Mae Pai - Mae Taeng diversion study

Mae Taeng Irrigation area of 148,000 rai is on the right bank of the Ping River in Chiang Mai province. The proposed dry season crop area 60,000 rai comparing with the actual growing area of 90,000 rai. Cropping pattern used in this study is based on the crop area of the year 1989 with the growing area of 143,663 rai (rice and fruit tree) and 91,780 rai (rice and upland crop) during the wet and dry season respectively. The calculated irrigation efficiencies are 30 % and 69 % for wet and dry seasons respectively. These calculated irrigation efficiencies are based on the following assumptions, for calculation of effective rainfall namely : STMIN = 50 mm, STO = 100 mm, and STMAX 120 mm for rice and STMIN = 0 mm and STMAX = 25 mm for upland crop. The hydrological data for the period of 1968 - 1989 is used for calculation of irrigation water demand. The assumed irrigation effciencies at the main canal intake of 45 % and 60 % for wet and dry season respectively are based on the calculation ones. The maximum, average and minimum water demands at the main canal intake are 28.58 cms, 10.46 cms, and 1.2 cms respectively. While the water demand shortage is varying from 0.7 cms to 8.3 cms and the average excess flow over Mae Taeng weir to the Ping River is 11.9 cms (maximum of 24.5 cms. and minimum of 1.6 cms.) Mae pai - Mae Taeng water diversion study by Hec - 3 programme is divided into 6 alternatives namely : PTO, PT1, PT2, PT3, PT4 and PT5. Input data are characteristic of Mae Pai Dam, monthly runoff at Mae Pai dam site and Mae Taeng headwork, hydro - meteorological data, Mae Taeng irrigation water demand. Minimun released from Mae Pai Reservoir of 2.0 cms and Chiang Mai domestic use of 1.0 cms are assumed. The result of the study reveals that the case of PT4 with the tunnel diameter of 4.5 m, reservoir capacity 30 mcm, the velocity of water in the tunnel of 2.6 m/s with corresponding to the average annual diverted water of 400.5 mcm is the best alternative. The effect of flooded area in the reservoir is also small. This diverted water can supplement the water shortage in the Mae Taeng Irrigation Project on the average of 10.4 mcm and 15 mcm in the wet and dry seasons respectively. The rest of the diverted water (375.1 mcm/year) flows into Ping River and finally becomes inflow of Bhumipol Dam.