THE EFFECT OF SEPARATING SNOWY AND RAINY SEASONS ON THE ACCURACY OF SUSPENDED SEDIMENT ESTIMATION IN GORGANROOD RIVER, IRAN

This study aims to investigate the effect of separating snowy and rainy seasons on the accuracy of suspended sediment estimations in Gorganrood River at the Gazaghly gauging site، Iran. In this site، there are about 2000 concentration samples taken during six years. In a comparative research، 20 sets of 52 weekly samples each year (including 22 samples during snowy-rainy seasons and 22 samples during rainy seasons) classified in four treatments and five replications were taken. Then، 20 sets of extra random samples in four different sizes (0، 25، 50 and 75) were extracted from high flow periods and were added to the respected treatments. With regards to the sampling strategies، 60 sample sets (including 20 for rainy seasons، 20 for snowy-rainy seasons and 20 mixed) were produced. Suspended sediment loads for each sample set were estimated using five different sediment rating curves including one curve (conventional)، parametric، non parametric، and FAO correction factors (PCF، NPCF، RCF) and logged mean load within discharge classes (LMLWDC) based on daily discharge data. Therefore، 100 (20×5) estimates for each treatment –rainy season، snowy-rainy season، mixed data، and sum of two seasons loads– (400 estimates altogether) were obtained. To evaluate different load estimation scenarios، three indices including percent error، coefficient of variation and normalized root of mean square deviation were calculated. The results showed that the LMLWDC and PCF rating methods produce better estimations with the lowest percent error، and more precise and accurate than other rating curves. The results also revealed that separation of rainy and snowy periods has no positive effect on the estimations. In addition، the results showed that the accuracy and precision of estimations in the rainy season were lower than the snowy-rainy one.