Analysis of Temporal Variation of Hydrological Processes from Experimental Small Plots in a Forest Watershed

Studying hydrological processes is important for effective management of soil and water resources. However، it has received less attention. Therefore، the present study was conducted to analyze temporal variations of hydrological processes in each of experimental plots in a forest watershed. Experiments were carried out with simulated rainfall intensity of 1. 6 mm min-1 and experimental plots of 30×30 cm in downstream of Educational and Research Forest Watershed of Tarbiat Modares University. The temporal variations of runoff depth، runoff commencement time and sediment yield were investigated for eight months from September 2008 to April 2009. Results indicated that the aforementioned variables have been changing over the time. Runoff depth was measured in March and November with respective value of 12. 5 and 10. 2 mm with the highest antecedent soil moisture of 25 and 23%، and the lowest surface litter cover. Sediment yield was at the maximum rate in April and May with rate of 21. 1 and 18. 9 mg due to reduction in soil surface cover. In addition، runoff commencement time was highly dependent on antecedent soil moisture if the soil surface was not covered well. October with low antecedent soil moisture of 9% had the maximum time of runoff generation of about 67 seconds. Runoff generation had the minimum time of about 32 seconds with the maximum soil moisture in early March. In other months، it was hard to find a reasonable relation in this way، whereby، it could be deduced that runoff beginning time was sensitive to environmental conditions.