Pieter Le Roux

Disaggregation of land types using terrain analysis, expert knowledge and GIS methods

Soil maps’ value is increasingly recognised for enabling the optimal management of ecosystems. Digital soil mapping (DSM) can overcome the cost constraints of traditional mapping methods, but requires local area-specific research. As South Africa is blessed with the land type survey, local DSM research should start with the disaggregation of this resource. This paper shows how two land types (Ea34 and Ca11) near Newcastle in KwaZulu-Natal were disaggregated using DSM methods. A series of soil maps were created. With each map, more information was incorporated when creating the map. For Map 1 only the land type inventory and terrain analysis were used. A reconnaissance field visit with the land type surveyor was added for the second map. Field work and a simplified soil association legend improved the map accuracy for Maps 3 and 4, which were created using 30% and 60%, respectively, of the observation points as training data. The accuracy of the maps increased when more information was utilised. Map 1 reached an accuracy of 35%, whereas Map 4 achieved a commendable accuracy of 67%. Thus DSM methods can be used to disaggregate land types into accurate soil association maps. Emerging principles include that lithology rather than hard geology should be used as parent material input, field work is critical to obtain acceptable results, and simplifying the map legend into soil associations improves the accuracy of the map.

Soil Indicators of Hillslope Hydrology

The demand for water doubles every 20 years which is more than twice the rate of the world’s population growth. New water resources are becoming scarcer and to treat and remediate existing sources more expensive (Clothier et al., 2008). The protection and management of surface and groundwater resources, especially in the highly variable water regime of semi-arid areas, requires accurate analysis of hydrological processes. This involves the identification, definition and quantification of the pathways, connectivities, thresholds and residence times of components of flow making up stream discharge. It is essential that these aspects be efficiently captured in hydrological models for accurate water resource predictions, estimating the hydrologic sensitivity of the land for cultivation, contamination and development, and for quantifying low flow mechanisms (Lorentz et al., 2007; Uhlenbrook et al., 2005; Wenninger et al., 2008). Ideally these hydrological models can best be developed using measurements of the surface and subsurface lateral flow paths, water table fluctuations, connectivity of the various water bodies and the residence flow time of water through the landscape. The landscape unit that is of particular importance is the hillslope (Karvonen et al., 1999; Lin et al., 2006; Ticehurst et al., 2007), hence the accent here on this landscape unit. The measurements named are however expensive and time consuming since these processes are dynamic in nature with strong temporal and spatial variation (McDonnell et al., 2007; Park & Van de Giesen, 2004; Ticehurst et al., 2007). The need for predictions of the named hydrological processes is becoming increasingly important and led to the launch an International Association of Hydrological Sciences (IAHS) initiative called Predictions in Ungauged Basins or PUB (Sivapalan 2003; Sivapalan et al., 2003) encouraging researchers and modellers to focus their efforts on predicting the hydrological behaviour of catchments based on physical principles without relying on calibrations of hydrological models. Soils integrate the influences of parent material, topography, vegetation/land use, and climate and can therefore act as a first order control on the partitioning of hydrological flow paths, residence time distributions and water storage (Park et al., 2001; Soulsby et al. 2008). The influence of soil on hydrological processes is due to the ability of soil to transmit, store and react with water (Park et al., 2001). Hydrologists agree that the spatial variation of soil properties significantly influences hydrological processes but that hydrologists lack the skill to gather and interpret soil information (Lilly et al., 1998). The relationship between soil and hydrology is interactive. Water is a primary agent in soil genesis, resulting in the formation of soil properties containing unique signatures of the way they formed. Almost every hydrological process of interest to hydrologists is difficult to observe and measure

The cropping potential of South Africa: land evaluation results obtained during the last 50 years

Reliable detailed information regarding the crop production potential of a country is necessary for the planning and execution of the proactive activities needed to avoid future food shortages due to increases in population and/or adverse climatic conditions. The land-type survey of the Republic of South Africa at a scale of 1:250 000 provides a sound basic framework for the assessment of cropping potential because the delineation criteria incorporate the three natural resource factors that determine cropping potential, namely climate, topography and soil. Assessing cropping potential at regional scales, resulted in estimates of arable area between 16.4 and 28.6 million ha. It is clear from these results that a more detailed countrywide assessment at a much larger scale is essential to provide reliable information. The procedure already demonstrated for an area of 2.7 million ha in the eastern Highveld of South Africa is recommended for this work. Modern soil survey techniques can facilitate this enormous task. Detailed assessment of the potential of the 15 million ha occupied by subsistence farmers needs to receive the highest priority. The ecotope is a suitable land unit for assessment of cropping potential. Information derived from ecotope delineation has potential to be useful in agriculture and forestry.

Interactions between stream channel incision, soil water levels and soil morphology in a wetland in the Hogsback area, South Africa

Wetland degradation in the form of channel incisioning can significantly alter the hydrological functioning of a wetland. In this study in a small headwater wetland in the Hogsback area, Eastern Cape province, the impact of channel incisioning on soil water levels and soil morphology was examined. A good correlation (R2 = 0.89) existed between the depth of channel incisioning and average water-table depths in most of the 21 installed piezometers. In localised cases the upslope supply of water was in equilibrium with drainage from the piezometers. Although all the studied soils showed hydromorphic characteristics, those continuously saturated close to the surface exhibited redox accumulations in oxygen-supplying macropores, whereas gleyic colour patterns occur deeper in soils where the water table has been lowered by channel incision. The nature and occurrence of different hydromorphic soil indicators observed confirmed the contribution of soil morphology as a valuable indicator of long-term averaged soil water conditions.