Surface runoff volumes from vegetated slopes during simulated rainfall events

The effect of rainfall intensity, slope gradient, initial soil moisture, and vegetation cover on runoff intensity and its progress were investigated using simulated rainfall. Various surface treatments of soil packed into bins were examined, namely bare ground and plantings with purple medic (Medicago sativa L.), spring wheat (Triticum aestivum L.), or ryegrass (Lolium perenne L.). Five different rainfall intensities (0.5, 0.75, 1, 1.5, and 2 mm min(-1) [0.02, 0.03, 0.04, 0.06, and 0.08 in min(-1)]) and four different slope gradients (5 degrees, 10 degrees, 15 degrees, and 20 degrees) were studied. The main results were as follows. (1) There was a positive linear relationship between mean runoff intensity and rainfall intensity, gradient, and initial soil moisture, while there was a negative exponential relationship between mean runoff intensity and vegetation cover. (2) The piecewise function approach was used to determine effective vegetation cover values; these were found to be 52%, 62%, and 73% for purple medic, spring wheat and ryegrass, respectively. (3) Runoff intensity increased with increasing rainfall duration; the relationship was logarithmic, and the time to stable runoff was proportional to gradient but inversely proportional to rainfall intensity and initial soil moisture: (4) The function describing the relationship between runoff intensity and rainfall duration was gamma = ae(b/x) (b < 0) for limited vegetation cover (<60%) and gamma = a + bln(x) for more dense vegetation cover (>60%).