No abstract

Accelerated gullying of dambos (seasonally waterlogged bottomlands) on commercial farms in Zimbabwe has resulted in legislation curtailing dambo cultivation, even in indigenous farming areas. The transfer of indigenous cultivation to interfluve areas and the resultant concentration of cattle grazing in dambos initiated similar gullying. Facilitated overland flow was perceived to be the cause of gullying both in the case of dambo cultivation and dambo grazing, this overland flow re-excavating what was believed to be fluviatile infill.Geomorphological evidence is assembled to show that dambos are not 'fossil' fluviatile systems, nor is their clay infill alluvial. They are the 'lows' of a landsurface irregularly lowered by differential leaching. The infill forms by precipitation of materials leached from the interfluve profiles and discharged into the dambos. The fluviatile-like configuration of dambos is attributed to integration of sub-clay groundwater movement, resulting from landsurface incision. Accelerated gullying in Zimbabwe is deduced to be caused by breaching of the dambo clay by subsurface water movement. The particular vulnerability of Zimbabwe dambos is attributed to: (1) the watershed situation; (2) shallow weathering and (3) the thinness of the dambo clay. Evidence from Malawi indicates that interfluve deforestation and cultivation of shallow-rooted crops conserve water, raising the groundwater level. Such an increased head of groundwater also appears to have occurred in Zimbabwe; this would strengthen upward discharge in the dambos, promoting gullying. Urgent land and water policy revision is required, allowing restoration of dambo cultivation and reafforestation of interfluves, if dambos are to be conserved and the resources which they offer satisfactorily utilized.

The epidemiologic necropsy measures the occurrence of unsuspected disease through the examination of necropsy records. The estimates of unsuspected disease should approximate what occurs in the living population. The necropsy records of the University of Kansas Medical Center (Kansas City) from 1950 to 1984 were examined for the occurrence rate of abdominal aortic aneurysms. Each adult patient was categorized as (1) without abdominal aortic aneurysm, (2) abdominal aortic aneurysm discovered as a necropsy surprise, or (3) abdominal aortic aneurysm diagnosed or suspected during life. Necropsy detection rates of unsuspected abdominal aortic aneurysms were compared with those found in five published screening surveys. The necropsy detection rate in men was 81 (0.019) of 4155 and was 28 (0.009) of 3142 in women, a difference that was statistically significant. When the necropsy series was adjusted to reflect the same demographic composition as the screening surveys, the results from necropsy and screening were statistically similar. In particular, two surveys from the United Kingdom showed screening detection rates among white men of 0.072 compared with a necropsy detection rate of 0.058. These results further support the use of the epidemiologic necropsy as a research tool for estimating the reservoir of disease in the population.

This paper summarizes the main results of recent geomorphological research on the hydrogeology of weathering profiles on the African erosion surface in Malawi. Deep regolithic profiles have developed by protracted and aggressive weathering and differential leaching. In the advanced stages of weathering, congruent kaolinite ‘dissolution’ causes the saprolite to collapse, forming a thick residuum of the most resistant materials, dominated by silica in the form of quartz and iron as goethite. Aluminium has been extensively leached. Such profiles pertain to a mechanism of land surface formation dominated by the activity of infiltrating water rather than direct surface runoff and it results in terrain with a generally basined configuration. Low-lying areas are occupied by dambos. These are clay-filled bottomlands which are seasonally waterlogged and are located where lithology favours leaching and saprolite collapse. The contemporary fluviatile-like configuration of the dambos is attributed to post-incision modification of the ancient land surface. As a result, their area is reduced and they become inset into it, following geological controls which express integrated groundwater movement within the saprolite. The infill is shown to be essentially an alumino-silicate evaporite rather than of alluvial origin. Analysis of dambo and dambo-peripheral profiles supports this genetic model, identifying the continuity of the contemporary dambo infill with a ‘palaeodambo’ clay wedge below the surficial sands in peripheral situations. The very low permeability of the clay is held to be responsible for the marginal seepage zone, fed by shallow throughflow from topographically higher profiles and augmented by upward discharge of deep water. Elements lost from the interfluves were identified in evaporites which occur on the dambo floors in the dry season. These include aluminium. Analysis of water discharging into the dambos failed to identify this element. There is a need for reassessment of technique to determines element mobilization in tropical groundwater, where organic binding is implicated.