JWSS - Journal of Water and Soil Science

Soil erosion by water is a serious environmental problem, particularly in semi-arid regions. In these areas, water loss strongly affects soil loss as well as soil productivity in the rainfed lands. Determination of appropriate seed density for each tillage direction is vital to achieve high crop yield and to prevent soil and water losses. This study was conducted to investigate the combined effects of tillage direction and plant density on the soil and water losses in a rainfed land. Twelve crop plots with the dimensions of 1.5 m × 5 m were installed to investigate the effect of two tillage directions (up to the down slope and on the contour line), two seed densities (90 and 120 kg h^-1), a three replications in a rainfed land with 10% slope steepness. Soil and water losses were measured in each plot during the wheat growth period (from October 2015 to June 2016). Significant differences were found between both tillage direction and plant density in the runoff (P<0.05) and soil loss (P< 0.001). Runoff and soil loss in the up to down slope tillage was 4.16 and 4.08 times bigger than the contour line tillage, respectively. Runoff and soil loss with the seed density of 120 kg h^-1­ ­­were 11.25 and 26.32% lower than those with 120 kg h^-1­, respectively. This result was associated with the increased cover crop and its control on water flow and the enhancement of water retention in the soil. There was no significant interaction between tillage direction and plant density in the runoff and soil loss. The importance of tillage direction in the soil and water loss was very larger than that of the plant density. The application of 120 kg ha^-1 seed density on the contour line could, therefore,   considerably prevent soil and water losses in the rainfed lands.

The objective of the present study was to investigate the performance of some of the extracted information for mapping land degradation using remote sensing and field data in Fras province. Maps of vegetation cover, net primary production, land use, surface slope, water erosion, and surface runoff indicators were extracted from MOD13A3, MOD17A3, Landsat TM, SRTM, ICONA model, and SCS model, respectively. The rain use efficiency index was obtained from the net primary production and rainfall map, which was calculated from meteorological stations. The final land degradation map was prepared by integrating all the mentioned indicators using the weighted overlay method. According to the ICONA model, 5.1, 9, 47.21, 27.91, and 10.73 percent of the study area were classified as very low, low, moderate, severe, and very severe water erosion; respectively. Overlaying the ICONA map with other indicators showed that very high and high classes, moderate, and low and very low classes of land degradation covered 1.3, 18.7, 70, 0.9, and 9.1 percent of the study area, respectively. According to the results, integrating remote sensing with ICONA and SCS models increases the ability to identify land degradation.

Splash (raindrop) erosion plays an significant role in soil loss, especially in arid and semi-arid regions with poor vegetation. In this paper, by analyzing the pattern of rainfalls that occurred during 26 years in four basins located in Semnan County, their effect on the pattern of eroded sediments from the basin was investigated. Sedimentary layers from the sampling of retarding reservoir sediments in 2017 were related to the corresponding precipitations. Due to the occurrence of the highest amount of rainfall in each quarter of rainfall, rainfall hyetographs were divided into four categories. Cumulative precipitation curves with similar quartiles were drawn in one shape and compared with sediment curves and vice versa taking into account the physical characteristics of the basin. The results showed that the Aliabad basin (with less slope and more elongation) with an effective quarter of type 3 had the highest similarity in precipitation and sediment patterns. Also, the Western Soldereh basin (with the highest slope and the least elongation) with an effective quarter of type 2 had the least similarity in precipitation and sediment patterns. The results indicate the vital role of rainfall patterns on the resulting sediment patterns, which show up to 85% similarity.