| Abstract: | Microtopography is an important surface characteristic of upland areas that affect erosion processes of detachment, transport, and runoff Yet, little quantitative information is available about the relationship between surface microtopography and sediment yield. A laboraory rainfall simulation study was conducted to determine for four different erosion susceptible soils the changes in surface microtopography and sediment yield during a series of 6 to 8 rainstorms of 0.75 h duration and 60 mm h-1 intensity each. The soils chosen were the Ap-materials of a Grenada sil (Glossic Fragiudalfs), Atwood sil (Typic Paleudalfs), and a Forestdale sicl (Typic Ochraqualfs), as well as the C-material, a Glauconitic sediment, of a Ruston sil (Typic Paleudalfs). Soil beds were prepared in a flume with a seedbed-like surface condition. Before all and after each rainstorm, the surface microtopography was determined using a laser microreliefmeter. Microtopography, expressed in terms of the mean local topographic gradient, and runoff data indicate a very similar pattern among the four soils. The data show an initially rapid increase in the sediment concenttation, which reached quickly a maximum and then gradually decreased to a near constant value at the end of the storm series. Sediment yield followed closely the sediment concentration trend due to a near constant runoff rate. Surface microtopography changed rapidly during the first rainstorm but then decreased more gradual to an approximate constant value for most of the later rainstorms in the sequence. Three distinct phases in the sediment yieldmicrotopography relationship are recognized: (1) a preponding phase, (2) a post ponding-increased sediment yield phase, and (3) a post ponding-decreased sediment yield phase. These phases reflect changes in the relative importance of soil erosion processes of roughness dissipation, rill development,and soil surface matrix stabilization. |
