Plot sizes dependency of runoff and sediment yield estimates from a small watershed

The spatial scale dependency of runoff generation and sediment yield and accuracy to extending the results of experimental plots to surrounding watersheds is rarely taken into account. The present paper aimed to evaluate the accuracy of soil erosion plots with different lengths in estimation of runoff and soil loss from a small watershed in Iran. Towards this attempt, two similar sets of experimental plots with lengths of 2, 5, 10, 15, 20 and 25 and fixed width of 2 m were installed on two north and south facing slopes (i.e. 6 plots on each slope and totally 12 plots) in Sanganeh watershed, Razavi Khorasan Province with an area of ca. 1 ha. Runoff and sediment was collected at the outlet of each plot as well as main outlet of the watershed associated with 12 storm events occurred during November 2006 to June 2007. The results verified that the accuracy of plot estimates on sediment and runoff improved while the plot length increased. The optimal lengths for estimation of sediment and runoff parameters were found 15 and preferably 20 m for plots located on southern slope. No significant difference was also proved among estimates obtained from plots established on north facing slope and those measured at the main outlet of the watershed.

Research Highlights► We explore the impact of plot size on estimation of a small watershed outputs. ► Different lengths and fixed width plots were installed on two slope aspects. ► The performance of two similar sets of experimental plots was examined. ► The optimal lengths for estimation of sediment and runoff were finally found.