نقش ته نشستها در تغيير برخي و تركيبهاي فيزيكي و شيميايي اراضي در شبكه پخش سيلاب موغار اردستان

Role of deposited sediments in changing physio-chemical properties of soils in the Moghar floodwater spreading system

Floodwater spreading is an efficient and appropriate method for the optimization of runoff utilization, particularly in arid and semiarid region. Besides the reduction of damage caused by the floodwater, this technique would also be useful in the artificial recharge of groundwater, rangeland rehabilitation, and desertification control. The objective of this study was to evaluate the physical and chemical characteristics of the sediments carried into the Moghar floodwater spreading system and the resulting problems. Samples of sediments and soils from different strips were taken and some of their physical and chemical properties, including texture, salinity, alkalinity and the amount of lime, nutrients and organic matter, were determined. Also, the infiltration rate was determined in both soils and sediments in different strips. The results show that more than 100000 tons of fine sediments have been deposited in the system as a result of four times flooding. The thickness of deposition varies from 91. cm in the first strip to I 1 cm in the fifth one. Moving from the first to fifth strip, the amount of sand in the sediment decreases, whereas the clay content increases. Soils of the area contain excessively higher sand content but much lower silt and clay as compared to sediment. While the sediment deposited have no salinity and alkalinity problem, the natural soils of the area are highly saline and alkaline. Furthermore, in contrast to low fertility of the soils, deposited sediments have more organic matter as well as higher total N and available K. Nevertheless, the deposition of the fine materials in different strips of the system has caused a great reduction in the infiltration rate, even in the fifth strips with only 11cm depth of deposit. It seems that a great portion of the water entered the system would be lost through evaporation if no measure is taken to reduce the floodwater turbidity. Deposition of a great deal of very fine sediments would also lead to reduction of system efficiency and lifetime.