Giovanni Ferrari

The Ziway–Shala lake basin (main Ethiopian rift, Ethiopia): a revision of basin evolution with special reference to the Late Quaternary

Abstract The Ziway–Shala basin, in the Main Ethiopian Rift (MER), is a reference site for regional to global paleoclimatic reconstructions. We undertook and interpreted a stratigraphical, pedological and geomorphological study, including a new geological map scale 1:250,000, to provide a Late Quaternary-centred revised geological history of the basin. 1 We mapped several Late Quaternary sedimentary units and arranged them in four major unconformity-bounded stratigraphic units (synthems), recording equivalent phases of geomorphic change. A new, extensive, soil survey allowed us to establish a pedostratigraphic unit, the T’ora geosol, as a distinctive marker of landscape stability and instability in the area during the Holocene. Climate change was a major control on geo-morphologic evolution of this area during the intense climate fluctuations of the last 100,000 years. Extensive lake systems developed during relatively humid Last Glacial interstadials and in the early-mid Holocene; this last was characterized by short, but high-amplitude, regressions during arid pulses. Major lakes’ lowering occurred in the terminal Pleistocene and in the last 5000 years. Evidences for high or very high terminal Pleistocene lake levels suggest possible non-climatic controls on changes in lakes’ extension and volumes between Late Pleistocene and Holocene. We suggest that modifications of hydrological thresholds, due to activity of structures parallel and transversal to the MER, established new lakes’ boundaries between terminal Pleistocene and early Holocene, setting the maximum level of Holocene lake systems at about 1670 m a.s.l. The integrated analysis of lacustrine, fluvial, slope and soil systems provided a basis for a general interpretation of relations between climatic changes and geomorphic processes at a basin scale.

Use of aerial photographs, Landsat TM imagery and multidisciplinary field survey for land-cover change analysis in the lakes region (Ethiopia)

Early multidisciplinary surveys in the Lakes region of central/south Ethiopia show a highly variable land cover pattern characterised by complex interactions between environmental parameters and socio-economic dynamics. From an ecological point of view the area is highly sensitive and both food security and soil conservation are becoming serious problems for the rapidly growing population. The intensive land cover changes observed in this area during the last few decades beg accurate analysis. Land-cover change analysis over a long time-span was performed. Interpretation (API) of aerial photographs dated 1972 and classification of a 1994 Landsat TM image were used. Problems due to the heterogeneous nature of the data were overcome with a method for quantifying land cover on aerial photographs, thus producing data comparable to TM classification results. As land cover is linked, through land use, to social dynamics, in ground control use was made of the results of parallel socio-economic investigations. From the analysis, a general trend of increase in cultivated surfaces was noted. Unique strategies of land allocation according to physical settings were observed. A trend in the evolution of badlands was identified: rapid reactivation of previous erosion in newly cropped areas occurred; within a few decades this erosion reached quasi-equilibrium. The methods adopted showed some accuracy limitations, but allowed land-cover change analysis over a 22-year time-span, providing important insight into recent phenomena and present trends.

Effects of gypsum on hydrological, mechanical and porosity properties of a kaolinitic crusting soil

Numerous papers have shown the effectiveness of gypsum addition in reducing crust formation on sodic or generally non-acid soils, while the effect on acid soils has been less frequently studied and is less known. The present paper reports the results of a series of field rainfall simulations on an acid red soil (Rhodic Kandiustalf) in Northeast Zimbabwe. In these experiments, gypsum effect on hydrological behaviour in subsequent rainstorms was verified, and the mechanical properties of the crusts so formed were assessed by a modified hand penetrometer. The results showed the ability of gypsum to improve overall infiltration, mostly by reducing the rate of infiltration decay in the non-steady portion of a Horton-type infiltration curve; such rate of infiltration decay was shown to be not constant in subsequent rainstorms, with a well defined increasing trend, that was, however, less marked for gypsum-treated plots. Effect on final infiltration rate was much less significant. Surface crusts formed on gypsum-treated plots were shown to be significantly harder than those formed on untreated plots. Both results could be explained by the observation that a substantial loss of clay from the soil surface took place in untreated plots, while this process was significantly reduced by gypsum treatment. This lesser clay depletion would be effective in slowing structural collapse during wetting, and was also observed to allow the regeneration of a better developed soil structure on drying, so further slowing soil response in subsequent rainstorms. The greater clay content in the surface layer could also induce a higher degree of cohesion, so explaining the greater surface crust strength induced by gypsum treatment.

Effect of gypsum on seedling emergence in a kaolinitic crusting soil

Abstract The emergence capability of maize seeds in a kaolinitic red tropical soil from Zimbabwe, subjected to rainfall simulation, was investigated under three kinds of treatment: untreated, 2.5 T·ha−1 and 5 T·ha−1 of phosphogypsum. Samples treated with gypsum exhibited a strong increase of crust strength and a decrease of the percentage of the cracked areas; they also showed a lower bulk density and a higher residual moisture content in air-dry condition. Gypsum translocation was limited to the first millimeters of the soil. A possible explanation advanced is the loss of brittle behaviour in treated samples, due to greater retention of clay in the surface soil layer; non-brittle behaviour by the treated soil would hamper soil cracking by emerging seeds and their emergence through cracks.

Pore space degradation in Zimbabwean crusting soils

Abstract Carnicelli, S., Ferrari, G.A. and Pagliai, M., 1994. Pore space degradation in Zimbabwean crusting soils. In: A.J. Ringrose-Voase and G.S. Humphreys (Editors), Soil Micromorphology: Studies in Management and Genesis. Proc. IX Int. Working Meeting on Soil Micromorphology, Townsville, Australia, July 1992. Developments in Soil Science 22, Elsevier, Amsterdam, pp. 677–686.

Geomorphology of the Archaeological Area of Aksum

Aksum is one of the most important archaeological and historical towns in Ethiopia. The archaeological area stretches on a plateau ranging from 2,250 to 2,460 m, and the general landscape consists of a few dome-shaped hills standing on a plateau crossed by small ephemeral streams. The main geomorphic features are the result of the emplacement of basalt flows; the intrusion of syenite sub-volcanic plugs; and domes, tectonics, weathering and its interaction with various surface erosion processes. In such a context, between 700 BC and 800 AD, the rise and the decay of the Aksumite Kingdom, one of the most known civilisations of East Africa, took place. The core of the archaeological area of Aksum, with tall carved obelisks, is a UNESCO World Heritage listed property. The integration of archaeological data with the analysis of soil conservation measures and agricultural practices, adopted till present, allowed to develop a plough marks-based methodology to assess the historical soil erosion rate and, hence, to understand historical landscape evolution within a man-controlled environment. The occurrence of archaeological evidence in such a characteristic geomorphological context provides an excellent opportunity to study and understand interaction between geomorphic and anthropic processes.