75 years of irrigation along the Lower Vaal River and tributaries in South Africa: major management interventions and current impacts on natural resources L.D. van Rensburg, J.H. Barnard & C.C. du Preez Department of Soil, Crop and Climate Sciences INTRODUCTION Natural resources • Rainfall 495 mm compared to a global average of 860 mm • Dryland 14.3 mil ha 7 mil ha- med pot 4.1 mil ha low pot • Irrigation 1.5 mil ha • Water management structure 1 • Water management structure • Aim Lower Vaal and tributaries 2 Vaalharts Vaalharts: Resource related research Irrigation belt West Average slope 1% Average slope 1% East Kalahari sand North Canal Dens permeable Vey permeable Harts River Clay – loam Sandy soil Calcrete Ventersdorp Lava Impermeable Dwyka shale & Tillite (Permeable only where weathered and fractured) 3 Vaalharts: 1st major interventions Vaalharts: 2nd major intervention 4 Vaalharts: Rise of water table levels Z (masl) Z(masl) Water level elevation (masl) Vaalharts: 3de Major interventions (5-7 year 5 Impact of irrigation: water quality of rivers (du Preez et al., 2000) Lower part of the Lower Vaal River system 6 Major interventions Impact of irrigation: water quality of rivers ( du Preez et al., 2000) 7 Impact of irrigation: water quality of rivers (du Preez et al., 2000) Impact of irrigation: soils du Preez et al., 2000; Le Roux et al., 2006) Virgin soils: > 4 ton salts / ha Irrigation removed salts 20% 60% 8 Impact of irrigation: soils (du Preez et al., 2000) Virgin soils: < 4 ton salts / ha Irrigation accumulated salts 1.3 – 9 times more than virgin soils: linked to soil types and I & D systems Impacts of irrigation: crops Ehlers et al., 2010 9 Impacts of irrigation: crops Ehlers et al., 2010 Peas 1.0 0.8 0.6 rel Peas BM 0.4 0.2 y = -0.00096x + 1.10116 R2 = 0.93 0.0 0 100 200 300 400 500 600 -1 Average ECsw (mS m ) Field crops 1.0 y = 1.0166x 2 0.8 R = 0.94 rel 0.6 BM 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Cum ETrel Vaalharts soils Taung Ñ Hartswater Ñ Pampierstad Ñ Harts River Ñ Jan Kempdorp Spitskop Dam Vaalharts Barrage W arenton Ñ Soils Hutton (dee per tha n 2 .44 m) Kimb erley Hutton/Mispah (on rock) N Dun dee Ka tspruit/Kroo nsta d Rock 10 Orange-Riet Conclusion • Protection of natural resources • Field scale interventions: -irrigation scheduling -site specific salt monitoring in crop lands 11.