Characterization of rainfall in the central South African Highveld for application in water harvesting

In-field rainwater harvesting (IRWH), a runoff farming system, is a beneficial water management technique for crop production in arid and semi-arid areas. In-field rainwater harvesting is influenced by rainfall characteristics, and hence this study aimed to identify and characterize rainfall events, and determine rainfall parameters that were of significance in in-field runoff. Two algorithms of event identification were developed. The algorithm that identified events spanning over a 24-h day limit as a single event, gave better identification results which were then characterized. This enabled systematic grouping of rainfall parameters. About 33% of the total rainfall amount received had zero potential to be harvested as runoff in the IRWH system. Therefore, a runoff harvesting practice needs to use the remaining 67%. Rainfall events that lasted 30 min or longer were of water-harvesting importance. This could be the minimum duration guideline when simulating rainfall for rainwater harvesting studies. Rainfall event amount and intensity were of significant importance for IRWH runoff determination

Characterization of rainfall in the central South African Highveld for application in water harvesting

In-field rainwater harvesting (IRWH), a runoff farming system, is a beneficial water management technique for crop production in arid and semi-arid areas. In-field rainwater harvesting is influenced by rainfall characteristics, and hence this study aimed to identify and characterize rainfall events, and determine rainfall parameters that were of significance in in-field runoff. Two algorithms of event identification were developed. The algorithm that identified events spanning over a 24-h day limit as a single event, gave better identification results which were then characterized. This enabled systematic grouping of rainfall parameters. About 33% of the total rainfall amount received had zero potential to be harvested as runoff in the IRWH system. Therefore, a runoff harvesting practice needs to use the remaining 67%. Rainfall events that lasted 30 min or longer were of water-harvesting importance. This could be the minimum duration guideline when simulating rainfall for rainwater harvesting studies. Rainfall event amount and intensity were of significant importance for IRWH runoff determination

Characterisation of plant water stress of greenhouse tomatoes (Lycopersicon esculentum) grown hydroponically in coir

Conference PaperOnly by characterising the growth medium's water retention characteristics and ability to supply water, can the irrigation technique for coir be improved. The study was conducted to describe the development of water stress of greenhouse tomatoes grown in coir, to identify different stages of water stress, and to determine the implication for irrigation management. From this study, mild water stress started as soon as the volumetric water content dropped below that of the well-watered mediums and lasted while the plant conditioned to the water stress. The onset of moderate water stress occurred from the point where the cucumber plants do not recover under continuous drying of the medium. Increased leaf water potential, stomatal resistance and the first signs of wilting confirmed moderate crop water stress. Severe water stress occurred where changes in the slope of transpiration of unwatered plants divided by reference transpiration became negligible over time and about 75% of all plants were irreversibly wilted. Based on plant response only, water depletion can be allowed to a point before moderate water stress is reached. Soil water sensors calibrated to measure volumetric water content in coir or tensiometer measurements converted to volumetric water content from a laboratory-determined retention curve for coir may be used to trigger irrigation

Yield and seasonal water productivity of sunflower as affected by tillage and cropping systems under dryland conditions in the Limpopo Province of South Africa

Sustainable food production in semi-arid tropical countries can be achieved through efficient utilization of rainwater. A field experiment to assess the grain yield, seasonal water use (WU), water use efficiency (WUE) and precipitation use efficiency (PUE) of sunflower (Helianthus annuus L.) intercropped with cowpea (Vigna unguiculata L.) on two tillage systems was conducted during the 2007/2008 and 2008/2009 cropping seasons at the University of Venda (22°58' S, 30°26' E at 596 m above sea level). The experiment was configured as a 2 x 2 x 2 factorial design with three replications. The tillage treatments were conventional tillage (CT) (control) and in-field rainwater harvesting (IRWH) system. The IRWH is a special crop production technique that promotes runoff on 2.0-m wide no-till strip between crop rows and collects the runoff water in basins where it infiltrates into the soil profile. The treatments in the cropping system (CS) consisted of a sole crop (sunflower or cowpea) and an intercrop (sunflower x cowpea). Results of the experiment revealed that IRWH led to a significant (P Cowpea Dryland Precipitation use Sunflower Tillage Water harvesting

Modelling the economic tradeoffs between allocating water for crop production or leaching for salinity management

Salinisation threatens the sustainability of irrigation agriculture and needs to be managed through leaching practices. Under conditions of water scarcity a tradeoff exists between allocating water for salinity management and production. Currently no model in South Africa is able to model explicitly the impact of salinity management through leaching on the economic efficiency of irrigation farming, taking the opportunity cost of water under limited water supply conditions into consideration. The main objective of this paper is to develop a robust non-linear optimisation model that is able to determine endogenously the impact of declining irrigation water quality on the economic efficiency of irrigation farming. A data envelopment framework was used to integrate recently developed soil water salinity crop-yield production functions and leaching functions to model the complex interactions involved in water allocation decisions. Results showed that it is profitable to reduce the area irrigated under limited water supply conditions in order to release water for leaching purposes. When more water, but still a limited amount of water, is allocated to the farmer, his willingness to pay for water will increase if irrigation water deteriorates. Thus, the conclusion is that leaching is profitable irrespective of the water supply conditions.Keywords: salinity, leaching, economics, trade-off, non-linear optimisation, data envelopment analysi

Soil microbial biomass in semi-arid-communal sandy rangelands in the Western Bophirima district, South Africa

Soil microbial biomass is considered as an important early indicator of changes that may occur in the long term with regard to soil fertility and constitutes an important source and sink of nutrients. In South Africa, rangeland monitoring has mostly focused on assessing changes of aboveground vegetation in response to land uses effects, but the associated changes at belowground soil level remain a topic of further research. The aim of this study was to explore soil microbial biomass at three sites under communal grazing management. Soils from grazed and adjacent ungrazed rangeland plots were collected at a depth of 0-25 cm towards the end of the rainy season in April 2005. The soil microbial biomass was characterized by analyzing the phospholipids ester-linked fatty acids. Soils were also analyzed for organic carbon, pH, and total phosphorus. Results showed no statistically significant differences in organic carbon and soil microbial biomass between the grazed and ungrazed plots at any of the sites. Both organic carbon and soil microbial biomass were low, ranging from 0.06 to 0.11% and 489.28 pmol g-1 to 1823.04 pmol g- 1 , respectively. Fourteen grass species were recorded during the vegetation surveys, and most occurred in low abundance. Plants supply organic materials as energy sources for microbial growth, so the low soil microbial biomass could be a reflection of the low vegetation abundance. This study provides essential baseline information regarding soil microbial activity never reported before in these rangelands. Further investigations are required for in-depth understanding of the underlying processes that regulate soil microbial biomass dynamics at these sites

Salinity guidelines for irrigation: Case studies from Water Research Commission projects along the Lower Vaal, Riet, Berg and Breede Rivers

A vast number of projects on salinity in irrigated agriculture were funded by the Water Research Commission (WRC) during the past 40 years. However, due to the diversity of the projects it is virtually impossible to cover all aspects thoroughly in a paper of limited length. Thus this review focuses mainly on projects along the Lower Vaal, Riet, Berg and Breede Rivers in South Africa. The results on the water quality of these rivers indicate that irrigation has led to the deterioration of water sources. There is a direct relationship between river water quality and soils irrigated. Fortunately, effective land-suitability guidelines were developed and applied during the establishment of the major irrigation schemes. This facilitated the management of soils under irrigation. The results from long-term irrigation case studies along the Lower Vaal River and Breede River show that the quality of soils can be improved. The opposite is also true where mismanagement occurred. Research on the salinity threshold of major crops (grapevines, wheat, maize, groundnuts, etc.) confirmed the empiric nature of the guidelines. It is suggested that a more dynamic approach be used for managing salinity under irrigation at farm level, i.e. the use of models. Amongst others, future research should focus on determining the spatial and temporal distribution of salt in irrigated soils

Precipitation intensity-duration-frequency curves and their uncertainties for Ghaap plateau

Engineering infrastructures such as stormwater drains and bridges are commonly designed using the concept of Intensity-Duration-Frequency (IDF) curves, which assume that the occurrence of precipitation patterns and distributions are spatially similar within the drainage area and remain unchanged throughout the lifespan of the infrastructures (stationary). Based on the premise that climate change will alter the spatial and temporal variability of precipitation patterns, inaccuracy in the estimation of IDF curves may occur. As such, prior to developing IDF curves, it is crucial to analyse trends of annual precipitation maxima. The objective of this study was to estimate the precipitation intensities and their uncertainties (lower and upper limits) for durations of 0.125, 0.25, 0.5, 1, 2, 4, and 6 h and return periods of 2, 10, 25, 50 and 100 years in the Ghaap plateau, Northern Cape Province, South Africa using the Generalized Extreme Value (GEV) distribution. The annual precipitation maxima were extracted from long-term (1918–2014) precipitation data for four meteorological stations (Postmasburg, Douglas, Kuruman and Groblershoop) sourced from the South African Weather Services (SAWS). On average, the estimated extreme precipitation intensities for the plateau ranged from 4.2 mm/h for 6 h storm duration to 55.8 mm/h for 0.125 h at 2 years return period. At 100 year return period, the intensity ranged from 13.3 mm/h for 6 h duration to 175.5 mm/h for the duration of 0.125 h. The lower limit of uncertainty ranged from 11.7% at 2 years return period to 26% at 100 year return period, and from 12.8% to 58.4% for the upper limit for the respective return periods. This methodology can be integrated into policy formulation for the design of stormwater and flood management infrastructures in the Ghaap plateau, where mining is the main economic activity