【正文】 HURULUWEWA and WAHALKADA, and many village tanks and ancient irrigation channels which are overhauled. The tanks that are functioning in a cascade fashion are very important water source for irrigation. Generally the local villages center around these tanks. The Yan Oya originated in the hilly areas of Dambulla and Sigiriya since it flows from southwest towards the northeast. In the middle river, there are Adappan Oya and Sellige Oya joining in the Yan Oya. The upper river is also known as Sigiri Oya and Habarana Oya. In the downstream, the Thawalam Halmilla Oya meets the main stream near the village Pangurugaswawa. The Yan Oya pours into the Indian Ocean in the southern region of the Pulmoddai, whose mainstream length is 150km. This river crosssection is uniform with the mild gradient of ‰ . The proposed dam site is Feasibility Study of Yan Oya Reservoir Project in Sri Lanka Chapter 2 China CAMC Engineering Co., LTD. 6 located in the down reaches of Yan Oya and the upper reaches of Pangurugaswewa, and the catchment area of dam site is 1341km2 Annexure figure SYS201 shows the Yan Oya cathment hydrographic s. Meteorology Sri Lanka is located in the tropics. Its position endows the country with a tropical maritime climate strongly influenced by monsoon. The Yan Oya Basin lies entirely within drought and water shortage zone of Sri Lanka. The rainfall pattern is influenced by the tropical monsoon from the Indian Ocean and Bay of Bengal and the seasonal variations for precipitation are more pronounced. During the period from October to next February, this region is controlled by the Siberian cold anticyclone produced in the Mongolian plateau of Asia. Influenced by the pressure gradient and Coriolisforce, the dry and cold northeast monsoon is prevailing. After ing from the Bay of Bengal, the northeast monsoon carries large amount of water vapor. As the winds encounter the warm and humid southwest air currents forming from subtropical low pressure or the mountain slopes of the Central Highlands, they unload heavy rains on the northeast areas of the island. This time is the main precipitation period, 70% of annual rainfall concentrate here. During the rest time, it is dry and only 30% of the rainfall is received. The annual range of temperature in the valley is small and the mean temperature of the area varies from 20℃ to 33℃ . The highest temperature occurs in the period from March to April as temperatures are lowest in January and February. Humidity is typically higher depending on the seasonal patterns of rainfall. The mean annual relative humidity is about 77%, which varies from about 80% during the northeast monsoon period to 65% in the period from March to September. Mean monthly wind velocities vary from 4 km/h to 14km/h, which are highest during the period from August to September and lowest in November. Evaporation in the project area is very high and the average annual evaporation Feasibility Study of Yan Oya Reservoir Project in Sri Lanka Chapter 2 China CAMC Engineering Co., LTD. 7 amounts to maximum evaporation during the period from July to September is about 200mm. It is minimum during November to next January which is of around half of the maximum value. Hydrological Information There are 15 raingauge stations, 6 rivergauge station and 2 evaporate station in and around the design watershed. Annexure figure SYS201 shows the locations of these gauge stations. Table 21 and table 22 gives situation of those gauges station in and around Yan Oya and Ma Oya. Most of the surrounding watershed rainfall station began to run successively in 1944. Now there are a total of 48 years precipitation series from 1944 to 1991, which can be used in designing. Although stations of Kanthalai, Maradankadawala, Mihintale and Minnerya are outside Yan Oya valley, the Yan Oya watershed boundaries are gentler and their own data are the longest. Therefore, four stations can be used as reference station for checking the homogeneity of their records through double mass analysis. Stations of Habarana, Kantaletank, Mihintale, Horowpothana, Maradankadawala, Kebithigollewa, Padaviya and Gomarankadawala have better dominating condition for the basin and relatively longer observation data series, so they can be the main basis stations for storm analysis and runoff analysis. There are two evaporation stations Padaviya and Kanthalai, of which the Padaviya station was considered as more representative for Yan Oya basin. And its data record is short, only three years from 1974 to 1977. Compared with this, the Kanthalai station established by the Department of Agriculture has a longer and more plete record and it can offer a mutual plementarily with the Padaviya station. The data record of stations mentioned above have been reviewed through reanization and published successively, whose accuracy can meet the engineering design requirements. Feasibility Study of Yan Oya Reservoir Project in Sri Lanka Chapter 2 China CAMC Engineering Co., LTD. 8 Table Raingauge Station List Station No. Name Location Period of Record Length of Record Latitude Longitude Year 137 Gomarankadawala 8176。40′23″N 80176。57′08″E 19441976 33 162 Habarana 8176。22′03″N 80176。45′00″E 19641991 27 198 Horowpothana 8176。32′44″N 80176。49′16″E 19441970 26 222 Kahatagasdigiliya 8176。25′31″N 80176。41′06″E 19441973 30 247 Kanthale Tank 8176。21′12″N 80176。58′36″E 19441991 48 261 Kebithigollewa 8176。38′20″N 80176。40′07″E 19441985 42 323 Medawachchiya 8176。32′28″N 80176。29′14″E 19441988 45 355 Maradankadawala 8176。07′49″N 80176。33′33″E 19441991 48 374 Mihintale 8176。21′16″N 80176。30′25″E 19441991 48 382 Minnerya Tank 8176。02′22″N 80176。53′35″E 19441991 48 445 Pad