The effect of flow variability on riparian vegetation in the lower Olifants River

Abstract

This study investigated the drivers of vegetation composition and the hydraulic niche requirements of riparian plants at two sites along the lower Olifants River within the Kruger National Park. Drivers assessed included substrate, water quality and hydraulic associations. The VEGRAI tool was applied as a supplementary assessment for the determination of the present ecological state. Hydraulic niche requirements of riparian vegetation communities were identified for their use as biological indicators of environmental flow determination. This was achieved through surveyed cross-sections with superimposed riparian plants and associated biological, substrate and hydraulic data. Hydraulic rating curves designed for individual cross-sections provided activation discharge (AD) values for individual plants. Activation discharge translates to the amount of discharge required to activate a plant at root level. These values were found to be useful for hydraulic niche determination on local scales as values were specific to site characteristics. In order to remove site-specific variation, AD values were applied to hydrological exceedance tables which provided values relating to the percentage of time that individual plants, populations and communities experienced hydraulic activation. This information was found to be transferrable across sites and therefore could be effectively used in the application of environmental flow determination.