Abstract
Mountain fynbos catchments in the Western Cape region of South Africa are prone to substantial changes in land cover due to invasion by exotic tree species (and their clearing), fires, and vegetation response to inter-annual variations in rainfall. While small catchment experiments and modelling studies have pointed to reductions in river flow as catchment biomass increases, there is little empirical evidence of land cover change affecting river flow in large catchments that are important sources of water for the region. Monitoring changes in above-ground green biomass in multiple large catchments is challenging, but may be accomplished using a remotely sensed spectral vegetation index. It was hypothesised in this study that annual river yield (river flow as a fraction of rainfall) in the Molenaars catchment near Paarl, South Africa co-varies with an index of green vegetation cover derived from satellite data (the normalised difference vegetation index, NDVI). The catchment was partitioned into ’upland‘ and ’lowland‘ zones and the relationship between annual river yield and summer NDVI was determined for each zone over an 18-year period. There was a statistically significant negative linear relationship between annual river yield and the NDVI of the lowland zone when three outliers were excluded from the analysis. These outliers corresponded to periods with prolonged drought conditions when river yield appeared to be decoupled from vegetation water use in the lowland zone. There was no relationship between river yield and changes in the NDVI in the upland zone where plants were unlikely to have sustained access to adequate soil water for transpiration. The importance of considering the location of land cover changes in a catchment, and inadequacies in high-elevation measurements of rainfall in this mountainous region, were highlighted in the study.
Highlights
Mountain catchments in the Western Cape Province of South Africa are important sources of water for the region, including the Cape Town metropolitan area
Estimates of river flow reductions have generally been made using empirical relationships between catchment biomass and flow reductions (e.g., Le Maitre et al, 2000), small catchment experiments where the fynbos shrubs have been replaced by tree plantations (e.g., Scott 1999), and measurements of river flow or evapotranspiration over short periods following the removal of invasive vegetation in specific locations (e.g. Dye and Poulter, 1995)
This study was designed to investigate the viability of relating annual river yields to remotely sensed changes in vegetation cover in a large mountainous fynbos catchment
Summary
Mountain catchments in the Western Cape Province of South Africa are important sources of water for the region, including the Cape Town metropolitan area. Fynbos vegetation covers many of these catchments, resulting in landscapes that are susceptible to frequent fires and invasion by exotic woody species. These woody species tend to be taller in stature than the native fynbos shrubs, causing an increase in above-ground biomass. Replacement of native fynbos by invasive species has been associated with reductions in river flow due to increased transpiration rates (Scott, 1999; Le Maitre et al, 2000; Dye et al, 2001). Little is known about the river flow response of large catchments to changes in above-ground biomass associated with invasion by exotic woody species or their clearing. Large catchments may have significant non-uniform spatial variations in land cover, soils, fire history, and rainfall. Siriwardena et al (2006) suggest that these heterogeneities are major factors contributing to the inconsistent response of different size catchments to changes in forest cover
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