Application of the Indicators of Hydrological Alteration method to the Mkomazi River, KwaZulu-Natal, South Africa
Hydrological regimes play a major role in structuring the biotic diversity within river ecosystems and hydrological variation is recognised as a primary driving force within those ecosystems. The US Nature Conservancy developed a method known as the Indicators of Hydrological Alteration, IHA (Richter et al. 1996, 1997, 1998), for assessing the degree of hydrological alteration attributable to human induced changes within an ecosystem. The method is based on the statistical analyses of 33 hydrological parameters representing five streamflow characteristics that can be attributed to playing major roles in determining the nature of aquatic and riparian ecosystems (Richter et al. 1996, 1997, 1998). The Range of Variability Approach, RVA, is an application of the IHA, incorporating the concepts of hydrological variability and river ecosystem integrity, and was developed to enable river managers to define and adopt preliminary flow management targets before conclusive, long-term ecosystem research results are available. This paper presents an application of the RVA to simulated streamflows at two of the four Instream Flow Requirements (IFR) Sites on the Mkomazi River in KwaZulu-Natal, South Africa, in order to assess the extent of alteration caused by human induced changes to the hydrological regime. The assessment was achieved by comparing the range of variation of the hydrological regime simulated under natural catchment conditions (pre-impact) with the variation resulting from catchment development (post-impact). The 25th and 75th percentile values of each of the 33 parameters were selected as the lower and upper thresholds within which streamflow management targets could be set. By setting preliminary streamflow management thresholds that can be modified and refined when ecological data and information become available, the RVA incorporates flexibility and adaptability.