The use of environmental isotopes, soil water measurements and soil water modelling to understand tree water use of an Acacia mearnsii (Black wattle) stand in KwaZulu-Natal.
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Date
2015
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Abstract
In Southern Africa commercial afforestation is an important agricultural activity and accounts
for a large portion of the gross agricultural production, However, there are concerns
regarding its possible detrimental impact on the hydrological system. Previous research in
the Two Streams catchment by Clulow et al. (2011) showed that a commercial forestry
species (Acacia mearnsii) was using more water than available through precipitation over a
30-month period (total evaporation was greater than rainfall) and they concluded that the
trees were drawing water from another source.
In this study, field measurements of stable isotopes of rainfall, soil water, stream water and
groundwater were collected and analysed in order to understand the deficit in the water
balance identified by Clulow et al. (2011). Experimental apparatus was used to extract
isotopes from soil water. Automated rainfall and streamflow samples were used to sample
rainfall and stream water (evaporation seals were designed to prevent fractionation). A
specific set of criteria was used to program the automated rainfall sampler to better
differentiate between event samples. HYDRUS 1-D model outputs of simulated total
evaporation and soil water fluxes were verified from total evaporation and soil water
measurements at the site.
Rainfall varied in isotope signature throughout the year ranging from -150 to -15 permil (δ2H)
and -20 to 2 permil (δ18O), these values were largely dependent on rainfall volume.
Groundwater isotope composition signature changed only slightly throughout the year
ranging from -12 to -5 permil (δ2H) and -4 to -1.5 permil (δ18O), with seasonality being the
driving variable. The results from the isotope signatures showed that the main contributor to
streamflow (-15 to -1.5 permil (δ2H) and -4.5 to -1.5 permil (δ18O)) was groundwater. Soil
isotope signatures varied with depth and season, ranging from -25 to -8 permil (δ2H) and -
1.5 to 4 permil (δ18O). Groundwater signatures were evident on three occasions within the
soil horizon (2.0 m and 2.4 m on 23/08/2013 and 1.6 m on 13/0/2013), where water was
moved by hydraulic lift or capillary rise and made available for uptake by rooting systems.
This was confirmed by Watermark and TDR-100 measurements, where there were upward
fluxes of deep soil water during the dry season. HYDRUS-1D results suggested that
simulated total evaporation (1052 mm) was similar to measured actual evaporation (1095
mm) during the wet season and dry season.
The results conclude that the Acacia mearnsii trees extracted soil water or deep
groundwater during the dry season, which allows for continuous growth throughout the year.
This supports the conclusion of Clulow et al. 2011 and confirms that commercial forestry
could have significant long-term impacts on catchment hydrology, particularly in dry season
low flows.
Description
M. Sc. University of KwaZulu-Natal, Pietermaritzburg 2015
Keywords
Acacia mearnsii--KwaZulu-Natal., Forests and forestry--Environmental aspects--KwaZulu-Natal., Forest hydrology--KwaZulu-Natal., Evaporation (Meteorology)--KwaZulu-Natal., Streamflow--KwaZulu-Natal., Groundwater--KwaZulu-Natal., Theses--Environmental science.