Measurement of the connectivity of runoff source areas on a bare soil surface in a laboratory experiment

As soil erosion has been severely constraining agricultural production and economic sustainability in the Loess Plateau in central China, study of hydrologic connectivity of tilled slope surfaces based on higher accuracy has major implications for prevention of soil erosion. This study explores the properties of hydrologic connectivity of microtopographic slope surfaces (contour tilled, at gradients of 15 degrees, 20 degrees, and 25 degrees in respective events) under artificial rainfall (at 90 mm h(-1)) produced in laboratory experiments.A spatial metric, flowlength, was acquired and tested to quantify the connectivity of runoff source areas and was calculated as the average lengths of the runoff pathways. The grid cells, which measured lengths of flow paths (6 to 10 mm) in runoff source areas at different slopes, accounted for over 46% of the total grid cells on the raster-based map. Flow path lengths increased when the slope gradient was steeper and decreased when the surface roughness increased.The ratio of the surface area connected to the outlet to the total surface area of the soil tank (C, %), which equals the parameter C, or the ratio of the surface runoff area connected to the downstream outlet to the total surface area of the study area, peaked quickly and then became stable under accumulative rainfall.The steeper the slope, the shorter amount of time it took for C to reach its peak; the greater the average C, the better hydrologic connectivity. Flowlength could be an important index for characterizing the structural properties of the hydrologic connectivity of a slope surface, and also simplified hydrograph and relative surface connection function can be used to characterize the runoff generation process and the mechanism of the soil surface and the properties of its hydrologic connectivity. These results are instrumental in providing guidelines on and insight into the role of hydrologic connectivity at a microtopographic scale.