Plot-scale sediment transport processes on a burned hillslope as a function of particle size

Abstract

Soil moisture, rainfall, runoff, and sediment transport data were collected from four 1-m 2 hillslope plots after the 2000 Hi Meadow Fire in Colorado. Data were collected daily during three summers, two of which were affected by drought. Maximum 30-minute rainfall intensities, I 30 , were less than 20 mm h -1 and the average runoff volumes per plot were less than 4.7 L per storm. The data were separated into three sediment transport processes based on rainfall intensity and runoff magnitude: (1) dry ravel, (2) rainsplash, and (3) rainflow and then into eight different particle size classes ( D i ). For each class, dry ravel transport had a non-linear dependence on initial soil moisture, θ i , with a maximum at intermediate values of θ i (5-9 % cm 3 cm -3 ). Dry ravel transport rates were small for low θ i , which may be caused by a cementation process, and also small for high θ i , which may be caused by increased surface tension. Rainsplash transport was confined to the I 30 domain from 1-7 mm h -1 was proportional to D i ( I max 30 ) 0.63 . Rainflow transport ( I 30 > 7 mm h -1 ) in shallow flows ( h < 2 mm) was most likely dominated by particles rolling. It had a non-linear dependence on D i with maximum transport of sediment in the 2-4 mm size class. Transport also depended on stream power, but critical stream power was essentially zero, which may indicate that the rainsplash preceding runoff detached soil for transport by overland flow.