Mahaweli Basin

THE MAHAWELI BASIN (SRI LANKA

    by
    Sena S. De Silva
    Department of Zoology
University of Ruhuna
     Matara
     Sri Lanka

ABSTRACT

The River (= Ganga) Mahaweli is the largest river in Sri Lanka and its basin covers approximately one-sixth of the island. The Mahaweli Ganga development was initiated in 1970 according to the Master Plan (1965–1969) of the United Nations Development Programme and the Food and Agriculture Organization, with a view to bringing about an increased area of the dry zone of the country under cultivation, to provide through the year irrigation facilities for existing land and to meet the increasing demand for hydroelectric power. In 1977 the plan was reviewed and in the light of the increasing cost of food importa and unemployment the Mahaweli Ganga Diversion and Development Programme was telescoped into a six-year period.

Although the Mahaweli basin accounts for 15–20% of the inland fish production in Sri Lanka, during the planning and execution of the project, particularly during the construction of the reservoirs,the development of the fisheries sector was not taken into consideration. This constribution discusses the effects of the Mahaweli Ganga development on the existing reservoir fishery and suggests ways for optimizing the fishery in reservoirs, villus (= flood lakes) and irrigation channels.

Figure 1. Major rivers and their basins in Sri Lanka.
1. INTRODUCTION
Sri Lanka is an island (6°–10° N; 80°–82° E) of approximately 63,000 km², Which is thought to have separated from the Indian mainland in the Micocene. A stretch of sea 32 km wide separates the two land masses now. The island has a coastline of 1,760 km and a continental shelf area of nearly 31,000 km² (De Silva and De Silva, in press). Nearly nine-tenths of the island's bedrock beneath the superficial deposits is composed of Pre-cambrian crystalline rocks, some of which are over 2 billion years old; over the rest of the area, chiefly along the north-west coastal belt sedimentry rocks of the Upper Gondwana (Jurassic) and Miocene age form the underlying bedrock (Cooray,1967).
Topographically, Sri Lanka may be divided into three peneplains, the first from 0–122 m, the second rising as a steep step with elevations from 364–762 m and the third rising as a further steep step from the second and having an elevation of 1,530–1,828 m. These peneplains are believed to be important with respect to the floral and fauna distribution on the island (Senanayake and Moyle, 1981).
Climatically, the island is characterized by the effects of the two monsoons - the Southwest and the Northeast and their corresponding intermonsoonal periods. Typically, the Southwest prevails from about May to September and the Northeast from October to March. On the basis of the rainfall pattern the island is divided broadly into two distinct zones, the wet and the dry zones, each covering approximately 25% and 75% of the island respectively, The dry zone receives on average below 1,900 mm of rainfall per annum and has only one rainy season from about October to March. More detailed classification based on the rainfall pattern has also been attempted (Domros, 1976). The mean annual precipitation in the dry zone is estimated to be around 70 x 109 m³ and that in the wet zone 39 x 109 m³ (Arumugam, 1969).
Historically, the major civilizations were almost always concentrated in the dry zone of the country. However, with the advent of foreign rule since 1505 and the consequent conquering of the whole island by the British, the settlement pattern and the agricultural traditions began to change. During foreign rule, emphasis began to be laid on cash crops, initially coffee and thereafter on tea, rubber and coconut, which are all wet zone crops. During the course of this century the population of Sri Lanka doubled from 6 million to 12 million in a span of 40 years (1931 to 1971), and now stands at 15.5 million. At the existing rate of growth of 1.8 the population of the island is projected (medium projection) to reach 22.7 million at the turn of the century, resulting in a mean density of 358 persons/km² (Jones and Selvaratnam, 1972).
The increase in the population has had obvious and glaring influences directly on the natural resources of the island as well as indirectly through the increased demand on amenities such as electricity, etc. The forest cover on the island has declined to 44.5% of the land area and the area under mono-cultural practices over the period 1871–1972 has increased from 435,000 ha to 1,710,000 ha, i.e. from 6.7% of the total land area to 26.4% (Domros, 1976).
Since independence in 1948, all governments have given top priority to food production, with the aim of being self-sufficient in rice, subsidiary crops, fish, sugar, etc. Consequently, apart from technological advances that have been achieved in bringing about an increased production, mostly through application of fertilizers, proper irrigation, selective breeding, etc., the area under cultivation of major crops has also been increasing, e.g. the area under rice-paddy increased from 370,000 ha to 586,000 ha during the period 1946 to 1972 (Domros, 1976).
Bringing an increased area under the plough has necessitated the provision of proper irrigational facilities. This has been achieved in two ways: restoring a number of ancient irrigation works (Brohier, 1934, 1935) since the second quarter of this century, and venturing on a number of new river basin development schemes since 1948. River basin development is not something alien to the people of Sri Lanka. The hydrological engineering feats of the ancient times remain a part and parcel of the life of the rural masses of Sri Lanka. The greatest era in the history of modern civilization of the island was during the reign of Parakrama Bahu I (1153–1186). It is recorded in the Mahavansa the Great chronicle of Sri Lanka (Geiger, 1960, in Arumugam 1969) that he was responsible for the construction, restoration and enlargement of 163 major reservoirs, 2,376 minor reservoirs, 165 anicuts and 3,910 channels from reservoirs and anicuts; the achievements of a ruler who believed that not even a drop of rain water should go to the sea, without benefitting man. The building of large reservoirs by damming of rivers and large streams resulted in the invention of a sophisticated sluice for the control of the outflow and the original valve-pit designed by the ancient Sinhala engineers for this purpose has remained almost unchanged (Brohier, 1934, 1935; Schnitter, 1967).
The backbone of the existing irrigational facilities constitutes over 10,000 man-made lakes (Fernando,1971) with an estimated water surface area of 130,000 ha (Fernando and De Silva, 1984; Mendis, 1977) or approximately 2.1 ha of water surface for every km² of Sri Lanka.
The increasing demand for agricultural produce and hydropower for the projected increase in the population, cannot be satisfactorily met with the existing irrigational and hydroelectric facilities. It is in this context that further major development of a river basin - the Mahaweli - has been planned and undertaken since 1970.
2. RIVERS - GENERAL CONSIDERATIONS
Sri Lanka has 103 perennial rivers, locally known as either Ganga, Aru, or Oya. Of these, only 23 river basins are larger than 500 km² and 80 of them are located in the dry zone of the country (Fig.1), covering approximately 73.2% of the island. Of the dry zone basins, the Mahaweli accounts for nearly 23%.
2.1 Run-off
The total river run-off from the dry zone and wet zone basins is 25.5×10.9×910.9 m³ and 25.8×109 m³ respectively. The run-off during the two main seasons for those individual basins over 500 km² and the others, grouped together, for the two zones is given in Table 1. The run-off from the rivers vary widely due to the pattern of rainfall, type of soil, slope, etc. Generally, rivers in the dry zone have a poor run-off, varying from 10% to 50% of the total precipitation. The annual run-off in the two climatic zones in relation to the precipitation is given in Table 2.
2.2.Floodplains
In any sort of river basin development the nature of the floodplain needs to be considered in detail. The floodplains are important from the point of view of fish production and from an important habitat, mostly temporary, for breeding populations of certain species, as well as nursery grounds and/or feeding grounds for others (Welcomme, 1979).

Table 1

River basin area, river discharge during the yala¹ and maha¹ seasons from the 23 major rivers and that of the other minor rivers (modified after Arumugam, 1969; 1 - refers to cultivating seasons which are based on the rainfall pattern)

River	Basin km3	Maha	Run-off in 106 m3 Yala	Total
Dry Zone
Walawe Ganga	2,442.2	1,389.5	812.0	2,201.5
Kirindi Oya	1,164.8	540.5	122.2	662.7
Kumbukkan Oya	1,218.6	636.7	86.4	723.1
Heda Oya	604.2	336.9	35.8	372.7
Gal Oya	1,792.0	1,632.6	117.2	1,749.8
Mundeni Aru	1,280.0	713.3	38.3	751.6
Maduru Oya	1,541.1	1,310.5	87.6	1,398.1
Mahaweli Ganga	10,327.0	3,499.6	1,814.0	5,313.61
Yan Oya	1,520.6	754.0	30.9	784.9
Ma Oya	1,024.0	459.0	14.8	473.8
Kanakanayan Aru	896.0	401.1	14.8	415.9
Aruvi Aru	3,246.1	1,187.1	51.8	1,238.9
Moderagama Aru	931.8	230.0	11.1	241.1
Kala Oya	2,772.5	981.0	79.0	1,060.0
Mi Oya	1,515,5	134.5	14.8	149.3
Deduru Oya	2,616.3	1,194.5	313.4	1,507.9
Others	11,591.8	5,939.2	441.8	6,381.0
Sub Total	46,484.5	21,340.0	4,085.9	25,425.9
Wet Zone
Kelani Ganga	2,265.6	2,927.0	4,238.8	7,165.8
Kalu Ganga	2,688.0	3,456.4	4,310.4	7,766.8
Bentara Ganga	622.1	818.1	942.8	1,760.9
Gin Ganga	921.6	905.8	1,006.9	1,912.7
Nilwala Ganga	960.0	771.3	721.9	1,493.2
Maha Oya	1,510.4	981.0	956.4	1,937.4
Attanagalu Oya	727.0	372.0	392.4	764.4
Others	2,369.3	1,632.6	139.8	1,772.4
Sub Total	12,064	11,864.2	12,709.4	24,573.6
Grand Total	58,548.5	33,204.2	16,795.3	49,999.5

¹ Elsewhere in the text, the discharge, based on 1950–1977 records, is given as 8.3 10⁹ m³, of which 5.8 10⁹ m³ is to be utilized. (Ed. note)

Table 2

Precipitation and run-off (in 10⁹ m³)

	Dry Zone	Wet Zone	Total
Annual precipitation	70.3	39.5	109.8
Run-off	25.5	25.9	51.3
	(37.5%)	(65.0%)	(47%)
Balance	44.8	13.6	58.5
	(62.5%)	(35.0%)	(53.0%)

The total floodplain area, inclusive of those lakes which will retain water through the year (locally known as villus) is estimated to be around 41,050 ha (Mendis, 1977), which is small in comparison to the extent of the river basins on the island. Perhaps, this limitation is brought about by the short length of rivers from the point of origin to the discharge into the sea and the steep gradient of the river beds.
2.3 Riverine ichthyofauna
Sri Lanka's geological history has resulted in an ichthyological diversity lower than would be expected for a tropical island very close to the mainland (Fernando, 1971). According to Darlington (1957) mountain stream fish did not reach the Indian mainland until the Pliocene and did not therefore reach Sri Lanka which separated from its mainland in the Miocene period. Nevertheless, some of these species, as well as secondary freshwater species were able to migrate from the Indian mainland across ‘bridges’ formed during depressions of sea level during the ice ages.
Fifty-one species of teleost fishes indigenous to the island have been recorded (De Silva et al., 1981). Of these species 42 are believed to be typically riverine and the rest to be marsh-dwelling (Fernando and Indrasena, 1969; De Silva, in press). There is uncertainty regarding the number of species endemic to Sri Lanka; according to Evans (1981) there are 17 constituting 33% of the indigenous fauna, whereas according to De Silva (in press) there are only 14 species which are taxonomically accepted.
Senanayake and Moyle (1981) recognised three ichthyological provinces based on the distribution of the indigenous fauna; the Southwestern Province - the richest, containing 15 endemic species of which 12 are restricted to the province, the Mahaweli Province with 4 endemic species (two confined to the province) and the dry zone province (Fig. 2). These provinces have a superficial relationship to the three peneplains of the island. The ichthyorichness of the Southwestern Province was attributed to high and more constant rainfall, the higher forest cover and the greater diversity of the available habitats.

Figure 2. Ichthyological provinces in relation to the three peneplains of the island (from Senanayake and Moyle, 1981)
2.4 Fish introductions
Since the first recorded introduction of Salmo trutta in 1882, eighteen species of food fishes have been introduced into the freshwaters of Sri Lanka (Table 3). Apart from these three transplantations, one typically estuarine and two catadromous species have been made. The role of exotics in the inland fishery of Sri Lanka has been aptly documented (Fernando, 1971; De Silva, in press). Of the introductions the cichlid, Oreochromis mossambicus (Peters) has not only been the most successful, but also has been singled out as the species which was indirectly involved in the establishment of a viable reservoir fishery on the island.
There are no records to show that the Indian and Chinese major carps have bred naturally, although they have been successfully bred in hatcheries (Weerakoon, 1979). A similar success has been achieved with Cyprinus carpio. As such, the role of these species in the fishery of Sri Lanka lies in their utilization, either in extensive and/or intensive cultural practices.
Of the transplanted species Etroplus suratensis has established itself in rivers (Willey, 1910) as well as in reservoirs (De Silva et al., in press), and contributes in varying extents to individual reservoir fisheries.
2.5 River basin development
Apart from the ancient development of some river basins, in the course of this century the following major river basins have been developed or are in the process of being developed, either solely, for irrigational purposes or for irrigation and hydroelectric power generation:

1. Gal Oya Basin;
2. Walawe Ganga Basin;
3. Mahaweli Ganga Basin;
4. Kirindi Oya Basin;
5. Kala Oya Basin.

Among these, the Mahaweli Ganga basin development is the largest from the point of view of capital works, area irrigated, hydropower and environmental manipulation. It is also associated and interconnected with the development of the Kala Oya and Maduru Oya basins. In the following chapters the Mahaweli Ganga basin and its development are considered in detail.
3. MAHAWELI GANGA (= RIVER)
The Mahaweli river is the largest and the longest river in Sri Lanka. It originates in the Adam's Peak range near Hatton, at an elevation of about 2,000 m. The Mahaweli Ganga which is 332 km long, flows north and then northeast and drains into the sea at Trincomalee Bay, the deepest and the largest bay of Sri Lanka (Fig. 3). The main river runs through six of the twenty-five administrative districts of Sri Lanka.
The headwaters of the Mahaweli originate on the Hatton plateau, into which enter seven subsidiary rivers, viz. Kurundu Oya, Belihul Oya, Maha Oya, Kotmale Oya and Nan Oya from the south, and Ma Oya and Hulu Ganga from the north. The upper catchment of the Mahaweli covers about 316,000 ha. In the lower basin there are three major tributaries that enter the

Table 3

Fish introductions into Sri Lanka with comments on their reproductive biology (updated from Fernando, (1971) * re-introductions; ** transplantations)

Species	Origin	Date	Stocking	Breeding	Remarks
Salmo trutta	Europe	1882–1893	Hill streams	+ (Fernando, 1971)	Stocks supplemented from the hatchery annually
Salmo gairdneri	N. America	1889–1893	Hill streams	+ (Fernando, 1971)	Stocks supplemented from the hatchery annually
Cyprinus carpio (2 strains)	Europe	1915	Streams/reservoirs	+ (Fernando, 1971)	Breeding not confirmed
Carassius carassius	Europe	1915	Streams/reservoirs	+ (Fernando, 1971)
Osphronemus goramy	Indonesia	1909	Streams/reservoirs	+ (Fernando, 1971; Ellepola & Fernando, 1968)	No longer important as a food fish
Ctenopharyngodon idella	China	1948/1973*	Reservoirs		Bred in fisheries station, Udawalawe (Weerakoon, 1979)
Hypophthalmichthys molitrix	China	1973			Bred in fisheries station, Udawalawe (Weerakoon, 1979)
Aristichthys nobilis	China	1978
Catla catla	India	1942/1981*			Reared in experimental stations
Labeo rohita	India	1982			Reared in experimental stations
Trichogaster pectoralis	Malaysia	1951	Lagoons/marshes	+ (Fernando, 1971; Indrasena, 1965)
Oreochromis mossambicus	E. Africa	1952	Reservoirs	+ (Fernando and Indrasena, 1969)	Very important in reservoir fisheries
O. niloticus		1956/1975a	Reservoirs	+ (personal observation)	Food fish common in certain reservoirs
Tilapia rendalli	E. Africa	1969	Reservoirs	+ (Chandrasoma and De Silva, 1981)	Food fish
O. hornorumb	E. Africa	1969	Experimental (?)		(no data available on the present status of these introductions nor have any of the species been reported in the catches
T. zillib	E. Africa	1969	Experimental (?)
Helostoma temmencki	Thailand	1951
Puntius gonionotusc	S.E. Asia
Etroplus suratensis	Lagoons	1910	Reservoirs/rivers	+ (Willey, 1910)	Important catch in certain reservoirs
Chanos chanos**d	Lagoons
Mugil cephalus**d	Lagoons

a - Mendis (1977);
b - no records of these introductions are available;
c - Jhingran and Gopalakrishnan (1974);
d - no organized transplantation carried out

Figure 3. The Mahaweli Ganga and its tributaries.

Figure 4. The topography of the Mahaweli Ganga Basin. Also indicated is the mean annual precipitation for different parts of the basin (modified from Hunting Survey Corporation, 1962).
Mahaweli: Amban Ganga, Ulhitiya Oya and Kaudulla Oya. The Mahaweli River Basin covers 10,420 km² - about one-sixth of the island area. The topographic features of the basin are shown in Figure 4, and the distribution of land area according to elevation is given in Table 4. The 150 m contour closely coincides with the edge of the lowland plain and the area enclosed by this contour constitutes nearly half of the basin.

Table 4

Distribution of land area according to elevation in the Mahaweli Basin

Elevation Zone	Area (km2)	% of Basin area
0 – 150	4,740	45.5
150 – 350	982	9.4
350 – 650	1,502	14.5
650 – 1,300	2,108	20.2
1,300 – 2,000	847	8.1
over 2,000	241	2.3
Total	10,420	100.0

3.1 Climate
The climate of the Mahaweli Basin is largely influenced by monsoons, with the single most important climatic parameter being the rainfall. The annual temperature variations are small, although there are significant differences between the up-stream and the down-stream areas.
3.1.1 Rainfall
The mean annual rainfall ranges from 1,650 mm in the downstream area to 5,300 mm in the upper-catchment (Fig. 4). The pattern is largely physiographically controlled. The highest parts of the hill country, which are exposed to the full force of the monsoons, experience the greatest rainfall, while the lowland plains and the ‘rain shadow areas’ in the hill country receive a much lower rainfall. The upper catchment receives the greatest rainfall during the southwest monsoon from May to September, and the northeast monsoon from December to January produces the least rainfall, February usually being the driest month of the year. The lower basin, on the other hand, receives the most amount of rainfall during the northeast monsoon (November/December to February/March) with adequate rainfall during October and November - the intermonsoonal rains of convectional and cyclonic origin. The two cultivation periods known as Maha and Yala correspond to the two monsoonal periods of northeast and southwest respectively.
The effective precipitation for the basin was found to range from 12 mm in September to 114 mm in November (UNDP/FAO, 1969). The annual total effective precipitation was 681 mm.
The island is divided into a dry and a wet zone by climatologists using the 75 inch average isohyet. Drawing of this single boundary, however, fails to emphasize the existence of a transition zone - the intermediate zone. The Mahaweli Basin contains large areas of wet, dry and intermediate zones. Hunting Survey Corporation (1962) reconstructed the zone boundaries taking into consideration annual rainfall, seasonal rainfall distribution, physiography, soils and the distribution of agricultural land uses and forest types (Fig. 5). They also divided the Mahaweli Basin into rainfall regimes taking into consideration the monthly rainfall regardless of the totals involved.
On this basis three regimes were recognised:

1. Regimes dominated by the southwest monsoon rainfall (SW>IM>NE) - only one area of the Mahaweli Basin belongs to this regime.
2. Regimes dominated by northeast monsoon rainfall (NE>IM>SW)-the largest area of the Mahaweli Basin falls into this regime and is represented in all three zones.
3. Regimes dominated by rainfall of the intermonsoonal periods (IM>SW>NE, IM>NE>SW>IM>NE>SW) - these regimes predominate in five areas of the basin.

3.1.2 Floods
The occurrence of floods in the lower Mahaweli Ganga is mainly confined to the periods of the northeast monsoons and the months immediately preceeding it. The major floods are mostly associated with tropical revolving storms or cyclones that are common during that period (Thambayahpillay, 1959). Because of the steep slopes and intense rainfall in the upper catchment area flood flows in Mahaweli Ganga are typically quick rising and also tend to regress rapidly (Fig. 6).
3.2 Soilsp>
Fifteen great soil groups have been identified in Sri Lanka (UNDP/FAO, 1969), of which the following six are reported to occur in the Mahaweli Basin: the reddish brown earth (RBE); the non-calcic brown (NCB); The low humic gley (LHG); the recent alluvials (RAL); the old alluvials (OAL); and the solodized solonetz (SS) soils.
In the lower basin area thirty-five major and minor soil series, representative within the above six great soil groups have been established (TAMS, 1980). Details of drainage and soil permeability classes, depth of the bedrock, water holding capacity, soil colour and electrical properties are known for all the minor soil groups (TAMS, 1980). The distribution of soil types in the Mahaweli Basin in relation to some other river basins are also dealt with by Moorman and Panabokke (1961) and Panabokke (1967).
3.3 Surface Water
3.3.1 Stream Flow
From the gauging records (1950–1977) at Manampitiya (Fig.3), it has been estimated that the mean annual flow of the Mahaweli Ganga is approximately 8.3 billion m³. The upper basin of the Mahaweli receives appreciable amount of rain throughout the year (Fig.5) while the lower basin receives most of its rainfall during the northeast monsoon. As such, the seasonal range of streamflow is not as large as that of the rainfall in the lowland area; for example, at Manampitiya the average flow during one dry season (March to September) is approximately 46% of that of the wet season, although the rainfall in the dry season is only 27% of that in the wet season.

Figure 5. Rainfall pattern of the Mahaweli Basin (from Hunting Survey Vorporation, 1962).

...

Figure 6. The Mahaweli Ganga - a river of extremes. A. The Ganga at full spate, December 1983 as seen from the Manampitiya Bridge to the north. B. The Ganga at its lowest, February 1983.

The mean monthly flow at Manampitiya for the 1950–1977 period and that for 1955/56 which is considered as a critical year is shown in Figure 7. The mean monthly flows at four other selected sites are given in Table 5 (after NEDECO, 1979).

3.3.2 Water quality

The water quality parameters of surface waters of the Mahaweli river and its tributaries have been measured since 1960 (Table 6). For convenience and clarity those data collected within a 5 km range have been pooled. In the main river the dissolved solids content, electrical conductivity, concentration of ions increase downstream. The same parameters have lower values in tributaries in the hilly country as compared to those in the plains

.

DDT has been in use in Sri Lanka for nearly 30 years, initially sprayed to control mosquito vectors of malaria parasities and later as a pesticide in paddy and other cultivations. Apart from DDT other organochlorine compounds such as heptachlor, endrin, aldrin, and dieldrin have been extensively used (Weeraratne, 1983). Organochlorine pesticides degrade extremely slowly, and tend to accumulate along the food chain. The residual levels of chlorinated pesticides were measured in surface waters of the Mahaweli Ganga, floodplain lakes, and major reservoirs (Table 7, from TAMS, 1980).
For the reservoirs and villus, the best water quality data are available for the reservoir Parakrama Samudra (Gunatilaka and Senaratne, 1981; Gunatilaka, 1983). The main physico-chemical data for the perennial as well as some seasonal or village tanks are summarized in Table 8. The analysis of the limnological information for the Parakrama Samudra suggests that the productivity of the lake is somewhat nutrient limited (Schiemer and Duncan, 1983). The phosphorus content showed strong diurnal dynamics, being limiting during the day (Gunatilaka, 1983). In addition, the high flushing rates also affect nutrient dynamics and productivity patterns. The Parakrama Samudra has a flushing rate that is nearly double that of Minneriya and Kaudulla (Table 12) and as such the influences of the flushing rate on the nutrient and phyto- and zooplankton dynamics of the latter reservoirs are likely to be far greater.

Table 5

Mean monthly flows of Mahaweli Ganga at selected, important sites in million cubic meters (after NEDECO, 1979)

Station	Jan.	Feb.	Mar.	Apr.	May	June	July	Aug.	Sept.	Oct.	Nov.	Dec.	Total
Polgolla	116	70	64	107	199	299	273	249	239	321	284	218	2,438
Minipe	408	264	172	247	334	415	400	368	349	501	528	538	4,524
Manampitiya	1,203	709	422	471	508	502	478	448	435	694	966	1,459	8,289
Elahera	151	93	55	49	46	27	25	22	21	57	104	178	829

Table 6

Water quality of the Mahaweli and its tributaries. The mean and the range for each parameter is given when available (modified after TAMS, 1980). The locations are given in Fig. 3.

Location	Year	pH	Hardness meq 1-1	EC us	TDS mg 1-1	meq 1-1 Ca
							Mg	Na	K	HCO3	Cl
Mahaweli	60,61	7.2	19.6	48.9	43	0.25		0.11	0.06	0.51	0.25
Polgolla/Peradeniya/ Getambe/Primrose Hills	79	(6.2e–7.7a)	(12–33)e	(20c–115e)	(24–74)e	(0.15–0.34)a		(0.08–0.14)a	(0.02–0.13)a	(0.02–0.80)c	(0.11–0.39)c
Victoria Dam Site	79c	6.3	-	60		0.35	0.13	0.12	-	0.61	-
Mohiyagama	79g	7.0	-	95		0.53	0.22	0.15	-	1.15	-
Manampitiya	79	6.8d–8.0g	-	90		0.53d–0.85g	0.16d–22g	0.21g	-	0.94g–1.29d	0.28d
Muttur, Chudankadu	79g	7.0	-	132–2120		0.66–1.23	0.34	0.22–13.2	-	1.35–1.81	-
Amban Ganga	60,61	7.4	-	169	107	0.52g	0.24g	0.20	0.05	0.98g	-
Elahera	79	(5.0–8.1)a	-	(75–270)a	(60–146)a			(0.14g–0.31a)	(0.03–0.09)a
Rattota	72,79	6.5f –7.0c	15c	83	-	-	-	-	-	-	0.14c
Kotmale Oya
Dam Site	63,77,78	7.0	10.5	33	22	0.23	0.06	-	-	0.21	0.20
	79	(6.4–7.2)d	(9–14)d	(19–50)d	(13–35)d	(0.10–0.46)d	(0.04–0.08)d			(0.16–0.30)g	(0.17–0.28)d
Talawakelle	79	7.0c,g	10c	22.5	-	-	-	-	-	-	0.14c

a to g - Sources;
a - U.S. Operation Mission (1961);
b - Pattiarachchi & De Silva (1972);
Ceylon Association of Science Presidential Address;
d - Government Analyst Department Records;
e - National Water Supply and Drainage Board Records;
f - Dr. G. Schut (field notes);
g - TAMS data

Table 7

Chlorinated pesticide residues in selected surface waters of the Mahaweli Ganga and associated reservoirs and villus in microgram per litre (TAMS, 1980)

Location	Aldrin	Dieldrin	Benzenehexachloride	Endosulfan	Total DDT
Mahaweli Ganga
Victoria Dam Site	0.01	0.06	0.17	-	0.05
Manampitiya	0.01	-	0.13	-	0.07
Mutur	0.07	-	0.22	-	0.41
Kotmale Dam Site	0.02	0.04	0.21	0.06	0.47
Flood Lakes
Karapola Villu	0.07	0.06	0.19	0.10	0.38
Velankada Villu	0.01	0.14	0.20	0.04	0.27
Reservoirs
Parakrama Samudra	0.04	-	0.22	-	0.13
Minneriya	0.04	0.03	0.17	-	0.25
Kaudulla	0.02	-	0.18	-	0.12

Seasonal reservoirs (village tanks), in comparison to the larger reservoirs, have a higher availability of nutrients as well as a higher potential production (Mendis, 1965; Oglesby, 1979, 1980).
3.4 Ground water
The availability of ground water depends on the amount of surface flow that percolates into the sub-soil, which in turn is dependent on the nature of sub-soil and the amount of rainfall. Very little information is available on the ground-water resources in Sri Lanka. In the dry zone the amount of rainfall available for infiltration into the sub-soil is limited due to low rainfall.
The major ground water resources of the Mahaweli Basin occur in the miocene limestone belt (Fig. 8) in the north-western, northern and north-eastern part of the basin. The annual recharge to ground water in this area is estimated at 150–250 mm per annum (Fernando, 1973). In this region the bed rock is covered by red alluvial soils which have a very high infiltration rate, and water is trapped in the solution cavities and in canal systems along joints and fractures (NEDECO, 1979).
3.5 Land use
The pattern of land use of the upper catchment of the Mahaweli Basin differs significantly from that of the downstream area. The overall land use pattern of the Mahaweli Basin is summarised in Table 9 and Figure 9.
The upper catchment has four major land use groups:
(a) the intensively managed lands, comprising 49% of the watershed area, of which two-thirds are under tea;
(b) 14% under dry land agriculture;

Table 8

Some water quality data for the major reservoirs and seasonal tanks of the Mahaweli Ganga basin

	Area	Mean depth at full supply level	pH	Conductivity (microsiemens)	Alkalinity mq/1	Plankton mg m3 mg m3
Major Reservoirs
Parakrama Samudra	2,662	5.3	7.0–8.9a	184a	1.7a	-
Minneriya	2,550	5.3	7.0–7.5b	221b /193c	1.4c	6.9d
Kaudulla	2,537	5.1	7.5–8.0b	252b /380c	1.8b	-
Giritale	356	8.6	7.5–8.0c	140–168c	4.1c	8.4d
Seasonal Tanks (Data extracted from Thayaparan, 1982, unless otherwise stated)
Kolkanaweliwewa	18.0	-	6	59	-	21
Dalukan Wewa	68.0d	-	7	161	-	6
Thimbirigaswewa	5.7	-	-	365	2.4	-
Kudahatawewa	6.5	-	-	-	2.7	-
Aralaganwila	158.0d	-	-	-	2.5	-
Moragaswewa	12.0	-	-	92	1.0	-
Floodplain Lakes
Gengala V.	256	-	6.7e	175e	-	-
Karapola V.	600	-	6.7e	260e	High HCO3
Mutugalla V.	420	-	6.8e	875e	High Saline

a - Gunatilaka and Senaratne, 1981;
b - Oglesby, 1979;
c - De Silva (unpublished)
d - Mendis, 1977;
e - TAMS, 1980.

Figure 7. Mean monthly flow of the Mahaweli Ganga at Manampitiya (1950-1977) as compared with that for the critical year 1955/56 (from NEDECO, 1979).

.
Figure 8. Ground water resources of the Mahaweli basin (from NEDECO, 1979)Table 9

Mahaweli Ganga Basin - summary of land use (modified from Hunting Survey Corporation, 1962)

Land Use	Wet Zone		Intermediate Zone		Dry Zone		Total
	ha	%	ha	%	ha	%	ha	%
Urban	1 650	0.6	500	0.2	306	0.2	3 060	0.3
Homestead Garden	32 490	10.8	22 390	10.0	15 280	2.9	70 150	6.7
Rubber	9 460	3.2	3 695	1.7	-	-	13 160	1.3
Tea	133 920	44.7	11 490	5.1	-	-	145 415	13.9
Paddy	21 902	7.3	21 731	9.7	28 490	5.4	72 126	6.9
Other permanent cultivation	7 270	2.4	11 860	5.3	15 203	2.9	34 330	3.3
Shifting cultivation	7 270	2.4	87 380	39.0	58 690	10.3	148 250	14.2
Grassland and Scrub	27 646	9.2	18 540	8.3	27 290	5.2	73 480	7.0
Forest	56 170	18.8	45 345	20.2	358 320	68.5	459 800	43.9
Unused land	573	0.2	370	0.2	5 370	1.0	6 313	0.6
Water and Marsh	1 126	0.4	760	0.3	18 620	3.6	20 508	1.9

Figure 9. Major patterns of land use in the Mahaweli Basin (from Ceylon 1969).
(c) 14% under forest, and
(d) 22% is misused land which is either in the form of scrub, chena (= small crop cultivation) grassland or abandoned tea (TAMS, 1980).
The upper catchment is characterized by steep slopes, with nearly 75% of the land having slopes of 21% to 40%. Over the last 25 years the land area used for intensive agriculture has decreased from 87,000 to 63,000 hectares, and the forest cover by almost half.
The downstream area covers approximately 500,000 ha and the extent of usage for different purposes is given in Table 10.

Table 10

Present land use in the downstream area of the Mahaweli Basin (modified from TAMS, 1980)

Land use	Extent ha
Rice Land	75,000
Chena	90,800
Other cropland	48,000
Pastures, Grasslands, etc.	24,000
Forest, scrub and woodland	140,000
Surface water	10,000
Other land	3,000
Total	390,800

The lower basin has been one of the most intensively used land on the island, due to the ancient irrigation works resurrected and improved in the course of this century.
3.6 Settlement patterns
Population density changes from 1871 to 1978 in the different administrative districts which fall within the Mahaweli Basin are given in Table 11.
Over the 100-year period the population density in the lower basin has increased by nearly ten-fold as compared to the six-fold increase in the upper basin area. The present population density distribution in those districts which fall within the Mahaweli Basin is shown in Figure 10. Although the lower basin has experienced higher degree of change, its significantly lower population density and the consequent availability of a greater land area makes this portion of the basin more suitable for future development.