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The development of micro-catchments in Sri Lanka
Journal of Hydrology, 80 (1985) 351--359
351
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
[1] THE DEVELOPMENT OF MICRO-CATCHMENTS IN SRI LANKA
U.D.S. UDAWATTAGE
Resources Development Consultants Ltd., 55, 2/1, GaUe Road, Colombo 3 (Sri Lanka) (Received February 20, 1985; accepted for publication March 8, 1985)
ABSTRACT
Udawattage, U.D.S., 1985. The development of micro-catchments in Sri Lanka. J. Hydrol., 80: 351--359. This paper gives an informatory account of the development of micro-catchments in Sri Lanka for irrigation. Information for the paper comes from the experience of detailed investigation and preparation of technical proposals for the improvement of 170 existing micro-scale reservoir schemes in the Puttalam and Kurunegala districts, over a period of three years (January, 1982 through December, 1984), under a World Bank-financed project called "Integrated Rural Development (IRD) Project". Investigations were carried out by a private body called Planning Consultancy, a professional staff comprising a team of eight engineers from the Resources Development Consultants Ltd., Sri Lanka, under a contract with the Ministry of Plan Implementation and under the supervision of the Irrigation Department of Sri Lanka and a World Bank-recommended body called Supervisory Consultancy. The writer is a member of the Planning Consultancy team who personally investigated and designed 30 of these schemes.
INTRODUCTION
Micro-catchments have n o t received much attention in hydrology, mainly because of the insignificant role they play, as far as their water resource is concerned, to a point of affecting a country's economy. However, in Sri Lanka, micro-catchments play a very significant role simply because the livelihood of a significant portion of the population is directly dependent on them. It has been estimated that there are more than t w e n t y thousand microcatchments in Sri Lanka, across which earthdams have been built to form mini-reservoirs (popularly known as "village tanks"), to store water during the rainy period, to be used for irrigation of paddy during the dry period of the cultivation season. A b o u t half of them have been systematically recorded by the government, with all technical and administrative details, in forms called "Tank Data Cards" (Ministry of Lands and Land Development, Water Resources Development Division). The village tanks represent a significant share of the country's rice production (Ariya Abeysinghe, 1982). In addition, the village tanks, b y intercepting a considerable portion 0022-1694/85/$03.30
© 1985 Elsevier Science Publishers B.V.
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of the catchments of the major river basins, reduce land erosion and flood inflow to major rivers. Most of the village tanks have been constructed during the times of kings, hundreds of years ago, and have been restored by the Irrigation Department (Ariya Abeysinghe, 1982). The sample of 170 tanks in the Puttalam and Kurunegala Districts for which detailed investigations were carried out by the Planning Consultancy is less than 5% of the total number of tanks in these two districts (Irrigation Department, 1975; Ministry of Plan Implementation, 1983), however, it can be considered to be an important sample, as it fulfilled certain selection criteria stipulated by the World Bank and the government of Sri Lanka, one of which is that the paddy cultivation under each tank should not be less than 8ha which eliminated a large number of tanks, permitting only the significant tanks to be taken up for improvement. The scatter on Fig. 1 shows the village tanks investigated by the Planning ~2
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Tcnk A r e a Catchment Cultivated Paddy
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Fig. 2. A minor tank across a major road.
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S a m Long
354 Consultancy under the IRD Project, in the Puttalam and Kurunegala districts and Fig. 2 shows a village tank, the Mahamanaweriya Tank, right across the Colombo--Puttalam main road (this is a minor village tank with a small e x t e n t o f cultivation, located in a fairly developed area close to the wet zone--dry zone boundary, and investigated under the IRD Project). The location o f this tank in the district is shown in Fig. 1. According to the statistics collected from the technical reports of the 170 tanks (Planning Consultancy, IRD Project, Puttalam and Kurunegala Districts, 1982--1984): (1) The individual c a t c h m e n t area of a significant village tank ranges from 0.5 to 5 km 2 but most of them are between 1.25 and 2 km 2. Some village tanks are in a cascade of relatively large gross catchments, however, the i n d ep en d en t n et - c a t c hm ent o f each tank in the cascade is small. (2) The mini-reservoir (the village tank), created by the earthdam across the t o r r e n t o f the micro-catchment covers an area o f between 8 and 50 ha but most o f them are around 12 ha. (3) The tank capacities range between 50,000 and 600,000 m ~ but most of them are between 74,000 and 185,000 m 3 . (4) The length of the earth dam (better k n o w n as tankbund) varies from 300 to 1500 m which contains an overflow spillway and one or two sluices to release water. (5) Areas o f paddy cultivation under a significant village tank vary from 8 to 40 ha but most of them are around 20 ha. (6) Each village tank has its farmer c o m m u n i t y the size of which depends on the cultivated pa ddy area. The n u m b e r of families directly benefited from the cultivation u n d e r a village tank varies f r o m 10 to 100. Despite the fact t hat these statistics were taken f r o m a very small sample, it can be reasonably said, taking into account the general knowledge acquired a b o u t village tanks during the period of the project, that t h e y cannot be very different f r o m what can be expect ed from a big sample of tanks covering a larger part of the country.
HYDROLOGY OF A VILLAGE TANK
Climatology Sri Lanka has been divided into two zones, called dry and wet zones, based on two monsoonal rainfall seasons: The northeast m o n s o o n (October to December); and the southwest m o n s o o n (April to June). The wet zone, which consists o f only a third of the land area of the c o u n tr y , receives bot h monsoons, and the dry zone, which forms the major part, receives only the northeast monsoon. For reasons o f high and fairly well distributed rainfall, no storage of water for irrigation is required for paddy cultivation in the wet zone, whose main
355
cultivations are export crops -- tea, rubber and coconut -- whereas the cultivation in the dry zone is mainly paddy, which forms the bulk of the country's rice production. There are a large number of major irrigation projects in the dry zone. Village tanks are mostly located in the dry zone, although, there are a few in the wet zone close to the dry zone boundary. Cultivation under a village tank generally begins in October or November with the arrival of rainfall. In general, the direct rainfall is sufficient to sustain the paddy cultivation until the end of December, thereafter, irrigation is required for the two m o n t h s January and February -- the harvest being in March. The rainfall between the period October to December occurs generally as intermittent, isolated showers, intervened by dry spells. Much of direct rainfall could be retained within the paddy field bunds in the cultivated area to be used during these intervening dry spells. The r u n o f f from the rainfall within the catchment area is collected in the village tank. Quite often, it is one or two heavy storms, occurring during the rainy season, which fills the tank, sometimes overnight. The storms are typical of those occurring in tropical areas -- localised, intense and of short duration. Intensity of rainfall largely varies temporally as well as areally, however, the areal distribution of rainfall within a micro-catchment is reasonably uniform. There are two paddy cultivating seasons in Sri Lanka: " M a h a " cultivation (October to March); and " Y a l a " cultivation (April to September). To do a Yala cultivation in the dry zone, it is necessary to conserve water in the tank during the rainy season (October to December) and carry it over to Yala. This is being done in large irrigation schemes with reservoirs across major river basins in the dry zone or schemes with diversion from perennial rivers. However, under a village tank, the catchment area and the tank capacity are so small t h a t the possibility of Yala cultivation is practically nil except under some tanks (very few in number) close to the wet-zone boundary, where the Yala season rainfall is adequate.
Estimation o f catchment area o f a village tank Although the catchment area is the most important hydrological parameter of the catchment, there is no way of directly measuring the catchment area of these micro-catchments. Engineering surveys of Sri Lanka are available in 73 maps, called " T o p o s " , to the scale of 1 mile to an inch ( 0 . 6 2 k m : 1 c m ) with contours at 1 0 0 f t ( 3 0 . 5 m ) intervals. In the dry zone, the interval between two contours covers a large area containing a number of village tanks. Therefore, the delineation of catchment area from the contours alone is n o t possible -except in areas where special engineering surveys with closer contour intervals have been done for major irrigation projects and the catchments of the village tanks within the area have been automatically covered (such cases are n o t many).
\/ _
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~- |HALAGAMA TANK
~
o
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VILLAGE TANK
PADDY CULTIVATION
CATCHMENT BOUNDARY
RAILWAY
MAIN ROAD
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Fig. 3. Topo inset showing demarcation of catchment boundary of Ihalagama tank -- Extracted from Galgamuwa topo sheet, coordinates F/18 (7,65 × 3.25).
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In view of the large number of tanks involved and the small extent of cultivation under each tank, investment of m o n e y for the survey of the catchment areas will not bring commensurate benefits, as the tanks are so developed, over a large number of years, to a point t h a t the knowledge of the exact catchment area does not help in improving the status of the scheme, other than helping to do an accurate study of hydrology of the catchment area. In a topo map, village tanks can be seen scattered all over and a number of village tanks can often be found on the topo within a very small area. Therefore, by separating the other tanks around, it is possible to demarcate the catchment area of a village tank indirectly and to determine the area with reasonable accuracy. Figure 3 shows such a demarcation of the catchment area of the Ihalagama Tank in the Kurunegala district, investigated under the IRD Project (see Fig. 1 for location in the district}. It is possible to refine the catchment boundary so marked on the topo with field investigations such as locating the ridges on the roads, cart tracks, etc., within the catchment and other visual field observations, as well as by local inquiry. Inflow to a village tank There are no stream-flow measurements in these micro-catchment streams, and there are no rain gauges within most of them. Although, it is possible to indirectly measure the inflow into the tank during a rainy period, by observing the water level and estimating the inflow through increase in tank capacity, such studies have n o t been done. For the purpose of design, the cultivable area and the required tank capacity have been decided rather on simplified, regionalised methods (Arumugam, 1957) as well as on trial and error, based more on experience, than on pure hydrological computations. Experience on filling of an existing tank, frequency of spilling, etc. have given good indication on the rich or poorness of the water resource of the catchment. Based on this experience, decisions have been taken to augment the tank capacity, from time to time, to create additional storage to conserve more water for irrigation. In the light of the limited data available, this seems quite a logical approach for the improvement of an cxi.~ting village tank. Floods in a village tank c a t c h m e n t Each village tank has its own spill -- which may be a concrete free overfall or natural ground with or w i t h o u t a concrete partition wall -- to discharge flood flows. The length of a spill varies from 10 to 30 m. The top of the earth bund is generally about a metre above the spilling level to allow freeboard for surcharge storage for floods etc. These spillways have been de-
358 signed with simplified hydrological techniques (Arumugam, 1957)as well as trial and error, based largely on experience. Village tanks are in flood following storms of short duration but with very high intensity, occurring when the tank is already full and the catchment is wet with antecedent rainfall. Over the last 30 years, village tanks have breached, in large numbers, during four major events: the two most recent being the cyclonic rainfall that occurred in November, 1978 and the high rainfall in April, 1984. The causes of failure are generally as follows: (1} the inflow is exceptionally high; (2) breach of an upper tank in the cascade, bringing a surcharge flood volume; (3} poor contact between concrete and earth at a structure such as the sluice or an a b u t m e n t of the spillway; (4) inadequate spillway or poor getaway or approach conditions of the spillway; and (5) a poorly constructed weak section in the bund giving way due to surcharge water pressure. Except in the cases 2 and 3, it is not easy to establish the exact cause of failure. Statistics were collected from the field offices of the Department of Agrarian Services, which maintains the village tanks, on the number of significant village tanks in the Put talam District, breached during the most recent high rainfall event which took place on 22 April 1984 {within the period of the IRD Project), mainly affecting the tanks in the Puttalam district (note: the recorded rainfall at Puttalam town on this day is 275 mm which is the highest recorded daily rainfall over the past 80 years). It was found that out of 488 tanks, only 78 tanks had breached. This is 16% of the total number of tanks. The fact that 84% of the tanks did not breach indicates that the floods of the village tanks are of "handleable" size and the simplified and the trial and error methods used in the design of village tanks, which are constructed with low-level technical resources available at the village level, do not drastically affect the economic cost of maintenance of village tanks. Since the bund height of a village tank is small {2.5--5 m) the refill of the breach, the length of which can vary from 10 to 25 m, is not a major problem. However, the breach of the tank bund causes the loss of a cultivation, due to the emptying of the tank. The study of flood in the design of spillway of a village tank should be viewed on this background and experience plays a major role.
CONCLUSION Micro-catchments in Sri Lanka are a fascinating domain in hydrology, both by their very number and their importance to the rural life and to the country's economy. They have not been taken seriously by the hydrologists, but an in-depth study of them will not only be interesting from the aca-
359
demic point of view but also helps to reduce the cost of improvement and maintenance, by refining the hydrological techniques used at present for their improvement, thereby, making a contribution to the social life of the rural c o m m u n i t y as well as to the country's economy.
ACKNOWLEDGEMENT
The writer wishes to thank Mr. D. Dantanarayana -- Chief Engineer (Water Resources) of the Resources Development Consultants Ltd. and the Team Leader of the Planning Consultancy, IRD Project -- for giving all assistance for the preparation of this paper and for reviewing the paper and making valuable comments. The writer also thanks the staff of the Planning Consultancy for their assistance.
REFERENCES Ariya Abeysinghe, 1982. Minor Irrigation in the Agricultural Development of Sri Lanka (Part I and II). Economic Review. People's Bank, Research Department. Vol. 8 (6): 25--28; (7 and 8): 22--28. Associated Newspapers of Ceylon, Colombo, Sri Lanka. Arumugam, S., 1957. Development of Village Irrigation Works. Government Press, Sri Lanka, chapters III and IV, pp. 13--24. Irrigation Department, 1975. Register of Irrigation Projects in Sri Lanka. (An Irrigation Department official publication, listing names of irrigation projects, their locations etc.), Ceylon Printers, Colombo, Sri Lanka, pp. 26--50. Ministry of Lands and Land Development (Sri Lanka), Water Resources Development Division. Tank Data Cards of Village Tanks. An official compilation (over a number of years} with a data card for each tank. Ministry of Plan Implementation, 1983. Puttalam District Sanwardanaya. A Sinhalese publication on the development of Puttalam District, Government Press, Sri Lanka, 15 pp. Planning Consultancy, IRD Project, Puttalam and Kurunegala Districts, 1982--1984. Reconnaissance and Technical Reports, submitted to the Ministry of Plan Implementation (Sri Lanka): a report for each tank.
351
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
[1] THE DEVELOPMENT OF MICRO-CATCHMENTS IN SRI LANKA
U.D.S. UDAWATTAGE
Resources Development Consultants Ltd., 55, 2/1, GaUe Road, Colombo 3 (Sri Lanka) (Received February 20, 1985; accepted for publication March 8, 1985)
ABSTRACT
Udawattage, U.D.S., 1985. The development of micro-catchments in Sri Lanka. J. Hydrol., 80: 351--359. This paper gives an informatory account of the development of micro-catchments in Sri Lanka for irrigation. Information for the paper comes from the experience of detailed investigation and preparation of technical proposals for the improvement of 170 existing micro-scale reservoir schemes in the Puttalam and Kurunegala districts, over a period of three years (January, 1982 through December, 1984), under a World Bank-financed project called "Integrated Rural Development (IRD) Project". Investigations were carried out by a private body called Planning Consultancy, a professional staff comprising a team of eight engineers from the Resources Development Consultants Ltd., Sri Lanka, under a contract with the Ministry of Plan Implementation and under the supervision of the Irrigation Department of Sri Lanka and a World Bank-recommended body called Supervisory Consultancy. The writer is a member of the Planning Consultancy team who personally investigated and designed 30 of these schemes.
INTRODUCTION
Micro-catchments have n o t received much attention in hydrology, mainly because of the insignificant role they play, as far as their water resource is concerned, to a point of affecting a country's economy. However, in Sri Lanka, micro-catchments play a very significant role simply because the livelihood of a significant portion of the population is directly dependent on them. It has been estimated that there are more than t w e n t y thousand microcatchments in Sri Lanka, across which earthdams have been built to form mini-reservoirs (popularly known as "village tanks"), to store water during the rainy period, to be used for irrigation of paddy during the dry period of the cultivation season. A b o u t half of them have been systematically recorded by the government, with all technical and administrative details, in forms called "Tank Data Cards" (Ministry of Lands and Land Development, Water Resources Development Division). The village tanks represent a significant share of the country's rice production (Ariya Abeysinghe, 1982). In addition, the village tanks, b y intercepting a considerable portion 0022-1694/85/$03.30
© 1985 Elsevier Science Publishers B.V.
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CO 01 b3
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of the catchments of the major river basins, reduce land erosion and flood inflow to major rivers. Most of the village tanks have been constructed during the times of kings, hundreds of years ago, and have been restored by the Irrigation Department (Ariya Abeysinghe, 1982). The sample of 170 tanks in the Puttalam and Kurunegala Districts for which detailed investigations were carried out by the Planning Consultancy is less than 5% of the total number of tanks in these two districts (Irrigation Department, 1975; Ministry of Plan Implementation, 1983), however, it can be considered to be an important sample, as it fulfilled certain selection criteria stipulated by the World Bank and the government of Sri Lanka, one of which is that the paddy cultivation under each tank should not be less than 8ha which eliminated a large number of tanks, permitting only the significant tanks to be taken up for improvement. The scatter on Fig. 1 shows the village tanks investigated by the Planning ~2
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MAHAMARAWERtYA TANK d
Tcnk A r e a Catchment Cultivated Paddy
Arec
IShc 2 Area 1"211 km
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Oc|cmba
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S e c t i o n Of E a r t h Dart UsedAue Cart Road Spillway
Fig. 2. A minor tank across a major road.
!
S a m Long
354 Consultancy under the IRD Project, in the Puttalam and Kurunegala districts and Fig. 2 shows a village tank, the Mahamanaweriya Tank, right across the Colombo--Puttalam main road (this is a minor village tank with a small e x t e n t o f cultivation, located in a fairly developed area close to the wet zone--dry zone boundary, and investigated under the IRD Project). The location o f this tank in the district is shown in Fig. 1. According to the statistics collected from the technical reports of the 170 tanks (Planning Consultancy, IRD Project, Puttalam and Kurunegala Districts, 1982--1984): (1) The individual c a t c h m e n t area of a significant village tank ranges from 0.5 to 5 km 2 but most of them are between 1.25 and 2 km 2. Some village tanks are in a cascade of relatively large gross catchments, however, the i n d ep en d en t n et - c a t c hm ent o f each tank in the cascade is small. (2) The mini-reservoir (the village tank), created by the earthdam across the t o r r e n t o f the micro-catchment covers an area o f between 8 and 50 ha but most o f them are around 12 ha. (3) The tank capacities range between 50,000 and 600,000 m ~ but most of them are between 74,000 and 185,000 m 3 . (4) The length of the earth dam (better k n o w n as tankbund) varies from 300 to 1500 m which contains an overflow spillway and one or two sluices to release water. (5) Areas o f paddy cultivation under a significant village tank vary from 8 to 40 ha but most of them are around 20 ha. (6) Each village tank has its farmer c o m m u n i t y the size of which depends on the cultivated pa ddy area. The n u m b e r of families directly benefited from the cultivation u n d e r a village tank varies f r o m 10 to 100. Despite the fact t hat these statistics were taken f r o m a very small sample, it can be reasonably said, taking into account the general knowledge acquired a b o u t village tanks during the period of the project, that t h e y cannot be very different f r o m what can be expect ed from a big sample of tanks covering a larger part of the country.
HYDROLOGY OF A VILLAGE TANK
Climatology Sri Lanka has been divided into two zones, called dry and wet zones, based on two monsoonal rainfall seasons: The northeast m o n s o o n (October to December); and the southwest m o n s o o n (April to June). The wet zone, which consists o f only a third of the land area of the c o u n tr y , receives bot h monsoons, and the dry zone, which forms the major part, receives only the northeast monsoon. For reasons o f high and fairly well distributed rainfall, no storage of water for irrigation is required for paddy cultivation in the wet zone, whose main
355
cultivations are export crops -- tea, rubber and coconut -- whereas the cultivation in the dry zone is mainly paddy, which forms the bulk of the country's rice production. There are a large number of major irrigation projects in the dry zone. Village tanks are mostly located in the dry zone, although, there are a few in the wet zone close to the dry zone boundary. Cultivation under a village tank generally begins in October or November with the arrival of rainfall. In general, the direct rainfall is sufficient to sustain the paddy cultivation until the end of December, thereafter, irrigation is required for the two m o n t h s January and February -- the harvest being in March. The rainfall between the period October to December occurs generally as intermittent, isolated showers, intervened by dry spells. Much of direct rainfall could be retained within the paddy field bunds in the cultivated area to be used during these intervening dry spells. The r u n o f f from the rainfall within the catchment area is collected in the village tank. Quite often, it is one or two heavy storms, occurring during the rainy season, which fills the tank, sometimes overnight. The storms are typical of those occurring in tropical areas -- localised, intense and of short duration. Intensity of rainfall largely varies temporally as well as areally, however, the areal distribution of rainfall within a micro-catchment is reasonably uniform. There are two paddy cultivating seasons in Sri Lanka: " M a h a " cultivation (October to March); and " Y a l a " cultivation (April to September). To do a Yala cultivation in the dry zone, it is necessary to conserve water in the tank during the rainy season (October to December) and carry it over to Yala. This is being done in large irrigation schemes with reservoirs across major river basins in the dry zone or schemes with diversion from perennial rivers. However, under a village tank, the catchment area and the tank capacity are so small t h a t the possibility of Yala cultivation is practically nil except under some tanks (very few in number) close to the wet-zone boundary, where the Yala season rainfall is adequate.
Estimation o f catchment area o f a village tank Although the catchment area is the most important hydrological parameter of the catchment, there is no way of directly measuring the catchment area of these micro-catchments. Engineering surveys of Sri Lanka are available in 73 maps, called " T o p o s " , to the scale of 1 mile to an inch ( 0 . 6 2 k m : 1 c m ) with contours at 1 0 0 f t ( 3 0 . 5 m ) intervals. In the dry zone, the interval between two contours covers a large area containing a number of village tanks. Therefore, the delineation of catchment area from the contours alone is n o t possible -except in areas where special engineering surveys with closer contour intervals have been done for major irrigation projects and the catchments of the village tanks within the area have been automatically covered (such cases are n o t many).
\/ _
a
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~- |HALAGAMA TANK
~
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4;
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,
~
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2km ]
VILLAGE TANK
PADDY CULTIVATION
CATCHMENT BOUNDARY
RAILWAY
MAIN ROAD
i I
Fig. 3. Topo inset showing demarcation of catchment boundary of Ihalagama tank -- Extracted from Galgamuwa topo sheet, coordinates F/18 (7,65 × 3.25).
N
di.~
357
In view of the large number of tanks involved and the small extent of cultivation under each tank, investment of m o n e y for the survey of the catchment areas will not bring commensurate benefits, as the tanks are so developed, over a large number of years, to a point t h a t the knowledge of the exact catchment area does not help in improving the status of the scheme, other than helping to do an accurate study of hydrology of the catchment area. In a topo map, village tanks can be seen scattered all over and a number of village tanks can often be found on the topo within a very small area. Therefore, by separating the other tanks around, it is possible to demarcate the catchment area of a village tank indirectly and to determine the area with reasonable accuracy. Figure 3 shows such a demarcation of the catchment area of the Ihalagama Tank in the Kurunegala district, investigated under the IRD Project (see Fig. 1 for location in the district}. It is possible to refine the catchment boundary so marked on the topo with field investigations such as locating the ridges on the roads, cart tracks, etc., within the catchment and other visual field observations, as well as by local inquiry. Inflow to a village tank There are no stream-flow measurements in these micro-catchment streams, and there are no rain gauges within most of them. Although, it is possible to indirectly measure the inflow into the tank during a rainy period, by observing the water level and estimating the inflow through increase in tank capacity, such studies have n o t been done. For the purpose of design, the cultivable area and the required tank capacity have been decided rather on simplified, regionalised methods (Arumugam, 1957) as well as on trial and error, based more on experience, than on pure hydrological computations. Experience on filling of an existing tank, frequency of spilling, etc. have given good indication on the rich or poorness of the water resource of the catchment. Based on this experience, decisions have been taken to augment the tank capacity, from time to time, to create additional storage to conserve more water for irrigation. In the light of the limited data available, this seems quite a logical approach for the improvement of an cxi.~ting village tank. Floods in a village tank c a t c h m e n t Each village tank has its own spill -- which may be a concrete free overfall or natural ground with or w i t h o u t a concrete partition wall -- to discharge flood flows. The length of a spill varies from 10 to 30 m. The top of the earth bund is generally about a metre above the spilling level to allow freeboard for surcharge storage for floods etc. These spillways have been de-
358 signed with simplified hydrological techniques (Arumugam, 1957)as well as trial and error, based largely on experience. Village tanks are in flood following storms of short duration but with very high intensity, occurring when the tank is already full and the catchment is wet with antecedent rainfall. Over the last 30 years, village tanks have breached, in large numbers, during four major events: the two most recent being the cyclonic rainfall that occurred in November, 1978 and the high rainfall in April, 1984. The causes of failure are generally as follows: (1} the inflow is exceptionally high; (2) breach of an upper tank in the cascade, bringing a surcharge flood volume; (3} poor contact between concrete and earth at a structure such as the sluice or an a b u t m e n t of the spillway; (4) inadequate spillway or poor getaway or approach conditions of the spillway; and (5) a poorly constructed weak section in the bund giving way due to surcharge water pressure. Except in the cases 2 and 3, it is not easy to establish the exact cause of failure. Statistics were collected from the field offices of the Department of Agrarian Services, which maintains the village tanks, on the number of significant village tanks in the Put talam District, breached during the most recent high rainfall event which took place on 22 April 1984 {within the period of the IRD Project), mainly affecting the tanks in the Puttalam district (note: the recorded rainfall at Puttalam town on this day is 275 mm which is the highest recorded daily rainfall over the past 80 years). It was found that out of 488 tanks, only 78 tanks had breached. This is 16% of the total number of tanks. The fact that 84% of the tanks did not breach indicates that the floods of the village tanks are of "handleable" size and the simplified and the trial and error methods used in the design of village tanks, which are constructed with low-level technical resources available at the village level, do not drastically affect the economic cost of maintenance of village tanks. Since the bund height of a village tank is small {2.5--5 m) the refill of the breach, the length of which can vary from 10 to 25 m, is not a major problem. However, the breach of the tank bund causes the loss of a cultivation, due to the emptying of the tank. The study of flood in the design of spillway of a village tank should be viewed on this background and experience plays a major role.
CONCLUSION Micro-catchments in Sri Lanka are a fascinating domain in hydrology, both by their very number and their importance to the rural life and to the country's economy. They have not been taken seriously by the hydrologists, but an in-depth study of them will not only be interesting from the aca-
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demic point of view but also helps to reduce the cost of improvement and maintenance, by refining the hydrological techniques used at present for their improvement, thereby, making a contribution to the social life of the rural c o m m u n i t y as well as to the country's economy.
ACKNOWLEDGEMENT
The writer wishes to thank Mr. D. Dantanarayana -- Chief Engineer (Water Resources) of the Resources Development Consultants Ltd. and the Team Leader of the Planning Consultancy, IRD Project -- for giving all assistance for the preparation of this paper and for reviewing the paper and making valuable comments. The writer also thanks the staff of the Planning Consultancy for their assistance.
REFERENCES Ariya Abeysinghe, 1982. Minor Irrigation in the Agricultural Development of Sri Lanka (Part I and II). Economic Review. People's Bank, Research Department. Vol. 8 (6): 25--28; (7 and 8): 22--28. Associated Newspapers of Ceylon, Colombo, Sri Lanka. Arumugam, S., 1957. Development of Village Irrigation Works. Government Press, Sri Lanka, chapters III and IV, pp. 13--24. Irrigation Department, 1975. Register of Irrigation Projects in Sri Lanka. (An Irrigation Department official publication, listing names of irrigation projects, their locations etc.), Ceylon Printers, Colombo, Sri Lanka, pp. 26--50. Ministry of Lands and Land Development (Sri Lanka), Water Resources Development Division. Tank Data Cards of Village Tanks. An official compilation (over a number of years} with a data card for each tank. Ministry of Plan Implementation, 1983. Puttalam District Sanwardanaya. A Sinhalese publication on the development of Puttalam District, Government Press, Sri Lanka, 15 pp. Planning Consultancy, IRD Project, Puttalam and Kurunegala Districts, 1982--1984. Reconnaissance and Technical Reports, submitted to the Ministry of Plan Implementation (Sri Lanka): a report for each tank.