly indicate lowering of water level even after rainfall. Though there were occasional rainfalls in the pre-monsoon period, no one-to-one relationship is observed between water level fluctuation and the rainfall, indicating heavy losses of the rainfall water. The water table rise is observed to be associated with a decrease in temperature, while pressure exhibits an inverse relation. The water table rise also exhibits an inverse relation with the temperature associated with a large lag time. The correlation analysis between the well hydrograph and related hyetograph in the study area reflects that the water levels in the phreatic aquifer respond prominently to monsoon rainfall recharge. The lag times were calculated from the well hydrograph and the corresponding rainfall hyetograph in different hydrogeological formations to understand its dynamic behaviour with time and space domains. Table 1 Hydrological heterogeneity of groundwater recharge. Sno Village Geological formation Infiltration capacity (cm/hr) Lag time in days Depth to water level before lag time (m) Depth to water level after lag time (m) Change in depth to water level (m) Rainfall (mm) period from 14-6-2007 to 24-6-2007 1. Erravani palem khondalite 12.11 10 4.037 0.686 3.351 245.2 2. Gollala palem khondalite 0.55 5 9.373 8.207 1.166 190.8 3. Narava Granite Gneisses 0.42 8 4.312 2.813 1.499 180.7 4. Chintagatla Agraharam Granite Gneisses 0.69 7 8.219 7.488 0.731 179.2 5. Paidawada Agraharam Granite Gneisses 1.384 8 5.934 3.58 2.354 205.6 The data on infiltration capacity, the derived parameters of the lag time, the gain in water level with response to rainfall and the total rainfall amount during the recharge period in different hydrogeological conditions are shown in Table 1. It can be clearly observed from Figs 3, 4 and Table 1 that hydrological heterogeneity, which reflects the varied behaviour of surface and atmospheric processes, plays a very significant role, irrespective of geological conditions. It can be seen that the water level in two bore wells located in Khondalitic formation responds differently to rainfall with different lag times, with a steady gain in the water table. On the other hand, three bore wells located in granite gneisses with comparable infiltration capacities and lag times, produce an entirely different gain in groundwater level. The schematic representation of the water level response and lithological data is shown in Fig.5 and. The lag time of 5 10 days between rainfall event and the shallowest groundwater level in the khondalite formation is shown in Fig. 5. Variations in lag time are attributed to the highly weathered kaolinised layer, which acts as an impermeable layer, even though the top soils have high infiltration capacity of 12.11 cm/h. The highly weathered kaolinised zone in both the formations result in 8 10 day lag times (Fig. 5 and ). Larger fluctuations in the water table are observed in the upstream of the basin.
Characterization of water level response to rainfall in Narava Micro Watershed, Andhra Pradesh, India 63 Fig. 5 Water level response to rainfall in the khondalite formation. Water level response to rainfall in granite gneisses.
64 P. Rajendra Prasad et al. Relation between the groundwater level and pressure Water level fluctuations in the aquifer system result in pressure changes. The pressure recorded from the observation wells reflects the recharge phenomena. Water level responses also can lag, because the drainage characteristics control the time necessary for the air to move through the unsaturated zone and affect the transfer of load to the water table (Weeks, 1979; Rojstaczer, 1988; USGS, 2005). The pressure is determined from the difference in pressure caused by water level (buoyancy) and the atmospheric pressure. Fig. 6 and relation between the water level and pressure in granite gneisses and khondalites, Narava Micro watershed. The data in Fig. 6 and show an inverse response between water level and pressure. The water table has an inverse relationship with the bore well pressure. It is observed that when the water level is at its lowest level in the pre-monsoon, the bore well pressure is also at its lowest. Groundwater temperature variation Groundwater temperature is higher than the atmospheric temperature during winter and lower during summer. Though the atmospheric temperature increases, fluctuations in the temperature of groundwater are subdued due to the insulating effect of the aeration zone (Karanth, 1987)
Characterization of water level response to rainfall in Narava Micro Watershed, Andhra Pradesh, India 65 rendering the variation in groundwater temperature to be very small. However, within the gneisses located in different parts of the basin the groundwater temperature is observed to vary marginally between 30.04 C to 30.62 C in contrast to a wider variation between 30.3 C to 32.08 C within khondalites in different locations of the basin. Fig. 7 and relation between the atmospheric temperature, groundwater temperature and depth to water level. Figure 7 and show the variation in the atmospheric temperature, groundwater temperature and depth to water level. The change in groundwater temperature associated with a rainfall event, though small, can be attributed to the infiltration characteristics and recharge phenomena. Further the groundwater temperature is not observed to be affected by atmospheric
66 P. Rajendra Prasad et al. temperature. The data on groundwater temperature, the change in groundwater temperature in response to a rainfall event and the total rainfall are presented in Table 2. It is observed that the groundwater temperatures are not influenced by the rainfall events. However, depending on the infiltration characteristics, proximity to recharge area appears to be influencing the groundwater level fluctuations and its temperature. For example, at Paidawada Agraharam the relatively steep decline of groundwater temperature as a sequence of the rainfall event is a cumulative effect of the nearby fracture and a small surface gravel layer which are responsible for the direct recharge of the well. Table 2 Change in the groundwater temperature at Narava micro watershed. Sno Village Name Groundwater temperature before rainfall C Groundwater Temperature After rainfall C Change in groundwater temperature C 1 Erravani palem 32.101 32.089 0.012 245.2 2 Gollalapalem 30.351 30.303 0.048 190.8 3 Narava 30.053 30.045 0.013 180.7 4 Chintagatla Agraharam 30.643 30.626 0.020 179.2 5 Paidawada Agraharam 30.538 30.234 0.304 205.6 Rainfall in mm period from 14-6-2007 to 24-6-2007 Fig. 8 and relation between the temperature of the groundwater and rainfall in Narava micro watershed.
Characterization of water level response to rainfall in Narava Micro Watershed, Andhra Pradesh, India 67 Similarly the relatively higher groundwater temperature in Erravanipalem is controlled by a very thick kaolin layer which is holding the groundwater limiting the freedom of its dynamics. Further, the groundwater temperature remains nearly constant in khondalites due to the presence of thick kaolin acting as an insulator for direct recharge. Figure 8 and represent the relationship between the rainfall and groundwater temperature in gneisses and khondalites, respectively. CONCLUSIONS Correlation and interpretation of the well hydrographs and the corresponding hyetographs in the study area has provided vital information relating to hydrogeological, hydrometeorological and recharge phenomena of the groundwater system. Rainfall events over 10 mm are observed to be critical in triggering measurable recharges in groundwater. Though the hydrographs reflect a oneto-one response to rainfall, varying lag times are observed in the recharge phenomena. Aquifer recharge response can be attributed more to the monsoon rainfall than to the total annual rainfall. Variations in recharge lag times are related to hydrogeological conditions consisting of weathered kaolinised zones and clay zones. Though marginal, variations in groundwater temperatures are observed to be more due to the recharge water temperature rather than the hydrogeological conditions. Acknowledgements The authors are thankful to the Department of Science & Technology (DST), NRDMS, New Delhi for financial support. REFERENCES Chachadi, A. G. & Choudri, B. S. (2004) Well hydrograph as tools for impact assessment of open cast mining on groundwater regime in Goa. J. Appl. Hydrol. 17(2 3). Karanth, K. R. (1987) Groundwater Assessment Development and Management. Tata-McGraw Hill Pub. Co. Ltd, New Delhi, India. Lee, L. J. E., Lawrence, D. S. L. & Price, M. (2006) Analysis of water level response to rainfall and implications for recharge pathways in the Chalk aquifer, SE England. J. Hydrol. 330, 604 620. Raghunath, H. M. (2007) Groundwater, third edition. New Age International (P) Ltd, New Delhi, India. Rojstaczer, S. (1988) Determination of fluid flow properties from the response of water levels in wells to atmospheric loading. Water Resour. Res. 24(11), 1927 1938. Sarma, V. V. J., Prasad, N. V. B. S. S. & Rajendra Prasad, P. (1983) A study of water level fluctuation and estimation of recharge and recession along the coastal strip of Visakhapatnam-Bhimilipatnam. J. Assoc. Exploration of Geophysics 4(1), 49 65. Todd, D. K. (2006) Groundwater Hydrology, second edition. Wiley India Pvt. Ltd, New Delhi, India. USGS report (2005) Analysis of ground-water levels and associated trends in Yucca Flat, Nevada Test Site, Nye County, Nevada, 1951 2003. Weeks, E. P. (1979) Barometric fluctuations in wells tapping deep unconfined aquifers. Water Resour. Res. 15(5), 1167 1176.