Investigation on the relationship between loess soil physico-chemical characteristic and erosional features in watersheds Atrac and Gorganrood in Golestan in province (2001-2003)

There are about 320,000 hectares of loess deposits in Golestan province in north east of Iran. Loess deposits are normally very fertile wind deposits and they are valuable land resources in agriculture and forestry production. However, soil erosion in these quaternary valuable land resources are very high in Golestan. The on-site effect of soil erosion in the loess deposits is loss of soil productivity and the off-site effect is sedimentation in the waterways and rivers valleys. The study of erosion features and their association with the loess properties could be important tool to be used in land resource management. Therefore, the study of erosion features and their relationships with loess properties in addition to the climate are important and set as objectives of this study. At first, the loess deposit distribution map in the province was made by using TM Landsat digital data in ILWIS-GIS environment. Using overlaying technique of two layers of the climate map (Demarten climate map) and loess deposit map, 6 areas based on the gully, rill and tunnel erosion features were selected for study in the field. In each area, three replicates of gully erosion, rill, and tunnel erosion type were selected for further study and analysis. Soil samples were extracted from each erosion type for analysis of percent of sand, silt, and clay, carbonate, ESP, ECe, pH, OM, gypsum percent, SP. For the erosion features, the following geomorphic variables were selected for measurements: length, height and volume were measured for gullies, depth of tunnel, track over the tunnels, area of the tunnel surface were measured. For rill erosion, only the ratio of width to depth was measured. In the regression analysis, Ca, Na content, and Salinity (ECe) had direct relationship with tunnel depth but it had inverse relationship with OM, pH, and SP. Length of track in tunnel erosion had direct relationship with SAR, Ca, gypsum, pH, ECe but it had inverse relationship with sand and OM percent. The length of tunnels also had inverse relationship with Demarten climate index. The area of tunnels had direct relation with sand and silt percent, and it had indirect relation with clay percent. In gully erosion, depth of gullies had direct relation with silt percent and inverse relation with clay and gypsum percent. Domarten index had also high relation ship with silt, clay and gypsum. Length of the gullies had direct relationship with sand percent and SAR and it had inverse relationship with gypsum and carbonates. The volume of the gullies had direct relation with SAR, sand and indirect relationship with caly, gypsum, carbonates and SP index. In rill erosion, correlation of width to depth ratio with carbonate percent was inverse and this correlation with Domarten index and clay percent was direct.