عنوان مقاله [English]
Evaporation is generally regarded as the most significant contributor to water loss from reservoirs in semi-arid regions of the world, such as Iran, and a precise estimate of this parameter is increasingly required for several agency functions, particularly water agencies. Monthly summer evaporation from the Saveh Dam reservoir, which accounts for around 50% of yearly water loss, was determined using the Bowen Ratio Energy Budget (BREB) method, and then compared and verified by results obtained by an automatic weather station and net
radiometer placed near the dam reservoir site.
In order to assess the data obtained from the adjacent weather station of the reservoir (Shah Abbasi weather station) and experimental equations, an automatic weather station (AWS) and a net radiometer were located near the reservoir for collecting parallel data, during four months of 2008 (July-October). With these data, all Shah Abbasi weather station data have been adjusted and the BREB method was modeled again for all of the study period.
These four months were divided into 12 energy budget intervals, and, for two thirds of them, the modified BREB results (MBREB) were larger than BREB. Furthermore, the largest difference between them occurred from the 9th to the 15th of July, with 11.2 mm, and there was at least 1.7 mm difference for the 8th to the 22nd of October interval. The maximum and minimum differences between results of evaporation using these two sets of data for these four months of field study were 15 and 3 percent, respectively, and the total evaporation difference was equal to 75 mm. Also, the yearly average BREB evaporation rate, using field study data (MBREB), was equal to 1620 mm, and the result was 1560 mm with Shah Abbasi station data. In addition, the accumulated volume of evaporation rate using field study data (MBREB) was equal to 83.7 MCM and the result was 87.0 MCM with Shah Abbasi station data for these 163 months. A sensitivity analysis for MBREB results showed that parameters, such as longwave and shortwave radiation, reflected longwave radiation from the water surface, air temperature, and water surface temperature variations have large effects on evaporation rates from this small reservoir. However, changes in stored energy in reservoir water, reflected longwave and shortwave radiation, Bowen Ratio and atmospheric pressure, caused small sensitivity in MBREB evaporation rates. Results of this study could explain the importance and sensible contribution of evaporation rates as a fundamental parameter in the hydrological cycle of lake ecosystems in the arid regions of Iran.