The high costs of measuring waterlogging and salt-affected soils on South African irrigation schemes, as well as inconsistencies in data collection and reporting methods have resulted in incomplete and often contradictory information on the extent and distribution of salt-affected and waterlogged soils. Since the late 1980’s no national effort has been made to quantify the extent of waterlogging and salt accumulation across irrigation schemes in South Africa. In order to identify soils for drainage and reclamation, the extent of waterlogging and salt accumulation has to be determined.
To address this, a research project by the ARC and Stellenbosch University, initiated and funded by the Water Research Commission (WRC), used various sources of data and techniques to compare empirical (reference) data to determine their potential for monitoring waterlogging and salt accumulation. Field verifications of the various satellite images were done at Vaalharts, Loskop (Olifants River), Vredendal (Olifants River), Makhathini, Sundays River, Tugela River, Limpopo River, Douglas (Vaal and Orange River) irrigation schemes. Soil samples were taken at the observation points for analysis and quantification of the salt content. In addition to this historical soil maps and reports were also used to identify problematic areas and to compare the change in salt-affected and waterlogged soils over time.
The within-field anomaly detection (WFAD) method quantified the extent of the affected areas on nine irrigation schemes. On average, 3.3% of the areas considered were found to be affected and this figure was adjusted to 6.27% to accommodate for abandoned fields. Although WFAD is very successful in identifying salt-affected and waterlogged areas, its main limitation is that it cannot discriminate such areas from anomalies that are caused by other factors (e.g. drought, flooding, soil compaction, disease, inadequate fertilizer application). Based on the field surveys conducted in the nine irrigation schemes, waterlogging and salt accumulation were the cause in 77.8% of cases. The WFAD method is a scoping mechanism that can direct attention to areas that are likely to be affected by salt accumulation and/or waterlogging. These areas should preferably be visited to investigate the probable causes.
If the figure of 6.27% of areas affected is applied to the 1.5 million hectares under irrigation in South Africa, the area that is salt-affected and waterlogged on South African irrigation schemes is 94 050 ha.
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