|Helming, K - ZALF UNIV|
|Prasad, S - UNIV OF MISSISSIPPI|
Submitted to: International Journal of Sediment Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 1999
Publication Date: March 1, 2000
Interpretive Summary: Soil erosion is a highly complex phenomenon involving many factors including soil, weather, topography, land management, cropping practices and soil control measures. Many of these factors are themselves determined by a host of other parameters, conditions, properties, etc. One of the most vexing problems in estimating soil loss from agricultural fields over the years has been the role of soil surface roughness. The traditional view has been that increased roughness reduces runoff velocities and increases surface storage and thus reduces the erosive power of runoff. Yet, little quantifying data are available. With the onset of new technology, especially laser technology, possibilities for determining more precisely the effect of surface roughness or topography on soil loss have substantially improved. This study was designed to determine this relationship for four, widely different Mississippi soils subjected to a series of simulated rainstorms of known characteristics. The results show that there is a characteristic relationship between sediment yield and the mean local topographic gradient. This relationship distinguishes three phases: a preponding phase in which the soil surface "levels" due to raindrop impact, a postponding phase with increasing sediment yield dominated by increased rilling due to increased surface runoff, and a post-ponding phase with decreasing sediment yield due to soil surface stabilization. These three phases which occur successively are in effect expressions of the soil surface dynamics in response to different erosive agents and changing soil surface characteristics.
Technical Abstract: Microtopography is an important surface characteristic of upland areas that affect erosion processes of detachment, transport, and runoff. A laboratory 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 66 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 yield, which reached quickly a maximum and then gradually decreased to a near constant value at the end of the storm series. 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 yield-microtopography 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.