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University of the Free State (2018)

Contribution of soil water and groundwater towards transpiration of tree species in the Ghaap plateau

Tfwala, Cinisani Mfan’fikile

Titre : Contribution of soil water and groundwater towards transpiration of tree species in the Ghaap plateau

Auteur : Tfwala, Cinisani Mfan’fikile

Université de soutenance : University of the Free State

Grade : Doctor of Philosophy 2018

Résumé partiel
Transpiration (T) by trees is a major route through which water from soils and groundwater aquifers re-enter the hydrologic cycle. It is therefore crucial to accurately quantify tree water use, with a full understanding of their environmental conditions, especially under the context of climate change. The ultimate aim of this study was to quantify T for selected indigenous tree species in an arid environment dominated by mining activities, and further partition it into soil water and groundwater. This involved a series of studies including a global review of whole tree water use, precipitation analysis, methods and instruments validation experiments prior to lysimeter and field tree T measurements. A meta-analysis was carried out on published whole tree water use studies with the aim of assessing the effects of morphological traits [height (H) and stem diameter at breast height (DBH)] and environmental controls [mean annual precipitation (MAP), mean annual air temperature (MAT) and elevation above sea level (Z)] on tree T at global scale. The study also aimed to analyse the techniques used for T measurement. The study revealed that log transformed T (ln T) was positively and significantly correlated with H (rs = 0.55) and DBH (rs = 0.62) at P < 0.1. A weak positive correlation was also found between ln T and MAP (rs = 0.16) at P < 0.1. The results further showed that 82% of the studies published during the period (1970 to 2016), used thermodynamic methods to measure T. It was concluded that the physiological traits play a pivotal role in whole tree water use, and hence should be incorporated in modelling T in forest ecosystems. Long-term precipitation data (1918-2014) was analysed with the aim of i) understanding the occurrence, severity and duration of droughts, ii) getting insights of the interannual variability of precipitation and iii) estimating precipitation intensities and their uncertainties for a range of storm durations (0.125-6 hrs) and return periods (2-100 years). Calculation of the Standardized Precipitation Index (SPI) showed that more droughts, which lasted for at most 2 years, occurred since the 1990s ; these were all moderate droughts with SPI between -1.03 and -1.46, except for the 1992 drought at Groblershoop which was severe (SPI = 1.74). Fitting of the precipitation data to a non-parametric spline smoother revealed that the total annual rainfall followed a secular pattern of fluctuations over the years, while the number of rainfall days and extreme rainfall events were essentially stable. Using the Generalized Extreme Value (GEV) distribution, the estimated extreme precipitation intensities for the plateau ranged from 4.2 mm hr-1 for 6 hours storm duration to 55.8 mm hr-1 for 0.125 hours at 2 years return period. At 100 year return period, the intensity ranged from 13.3 mm hr-1 for 6 hours duration to 175.5 mm hr-1 for the duration of 0.125 hours. The uncertainty ranged from 11.7% at 2 years return period to 58.4% at 100 years return period. These results can be integrated into policy formulation for the design of ecosystem water balance management as well as stormwater and flood management infrastructures.

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