Quality of Irrigation Water in Sri Lanka Status and Trends

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1 Asian Journal of Water, Environment and Pollution, Vol. 1, No. 1 & 2, pp Quality of Irrigation Water in Sri Lanka Status and Trends E.I.L. Silva Institute of Fundamental Studies Hantana Rd, Kandy, Sri Lanka * sil@ifs.ac.lk Received April 12, 2004; revised and accepted June 1, 2004 Abstract: Already available and newly collected data sets on water chemistry of twenty reservoirs located in seven river basins in Sri Lanka were analyzed to determine the status and trends in irrigation related water quality characteristics. ph of irrigation water varied within a wide range but the upper limit did not exceed the proposed level suitable for irrigation water. Salinity was found beyond the threshold level (1.250 g l 1 ) only in two reservoirs located downstream of the Malwathu Oya basin and the electrical conductivity had never reached above 1 ms cm 1. Sodium absorption ratio (SAR) was well below the threshold value of 6 meq l 1 in the study reservoirs. Least square regression model showed that electrical conductivity was a good predictor of salinity and SAR. Time series data showed no marked changes in irrigation water quality of the reservoirs located in the Mahaweli river basin over the last forty three years with respect to EC levels but the EC of those proximate reservoirs receiving additional transfer water from the trunk stream of the Mahaweli river showed a progressive decrease following the trans basin diversion. Key words: Irrigation reservoirs, salinity, SAR, basin transfer, Sri Lanka. Introduction Water contains dissolved minerals in changeable concentrations and compositions in space and time. When water is used for irrigation, these mineral salts are left in the soil after the crop has used the water. Most of these mineral salts are beneficial to crop growth and soil conditioning, but in some cases they may be harmful due to their high concentration, development of sodic soils (slick spots) or accumulation of toxic trace elements such as boron. There are four basic criteria for evaluating irrigation water quality (Ayers & Westcot, 1989), namely total soluble salt content (salinity hazard), relative proportion of sodium ions to other cations (sodium hazard - soil permeability effects), total alkalinity as related to calcium plus magnesium concentration (alkalinity), and the concentration of elements that may be toxic (toxicity). The first two criteria are of major concern because water high in salinity and/or sodium causes most problems in many parts of the world with respect to crop yield. Salinity hazard is related to osmotic pressure of the soil-water interface resulting in a physiological drought condition while the main problem with sodic soils is its effect on soil permeability and water infiltration. Sodium also contributes directly to the total salinity of the water and may be toxic to sensitive crops such as fruit trees. The sodium hazard of irrigation water is estimated by the sodium absorption ratio (SAR). This is the proportion of sodium to calcium plus magnesium in the water. High total alkalinity in water essentially increases the sodium hazard of the water to a level greater than that indicated by the SAR. It also tends to precipitate calcium and magnesium as carbonates under dry weather. Some elements in irrigation water may be directly toxic to crops but establishing toxicity limits in water is complicated by reactions which take place once the water is applied to the soil. However, excessive levels of boron are nearly always associated with deep groundwater which also has a high salinity hazard.

2 6 E.I.L. Silva Salinity hazard is most often associated with arid or semi-arid regions of the world and problems are common in the rice producing regions. The poor drainage characteristics of the soils that allow them to be efficient for rice production also contribute to the problems associated with salinity. Salinity results from adding salt to soils, usually in irrigation water, faster than it is removed by natural processes, such as surface runoff and downward percolation. Irrigation water is the major contributor of soluble salts in many parts of the world but excessive nutrient additions from fertilizers, manures or waste materials may also contribute to the accumulation of salts. The types of soluble salts that usually contribute to salinity problems include calcium, magnesium, sodium, chloride, sulfate and nitrate. Hydraulic civilization in Sri Lanka dates back to the pre-christian era and at present irrigated agriculture accounts for around 96% of water withdrawal which is equivalent to 9.4 km 3 per annum. Although the importance of quality assessment and monitoring of irrigation water has been addressed in many instances (IIMI, 1995; Silva, 1996; Madduma Bandara, 2000; Shortt, 2001; SOE, 2001), the status and trends in irrigation water quality in Sri Lanka is poorly understood except for scanty information available since the early 1960s (US Mission, 1961; Amarasiri, 1973; TAMS, 1980; Gunawardena & Adhikari, 1981; Roonage et al., 1995 and Matsuno et al., 2000). Nevertheless, a fair number of data sets on water quality of dry zone irrigation reservoirs have been collected under limnological investigations (Amarasinghe et al., 1983; Schiemer, 1983; Silva & Davies, 1986 and 1987; Chandrasoma et al., 1986; Silva & Wijeyaratne, 1999; Silva & Gamlath, 2000; Nissanka, 2002; Silva et al., 2002). Further, National Water Supply and Drainage Board (NWS&DB) of Sri Lanka undertakes regular analyses of basic drinking water quality parameters in some irrigation reservoirs which are being used as a drinking water supply (e.g., Parakrama Samudraya, Giritale, Minneriya, Nuwerawewa, Tissawewa, Kantale, Uda Walawe and Chandrikawewa). Some of these water quality parameters available in these data sets (e.g., specific conductivity, alkalinity, major cations) can be used to characterize irrigation water quality. Therefore the objective of this paper is to highlight the status and trends in irrigation water quality in Sri Lanka using already available and newly collected data sets. Material and Methods Present study was confined to 20 irrigation reservoirs located in seven river basins in the dry zone of Sri Lanka covering six major rice producing districts namely Polonnaruwa, Anuradhapura, Trincomalee, Ratnapura, Moneragala and Hambantota (Figure 1). Already available information related to irrigation water quality of each reservoir were collected from the literature. Figure 1. Map of Sri Lanka showing major rivers. Several data sets were also obtained from NWS&DB where they have collected basic water quality parameters from irrigation reservoirs. In most cases, already available data sets were carefully examined for reliability and compatibility considering the methodologies used for data collection prior to integrating them for the assessment of irrigation water quality related parameters (e.g., Total Dissolved Salts, SAR) or trend analysis. Basic parameters considered in this study were electric conductivity, total alkalinity, ph, major cations (i.e. Na +, K ++, Ca ++ and Mg ++ ) and anions (i.e. Cl and SO 4 2 ). Whenever data is available, total dissolved salts (TDS) were calculated as a cumulative weight of total cations

3 Quality of Irrigation Water in Sri Lanka Status and Trends 7 and anions per unit volume which is also expressed as salinity. In addition to available data sets, new data sets on irrigation water quality were collected from eight major irrigation reservoirs located in the Mahaweli River (Parakrama Samudra and Minneriya reservoirs), Kala Oya (Kalawewa and Rajangana tank), Malwathu Oya (Nachchaduwa tank and Tissawewa) and Walawe River (Udawalawe Reservoir and Chandrikawewa) basins during Irrigation water quality-related parameters such as EC, TDS, total alkalinity, dissolved cations and anions, and ph were determined using standard methods (APHA, 1989) during this survey. The study was not extended to determine the concentration of trace metals such as boron. Using the concentration of Na +, Ca 2+ and Mg 2+, SAR was calculated as an equivalent ratio of Na +, to the average value of the square root of Ca 2+ plus Mg 2+. Adjusted SAR was not calculated since the concentrations of bicarbonate and carbonate ions are low in Sri Lankan surface water. Interrelationships between independent and dependant variables were computed using least square linear regression model. conductivity and SAR under low water conditions [SAR = 3.54(EC) ; R 2 = 0.816] of study reservoirs. In these regression models EC is used as a predictor of Salinity and SAR values and salinity and SAR values were estimated for a given EC value. Results Table 1 shows the general characteristics of 20 reservoirs subjected to the present study. Except Samanalawewa, Udawalawe and Rantambe reservoirs, the others are primarily used for irrigation purposes in the dry zone of the country. Most of them are located in the dry zone but the headwaters of the feeding rivers are located in the uplands of the wet zone except in Malwathu Oya, Kantale Oya, Maduru Oya and Yan Oya basins (Figure 1). However, some of the rivers draining only the dry zone watersheds (Kala Oya, Kantale Oya, Maduru Oya) receive transfer water from the Mahaweli, largest river in the country whose headwaters are confined to the wet zone. Elevation of the reservoirs ranges from 57.3 m amsl to 460 m amsl but 57 % of the study reservoirs are located below 100 m amsl. Local watershed of the reservoirs varies between 9 km 2 and 1095 km 2 while reservoir area ranges from 182 ha to 6280 ha. Table 1 also shows a wide range of maximum depth and capacity of the reservoirs. Catchment area per unit reservoir area which indicates the potential material influx also varies between 3.6 and 548. Figure 2 shows significant relationships between electrical conductivity and salinity [Salinity = ln (EC) 0.531; R 2 = 0.879] and the electrical Figure 2. Relationship between electrical conductivity vs total Dissolved Salt (a) and electrical conductivity vs SAR (b). Irrigation related water quality characteristics for 20 reservoirs are given in Table 2. In the case of SAR, two values are given for each reservoir as SAR at high and low water levels. Table 2 also contains the concentration ranges of chloride and sulfate ions for each reservoir. Lower limits of the ph of reservoirs were around or less than 7.50 except in a few cases and there were instances in which slightly acidic values were reported. Upper limit of ph values hardly exceeded 8.50 but never reached more than 9.00 in the study reservoirs. The range of alkalinity of study reservoirs was also relatively narrow. The lower limit ranged from 30 mg l 1 to 121 mg l 1 while the upper limit varied between 38 mg l 1 and 388 mg l 1. A more or less similar trend was found with respect to specific electrical conductivity of the study reservoirs. The lower limit of EC ranged from 0.60 ms

4 8 E.I.L. Silva cm 1 to ms cm 1. However, the upper limit was less than ms cm 1 in seventeen of the twenty one study reservoirs (Table 2). Total dissolved salts or salinity of the study reservoirs varied from to g l 1 and 60 % of the study reservoirs had salinity less than g l 1. SAR values of the study reservoirs during high water level ranged from meq l 1 to meq l 1 while it varied between meq l 1 and meq l 1 under low water condition. Estimated SAR values were less than 1.00 meq l 1 in the study reservoirs under low and high water conditions except the reservoirs located in the Yan Oya, Malwathu Oya, and lower reaches of the Kala Oya basins. As indicated in Table 2 the concentration of chloride ions in study reservoirs was higher than sulfate ion in most cases. The concentration of chloride and sulfate ions were relatively high in reservoirs located in the Malwathu Oya, Yan Oya, and the lower reaches of the Kala Oya basin. Long Term and Seasonal Trends Long term trends were analyzed only for seven irrigation reservoirs of the ancient category (Parakrama Samudra, Minneriya, Kalawewa Rajangana, Nuwerawewa, Nachchaduwa and Hurulwewa) located at four river basins in the north central dry zone of the country (Figure 1). However, scattered time series data from early 1960s are available only for Minneriya tank and Parakrama Samudra. The upper levels of electrical conductivity values of seven irrigation reservoirs subjected to time series analysis are shown in Figure 3. Parakrama Samudra and Minneriya reservoirs fed by the Mahaweli river and its tributaries did not show any marked variation in electrical conductivity over the last forty three years. In contrast, a marked decrease in electrical conductivity is noticeable in the reservoirs which receive transfer water from the Mahaweli river since 1979 to 2003 (Figure 3). Figure 4 depicts the monthly variation of electrical conductivity in eight irrigation reservoirs located in four river basins (i.e. Mahaweli, Kala Oya, Malwathu Oya and Walawe). Annual ranges of electrical conductivity were relatively narrow in the reservoirs located in Mahaweli (Parakrama Samudra and Minneriya), Walawe (Udawalawe and Chandrikawewa) basins and in the Kalawewa which is located at the upper reaches of the Kala Oya basin. In contrast, the annual ranges of EC were relatively high in Rajangana tank and the two reservoirs located in the Malwathu Oya basin (Figure 4). In general, EC values were lowest during the northeast monsoon (Dec-Feb) and gradually increased towards the end of southwest monsoon (Aug-Sep) and decreased again with the onset of second inter-monsoonal rains in October and November. Electrical conductivity in Minneriya and Parakrama Samudra reservoirs ranged from approximately 100 ms cm 1 in high water levels (January-February) to 250 ms cm 1 during low water level (August-September). A more or less similar trend was observed in Kalawewa (Figure 2) but in the case of Rajangana tank, EC ranged around ms cm 1. The annual ranges of electrical conductivity in Nachchaduwa reservoir and Tissawewa, which are located in the Malwathu Oya basin, were also relatively higher compared to those reservoirs located in the Mahaweli basin, the upper reach of Kala Oya and the Walawe River basins. Although the trend in seasonal pattern was more or less similar, Udawalawe reservoir and Chandrikawewa showed a narrow range of electrical conductivity compared to the other reservoirs (Figure 4). Nevertheless, the upper limit of the electrical conductivity did not reach 1000 ms cm 1 in any of the study reservoirs. Figure 3. Long term changes of electrical conductivity in P Samudra, Minneriya, Kalawewa, Rajangana, Nachchaduwa, Nuwerawewa and Tissawewa. Figure 4. Seasonal changes of electrical conductivity in eight reservoirs located in four river basins.

5 Quality of Irrigation Water in Sri Lanka Status and Trends 9 Table 1: General characteristics of 20 reservoirs studied Name Basin YOC/R FSL (m) CA (km 2 ) RA (ha) D max (m) V (MCM) CA:RA Samanalawewa Walawe 1992 n Udawalawe Walawe 1968 n Chandrikawewa Walawe 1963 n Maduru Oya Maduru Oya 1982 n Rantambe Mahaweli 1992 n Ulhitiya/Rathkinda Mahaweli 1968 n Bowatenna Mahaweli 1963 n Nalanda Mahaweli 1982 n Giritale Mahaweli 1905 a Minneriya Mahaweli 1906 a Kaudulla Mahaweli 1958 a P Samudra Mahaweli 1952 a Kantale Kantale Oya 1869 a Huruluwewa Yan Oya 1953 a Nachchaduwa Malwathu Oya 1906 a Nuwerawewa Malwathu Oya 1890 a Tissawewa Malwathu Oya 1889 a Kandalama Kala Oya 1957 n Kalawewa Kala Oya 1887 a Rajangana Kala Oya 1951 a (n and a denote newly built and ancient respectively; YOC/R, year of construction or restoration; FSL, Full Supply Level; CA, Catchment Area; RA, Reservoir Area; D max, Maximum Depth and V, Volume) Table 2: Irrigation-related water quality characteristics of 20 reservoirs studied Reservoir ph EC ms Salinity SAR H SAR L Cl mg l 1 SO 4 mg l 1 Samanalawewa Udawalawe Chandrikawewa Maduru Oya Rantambe Ulhitiya Bowatenna Nalanda Giritale Minneriya Kaudulla P Samudra Kantale Huruluwewa Nachchaduwa Nuwerawewa Tissawewa Kandalama Kalawewa Rajangana (L and H denote SAR value under high and low water levels).

6 10 E.I.L. Silva Discussion The watershed geochemistry has been shown to be the most important determinant of chemical composition and concentration of surface water (Douglas, 1968; Webb & Walling, 1974). Gibbs (1970) coined rock-dominance, precipitation and evaporation-crystallization processes to explain the chemical composition of epi-continental surface waters. Altitude can also influence the water chemistry due to variations in water balance (Vitousek, 1977). Nothing is definitive, spatial and temporal variations in water chemistry conditioned by the overall framework within which the hydrogeochemical system functions but, within a particular climatic region, rainfall, watershed characteristics, vegetation and soil become more significant (Gower, 1980). With respect to irrigation-related water quality of the study reservoirs, only Nachchduwa and Nuwerawewa have exceeded the threshold value of electrical conductivity (0.750 ms cm 1 ) and salinity (0.500 g l 1 ). Of the twenty study reservoirs, a majority has exceeded the upper limit of threshold value of ph (8.50) proposed for water for irrigation but it varied within the limiting range (ph ) stipulated by the International Standards. Estimated values of SAR were less than the threshold concentration (6.0 meq l 1 ) in all irrigation reservoirs. The first study on irrigation water quality conducted in 1965 by Amarasiri (1973) demonstrated the suitability of EC, TDS and SAR values of fifteen irrigation reservoirs in the North Central dry zone of the country. The waters of that study had SAR values ranging from 0.4 meq l 1 at Minneriya to 2.1 meq l 1 at Giant s tank while mean EC values during the dry season in Pavatkulam, Giant s tank, Maha Wilachchiya Nuwerawewa and Kalawewa were 0.707, 0.908, 0.604, and ms cm 1 respectively. Further, studies conducted in Kalawewa and associated irrigation outflows in by Gunawardena and Adhikari (1981) conclude that SAR was less than 2.0 meq l 1. The results of this study shows SAR values of 1.02 meq l 1 and 2.25 meq l 1 for Kalawewa water under high and low water conditions respectively. Threshold values at which the irrigator might become concerned about the water quality might consider supply of additional water for leaching. Below these values, water should be satisfactory for almost all crops and almost any arable soils. In general, surface waters in Sri Lanka are rich in Na + and the concentrations of Ca 2+ and Mg 2+ are more or less similar but K + has the lowest concentration (Dissanayake & Weerasooriya, 1982). Even though the HCO 3 concentration is higher compared to Cl and SO 4 2, a substantial concentrations of Cl are present in some waters (Silva & Gamlath, 2000). Ionic composition and ratios in lowland dry zone rivers and reservoirs are also more or less similar to that of the highland/midland rivers and reservoirs but the concentrations are relatively higher. The variation of Ca 2+ with Mg 2+ and Na + with K + along the rivers shows a marked similarity. The concentrations of Na +, K + and HCO 3 tended to increase in the direction of the flow of the river with the increasing stream order. In the case of the Mahaweli River, the largest and the longest river which is allogenic and traverses the north east dry zone of the country, it is seen that the headwaters of the river have the calcium type of water and gradually changes to the non-dominant type before changing to the sodium-potassium type, closer to sea mouth (Dissanayake & Weerasooriya, 1982). Available information on salinity or total dissolved salts and estimated SAR reveals that extreme salinization or sodium hazard of these reservoirs are very unlikely although salinity of some reservoirs located in the proximity of Malwathu Oya (Nuwerawewa and Nachchaduwa) and Kala Oya (e.g., Rajangana) was relatively high and exceeded the threshold value. Relatively low SAR values of irrigation water in Sri Lanka may be attributed to low concentration of sodium ion. In contrast, salinity of the Kantale tank located at the lowest elevation was relatively low. Low salinity of and corresponding low SAR values in the Kalawewa and Kantale tank can be attributed to dilution resulting from transfer of Mahaweli water via Bowetenna. Although the reservoirs located in the Malwathu Oya basins receive transfer water from Mahaweli trunk stream, the volume of water reaching those water bodies may not be sufficient to reduce the salt content to the suitable range. Rajangana tank does not receive transfer water of Mahaweli except the irrigation drainage. It is evident that the electrical conductivity, a predictor of TDS/ salinity or SAR has decreased since the trans basin diversion of Mahaweli River water into north central dry zone under the first phase of the Mahaweli Development Project implemented during mid 1970s. The threshold value for chloride (100 mg l 1 ) for irrigation water has exceeded in Nuwerawewa (Malwathu Oya basin) but it is within the limiting range according to international standards. Oceanic spray is reported as a major source of chloride ions in island countries and the rain water in Sri Lanka is also rich in chloride ions (Silva, 1998). Concentrations of sulfate have never reached the threshold value of 200 mg l 1 in irrigation reservoirs

7 Quality of Irrigation Water in Sri Lanka Status and Trends 11 according to the available data. No data is available on the concentration of arsenic, boron and copper for irrigation reservoirs in Sri Lanka, which are important water quality variables with respect to irrigation use and crop yield. Time series data related to irrigation water quality is available, although on a regular basis, for only a limited number of parameters for a few reservoirs in Sri Lanka. There is no systematic national water quality monitoring programme in the country, and reliable data for many of the study reservoirs and tanks are either lacking or fragmentary. Available long-term data on electrical conductivity of Parakrama Samudra and Minneriya reservoirs indicate no significant increase in total dissolved salts in these reservoirs over a forty three-year period and they were well below the threshold value. The total dissolved salt contents of those proximate reservoirs receiving transfer water with low electrical conductivity have decreased noticeably over the last decades. The volume of transfer water may not be sufficient to dilute the TDS levels in the reservoirs experiencing high evaporation in the downstream. Seasonal changes in irrigation-related water quality parameters are strongly linked with the annual rainfall pattern of the country. Almost all lowland water bodies are filled up to their full supply level with the onset of Northeast monsoonal rain (Nov.-Jan.). Rainwater contains a very small amount of measurable total dissolved salts and acidic in most cases (Silva & Manuweera, ibid). Therefore the dissolved salt concentration and related parameters in these reservoirs declined down to the lowest level at the end of the rainy season. TDS starts to increase with the release of water to the command area for Yala crop at the end of April. As a result of high evaporation loss and low volume available in the reservoir, TDS reach its maximum during August-September (Silva & Gamlath, 2000). Seasonal pattern in TDS or EC changes in the reservoirs in the Walawe river basin was quite different from those reservoirs in the North Central dry zone. This is mainly due to the bi-model rainfall pattern that is experienced by the reservoirs located in the Walawe basin which receives rainwater during peak southwest and northeast monsoons. Salinity problems have been reported in number of irrigation areas in the dry zone of Sri Lanka (Roonage et al., 1995; Matsuno et al., 2000) but not addressed correctly with appropriate determinants. Sri Lanka, primarily, is a rice producing country. Certainly highyielding varieties do not perform well under salt stress condition. The occurrence of different topographical and hydrological conditions is not in an extreme condition in Sri Lanka when compared with that in other countries in the region with vast arid areas (Pakistan and India). Even though less attention is paid at local level, farmers have often faced significant reduction in rice production in the semi arid areas of the country perhaps owing to the salinity problem (Matsuno et al., 2000). It is possible that the extremely high salinity or dissolved salt concentration could occur in the water logged areas of the irrigation command as a result of high evaporation losses. Therefore it is necessary to examine irrigation water quality characteristics in detail in the areas such as downstream of Kirindi Oya and Malwathu Oya basins. It may also require paying greater attention during the dry season drainage of the low-lying lands to prevent a build up of salinity (Amarasiri, 1973). This high salinity levels in the irrigation fields can be mediated by proper irrigation management practices which is beyond the scope of this report. Rhoades (1990), however, argues that numerous schemes for classification of water for irrigation use are essentially empirical and have some problems. For example, the substantial experience in using brackish water for irrigation shows that existing classifications (Ayers & Westcot, 1989) have severe restrictions. Brackish water is being successfully used in numerous places throughout the world under widely varying conditions of soil, climate, irrigation techniques and cropping systems. In Sri Lanka, if hydrological and water quality data related to irrigation are available adequately, models can be derived to predict status and trends. Acknowledgements This research was funded by the Institute of Fundamental Studies, Sri Lanka. M/s Namal Athukorale, Waruna Bandara, Sarath Dharmaratne, Charukshi Karunarathne and Fathima Farveen helped in field sampling and laboratory analysis. Iranganie Thumpela helped in preparation of the manuscript and computer graphics. Prof. U.S. Amarasinghe and Dr. Shirani Nathaneal made constructive comments on the manuscript. References Amarasinghe, U.S., Costa, H.H. and M.J.S. Wijeyaratne (1983). Limnological and fish production potential of some reservoirs in Anuradhapura District. Sri Lanka Journal of Inland fisheries, 2:

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