Contribution of soil water and groundwater towards transpiration of tree species in the Ghaap plateau
Tfwala, Cinisani Mfan'fikile
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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. A procedure of transplanting grown trees into lysimeters to study their water use was developed and implemented on four indigenous trees (Vachelia karoo, Olea Africana, Sersia lancea and Ziziphus mucronata). These trees were sampled together with 1.2 m3 soil monolith using the locally designed sampler. Three years after transplanting, the water use ranged between 7 and 14 L day-1, which was within the range for other similar trees of the same size growing under natural conditions. Accompanying this transplanting were calibrations of a newly developed HydraSCOUT (HS) capacitance soil water measuring probe and the compensation heat pulse velocity (CHPV) sap flow measurement technique. For the HS probe, the aim was to compare laboratory and field developed calibration equations for the estimation of volumetric soil water content (θv) on the soils of the tree sampling sites. Laboratory equations estimated θv better (RMSE=0.001 m3 m-3 - 0.015 m3 m-3) than field calibration equations (RMSE=0.004 m3 m-3 - 0.026 m3 m-3). The HS probe was confirmed as a good candidate for θv measurement. Against pre-calibrated loadcells, the accuracy of the CHPV technique to estimate water use of the sampled trees was established. Good agreement indices between CHPV and load cells were obtained across species (D = 0.778-1.000, RMSE = 0.001-0.017 L hr-1, MAE < 0.001 L hr-1 and MBE = -0.0007-0.0008 L hr-1). It was concluded that the CHPV method can accurately measure tree water use, and therefore can be useful for water resources management in forested areas. The total T of the trees in the lysimeters measured by the CHPV method was partitioned into groundwater and soil water using a water balance approach. The daily soil water depletion was measured using the HS capacitance probe while the groundwater depletion was measured from a graduated water supply bucket. The contribution of groundwater towards T amongst all the investigated trees ranged from 31% when the top soil was wet to 97% when the top soil was dry. The contribution of soil water ranged from 3% when the top soil was dry to 69% soon after irrigation. It was concluded that trees switch to source water from different pools depending on availability. The water balance approach was shown to be a good method for determining the water source for trees. Field experiments were conducted in the arid Northern Cape Province of South Africa and aimed to i) assess the trends of T for selected tree species (Vachellia erioloba, Vachellia karoo, Boscia albitrunca and Z. mucronata) across a range of soil water content conditions and ii) partition the total T of the selected tree species into soil water and groundwater. The soil water content within the upper 0.5 m soil profile ranged from 11 mm during the dry season to 20 mm during the wet season, and was monitored using DFM capacitance probes. The deeper soil layer (0.5-1.2 m) was generally wetter compared to the top layer with water content of up to >30 mm during the wet season. Measured using the CHPV method, the water use ranged from 6 L day-1 on Z. mucronata during the dry season to 125 L day-1 on V. erioloba in summer. The largest (diameter at breast height = 460 mm) V. erioloba tree in the experiment was not responsive to seasonal variations of soil water availability as it constantly used about 80 L day-1 throughout the year. Diurnal patterns of water use did not cause any concurrent changes on the soil water depletions within the top 1.2 m soil profile, which indicated that the trees sourced water from deeper pools. It was concluded that the water use of trees was inclined to the seasonal variations, which however was not the case in old trees. Almost all the water transpired by trees in the study area was sourced from groundwater reserves. It was recommended that tree water use studies should be extended to other species for comprehensive catchment tree water use calculations to inform water budgets.