elias khajavi

The amount of country's water erosion is estimated at a rate of 976 million t.yr-1 by Erosion Potential Method (EPM) based on the national erosion map prepared at the Soil Conservation and Watershed Management Research Institute. This map is suitable for identifying the most sensitive areas to erosion in each region, but the estimated values ​​of the model need calibration, which was estimated ​​based onthe observed sediment yield data of 276 watersheds. The minimum and maximum annual observed sediment yields were 4 and 5591t.km-2, and estimated by the model was 31 and 2946 t.km-2, respectively. Correction ratio in the eight homogeneous regions was between 0.57 to 1.43 by dividing the median of the observed sediment yields to the corresponding ​​estimates in the studied watersheds. These ratios then were used to improve the erosion values ​​for homogeneous regions between 1063 and 130 t.km-2.yr-1, belonging to the Oman Sea (Region 8) and the internal desert (Region 6) basins, respectively. The annual erosion of the country was estimated to be million 895 t.yr-1. The(5.5 t.ha-1.yr-1), with the highest and lowest contribution, respectively to the Region 8 (33%) and the Region 1 or Caspian Sea coastal basins (2.5%). It was also found that the observed specific sediment yield had a positive correlation with an increasing area unlike the expectation, with its probable reasons have been explained in the paper.

Soil erosion is a serious issue in Iran. Determining the rate of soil erosion in various regions could help for better understanding of the erosion condition and soil conservation planning as well. In accordance with this constraint, more than 840 measured values derived from Cs-137 method and experimental plots in 31 research sites countrywide were collected, and analyzed related to different land uses, climates and slope classes. The existence of a few hundred times difference between cesium- 137 derived soil erosion and experimental plot's soil loss prevent us to combine these two sets of data. Comparison of the soil erosion and soil loss value in different land uses characterized the forest with minimum rate followed with ranges and dry farming respectively. Nevertheless, soil loss from poor ranges on marly formations was analyzed in a separate class due to nearly ten-fold differences with other ranges. Surface cover, tillage, and land sensitivity to the erosion are some factors that affect these differences. Comparison of two measured data sets in different climates indicated that the highest rates are generally matched to the Mediterranean climate. Lower soil loss in other regions could be related to the dense vegetation cover in humid area and low rainfall in arid climates correspondingly. Also, it is noted that apart from exceptional cases, the highest rate of soil erosion and soil loss are measured on gentle slopes between 1.5-12 percent which reduces by an increase in slope degree. The reduction of erosion rate in steeper slopes most probably relates to the abundance of surface gravel cover.