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Wageningen University (2011)

A hydrological assessment of the spekboom thicket in Baviaanskloof, South Africa

Luijk, G.M. van

Titre : A hydrological assessment of the spekboom thicket in Baviaanskloof, South Africa

Auteur : Luijk, G.M. van

Université de soutenance  : Wageningen University

Grade : Master of Science (MS) 2011

Résumé partiel
Vegetation cover is seen as an important factor in soil and water conservation, especially for dry regions, such as the Mediterranean.Theoretical, vegetation develops a soil and decreases runoff and erosion. Despite the positive stories about environmental restoration, its hydrological effects remain uncertain. Many studies have shown that revegetation does decrease peak flow, but also decrease baseflow. The fall in baseflow result in dry river beds and a significantly decrease of water resources downstream. The study presented in this report focused on the effects of revegetation in the Baviaanskloof, South Africa, using Spekboom (Portulacaria afra). The Baviaanskloof has been used for intensive grazing with goats, that destroyed the original thicket and left the land severely degraded. At the start of the new millenium, a project was started to revegetate with Spekboom to restore the natural environment. Evaluating its ecohydrological effects, measurements were carried out in the summer of 2010/2011 focussing on rainfall dynamics, soil moisture evolution and runoff events. Rainfall and throughfall were measured using six tipping buckets, four interception troughs and a manual rain gauge. Stemflow was measured using a rubber collar. Soil moisture content was measured with six 60 cm soil moisture probes and four individual probes at shallow and deep depth for Spekboom and degraded land. Infiltration measurements were carried out with a minidisc infiltrometer and runoff was monitored with Gerlach troughs. A desktop study was done, predicting interception and infiltration as result from rainfall, using the GaIn3a model, which combines the sparse Gash model with a simple bucket model for runoff. Results from the monitoring site, showed that around 40% of the gross rainfall is intercepted in Spekboom with a canopy closure of 75%. Minor rainfall events (< 5 mm) intercept significantly more rainfall compared to major rainfall events : 55% versus 35%. The interception rate found in this study is comparable to results from a similar study on Spekboom, but much higher than other Mediterranean shrublan with typical rates between 22% and 33%. Results of stemflow measurements were less clear, showing two significantly different rates (7% versus 1% of gross rainfall). The Gash model predicted rainfall dynamics for the given rainfall dataset good, overestimating interception with 132%.

Mots clés : revegetation / portulacaria afra / ecohydrology / hydrology / ecological restoration / south africa

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