The study area is the Upper and Middle Modder River basin situated in a semi-arid area of central South Africa. This is an important catchment because of the relatively large nearby towns of Bloemfontein, Botshabelo and Thaba Nchu. Crop production in the basin using conventional production techniques is currently not suitable due to marginal and erratic rainfall, and high evaporative demand, as well as low precipitation use efficiency on the clay and duplex soils caused by large runoff and evaporation losses. A labour-intensive in-field rain-water harvesting (IRWH) technique for crop production recently introduced into a part of the basin occupied by communal farmers has been shown to increase maize and sunflower yields by 30 to 50% compared to conventional tillage, making crop production utilising this technique a feasible proposition for these farmers. The area of land suitable for the IRWH in the basin is estimated to be 80 667 ha, of which 15 000 ha is located in the communal land. The two catchment management options compared in this paper are: • Allowing the 80 667 ha to remain under grassland and utilising the runoff downstream for irrigating maize • Utilising the 80 667 ha for maize production in the basin using the IRWH technique. Results showed that the expected maize production from the options shown above were 23 040t and 137 134t respectively. The large unproductive water losses during storage and conveyance to downstream use points are probably the main reason for this large difference in production. An economic analysis, which enabled the grazing benefit to be included in the first option, shows that the gross margin of this option, expressed as R/m 3 of rain water utilized, could be expected to be between 0.0234 to 0.0254 under current conditions, of which irrigation contributed about 25% or less. The comparable value for the IRWH option was 0.0354. The second option is clearly shown to be the most preferable, with high socioeconomic benefits for the communal farmers who are currently struggling to achieve sustainable livelihoods.

In the semi-arid part of central South Africa, population growth and industrial development are the driving forces for an increased demand for water. This accentuates the need for wise decisions by catchment management agencies (CMAs), especially in water-scarce semi-arid areas. These decisions become more and more complex as the range of demands widens over the spectrum of water consumers, i.e. municipal, industrial, irrigation and rain-fed farming. A study was conducted in the Upper Modder River catchment, which is situated in the semi-arid area of central South Africa, where crop production in the catchment using conventional production technique is currently not suitable due to marginal and erratic rainfall. Moreover, the area is characterised by low precipitation use efficiency because of large runoff and evaporation losses on clay and duplex soils. A labour intensive in-field rainwater harvesting (IRWH) technique recently introduced into a part of the basin occupied by communal farmers has been shown to increase maize and sunflower yields by 30 to 50% compared to conventional tillage, making it a feasible option for the subsistence farmers in the catchment. The area of land suitable for the IRWH located in the communal land is estimated to be 23 000 ha. Two catchment management options presented in this paper are: • Option1: allowing the IRWH suitable land in the communal farming area to remain under grassland and utilising the runoff downstream for irrigating maize • Option 2: utilising the IRWH suitable land for maize production in the basin, using the IRWH technique Results showed that the expected maize production from Option 2 was higher than from Option 1. A financial analysis also showed that gross margin of option, expressed as R/ m 3 of rainwater utilised, was estimated to be between 0.0234 to 0.0254 under Option 1 and 0.0354 for Option 2. This clearly shows that use of rainwater where it falls has high socioeconomic benefits for the communal farmers who are currently struggling to achieve sustainable livelihoods.

In arid and semi-arid regions irrigation tends to degrade soil and water quality through salt accumulation with devastating effects on some crops. This is, according to irrigators, also the case along the lower Vaal River in South Africa. Properly calibrated and tested salinity models could assist the agricultural community in improving salinity management under irrigation.This paper reports on, firstly, salt balances of soils in this region being irrigated for different time periods, and secondly, salt content changes that can be expected as a consequence of future irrigation. Two empirical models, viz. a general and specific salt-balance model were used together with existing water-and soil-quality data to generate such information. The soils selected for this study had been irrigated for periods of between 17 to 53 years. Over these periods addition of salts as a result of farming practices varied between 79 and 280 t·ha -1 , with irrigation water being the major contributor. Between 78% to 87% of the salts added to the soils had been leached from the root zone Despite these large amounts of salts that have been removed, certain irrigation practices have promoted the buildup of salts in some of the soils. The freely drained sandy soils irrigated by centre pivot are of particular interest. Poor management of this system can reduce crop yields. On account of inadequate leaching salts are building up to levels that impair the potential evapotranspiration level of maize. Predictions also show that irrigation should rather be withdrawn from soils with poor internal drainage properties, such as the Arcadia soil at Spitskop. In contrast, flood irrigation on certain duplex soils, such as the Valsrivier at Vaalharts, with relatively good internal drainage properties, can improve their quality.

No abstract

In South Africa canola (Brassica napus L.) is cultivated in rotation with wheat under winter rainfall in the Western Cape Province, primarily for seed to make oil. Expansion of the crop to the other 8 provinces is proposed to reduce shortages of locally produced plant oils. At the same time, canola can serve as a rotational crop for wheat in these summer rainfall provinces. In central Free State, information on evapotranspiration and various water use indicators for canola as influenced by sustained deficit irrigation and plant density is lacking. An experiment with a line source sprinkler irrigation system was therefore conducted, comprising of full irrigation as a control with 4 sustained deficit irrigation levels (mean reduction in irrigation depth per event of 67%, 52%, 34% and 19%) and 5 plant densities (25, 50, 75, 100 and 125 plants‧m−2). Mean seasonal maximum evapotranspiration amounted to 429 mm across plant densities. Plant density did not significantly influence seasonal evapotranspiration. Reducing the irrigation depth per event by more than 20% decreased seasonal evapotranspiration by a mean 3.5 mm per percentage increase in irrigation depth. A maximum biomass water productivity of 22 kg‧ha−1‧mm−1 was measured with full irrigation and a plant density of 75 plants‧m−2. Seed water productivity amounted to a high of 11 kg‧ha−1‧mm−1 with full irrigation and a plant density of 25 plants‧m−2. A percentage reduction in irrigation depth and increase in plant density above 25 plants‧m−2 will reduce seed water productivity by 0.071 and 0.033 kg‧ha−1‧mm−1, respectively. Sustained deficit irrigation increased water use efficiency by a mean 0.5% per percentage reduction in irrigation depth per event.