DETERMINATION OF THE WATER YIELDS FOR SMALL BASINS IN SEMI-ARID AREAS: APPLICATION OF THE MODIFIED TURC METHOD TO THE TURKEY’S CONDITIONS
2005, 6 (3) p. 263-268
The Turc Method is used widely in Turkey to determine runoff depths therefore, water yield from a particular watershed and subsequently the reservoir’s volume by Turkish General Directorate of Rural Services which is responsible for the investments on agricultural and rural infrastructures. However the method over predicts the water yield markedly when compared to the directly measured long-term water yields, which increases the total cost for the instruction of reservoirs and leads to environmental hazards due to disturbing more agricultural areas. In this research, the Turc Method was modifi ed through replacing the new coeffi cients with the original coeffi cients of the 300 and 0.9 by fi tting the calculated values to the directly measured long-term, a total of 223 years, in 22 sub-basin distributed throughout Turkey. Coeffi cients 566 and 0.68 were proposed as average values for Turkey in general instead 300 and 0.9, respectively, though the new coeffi cients for a particular watershed varied widely from 20 to 1135 and from 0.4 to 1.32, respectively. The country’s sub-basins divided into three groups in terms of basin characteristics affective on these coeffi cients and new coeffi cients were also suggested for each group. Employing the modifi ed Turc Method with these new coeffi cients for the research sub-basins can reduce the reservoir’s volume by 45 % and this may decrease the total cost of the reservoirs by about 20-25 % through reducing occupied surface area, embankment and crest height.