Effects of subsequent rainfall events with different intensities on runoff and erosion in a coarse soil

A deeper understanding of the hydrological response to subsequent rains would be useful in the prediction of runoff production for planning vegetation restoration and assessing flood risks. We used subsequent rains to study the role of rain intensity and antecedent soil moisture content (ASMC) on runoff and erosion for coarse soil of the semiarid Loess Plateau in China. The study used a rain simulator in a field planted with alfalfa (Medicago sativa), which is widely grown for animal feed to develop livestock operations, reduce soil erosion, and improve soil fertility/quality. A slope of 18% was selected because most of the land with slopes <18% in the region is used for cropland. We tested three rain intensities (20, 40, and 60 mm h−1, corresponding to low, moderate, and high intensities, respectively) with five successive rains (an initial and four subsequent rains) in triplicate. We quantified the changes of runoff depth (RD), sediment yield (SY), and sediment concentration (SC) over time and then analyzed the relationships between ASMC and runoff in 0-50 cm soil layers for all 45 simulated rains. Runoff commencement time (RCT) was shorter, the runoff coefficient (RC) was larger, and runoff was higher for the moderate and high intensities than the low intensity. Intermittency and the characteristics of the sequential rains also influenced these processes. A general linear model identified significant effects of rain sequence and intensity on RCT, RD, RC, SY, and SC (P < 0.01), but their interaction did not have a significant effect on RCT and SY. An exponential fit between ASMC and RC was best for the 0-10 cm and 10-20 cm layer (R2 = 0.38, P < 0.000), and R2 decreased from the 0-20 cm to the 30-40 cm layers. Soil moisture content (SMC) was an important factor controlling runoff, and the sequential rains led to high runoff and sediment transport, because runoff from storms on highly permeable soils is controlled by the saturation of the topsoil horizon and is more dependent on initial conditions.