River flow response to changes in vegetation cover in a South African fynbos catchment

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.

Synthetic monthly flow duration curves for the Cape Floristic Region, South Africa

“…For example, transpiration associated with phreatophytic vegetation located within the riparian zone could be expected to impact low flows more than transpiration from vegetation on the hill slopes. This was demonstrated by Hope et al (2009), who found a significant relationship between a spectral vegetation index (NDVI) in the lowland area of a CFR catchment (Molenaars) and annual flow volume, but no relationship when the index was calculated for the upland areas.…”

Predicted impacts of land use change on groundwater recharge of the upper Berg catchment, South Africa

“…Coniferous forests, for example, consume more water than deciduous forests, while shrubs and grasslands use less water than forests (Bosch and Hewlett, 1982;Brown et al, 2005). Hope et al (2009) investigated the association between annual river yields and AVHRR remotely-sensed changes in vegetation cover in a large mountainous fynbos catchment for 18 years. From the results it is concluded that the spectral vegetation index NDVI (normalised difference vegetation index) has a negative relationship with the river yield at low altitudes.…”

Chlorophyll fluorescence of nitrogen fixing soybean plants (Glycine max L.) under stress conditions

“…The ability of the chlorophyll molecule not only to absorb light and to carry out the primary photochemical reaction, but also to dissipate part of this energy as fluorescence, makes the fluorescence a very convenient probe for the state of the photosynthetic apparatus [7]. Luminescence methods based on registration of light emitted by Photosystem II, as well as the both prompt and delayed chlorophyll fluorescence, are among the most perspective biophysical methods to evaluate the physiological state of the plants [5,8].…”