Johan van Tol

Soil erosion and dam dividends: science facts and rural ‘fiction’ around the Ntabelanga dam, Eastern Cape, South Africa

Large dams can play an important role in rejuvenating economic and social development but are often associated with environmental degradation. The Mzimvubu Water Project will involve the building of two large multi-purpose dams, namely Ntabelanga and Laleni. A key anticipated benefit of the former is the expansion of irrigated agriculture. This article reveals that large areas surrounding the dam are, however, severely affected by soil erosion and that irrigation on these areas is likely to be unsustainable. Focus Group Discussions (FGDs) and interviews were held with members of two communities that will be affected by the dam to gauge community sentiments and anticipations. The FGDs revealed that despite respondents lived experiences of soil erosion, local narratives about it appear irreconcilable with geophysical facts. The one crucial anticipated dam dividend appeared to be support for large-scale irrigated agriculture. Rural anticipations that irrigated agriculture can counteract soil degradation caused by erosion are not supported by scientific evidence, and without appropriate community awareness programmes by the dam developers, the divergence between science facts and local views may in the future engender conflict between the community and other stakeholders.

Soil properties influencing erodibility of soils in the Ntabelanga area, Eastern Cape Province, South Africa

ABSTRACT Soil erosion has serious off-site impacts caused by increased mobilization of sediment and delivery to water bodies causing siltation and pollution. To evaluate factors influencing soil erodibility at a proposed dam site, 21 soil samples collected were characterized. The soils were analyzed for soil organic carbon (SOC), exchangeable bases, exchangeable acidity, pH, electrical conductivities, mean weight diameter and soil particles’ size distribution. Cation exchange capacity, exchangeable sodium percentage, sodium adsorption ratio, dispersion ratio (DR), clay flocculation index (CFI), clay dispersion ratio (CDR) and Ca:Mg ratio were then calculated. Soil erodibility (K-factor) estimates were determined using SOC content and surface soil properties. Soil loss rates by splashing were determined under rainfall simulations at 360 mmh−1 rainfall intensity. Soil loss was correlated to the measured chemical and physical soil properties. There were variations in soil form properties and erodibility indices showing influence on soil loss. The average soil erodibility and SOC values were 0.0734 t MJ−1 mm−1 and 0.81%, respectively. SOC decreased with depth and soil loss increased with a decrease in SOC content. SOC significantly influenced soil loss, CDR, CFI and DR (P < .05). The soil loss rate was 5.60 t/ha per 8 minute rainstorm of 360 mmh−1. Addition of organic matter stabilize the soils against erosion.

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

Digital Soil Mapping for Hydrological Modelling

Digital soil mapping approaches can play a role in providing soil information in a format useful to hydrological modellers, thus filling a void in the current state of hydrology. In this paper, it is shown how an expert knowledge-based digital soil mapping approach was used to provide the soil-related input needed for a process-based hydrological model (ACRU) of the Stevenson Hamilton Research Supersite (SHRS) in the Kruger National Park, South Africa. First, a soil map was created for the entire 4001 ha study area. This soil map had a validation point accuracy of 73 %. Thereafter, the study area was divided into hillslopes. The hillslopes combined with the soil map were used to create a map showing the size and position of the hillslope-specific conceptual hydrological response models (CHRMs). The CHRM map was then used to configure ACRU and to model stream flow in a first-, second- and third-order catchment within the larger area. The stream flow modelling proved successful for the second- and third-order catchments, with Nash–Sutcliffe model efficiency coefficients (NS) of 0.79 and 0.73 for the two catchments, respectively. That the first-order catchment did not model well was explained by the level of detail of the soil mapping which was too coarse to model such a small catchment successfully. All configurations of ACRU modelled the third-order catchment very well (NS between 0.75 and 0.79), but failed to map single rain events consistently. This work showed that digital soil mapping can provide the soil information necessary to configure a process-based stream flow model successfully, provided that the scale of the mapping corresponds with the scale of the first-order controls of the process being modelled. It was indicated that the optimal time frame for this form of hydrological modelling is a hydrological season.

The nature of soil erosion and possible conservation strategies in Ntabelanga area, Eastern Cape Province, South Africa

ABSTRACT Soil erosion is a major land degradation problem in South Africa (SA) that has economic, social and environmental implications due to both on-site and off-site effects. High rates of soil erosion by water are causing rapid sedimentation of water bodies, ultimately leading to water crisis in SA. Lots of financial and human resources are channelled towards controlling of soil erosion but unfortunately with little success. The level of soil erosion in a particular area is governed by the site properties. Therefore, it is inappropriate to generalize data on soil erosion at a large-scale spatial context. The literature on soil erosion in SA classifies Eastern Cape Province as a high-erosion-potential area using data collected at a large-scale spatial context. Collecting soil erosion data at a large spatial scale ignores site-specific properties that could influence soil erosion and has resulted in failure of many traditional soil erosion control measures applied in the province. Moreover, scientific principles underlying the processes and mechanisms of soil erosion in highly erodible soils are missing in SA. This review was to find effective soil erosion control measures by having an insight on what happens during soil erosion and how soil erosion occurs in Ntabelanga. The literature suggested that erosion in Ntabelanga could be influenced by both the erosivity and erodibility factors though the erodibility factors being more influential. Soil permeability contrast between the horizons could be influencing the rate and nature of soil erosion. To mitigate the impact of soil erosion in Ntabelanga, efforts should aim to improve the vertical flow capacity in the B horizon. Clay spreading, clay delving, addition of gypsum, deep ploughing and mulching could aid the water permeability problems of the subsurface horizons. However for effective soil management and control option, detailed studies of specific site properties are needed. The generated information can assist in formulating soil erosion policies and erosion control strategies in the Ntabelanga area and SA at large.

Application of Hydropedological Information to Conceptualize Pollution Migration From Dry Sanitation Systems in the Ntabelanga Catchment Area, South Africa

The hydrological response of catchments is determined by the combined hydropedological response of hillslopes. In the Ntabelanga area, 56% of the households use pit latrines and untreated drinking groundwater supplies. Soil morphological properties and their spatial distribution were used to conceptualize hillslope hydropedological behaviour to determine the fate of Escherichia coli and faecal coliform from 4 pit latrines. Four hillslopes below the pit latrines (MT1, MT2, MT3, and MT4) occur above first-order tributaries to the Tsitsa River, South Africa, were studied. The studied sites are adjacent to the proposed footprint of a planned multi-purpose storage dam, Ntabelanga. Apedal soils, without morphological evidence of saturation, dominated the upper slopes of MT1 and the lower slopes of MT2, thus promoting vertical drainage. Hydromorphic properties were observed at the soil/bedrock interface in the lower parts of MT1 and the entire slope of MT4. This signifies slowly permeable bedrock and the occurrence of lateral flow. High clay contents and strong structured soils were dominant in MT3, indicating slow internal drainage with a large adsorption capacity. The conceptual models derived from morphological properties were verified using soil physical and organic pollutant measurements. In general, hydraulic conductivity values support the interpretations made from soil morphological measurements. Faecal coliforms and E coli bacteria counts were mostly <1 CFU/g soil in MT1, MT2, and MT4; hillslope migrations were detected in MT3 posing pollution risks.

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.