Quaternary Catchments Research Articles

Large-scale estimators of threatened freshwater catchment species relative to practical conservation management

Freshwater ecosystems are among the most threatened in the world. In light of the threats to freshwater biodiversity, it is essential to map the distribution and status of species to ascertain their threat status for prioritizing conservation action. However, while there is agreement that the conservation of freshwater ecosystems depends on whole-catchment management, there are still a wide variety of large-scale mapping methods in use, the advantages and disadvantages of which have not been fully explored. This study shows that area estimation based on minimum convex polygons should not be encouraged for aquatic species. The IUCN definition of area of occupancy (AOO) is a useful term, albeit highly scale-dependent, for assessment of the total approximate area over which a species occurs. However, for aquatic fauna, and perhaps many other organisms, assessment of occurrence should be based on the more accurate point-locality presences only. The IUCN extent of occurrence (EOO), for freshwater catchment species, should be redefined as ‘the sum of the smallest hydrological units identified, of presently known, inferred or projected occurrences of a taxon, excluding cases of vagrancy, that are used to estimate the threat to a taxon’. A single hydrological unit is also the conservation or management unit. Here we suggest that this unit is the quaternary catchment. This new mapping approach is more appropriate and practical for use in both management planning and conservation action. We suggest that conservation managers and decision makers facilitate co-operation in freshwater mapping efforts by working at the same spatial scale, i.e. the same hydrological unit.

A GIS-based decision support system for rainwater harvesting (RHADESS)

Rainwater harvesting (RWH) is an unconventional water source that is increasingly adopted in South Africa. Its implementation is promoted by non-governmental organisations and government programmes to alleviate temporal and spatial water scarcity for domestic, crop and livestock production and support the overall water resources management. Unreliable water supply is one of the elements central to the poverty level of rural population. As the potential of RWH to improve water access for drinking and other basic human needs is still untapped, the technique will spread further in the coming decades. Studies on the hydrological impacts of RWH are focused on plot scale and very little is known about its impacts at catchment scale. To integrate RWH into the development and management of water resources in South Africa, there is a need to develop tools and methodologies that not only assist planners with the identification of areas suitable but also quantify the associated hydrological impacts of its wide scale adoption. This paper presents the rainwater harvesting decision support system (RHADESS) that was built to assist decision makers and stakeholders by indicating the suitability of RWH in any selected part of South Africa and quantifying the potential impacts associated with its adoption at catchment scale. RHADESS is GIS-based and uses ArcView 3.3 as a platform to assess the RWH suitability of any given area of South Africa. Results are thereafter exported into an Excel spreadsheet that contains the hydrological impact, as runoff reduction, of different levels of adoption of RWH assessed by using the Pitman model. The decision support system guides the implementation of the following RWH categories: Infield RWH and ex-field RWH and domestic RWH. RHADESS was tested in two selected quaternary catchments C52A and V13D located in the Upper Orange water management area and the Thukela water management area, respectively.

Crop yield risk analysis and mitigation of smallholder farmers at quaternary catchment level: Case study of B72A in Olifants river basin, South Africa

Currently, Sub-Sahara is experiencing increased frequency of disasters either as floods or droughts which depletes the scarce resources available to sustain increasing populations. Success in preventing food shortages in the African continent can only be achieved by understanding the vulnerability and risk of the majority of smallholder farmers under rainfed and supplementary irrigation coupled with appropriate interventions. Increased frequency of floods, droughts and dry spells pose an increasing threat to the smallholder farmers’ food security and water resources availability in B72A quaternary catchment of the Olifants river basin in South Africa. This paper links maize crop yield risk and smallholder farmer vulnerability arising from droughts by applying a set of interdisciplinary indicators (physical and socio-economic) encompassing gender and institutional vulnerabilities. For the study area, the return period of droughts and dry spells was 2 years. The growing season for maize crop was 121 days on average. Soil water deficit during critical growth stages may reduce potential yields by up to 62%, depending on the length and severity of the moisture deficit. To minimize grain yield loss and avoid total crop failures from intra-seasonal dry spells, farmers applied supplementary irrigation either from river water or rainwater harvested into small reservoirs. Institutional vulnerability was evidenced by disjointed water management institutions with lack of comprehension of roles of higher level institutions by lower level ones. Women are most hit by droughts as they derived more than 90% of their family income from agriculture activities. An enhanced understanding of the vulnerability and risk exposure will assist in developing technologies and policies that conform to the current livelihood strategies of smallholder, resource-constrained farmers. Development of such knowledge base for a catchment opens avenues for computational modeling of the impacts of different types of disasters under different scenarios.

Climate change impacts on agro-ecosystem sustainability across three climate regions in the maize belt of South Africa

The Highveld region in South Africa is an important area for its food production for the nation, as 70% of country's cereal crops and 90% of the commercially grown maize is cultivated there. The sustainability of these agro-ecosystems is, therefore, of vital importance for the nation's food security. The western part of the Highveld is characterised by relatively low mean annual precipitation (MAP) and highly variable yields, and while rainfall increases towards the east, inter-annual yield variability remains high. Variability of yields is already a concern for agro-ecosystems and it is hypothesised that it could be exacerbated by future climate changes. A sustainability framework was used to assess the sustainability agro-ecosystems under plausible future climate scenarios. Three Quaternary Catchments were assessed ranging from relatively dry (MAP 432 mm) to relatively moist (MAP 903 mm). A sensitivity analysis of plausible scenarios was performed with incremental increases in temperature by 1, 2 or 3 °C, increases/decreases of rainfall by 10% and a doubling of pre-industrial atmospheric CO 2 concentrations to 555 ppmv. From the present and nine plausible future climate scenarios which were modelled using CERES-Maize over a 44-year period, it is shown that climatic changes could have major negative effects on the already drier western, and therefore more vulnerable, areas of the South African Highveld. An increase in temperature increases the variability of yields in the relatively moist Piet Retief area (MAP 903 mm), while at the more sub-humid Bothaville, with a MAP of only 552 mm, the inter-annual variability remains the same but mean yield over 44 seasons is reduced by 30%. A simulated increase in temperature coupled with a doubling of CO 2 increases the rate of soil organic nitrogen depletion from the agro-ecosystem. Therefore, long-term perspectives in regard to human well-being and ecological integrity need to be applied to policies and actions for sustainability of both commercial and smallholder agro-ecosystems, particularly, in the western Highveld.

Managing the hydrological impacts of South African plantation forests: An overview

South Africa is poorly endowed with natural forests, and is heavily dependent on plantations of exotic forestry species to meet its timber needs. The first forest plantations of exotic trees were established in South Africa in 1875. Since then, there has been a steady growth in the total area of forest plantation, culminating in the approximately 1.5 million hectares that are today spread over the higher-rainfall eastern and southern regions of the country. Concerns over the effects of these forest plantations on streamflows and catchment water yields arose as far back as 1915, and were thoroughly debated during the Empire Forestry Conference that took place in South Africa in 1935. A decision taken at this conference led to the establishment of a network of long-term paired catchment experiments in various catchments located in the major forestry areas of South Africa. Since the mid 1980s, these experiments have been supplemented by a wide variety of process studies, where transpiration or evapotranspiration in particular have been directly measured above forest, grassland and other vegetation types, using heat pulse and micrometeorological techniques. Information from all these studies has been used to calibrate catchment hydrological models that have provided estimates of the hydrological impacts of forest plantations in all quaternary catchments in which some degree of afforestation has taken place. Outputs from such studies have been simplified and tabulated to permit planning authorities to predict the likely hydrological impacts of afforestation in any given catchment, and to limit the spread of further afforestation in catchments where available water resources are fully or over-committed. This paper reviews the present South African forest water use situation, and in particular the attitude towards new afforestation, outlining the requirements of the National Water Act that was passed in 1998. Recent efforts to implement an integrated water resource management system that meets the requirements of this Act are described, as are the new institutional arrangements in the process of being implemented. The concept of streamflow reduction activities is presented, and the process by which the national Department of Water Affairs and Forestry currently regulates plantation forestry in the context of all forms of land and water use, is described. Additional aspects of the new system, such as the protection of an ecological reserve (to maintain aquatic systems and ensure supply for basic human needs), water pricing and trading, Compulsory Licensing and the need for equity in the allocation of water resources are highlighted, as well as some of the difficulties hindering implementation efforts.

Invasive alien plants and South African rivers: a proposed approach to the prioritization of control operations

Summary1. A number of parallel initiatives in South Africa have been addressing the prioritization and management of invasive alien plant species, the prioritization of rivers for the conservation of biodiversity, and broad‐scale planning for water resource management. This paper has combined aspects of these approaches to develop a composite index of prioritization of quaternary catchments for alien plant control purposes.2. We calculated, for each quaternary catchment, a simple composite index that combined estimates of (i) the number of invasive alien plant species present; (ii) the potential number of invasive alien plant species that would be present if they occupied the full range as determined by climatic envelope models; (iii) the degree of habitat loss in rivers; and (iv) the degree of water stress. Each of the four components contributed between one and four to the combined index, which had a range of values between four and 16.3. We used a geographic information system to map the distribution of priority catchments for invasive alien plant control. Of the 1911 quaternary catchments in South Africa and Lesotho, just over one‐third (650) were in the highest priority category with an index of 13 or more. A relatively small proportion (273, or 14%) of the catchments had the maximum scores of 15 or 16.4. The approach identified priority areas that have not currently been identified as such, and should provide decision makers with an objective and transparent method with which to prioritize areas for the control of invasive alien plants. We anticipate debate about the way in which components of the index are calculated, and the weight given to the different components, and that this will lead to the transparent evolution of the index. Improvements would also come about through the addition of a more comprehensive list of species, and through the addition of further components.

Continuous simulation modelling for design flood estimation in South Africa: Preliminary investigations in the Thukela catchment

Several recent literature reviews highlight the need for improvements in procedures for design flood estimation [Cordery, I., Pilgrim, D.H., 2000. The state of the art of flood prediction. In: Parker, D.J. (Ed.), Floods. vol. II. Routledge, London, UK, pp. 185–197; Smithers, J.C., Schulze, R.E., 2001. Design runoff estimation: a review with references to practices in South Africa. In: Tenth South African National Hydrological Symposium. SANCIAHS, Pietermaritzburg, RSA]. In general, these reviews indicate that internationally the trend is to adopt a continuous simulation modeling approach for design flood estimation. The continuous simulation modelling (CSM) approach to design flood estimation has many advantages and has the potential to overcome many of the limitations of the often used design event approach. A pilot study into the development of a continuous simulation modelling system for design flood estimation is being undertaken in the Thukela catchment in South Africa. Preliminary studies using the ACRU agrohydrological modelling system are detailed in this paper and include investigations into the appropriate scale and levels of soil and land cover information required for use in a CSM approach for design flood estimation. Results indicate that CSM with the ACRU model requires quaternary catchments to be further divided into sub-quaternary catchments, and that using area weighted soils and land cover information gives better results than using modal soils information or single dominant soil or land cover information.

Problems of estimating hydrological characteristics for small catchments based on information from the South African national surface water resource database

Rapid assessments of water resource availability in South Africa have been facilitated by the availability for a number of years of a national data set of naturalised monthly flow time series. However, these data are only available for moderate to large catchments (referred to as quaternary catchments). In the absence of further information it has often been the practice to apply a simple catchment area-based scaling factor to estimate subquaternary scale flow characteristics. This has proved to be problematic in many studies. The paper presents a comparison of quaternary and subquaternary flow data using 41 gauged catchments and develops a simple approach to scaling based on estimates of the mean annual rainfall characteristics for the two areas. The use of the scaling method in a model designed to provide preliminary, low-confidence, estimates of environmental flow requirements suggests that it represents an improvement. However, there is still a need for a method that allows flows of different magnitudes and frequencies of exceedance to be scaled differentially.

LAND REFORM IMPLICATIONS OF THE DISTRIBUTION OF BADLANDS IN THE MFOLOZI CATCHMENT, KWAZULU-NATAL

ABSTRACT Several studies have shown that gully erosion poses a problem to South African peasant farmers. Land that is already severely affected by and/or susceptible to this form of erosion is therefore not suitable for transfer to such beneficiary communities under the Land Reform Programme. Concern that the transfer of such land in the Mfolozi catchment would exacerbate its already serious soil erosion problem motivated Watson and Ramagopa (1997) to identify unsuitable land types and subcatchments in it from this gully erosion perspective. It has subsequently been established that badlands pose as much a problem as do gullies to the Mfolozi's peasant farmers. By analysing the functional relationships between data sets on the biophysiographic characteristics of land types and on the location and spatial extent of badlands, this study identified unsuitable parts of the catchment from a badland erosion perspective. Conclusions reached from integrating the findings of this study with those of Watson and Ramagopa (1997) have important implications for the future decision making process regarding land that becomes available for potential transfer to peasant farmer communities in the Mfolozi catchment. Firstly, the current erosional status of any land in subcatchments W224, W226, W231, W232, W241, W242, W243 and W244 (Department of Water Affairs quaternary catchment numbers) should be assessed at a very early stage in the process because of the very high probability that it is already seriously affected by gully and/or badland erosion. Secondly, land within land types CA, BD, DB, FA and FB is most unsuitable and should be exempt from transfer. Land within land types AC, BB, EA and AB is unsuitable and should only be considered if there is not a more suitable alternative. However, should other socio-political considerations make the transfer of land within both these categories of land types inevitable, they should respectively receive a very high and high rating on the priority list for both support services and monitoring and evaluation.

Adjustment of a drainage network to capture induced base-level change: an example from the Sorbas Basin, SE Spain

Quaternary catchments in the south of the Sorbas Basin, SE Spain have been affected by two regionally significant river captures. The river captures were triggered by changes in regional gradients associated with sustained Quaternary uplift in the region of 160 m Ma −1. The first capture occurred in the early Pleistocene and re-routed 15% of the original Sorbas Basin drainage into the Carboneras Basin to the south. The second occurred in the late Pleistocene and re-routed 73% of the original Sorbas Basin drainage to the east. This latter capture had dramatic consequences for base-level in the Sorbas Basin master drainage. Local base-level was lowered by 90 m at the capture site, 50 m at 7 km upstream and 25 m at 13 km upstream of the site. The base-level change instigated a complex re-organisation of the drainage networks in systems tributary to the master drainage over the ensuing period (some 100 ka). After the capture, drainage systems closer to the capture site experienced a tenfold increase in incision rates over most of their network. Those located some 13 km upstream of the capture site experienced a fivefold increase in incision, although in this instance, the changes do not appear to have propagated to the headwater regions of the drainage nets. The sensitivity of individual catchments was largely governed by geological controls (structure and lithology). The detailed network evolution in the most sensitive areas can be traced by reconstructing former drainage pathways using abandoned drainage cols and the alignment and degree of incision of the drainage networks. Three main stages of evolution can be identified which record the progressive spread of base-level changes from the master drainage. These are Stage 1 (pre-capture): original south-to-north consequent drainage; Stage 2 (early stage, post capture): aggressive subsequent southwest-to-northeast and east–west drainage developed along structural lineaments first in the east of the area (Stage 2a), and later in the west of the area (Stage 2b); and Stage 3 (late stage, post capture): obsequent drainage developing on the topography of the Stage 2 drainage. All stages of the network evolution are associated with drainage re-routing as a function of river capture at a variety of scales. The results highlight the complex response of the fluvial system, and the very different geomorphological histories of adjacent catchments, emphasising the need for regional approaches for examining long-term changes in fluvial systems.