Evaluation of watershed management measures effects on particle size distribution and phosphorus and organic carbon amounts in sediments (Case study: Zidasht Taleghan paired watersheds)

Soil erosion is one of the factors that can reduce soil permeability in watersheds and thus increase the risk of floods. Plant biodiversity is weakening and the sediments pollute the surface water and fill dam reservoirs. In recent decades, various watershed management measures have been taken, including the construction of sediment dams and conservation measures such as exclosure in watersheds to reduce the effects of erosion throughout the country. The present study was conducted to investigate the effect of watershed management measures on vegetation, sediment and phosphorus content, organic carbon and sediment granulation in Zidasht Taleghan paired watershed.

The study area is Zidasht Taleghan paired watersheds located in Alborz province, which includes control watershed and treated watershed with an area of 92 and 104 hectares, respectively. Using 1: 25000 topographic maps and field survey in rangeland areas, vegetation types were identified and studied. Also, percentage of canopy cover, rocks and, biomass and bare soil percentage were measured in 35 plots in sample and control watersheds and the average of each factor was calculated. The amount of sediment was also measured by measuring the total volume of sediment accumulated in the reservoir tank of both areas. Three samples were taken from the sediments of each outlet of the watershed and the amounts of total phosphorus, organic carbon and particle size distribution were measured in them. The total amount of phosphorus and organic carbon output from the watersheds was also calculated.

The results of vegetation assessment showed that the percentage of vegetation in the treated watershed (53.9) was about 8.7% higher than the control watershed (45.2). The results of evaluation of outflow sediments showed that the amount of outflow sediment in the treated watershed (520.5 tons) was about 72% less than the control watershed (34.06 tons). In other words, the portion of the control watershed was 86.05% and the treated watershed was 13.95% of the total reservoir sediments in the output of the watersheds. The results of particle size distribution also showed that the percentage of particles with a diameter less than 0.5 mm in the treated watershed (26.8) was 18.6% higher than the control watershed (8.2). Also, the amount of total organic carbon (40.26 kg) and total phosphorus (13.44 kg) in the sediments of the treated watershed were 11.81 and 70.88% less than organic carbon (51.9 kg) and total phosphorus (78.82 kg) of control watershed, respectively.

Watershed management measures have prevented the transfer of sediment and fertile soil to downstream, especially in the reservoir of Taleghan Dam. Although these elements in the watershed increase soil fertility, improve plant nutrition, increase soil permeability and create strengthening conditions for plants and soil, if they enter the downstream and the main river due to erosion, they will have the opposite role and cause pollution and reduction. The cost of improving water quality will be compensated with implementing watershed management measures, due to the low cost of implementing watershed management measures.