Application of vegetation cover seasonality, temporal variation of rainfall erosivity and sediment yield for land utilization planning

Erosion, sediment yield (SY) as well as rainfall erosivity and vegetation cover show seasonality during a year. Basically, most SY occurs during high erosivity and poor soil cover periods. Therefore, in selecting crop and vegetation management programs at the basin level, it is necessary to underestand the temporal changes of erosivity and SY.

Two watersheds, Kasilian in Northern Alborz and Latyan in Southern Alborz were selected and their landuse maps were extracted. Then, spatiotemporal variations of NDVI for three hydrological years including dry, normal and wet years, were derived bimonthly form MODIS data. For estimating suspended sediment, we used a combination sediment rating curve and average daily discharge records. Erosivity factor, EI30, was calculated in the nearest gauging sites for both basins based on 1-minute interval rain records. Simultaneous to the NDVI, suspended SY and rainfall erosivity were computed. Then, based on the correlation of SY with erosivity and vegetation, the relationship was analyzed and finally, the appropriate time of exploitation for forests, rangelands and suitable crops with best ground cover during the erosion risk peaks was proposed.

In Kasilian, four main landuses were forest (97%), orchards, croplands and rangeland, and in Latyan, two landuses of rangeland (94%) and orchards were dominat. The average NDVI of Kasilian and Latyan was 0.67 was 0.13 respectively. However, the NDVI of similar landuses was measured to be about half and less in Latyan than Kasilian. For all landuses, NDVI reaches its lowest value in the colder months. Erosivity did not indicate a clear seasonal cycle, possibly due to occurring erosive storms even during the dry periods. The peaks of the SY did not occur at the same period of time in the studied years in Kasilian basin; while, for Latyan basin, the peaks were concurrent with snow melting and percipitation in the beginning of spring in all three years. In studied dry, normal and wet years, high negative correlations (-0.54, -0.45 and -0.85 respectively) were found between the NDVIs and SYs in Kasilian basin. In contrast, no significant correlation was observed between the two aforementioned factors in the studied years of Latyan basin. There was also no correlation between erosivities and basin SYs. Finally, according to the SY cycle, June and July were proposed as the best time for grazing of rangelands and exploiting the forests in Kasilian basin, and early June to September were proposed in the Latyan basin to minimize the risk of erosion. Due to the high sediment production in the Latyan basin, the number of livestock entering the area and the capacity of the grazing should be more closely monitored. From the point of view of erosion risk, summer Soybeans, Canola and Wheat and Barley, were introduced as suitable and spring Soybeans as unsuitable crops for Kasilian based on to their growth stages.

For soil conservation and management and sediment yield prevention, it is essential agricultural products and the time of exploitation of rangelands and forests be planned and managed based on the erosivity and sediment yield cycles in a way to limit soil erosion.