Water SA

Plantwide modelling - anaerobic digestion of waste sludge from parent nutrient (N and P) removal systems

Wastewater treatment plant (WWTP) mathematical models are based on the behavioural patterns of microorganisms involved in the treatment process. These microorganisms are assumed incapable of thinking or planning but simply act according to the capabilities afforded to them by their surrounding conditions - hence different microorganisms pre-dominate different WWTP zones according to how well the conditions suit them. When waste activated sludge (WAS) from biological nutrient removal (BNR) activated sludge (AS) systems, containing phosphorus-accumulating organisms (PAOs), is fed to an anaerobic digester, there is a release of high quantities of metals, phosphorus (P) and nitrogen (N). The manner in which we model the release of these metals and nutrients significantly affects the accuracy of predicted anaerobic digestion (AD) outcomes. Previous studies of PAOs show that in the anaerobic zone of the AS system, they can form energy-rich poly3-hydroxybutyrate (PHB) at the expense of their aerobically generated polyphosphate (PP). Thus, it is expected that the PAOs containing PP sent into an anaerobic digester with volatile fatty acids (VFAs) present, would utilize their PP reserves as they would in the anaerobic zone of an AS process ending up with formation and storage of some PHB. Ultimately, all the stored products of the PAO get released, since there is no alternating aerobic environment to cater for their growth. Since it has been established that the PP release in the AD occurs much faster than the PAO biomass hydrolysis rate, it is modelled as a separate process. Steps are presented in the development of this PP release mass-balanced stoichiometries that occur with AD of PAOs. By comparing outcomes from these proposed stoichiometries against measured experimental data, it is noticed that better predictions are obtained with acetate uptake for PHB formation than when modelling the AD PP release to occur with PAO death and hydrolysis.

Modelling mesophilic-thermophilic temperature transitions experienced by an aerobic membrane bioreactor treating furfural plant effluent

A mathematical model was developed of an aerobic membrane bioreactor (MBR) treating effluent from a by-products facility at a sugar mill producing furfural, based on measurements of microbial kinetics and stoichiometry at different temperatures. The model was calibrated and validated against plant data using volumetric flow into the MBR and volumetric sludge wasting from the MBR as inputs. The model is able to predict steady-state and unsteady-state operation of the MBR under both mesophilic and thermophilic conditions, and the transitions between the two regimes. Comparison of model simulations and plant data suggests that thermophilic operation is advantageous, but it is less stable than mesophilic operation and frequent feed disruptions can have detrimental effects on MBR operation.

Reconsideration and upgrading of sampling and analysis methods for avoiding measurement-related design and operation failures in wastewater treatment

Success of design and high operational efficiency may basically stand or fall on the quality of measured (or estimated) input data. Even small mistakes committed in the initial steps of sampling and analysis may become large once scaled up in the design process or during full-scale operation. The paper provides several experiment-based practical recommendations and easily implemented, powerful methods for appropriate sampling and analysis practice in wastewater treatment. Representative wastewater characterization is crucial for satisfactory design and cost-effective operation. The paper highlights hidden problems and challenges of sampling and analysis in activated sludge wastewater treatment which may strongly affect the quality of input data, and thus basically determine the modelling outputs. Full-scale results proved that wastewater quality may change significantly in the sampling tubing and vessels; during the sampling process even nitrification can happen. Regarding sludge settling measurements, effects of dilution, temperature, floc structure, nitrate and dissolved oxygen concentrations as well as current biochemical condition of the sludge sample have been studied and important recommendations provided. A combined comparative method including SVI and DSVI measurements has been elaborated for indication and early warning alert of undesired floc structure transformations. Influent BOD5 concentration is a key factor for describing biodegradability and denitrification capacity of wastewater to be treated. Results of the two most commonly used BOD testing methods were compared for preclarified wastewater. An electrochemical measurement technique provided significantly lower BOD5 concentrations compared to manometric analysis results with a difference of 23% and 15% on average for unfiltered and filtered samples, respectively. Effects of BOD-based fractionation deviations on predictable denitrification efficiency were studied at different inlet C/N ratios by simulating existing full-scale wastewater treatment plants resulting in remarkable differences in effluent nitrate concentrations. Based on the results, application of the manometric BOD measurement method proved to be preferable.

Physico-chemical properties and bacterial community structure dynamics during the mesophilic anaerobic digestion of pit latrine faecal sludge

The study characterized the changes in physico-chemical properties and bacterial community structure during mesophilic anaerobic digestion (AD) of pit latrine sludge. The sludge was sampled from six different pits six times at an interval of 40 days. Standard techniques were used to assess the changes in pollution indicators including COD and faecal coliforms. Metagenomic DNA from a composite sample from the six pits' sludge was then extracted at Days 0, 14 and 35 and directly sequenced followed by analysis of the microbial structure using the Ribosomal Database Project tools. Multivariate analyses were used to identify the main determinants of microbial community structure during the digestion process. AD significantly reduced the levels of pollution indicators (p < 0.05). Total solids, volatile solids and COD were reduced by 17-27%, 52-79%, and 42-63%, respectively. The indicator pathogenic microorganisms FC and E. coli were reduced by 34-54% and 35-60%, respectively. The reduction in terms of COD and BOD were, however, not sufficient to satisfy the standards for safe disposal into the environment. Proteobacteria were the most dominant bacterial phylum in the undigested sludge (24.1%) and were significantly reduced to 2.5% at the peak of the AD (Day 14) up until Day 35. Firmicutes significantly increased (p < 0.05) from 22.4% to 28.8% at Day 14 before being reduced to 11.6% at Day 35. This study contributes to our understanding of AD of pit latrine faecal sludge through mesophilic AD as a baseline study, and helps to inform future research on mesophilic AD.

Effects of nitrogen loading from domestic wastewater on groundwater quality

In this study the effects of nitrogen effluent from a permeable constructed sewage tank on groundwater quality were investigated. Sampling took place before and 1.5 years after the closure. Using a 3D hydrogeological model, the spatial distribution of dissolved inorganic nitrogen (DIN), comprising the species NH4+, NO2− and NO3−, was modelled in the saturated zone and the amounts and changes in the ratio of NH4-N, NO2−N, NO3−N were determined. The first part of our general hypothesis, that the groundwater was heavily contaminated in the area surrounding the sewage tank, was clearly verified, since every investigated nitrogen compound exceeded the contamination limit values; but the second part of our hypothesis, that the degree of contamination would significantly decrease after the sewage tank was taken out of use has not been confirmed, since the amount of nitrogen present in inorganic forms increased in the modelled zone. The increase in DIN and the relatively high concentration of NH4+ (35 mg/L) in the immediate vicinity of the tank can be explained by the fact that organic matter (OM) accumulated over the decades provided a constant supply of inorganic nitrogen forms.

The feasibility of wastewater recycling that includes residue from dissolved air flotation within a drinking water treatment plant: case study of Midvaal Water Company, South Africa

When purifying water for potable use, wastewater is generated, due to the class of the water treatment plant and the quality of the source water. Midvaal Water Company recycled wastewater that included residue from the dissolved air flotation (DAF), sedimentation and filtration processes in an attempt to save water and reduce costs. The aim of this study was to determine functionality and water quality of such a wastewater recycling system. Samples were collected for analysis, at the sections that contributed to the total wastewater system as well as after various treatment processes. The water quality of these samples was determined, as well as the incidences of water quality failures of the final water, to establish whether the recycle stream that enters the plant together with the source water had any impact on the water quality after the different treatment processes. Data were grouped into periods prior to, during and after recycling to enable comparisons. The water quality of the recycle stream was poorer than that of the source water from the Vaal River with regard to the mean values for total chlorophyll, suspended solids, turbidity and dissolved organic carbon, but the sedimentation process of the wastewater system improved the wastewater quality by drastically reducing total chlorophyll, suspended solids and turbidity. The risk-defined compliance for the final water was excellent (≥95%), despite aluminium, turbidity and total chlorophyll failures of the final water quality during the recycling period. Total chlorophyll was identified as the largest risk during wastewater recycling, especially after the filtration process. It is evident from the data that wastewater recycling, which included wastewater from the DAF, into the main inlet stream of the water treatment plant proved to be effective, based on compliance with national legislation, and had no detrimental impact on overall treatment processes or final water quality.

Membrane surface properties and their effects on real waste oil-in-water emulsion ultrafiltration

Membrane surface properties and their effect on the efficiency of ultrafiltration (UF) of real waste oily emulsions was studied. Experiments were performed in cross-flow operation at total recycle condition in a lab-scale system. The ceramic UF membrane in the tubular type module was employed. During the experiments permeate flux was measured. The most important influential factors, such as temperature, TMP, and pH, were considered during the experiments. Zeta potential was measured in order to explain the phenomena on the membrane surface. The isoelectric point of the fouled membrane was shifted to the alkaline range. COD removal efficiency reached 89%. Gas chromatography measurements were performed in order to determine the composition of waste emulsions. SEM micrographs showed the formation of calcite on the membrane, which contributed to membrane fouling. Chemical cleaning was examined using alkaline and acid solutions, and a cleaning strategy was determined.

Removal of lead in water using activated carbon prepared from Acacia catechu

The adsorption of Pb (II) on bicarbonate-treated Acacia catechu carbon (BTACC) and commercial activated carbon (CAC) was investigated to assess the possible use of this adsorbent for Pb (II) removal from aqueous solutions. The results obtained from batch studies showed that 98% Pb (II) adsorption for BTACC occurs within 4 h of contact time, within a pH range of 4-10 and a carbon dosage of 100 mg with initial Pb (II) concentration of 10 mg∙L−¹, whereas for CAC 52% Pb (II) adsorption occurred within 5 h of contact time and a narrow range of pH 5 and carbon dosage of 100 mg. Adsorption followed pseudo-second-order kinetics for both carbon sources, with the mechanism of adsorption being intra-particle diffusion and film-diffusion. The best fitting adsorption isotherm for both BTACC and CAC were the Langmuir, Freundlich and Temkin models. Surface characteristics were studied using FT-IR, SEM, EDX.

Smallholder willingness to pay and preferences in the way irrigation water should be managed: a choice experiment application in KwaZulu-Natal, South Africa

Efficient and sustainable utilization of irrigation water is the key to realizing the objective of enhancing agricultural productivity and commercializing smallholder irrigation farming. Valuing and recognizing the scarcity of irrigation water is essential for its sustainable use. Using cross-sectional data from 328 smallholders in and around Makhathini and Ndumo-B irrigation schemes in KwaZulu-Natal Province, the study aimed to assess smallholder farmers' preferences for the way irrigation water resources should be managed and their willingness to pay for irrigation water. This was done employing a choice experiment method. The results suggest the need for irrigation water pricing to reflect irrigation intensity. They also show that improving agricultural production and productivity, with market access can enhance farmers' willingness and ability to pay for irrigation water. The need to consider multiple uses of irrigation water for sustainable utilization of water resources is evident, while supporting women smallholders will have a positive impact on their willingness to pay for irrigation water. The paper recommends a shift towards on-farm volumetric water pricing in the irrigation schemes. The schemes should also have clearly defined boundaries and enforceable rules on collective use of water. The design of irrigation infrastructure should integrate other uses of irrigation water such as domestic and livestock purposes. Consequently, there is a need for further research to ensure that irrigation water prices reflect the marginal value of irrigation water use. Policies should address factors that inherently result in gender differences in terms of access to productive resources which negatively affect sustainable water utilization.

The role of water conservation strategies and benchmark ecotopes for increasing yields in South Africa's semi-arid croplands

Recently published results regarding South Africa's cropping potential show that about one third of the arable land is of low potential, located mainly in semi-arid areas, with the main problem being water shortage. This is therefore an appropriate time to review priorities and procedures, for selecting benchmark ecotopes to represent marginal areas, and for research needs with regard to water conservation strategies to mitigate the problems of low yields. Relevant international principles encapsulated in the words agro-ecology, sustainability and socio-economic conditions, are discussed. Relevant new technologies are described, namely: digital soil mapping that will facilitate the identification of benchmark ecotopes; a stochastic procedure to predict rainfall intensity data from daily rainfall that will facilitate runoff predictions; a crop yield cumulative probability procedure that enables sustainability to be described quantitatively. As a case study, results from a successful field experiment using the infield rainwater harvesting production technique on benchmark ecotopes in a semi-arid area, inhabited by subsistence farmers, are presented. The objectives of the study, procedures used and the method of expressing the results are recommended as guidelines for contributing towards mitigating the problem of low crop productivity across a large portion of the arable area in South Africa.

Developing a fuzzy logic model for predicting soil infiltration rate based on soil texture properties

The prediction of the soil infiltration rate is advantageous in hydrological design, watershed management, irrigation, and other agricultural studies. Various techniques have been widely used for this with the aim of developing more accurate models; however, the improvement of the prediction accuracy is still an acute problem faced by decision makers in many areas. In this paper, an intelligent model based on a fuzzy logic system (FLS) was developed to obtain a more accurate predictive model for the soil infiltration rate than that generated by conventional methods. The input variables that were considered in the fuzzy model included the silt and clay contents. The developed fuzzy model was tested against both the observed data and multiple linear regression (MLR). The comparison of the developed fuzzy model and MLR model indicated that the fuzzy model can simulate the infiltration process quite well. The coefficient of determination, root mean square error, mean absolute error, model efficiency, and overall index of the fuzzy model were 0.953, 1.53, 1.28, 0.953, and 0.954, respectively. The corresponding MLR model values were 0.913, 2.37, 1.92, 0.913, and 0.914, respectively. The sensitivity results indicated that the clay content is the most influential factor when the FLS-based modelling approach is used for predicting the soil infiltration rate.

Experimental investigation of time-dependent local scour downstream of a stepped channel

In this study, temporal variation of local scour occurring at the downstream part of the stepped channel were investigated experimentally. The experimental tests were carried out in a stepped flume with a height of 2.4 m. The width of the rectangular flume was 0.10 m and the length of the stilling basin was 2.12 m. Bed material was placed in a sediment box with a height of 24 cm and length of 2.48 m, without any compaction. Experiments were carried out by using bed material of 4 different grain size distributions, 2 different sill heights and 6 different flow rates. Two empirical equations which include Shields parameter (θ) and densimetric Froude particle number (Fd) were proposed by using the experimental findings to predict the temporal variation of the scour depth. The R² (coefficient of determination) values were computed for both proposed equations as 0.866 and 0.865. The scatter index (SI) values were also determined and computed as 8.73% and 8.25%. The fit of the equations was also determined by means of Fisher's test.

Assessment of CFD turbulence models for free surface flow simulation and 1-D modelling for water level calculations over a broad-crested weir floodway

Floodways, where a road embankment is permitted to be overtopped by flood water, are usually designed as broad-crested weirs. Determination of the water level above the floodway is crucial and related to road safety. Hydraulic performance of floodways can be assessed numerically using 1-D modelling or 3-D simulation using computational fluid dynamics (CFD) packages. Turbulence modelling is one of the key elements in CFD simulations. A wide variety of turbulence models are utilized in CFD packages; in order to identify the most relevant turbulence model for the case in question, 96 3-D CFD simulations were conducted using Flow-3D package, for 24 broad-crested weir configurations selected based on experimental data from a previous study. Four turbulence models (one-equation, k-ε, RNG k-ε, and k-ω) ere examined for each configuration. The volume of fluid (VOF) algorithm was adopted for free water surface determination. In addition, 24 1-D simulations using HEC-RAS-1-D were conducted for comparison with CFD results and experimental data. Validation of the simulated water free surface profiles versus the experimental measurements was carried out by the evaluation of the mean absolute error, the mean relative error percentage, and the root mean square error. It was concluded that the minimum error in simulating the full upstream to downstream free surface profile is achieved by using one-equation turbulence model with mixing length equal to 7% of the smallest domain dimension. Nevertheless, for the broad-crested weir upstream section, no significant difference in accuracy was found between all turbulence models and the one-dimensional analysis results, due to the low turbulence intensity at this part. For engineering design purposes, in which the water level is the main concern at the location of the flood way, the one-dimensional analysis has sufficient accuracy to determine the water level.

Development of a design and implementation process for the integration of hydrokinetic devices into existing infrastructure in South Africa

In South Africa there is currently no notable use of modern small-scale hydrokinetic (HK) energy systems, mainly due to formerly low-cost coal-powered electricity. This renewable energy option makes use of the kinetic energy from flowing water, rather than potential energy, which is more often used in conventional hydropower. Updated refined versions of this technology are now being investigated and manufactured due to the global drive towards reducing carbon emissions and increasing energy efficiency. These modular units allow for installation of HK turbines into existing water infrastructure with very little civil works. The study's objective was to develop a simplified design and implementation process for HK devices within the South African legislative and regulatory environment. Approximately 66% of South Africa's water supply is used by the agricultural sector with more than 6 500 km of canal systems running through many areas which could benefit from alternative energy sources. The recent electricity crisis in the country allowed for problem resolution through funding opportunities and thereby an introduction of an innovative and sustainable technology to provide renewable electricity where otherwise not feasible. A pilot HK project was implemented in an applicable section on the Boegoeberg irrigation canal in the Northern Cape Province and tested for optimum functionality and correct application. This process allowed evolution of a development process for the implementation of HK devices in existing water infrastructure.

Garden irrigation as household end-use in the presence of supplementary groundwater supply

Garden irrigation is a significant and variable household water end-use, while groundwater abstraction may be a notable supplementary water source available in some serviced residential areas. Residential groundwater is abstracted by means of garden boreholes or well points and - in the study area - abstracted groundwater is typically used for garden irrigation. The volume irrigated per event is a function of event duration, frequency of application and flow rate, which in turn are dependent on numerous factors that vary by source - including water availability, pressure and price. The temperature variation of groundwater abstraction pipes at residential properties was recorded and analysed as part of this study in order to estimate values for three model inputs, namely, pumping event duration, irrigation frequency, and flow rate. This research incorporates a basic end-use model for garden irrigation, with inputs derived from the case study in Cape Town, South Africa. The model was subsequently used to stochastically evaluate garden irrigation. Over an 11-d period, 68 garden irrigation events were identified in the sample group of 10 residential properties. The average garden irrigation event duration was 2 h 16 min and the average daily garden irrigation event volume was 1.39 m³.

Do rope and washer pumps provide safe water and satisfied users? A case study piloting new rural water supply technology in Rumphi District, Malawi

Innovation is needed to develop rural water supply to support sub-Saharan Africa communities that are hard to reach. The purpose of this study was to critically review rope and washer pumps that have been installed on manually drilled boreholes in 48 communities as part of a pilot project in Rumphi District, Malawi, and which serve as a sustainable source of drinking water from both technical (water quality and functionality) and social (user satisfaction) perspectives. At each water source, an infrastructure checklist was used (n = 48); 10 users were interviewed (n = 472); and, if the pump had water, water quality samples were collected (n = 24). The results show that use of a professional driller does not guarantee a functioning rope and washer pump that produces safe water. Where the pumps were functional, most provided safe drinking water. However, only 8% (4/48) of pumps had good water quality, a flow rate of >20 L/min and a full consensus of positive satisfaction among users. Pumps are not necessarily working better or worse in more remote areas. A process of introducing and creating evaluative guidelines for new (approved) technologies for rural water supply has not been established in Malawi. Sub-Saharan African governments need to be open to innovative solutions while making sure that standards, including those for functionality, water quality, user satisfaction, private operators, and human capacity for local government regulators, are being followed to ensure safe water for rural communities.

Prevalence of bromide in groundwater in selected regions in South Africa

Many regions across South Africa are dependent on groundwater as the only water source for livestock watering and domestic use. This paper presents an analysis of 350 water samples from collated data of 5 reports published between 2001 and 2016 that show the vast range of 0-132.68 mg/L bromide (Br−) present in South African groundwater. It further highlights that Br− may be a greater toxicity risk factor to livestock production and human health as an endocrine disrupting chemical (EDC) and through accumulation in organs than previously considered. Further validation is required of the physiological effects of Br− for inclusion in water quality guidelines (WQG). Attention is drawn to the importance of site-specific water quality (WQ) monitoring and identification of vulnerable populations to enable adequate risk assessment and implementation of mitigating strategies to lower exposure risk in a specific area.

Organophosphorus flame retardants in surface and effluent water samples from the Vaal River catchment, South Africa: levels and risk to aquatic life

The occurrence and risk assessment of seven organophosphorus flame retardants (OPFRs) in surface water samples within the Vaal River catchment in South Africa were investigated. Wastewater treatment works effluents as the potential sources of OPFRs in surface water were also analysed. In surface water, tris-(chloro-propyl)-phosphate (TCPP) - the total of the three TCPP isomers studied, and tris-(2, 3 dibromo-propyl)-phosphate (TDBPP) were the most abundant OPFRs, with mean concentrations of 276 ng/L and 227 ng/L; respectively. In effluent water samples, the most abundant OPFR was TCPP with a mean concentration of 700 ng/L. A high detection frequency (> 80%) was observed for six of the seven OPFRs with tris-(1, 3- dichloro-propyl)-phosphate (TDCPP) detection frequency being the lowest at 17%. Assessment of risk to aquatic organisms using risk quotients based on measured environmental concentrations (MEC) and predicted no-effect concentrations (PNEC) ranged from no significant risk (for algae, daphnia and fish) to low potential for adverse effects (for algae and fish).

Evaluating the effectiveness of freshwater fishes as bio-indicators for urban impacts in the Crocodile (West) catchment, South Africa

Urbanisation in South Africa has resulted in the degradation of aquatic ecosystems across a rural-to-urban gradient; impacting the availability of clean water. Biological organisms, including fish assemblages, have been used as indicators of environmental change, as part of monitoring programmes designed to protect and improve aquatic ecosystem conditions. However, the effectiveness of individual freshwater fish species as bio-indicators for urban impacts has not yet been evaluated. This study investigated the occurrence of freshwater fish species across three urban gradients within the upper Crocodile River sub-management area as potential bio-indicators. Having collected presence and absence data, five native fish species were determined to be widespread. Their effectiveness as bio-indicators for six environmental drivers, identified through principle component analysis, was assessed using species stressor-response curves derived from logistic regression analysis. Of the five species, the largescale yellowfish (Labeobarbus marequensis) and stargazer catfish (Amphilius uranoscopus) showed potential to be effective bio-indicators for urban impacts on aquatic water quality and instream habitat. These taxa, as effective urban bio-indicators, have the potential to improve the efficiency of urban river health assessments through reducing data gathering and staff training requirements.

Water quality in non-perennial rivers

More than half the river-lengths of rivers in southern Africa dry up occasionally or - more commonly - seasonally. Here we review the literature on water quality (WQ) in non-perennial rivers (N-PRs), with emphasis on river management and southern African systems. Hydrological regimes cover a spectrum from relatively predictable and unvarying in perennial rivers, to unpredictable and highly variable in non-perennial rivers, which are complex, continually shifting mosaics of flowing water, standing-water pools and terrestrial habitats. N-PRs are uncommonly difficult to manage because they represent a limited source of water that is renewed unpredictably and is competed for by local people as well as being required by wildlife. Groundwater, and therefore its chemical and physical features, contributes significantly to base flow and to the maintenance of pools remaining in the bed when the river is not flowing. Water chemistry reflects catchment geology except in polluted systems. Salinity varies temporally, and spatially over three dimensions, and is the variable controlling the composition of the biotas of many N-PRs. Hydrological regimes are seldom predictable with any certainty; WQ varies naturally over time and space; groundwater often determines the WQ of surface water, especially in pools; and WQ in non-perennial rivers and pools may be affected by activities far upstream in the catchment. As yet we have no more than a sketchy understanding of the extent to which data on any one system can be applied to any other. Until we have a better understanding of these systems, the following basic principles should guide the management of WQ in N-PRs: (i) Rivers need to be assessed on a case-by-case basis. (ii) Understanding of the groundwater regime, including its chemistry, is crucial. (iii) Effluents need to be controlled by conservative effluent standards set for both ground and surface waters. (iv) Flows may need to be augmented at certain times of the year.

Methods for the estimation of extreme rainfall events

The estimation of design rainfalls is necessary to estimate the exceedance probabilities of extreme floods required to design hydraulic structures and to quantify the risk of failure of these structures. New approaches to estimating extreme rainfall events are being developed internationally. This paper reviews methods for estimating design rainfalls, particularly extreme events, in South Africa and internationally, and highlights the need to update methods used for estimating extreme rainfall events in South Africa as a platform for future research.

Climate change and the water footprint of wheat production in Zimbabwe

Reductions in the water footprint (WF) of crop production, that is, increasing crop water productivity (CWP), is touted as a universal panacea to meet future food demands in the context of global water scarcity. However, efforts to reduce the WF of crop production may be curtailed by the effects of climate change. This study reviewed the impacts of climate change on the WF of wheat production in Zimbabwe with the aim of identifying research gaps. Results of the review revealed limited local studies on the impacts of climate change on the WF of wheat production within Zimbabwe. Despite this, relevant global and regional studies suggest that climate change will likely result in a higher WF in Zimbabwe as well as at the global and regional level. These impacts will be due to reductions in wheat yields and increases in crop water requirements due to high temperatures, despite the CO2 fertilization effect. The implications of a higher WF of wheat production under future climate change scenarios in Zimbabwe may not be sustainable given the semi-arid status of the country. The study reviewed crop-level climate change adaptation strategies that might be implemented to lower the WF of wheat production in Zimbabwe.

Sensors for the improvement of irrigation efficiency in nurseries

Traditional timer-based systems for irrigation management, which are more commonly used in commercial nurseries in South Africa, are not ideal as they may not irrigate seedlings efficiently. A sensor-based irrigation system is presented as an alternative, as this can provide several benefits to nurseries and nursery-grown seedlings. Small-sized soil water sensors that could fit in small-volume nursery containers (25 to 100 mL), and could be integrated into an automated irrigation system, are reviewed. Several experiments have been conducted internationally to measure soil water status of small-volume containers in soilless substrates, and a large body of knowledge is now available. In this review, we describe the principles of several currently commercially available sensors that can be adapted to this purpose, giving advantages and disadvantages of each type. We conclude that a sensor-based irrigation system has great potential to address the challenges associated with irrigation scheduling, while improving water usage in most nurseries.