Drying Up of Groundwater Wells and Sustainable Development Options for Preservation of Groundwater in Sri Lanka

Transcription

1 Drying Up of Groundwater Wells and Sustainable Development Options for Preservation of Groundwater in Sri Lanka A.C.Dahanayake 1 and R.L.H.L.Rajapakse 1 1 Department of Civil Engineering, University of Moratuwa, Sri Lanka. E -mail: amalidahanayake@yahoo.com Abstract: Groundwater has been one of the most indiscriminately used natural resources in Sri Lanka and it has led to the occurrence of declining groundwater tables and drying up of wells in both wet and dry zones of the country. It is necessary to study the variation of groundwater storage with time to identify the trends of well drying up scenario. Due to the absence of long term groundwater monitoring, management, and usage data, a precise analysis is not feasible and, a general water balance model was formulated in the study, to carry out a quantitative analysis of groundwater storage and identify the interaction with land use pattern and impending climate change impacts. This model was compiled based on the water balance equation as the governing equation and using gathered and simulated usage and recharge data of the surface and groundwater basins, focusing on Dunamale catchment of the Attanagalu Oya watershed in Gampaha district. In this research, the variation of groundwater storage in response to several parameters such as population growth, deforestation, reduced precipitation due to climate change, and with the effect of rain water harvesting was studied. It was found that the groundwater storage depletes with the increasing population, reduced precipitation and decreasing forest coverage. It was also found that the adverse effects of groundwater depletion can partly be minimized by implementing rain water harvesting. Remedial measures to overcome the problem and sustainable methods to preserve groundwater for the future generation are proposed based on the findings of the study. Keywords: groundwater, well drying up, scenario analysis, water balance model. 1. INTRODUCTION Groundwater, being the most widely used source of obtaining water in Sri Lanka (Hettiarachchi, 2008), has now become a limited resource due to the adverse impacts of various natural and anthropogenic causes. Due to the increasing population and rapid urbanization, the demand for groundwater has been increasing drastically. Therefore, it can be noted that the number of shallow and deep wells extracting groundwater has increased during the last few decades (Panabokke and Perea, 2005). In consequence of unplanned and excessive use of groundwater resources, and due to the absence of means to regulate the usage of groundwater, drying up of wells have occurred (Endersbee, 2005). In Sri Lanka, since most of the groundwater wells were constructed neglecting the appropriate technical norms, drying up of those have been experienced very often, along with a lowering of the groundwater tables in the respective areas (Jayakody, 2006). The recent studies show that, since groundwater is a limited resource, sustainable methods of extracting this invaluable resource with proper utilization control and future development plans must be adopted (International Water Management Institute, 2005). 2. METHODOLOGY A general water balance model was initially developed based on rainfall data, evaporation data and land use maps, for the Attanagalu Oya catchment area by using governing equations (Jayarathna, 2013). Surface water balance model has been linked with the groundwater storage to find out the correlation of the fluctuations in groundwater storage with the land use pattern and climate change. Since the developed water balance model had already been calibrated and verified using the available stream flow data at Dunamale station, the same water balance model was used for conducting the scenario analysis in this research. Subsequently, the variation of the groundwater storage with the population growth, deforestation, reduced precipitation due to climate change, and with the effect of rain water harvesting were studied. 109

2 2.1. Study Area Surface area of the total catchment of Attanagalu Oya is nearly 736 km 2. (Perera and Samarakoon, 2008). The entire catchment was divided into five (5) sub basins in accordance with the terrain and distribution of stream paths. Dunamale catchment was situated entirely within sub basin 3 (refer Figure 1) and has a total area of km 2 (Jayarathna, 2013). Figure 1. Location of Attanagalu Oya watershed and delineation of sub basins 2.2. Water Balance Model It was conceptualized that, flow through the catchment is similar to the reservoir routine concept, so the concept of reservoir routine was used to develop the computerized model. The dynamic water balance of a river basin follows the fundamental equation of hydrology; I-Q= ds dt (1) where I and Q denotes the inflow and outflow, respectively. The term ds/dt is the rate of change of all water stored within the basin (Ponrajah, 1984). Basic inflow components for the surface water storage are; direct runoff from rainfall and recharge from the groundwater storage. Outflow components are; overflow from the imaginary reservoir, surface water usage, percolation and evaporation from the reservoir. By adding all these components following water balance equation was expanded. S (t+1) Final storage (m 3 ) S (t) Initial storage I 1 (t) Inflow from the direct runoff I 2 (t) Surface water recharge Q (t) Outflow WS (t) Surface water usage P (t) Percolation S(t+1) = S(t) + I 1 (t) + I 2 (t) Q(t) WS(t) P(t) E(t) (2) For t = 1, 2, 3 n 110

3 E (t) Evaporation Evaporation Direct runoff (Q= C 1 IA) Surface water basin (Q ) Water usage Out Flow (Q=C 3 Lh 3/2 ) Seepage (Q = Q ) Recharge (Q= C 2 Q ) Direct Infiltration (Q= (1- C 1 ) IA) Groundwater basin (Q ) Groundwater usage Figure 2 Water balance model Conceptual Water Tank A hypothetical water tank (Sugawara, n.d) was assumed at the end of the catchment area considered. Flow rates were calculated as the spill discharge from the imaginary tank by using the weir equation (Ponrajah, 1984). Q = CLH 3/2 (3) Q C L H Spill discharge Weir coefficient Spill length Flux height Direct Runoff Thiessen method was used to calculate the relevant daily rainfall for sub basins. Part of this precipitation flowed into the reservoir as direct runoff and the remainder was infiltrated directly to the groundwater storage. Direct runoff generation due to the rainfall in the catchment is the main input to the surface water basin and was obtained by estimating the volumetric accumulation using a variant of the rational equation, as follows. Q = CIA (4) The runoff coefficient was calculated by taking the area weighted average of the runoff coefficient for each land use and it was assumed to be a constant throughout the catchment area considered (Nandalal, 2010) Recharge from the Groundwater Storage A component of the total precipitation was directly infiltrated to the groundwater and after few days a fraction of that water is added again to the surface water. In this model it was assumed that a fraction of direct precipitation is added to the surface water with 2 day time gap Direct Infiltration A part of the precipitation was infiltrated directly to the groundwater storage. It was calculated as the remainder from precipitation after the runoff. 111

4 Direct infiltration = (1 - C) IA (5) Surface and Groundwater Usage Total amount of water supplied from the eight intakes and springs in Attanagalu Oya basin were obtained from literature. By considering the population growth, total water demand was calculated for all sub basins for both surface and groundwater Evaporation from the Surface of the Conceptual Water Tank The evaporation losses were estimated based on the surface area of the imaginary water tank. Daily pan evaporation data had been obtained from The Meteorology Department, prior to this research Seepage from the Conceptual Tank The monthly seepage loss was assumed to be 0.5% of the volume of water stored in the reservoir (Ponrajah, 1984) Calibration and Verification of the Model Values of the mean squared error, root mean squared error, correlation coefficient and Nash Sutcliffe model efficiency coefficient were used for the calibration and validation of the developed model. Runoff coefficient (C_1), dimensions of the conceptual reservoir, weir release coefficient (C_3) and surface water recharge coefficient (C_2) can be adjusted such that the error was minimized. (Jayarathna, 2013) Scenario Analysis from the Water Balance Model Runoff coefficient is a function of the land use pattern. The variation of the groundwater storage with the land use pattern can be studied by the water balance model. For further simplification, it has been assumed that the area consists only of forest area and urban area. An estimation of the forest and urban area was found by using Google maps and the GIS maps developed using ArcMAP (version 10.1). The data from year 2005 to 2010 were used for the condition model. It was found that approximately the forest cover is 35% for this catchment ( condition). Therefore, the urban area was presumed to be 65%. The runoff coefficient (C) values for forest and urban areas are 0.2 and 0.55, respectively (Chow, Maidment and Mays, 1988). Then the area weighted average runoff coefficient was calculated. The average C value was found to be 0.43 for the condition. It was considered 20 years to the past (year 1985 to 1990) for the past condition model and 20 years to the future (year 2025 to 2030) for the future condition model. The forest cover in the past and future were obtained by considering the annual deforestation rate. The annual deforestation rate of Sri Lanka is 3% (Newman and Ranasinghe, n.d.). This rate was for the entire island and it was based on past data. Hence, for the Dunamale catchment area, which is situated in the wet zone of the country, the deforestation rate was conservatively taken as 1%. Then the forest area and urban area were calculated accordingly, and then the area weighted average C value for the past and future were found to be 0.4 and 0.45, respectively. The surface water usage and the groundwater usage values were obtained from literature (Jayarathna, 2013). The mean value of population growth was obtained and the water usage values were changed according to the population growth. The mean value of population growth rate between 1985 and 2010 is 0.9% (Anon., n.d.) and, between 2005 to 2030 (data is available from 2005 to 2013 only), it is 1.015% (Anon., n.d.). It is shown that there s a possibility of a 9% reduction of the precipitation values in Sri Lanka in the 112

5 future (De Silva, et.al., 2007). Therefore, in the study, the precipitation values were reduced by 5% and those values were applied in the future condition model, to study about the impacts of the impending climate change on groundwater. Consequently, the effect of rain water harvesting was studied using the future condition model. It was estimated that the roof area is 2% from the urban area, and 70% of the collected rainwater was conservatively assumed to contribute to the groundwater storage. Furthermore, to study the effect of rain water harvesting in the worst case scenario, the above described modification has been introduced to the future condition model with the reduced precipitation values. Then the water balance models were analysed and the average value of final storage was obtained for each scenario. 3. RESULTS AND DISCUSSION 3.1. Variation of Groundwater Storage with Time and Precipitation Groundwater storage showed a similar variation with time, in all scenarios. It showed a decreasing trend with time. It consisted of a fluctuation with peaks and troughs, with the peaks of groundwater storage corresponding to the peaks of precipitation values (Figure 3). Final Groundwater Storage (m3) Millions /14/2004 5/28/ /10/2006 2/22/2008 7/6/ /18/2010 Date Final Groundwater Storage(m3) Precipitation (mm) Figure 3 Final groundwater storage (m 3 ) ( condition) The groundwater storage decreases due to; the reduced amount of precipitation received, the amount of extraction from the wells, as well as due to the other losses in the groundwater storage. The increase in the groundwater storage in some periods of the year can be explained similarly Precipitation (mm) 3.2. Comparison of Groundwater Storage in Different Scenarios Graphical Comparison of Groundwater Storage in Different Scenarios Figure 4 illustrates the variation of groundwater storage in different scenarios that have been analyzed. 113

6 FINAL GROUNDWATER STORAGE (M3) MILLIONS DATE past future reduced precipitation in future RWH in future normal precipitation values RWH in future reduced precipitation values Figure 4 Graphical comparison of groundwater storage in different scenarios Comparison of Average Values of Final Storage in Different Scenarios Table 1 shows the numerical comparison between the average values of final storage and figure 5 illustrates a graphical comparison of the same. Table 1 Numerical comparison between the average values of final storage Comparison between Percentage difference (value) (%) Increase or decrease with respect to the base value considered Present AND past decrease with respect to past Present AND future decrease with respect to (Reduced precipitation in future condition) AND (RWH in future normal precipitation) AND (RWH in future normal precipitation) AND (future normal precipitation) (RWH in future reduced precipitation) AND (future reduced precipitation) decrease with respect to decrease with respect to increase with respect to future normal precipitation increase with respect to future reduced precipitation 114

7 Present wrt Past Future wrt Present (Reduced precipitation in future condition) wrt Present (RWH in future normal precipitation) wrt (future normal precipitation) (RWH in future reduced precipitation) wrt (future reduced precipitation) Figure 5 Graphical comparison between the average values of final storage At, the groundwater storage has been reduced by 36.05% with respect to the situation 20 years ago. If this rate of deforestation continues, a 58% reduction in the groundwater storage in another 20 years of time can be expected. There can be many reasons for this decreasing trend that has been observed. Due to the increasing population with time, the water usage values will also increase. More dug wells will be used in order to satisfy the water needs. The extraction rates from the wells will also increase, thus causing a decrease in the groundwater storage with time. In addition to that, continuation of the removal of the forest cover and transferring them into urban areas, in order to facilitate and cater for the increasing demand in human needs would be inevitable. Decrease in the forest area and increase in the urban area will result in an increase in the value of the runoff coefficient. In fact, it was found that the runoff coefficient value related to the forest area as 0.2 and for urban area as Therefore, increase in the urban area has a major impact on the increase of the runoff coefficient. This increase in the runoff coefficient has resulted in decreasing the average value of the groundwater storage with time. Changes in the climate patterns can also be expected in the future, due mostly to the impacts on the nature caused by human activities. As a result, a reduction in the precipitation in future can be expected. Reduction of rainfall will affect to the groundwater storage in a drastic amount. It was found that the reduction of 5% in precipitation has caused a huge decrease in the final storage, numerically, 59.56% with respect to the situation. This might be named as the worst case scenario of the cases that have been studied under this research, since this has the maximum amount of decrease with respect to the condition. The remedial measures that can be taken to reduce the reduction in the groundwater storage were investigated. Rain water harvesting has come up as one of the solutions for this problem. Therefore, the effect of rain water harvesting on the groundwater storage has been studied. It was assumed that, in the future, the roof area can be used to collect and store rain water. The rain water that has been collected can be used to cater the needs and the remaining or used rain water can be used to replenish the groundwater as well. It was assumed that 70% of the collected rainwater contributes to the replenishment of groundwater storage, since the horizontal flow of the water that has been added to the ground had to be taken into account. 115

8 It was found that an increase of 29.13% in the average value of final storage, can be obtained if we introduce rain water harvesting in the future condition model. Further, by the introduction of rain water harvesting, the drastic decrease in the final storage that was observed in the worst case scenario, can be transformed into an increase by 28.74%. This is an acceptable value because that is the condition with the reduced precipitation values as well as the reduced forest cover and increased water usage due to the increase in population in future. 4. CONCLUSIONS This research mainly focused on the studying of the effects of aquifer characteristics on the recent depletion of groundwater as observed in several aquifer basins in Sri Lanka. The better awareness of depletion-recharge characteristics and role of aquifer characteristics in governing well behaviour is expected to be extremely useful in proposing sustainable new methods of preserving invaluable groundwater resources for future generations. The different aquifer types in Sri Lanka, their main characteristics and the existing recharge patterns were studied. Also the places where the problem of drying up wells exist have also been investigated. Several possible reasons for the drying up of well scenario have been identified, concentrating on the areas where the majority of dried up wells have occurred. Catchment characteristics which are affecting to the groundwater recharge pattern of the basin should be recognized for further studies on dried up wells. In this study; runoff coefficient and surface water recharge coefficient were considered as two major catchment characteristics. In the absence of long term groundwater monitoring, management, and usage data, a general water balance model was formulated in order to identify the values of this characteristics and their relationship to the quantitative replenishment of groundwater. Then several scenario analyses have been conducted based on that water balance model, in order to find out the variation of groundwater storage with; the population growth, deforestation, reduced precipitation due to climate change, and with the effect of rain water harvesting. Finally, by considering all the above aspects, it can be concluded that it is necessary to take remedial actions to preserve the groundwater storage to our future generations. By implementing suitable measures to overcome the groundwater problems that have been encountered and managing the utilization of groundwater in a sustainable manner, we could expect satisfactory changes that would contribute to the preservation of this valuable resource in a more pragmatic way. REFERENCES Annon., n.d., Population growth (annual %) in Sri Lanka. [online] Available at: < [Accessed 25 December 2013]. Chow, V.T., Maidment, D.R., Mays, L.W., Applied Hydrology. New York: McGraw-Hill. De Silva, C.S., Weatherhead, E.K., Knox, J.W., Rodriguez-Diaz, J.A., Predicting the impacts of climate change-a case study of paddy irrigation water requirements in Sri Lanka. [online] Available at: < [Accessed 25 December 2013] Endersbee, L., A Voyage of Discovery: A History of Ideas About the Earth, with a New Understanding of the Global Resources of Water and Petroleum, and the Problems of Climate Change. Frankston: Lance Monash University Bookshop. Hettiarachchi, I., A review on groundwater management issues in the dry Zone of Sri Lanka. In: Third International Scientific Conference BALWOIS Ohrid, Republic of Macedonia, May International Water Management Institute (IWMI), Planning groundwater use for sustainable 116

9 rural development. [online] Colombo, Sri Lanka: International Water Management Institute (IWMI). Available at: < [Accessed 25 December 2013]. Jayakody, A.N., Large Diameter Shallow Agro-Wells - A National Asset or a Burden for the Nation?. The Journal of Agricultural Sciences, [online] Available at: < [Accessed 25 December 2013]. Jayarathna, K.A.R.N., Drying up of groundwater wells in the wet zone of Sri Lanka and effects of aquifer characteristics and recharge patterns. BSc Thesis, University of Moratuwa. Nandalal, K.D.W., Hydrology, CE 205 Engineering Hydrology. University of Peradeniya, unpublished. Newman, S.W., and Ranasinghe, D.M.S.H.K., n.d, Preparation of a Country Environmental Profile for Sri Lanka for European Union (EU) Sri Lanka economic cooperation. [online] Available at: < [Accessed 25 December 2013] Panabokke, C.R. and Perera, A.P.G.R.L., Groundwater Resource of Sri Lanka. Colombo: Water Resources Board Sri Lanka. Perera, A. P. G. R. L., and Samarakoon, A. S. M. N. B., Application of Remote Sensing and GIS Techniques in Groundwater Exploration in Attanagalu Oya Basin. In: Asian Association on Remote Sensing, 29th Asian Conference on Remote Sensing 2008 (ACRS 2008). Colombo, Sri Lanka, November Colombo: Asian Association on Remote Sensing. Ponrajah, A.J.P., Design of Irrigation Systems for Small Catchments. 2nd ed. Colombo: Irrigation Department of Sri Lanka. Sugawara, M., n.d. Tank Model. [online] Available at: < Model/TankModel.pdf> [Accessed 25 December 2013] 117