لزوم بازنگري بر مدلهاي برآورد تبخير از سطوح آزاد آب در مناطق خشك داراي اثر واحه اي منطقه اي : مطالعه موردي در مخزن چاه نيمه

Necessity of Revision on Free Water Surface Estimation Models in Arid Areas with Regional Advection: Chahnimeh Reservoirs as a Case Study

Chahnimeh Reservoir as a source of water supply for Zabol and Zahedan cities and about 80,000 ha irrigated area of Sistan is the most important fresh water resource in south east of Iran. Operation programming and management of this reservoir is very difficult because of high rate of evaporation in this area and high risk of water delivery due to recent drought and also source of Hirmand River in Afghanistan. In this paper, specific hydroclimatological characteristics of Chahnimeh Reservoir are described and necessity of investigation on evaporation mechanisms in this area of Middle East is introduced. A volume balance study was carried out and fundamentals of energy balance and mass transport theories were assessed by evaporation computed on the base of volume balance. Results of this study showed that most of methods are not suitable to estimate evaporation from Chahnimeh Reservoir due to regional advection. Priestley-Taylor and Penman-Monteith models showed errors of estimate of evaporation between 32 to 66 percent and 45 to 73 percent, respectively. Mass transport method showed water loss of more than 8 mm/day from reservoir when multiplication of wind speed to vapor pressure deficit is zero. It is necessary to remark that field studies showed that there is not a significant seepage from bottom and banks of reservoir. Calibration of pan coefficient showed a coefficient up to 0.9 among advection period.