Erodible Substrate Research Articles

Playas and Lunettes on the Southern High Plains: Morphometric and Spatial Relationships

AbstractThe surface of the Southern High Plains is dotted with thousands of small (<5 km2), circular, seasonally dry depressions, or playas, some of which are accompanied by a crescentic dune ridge, or lunette. The plays are inset into the sheets of eolian sediment that blanket the region (Blackwater Draw Formation) and the lunettes rest on the eolian mantle. Approximately 25,000 small playas, 1,100 of which have a lunette, were counted from all 540 topographic quadrangles covering the region (verified using data from soil surveys). Statistical analyses of the physical and spatial relationships of playas and lunettes were undertaken to better understand the origins and development of playas, and their relationship to lunettes. Data were collected from a subset of 40 topographic maps, digitized on a GIS, and analyzed using a nonparametric statistical procedure. Variation in texture of the Blackwater Draw Formation exerts considerable control on the distribution and size of playas. Playas are most numerous in the coarsest (sandy), most easily deflated substrate, followed by the finest substrate which produce erodible, sand‐size aggregates. The deepest playas and those with largest area also are in the finest, easily erodible substrate. These data support the hypothesis of playa development by erosion rather than by dissolution and subsidence. Lunettes most often are associated with deeper and wider playas. Presence or absence of lunettes probably is related to the nature of the playa fill and is not a function of substrate texture. Lunettes form from deflation of calcareous playa fill which is more common in larger playas. Otherwise, the controls on the distribution of this fill are unknown.

Sediment budgeting: A case study in the Katiorin drainage basin, Kenya

AbstractThe primary objective of this study was to compute a detailed budget for a small semiarid tropical drainage basin in Kenya. Results indicated that transfer of sediments (‘inputs’) from primary source areas was minor in comparison to changes in storage. The major sediment source area within the Katiorin drainage basin was the colluvial hillslope zone. The net change in storage within this zone was approximately 2100 Mg yr−1. Surface wash and rilling were the dominant transport processes responsible for the remobilization of colluvial sediments. Sediment storage within the in‐channel reservoir increased by 60 Mg yr−1, which was minor when compared to the total store of sediment in this reservoir. During 1986, the channel network stored only a small fraction ( < 3 per cent) of the sediment delivered from the hillslope subsystem. Therefore, the in‐channel reservoir had limited influence on sediment conveyance to the basin outlet. These data indicate that a static equilibrium condition cannot be assumed within the Katiorin drainage basin. Such an assumption would result in erosion estimates of approximately 5.5 mm yr−1 for the entire basin (based on a sediment output of 7430 Mg km−2 yr−1 and a measured bulk density of 1.35 Mg m−3). However, this masked the actual rates of 1.2 to 7.1 mm yr−1 in subbasin primary source areas, and rates of 0.6 to 17 mm yr−1 for colluvial material in the various subbasins. The extreme accelerated erosion rates resulted from minimal ground vegetation, steep slopes, soil crust formation, an erodible substrate, and a well‐integrated drainage network for rapid conveyance of sediments from the hillslope subsystem to the basin outlet.