The influence of the irrigation on groundwater at the Vaalharts Irrigation Scheme

English: Vaalharts Irrigation Scheme is not only the largest in the country it is known as "The Food Basket". In 1875, Mr Ford, a Government Surveyor got the idea that the Vaalharts area has irrigation scheme possibilities due to the topography of the area. The proposal was approved by government in 1833. Planning, soil tests and more surveys to investigate the possibility were done. A weir was constructed, in the Vaal River, 8.5 km upstream from Warrenton, to deviate water to the Jan Kempdorp/ Hartswater area. In 1938 the first farmers received plots. Today there are almost 1200 plots vary in size from 25 - 75 ha it cover a total area of 35 302 ha. At the start of the irrigation project the water table was 24 mbgl by 1971 it has risen to 1.5 mbgl and waterlogging was experienced. Streutker studied what the cause of the watertabie rising were. The feeder canals were ground canals and it leached to the water table causing the rise, the canals were lined. The water table remained high, in 1976 Gombar & Erasmus investigated the possibility to drain the area with boreholes. It was a solution but to expensive, The water in the Spitskop dam in the Harts River, were all the drain water flow to do not show parallel deterioration and accumulation of salt as the groundwater in the irrigated areas. A research done by Haroldt & Bailey investigated where does the salts and water go. Findings was that there are a "salt sink" present, mainly due to a perched water table and if at some stage the sink will be exhausted it would have severe effects. A 2004 research was done to find the "salt sink". Boreholes were drilled to study the groundwater characteristics, piezometers were installed, to check the possibility of two aquifers. The study concluded that water levels do not differ more than centimetres in the deep and shallow water systems. Water quality as profiled in piezometers indicated no major stratification of groundwater. The deep lying aquifer does not perform separately, thus no "salt sink". This study was done to conclude what is the effect of the irrigation on the groundwater and the following was done: • Planning and Installation of piezometer network • EC profiling of the piezometers • Monitor groundwater levels and EC's • Determine Hydraulic Conductivity • Sample collection and chemical analyses • Monitor flow of drains in the K block • Develop groundwater level contour maps • Develop and run a model to estimate drainage needs • Calculate salt and water balance A Piezometer network consisting of 246 piezometers were installed between Taung in the North and Jan Kempdorp in the south, 208 were surveyed for XYZ coordinates and used for monitoring. The water levels and EC values were measured four times over a period of a year to cover all seasons. The average water level was 1.63 mbgl and the EC average were 191.5 mS/m. Twenty five piezometer sites were selected to cover as much of the soil types present as possible, to determine the hydraulic conductivity. It was between 0.002 and 5.2 m/d. A map was generated to visualize it, and the values were used in the modeling of the drain zones. Water and salt Balance: The leaching requirement to ensure sustainable farming is 611.5 mm/a. According to the water balance it is 562 mm/a. Incoming salts through irrigation water = 4.65 tlha/a. The TDS determined in 1976 averaged 1005 mg/I, in 2004 it was 1350 mg/I, an average increase of 13 mg/I/a. During the research period it were 1476 mg/I, an increase of 96 mg/I in 5 years an average increase of 19.25 mg/I/a. Irrigation salt not drained = 0.8 t/ha/a Upgrading of all infra structure is essential. Internal subsurface drainage should be cleaned and replaced and the spacing should be decreased to drain the area more effective. Effective drainage would minimize the salt loss prevent a salt build up and have a positive influence on farming and crop quality in the area. The drained water can be reticulated to a transpiration pond to recover the salt thus preventing it from influencing nature and activities downstream.