Research Articles (Soil Science)

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    Estimating evapotranspiration in a semi-arid catchment : a comparison of hydrological modelling and remote-sensing approaches
    (Water Research Commission, 2020-04) Bugan, Richard; Garcia, Cesar Luis; Jovanovic, Nebo; Teich, Ingrid; Fink, Manfred; Dzikiti, Sebinasi
    Reliable spatial data of evapotranspiration (ET) in support of water resources management are limited. ET is a major component of the water balance, in many regions, and therefore it is critical that it be accurately quantified. To identify a product that accurately estimates spatially distributed ET for application in data-scarce regions, an inter-model comparison was conducted between the MOD16 ET dataset and the ET calculated with the calibrated and validated JAMS/J2000 hydrological model in the Sandspruit catchment (South Africa). Annual JAMS-ET and MOD16-ET data were generally consistent. Monthly JAMS-ET and MOD16-ET dynamics are influenced by the response of vegetation to precipitation as well as the atmospheric evaporative demand. The maximum correlation coefficient between JAMS-ET and MOD16-ET was 0.82 and it was evident at Lag 0, showing that both ET estimates are in phase when evaluated at the basin scale. The maximum correlation coefficients between the ET estimators and precipitation were 0.67 and 0.70 for JAMS-ET and MOD16-ET, respectively, and this was evident at Lag 2 (1 lag is 1 month) for both methods. This suggests that there is a 2-month delay in the maximum response of ET to precipitation. The models did not exhibit significant dependence on the seasonal distribution of precipitation. The complementary use of hydrological modelling and satellite-derived data may be greatly advantageous to water resources management, e.g., water allocation studies, ecological reserve determinations and vegetation water use studies. The results of the inter-model comparison also provide motivation for the use of the MOD16 ET dataset to estimate ET in data-scarce regions. Additionally, this study provides evidence for the potential use of validated satellite-based ET data as inputs in hydrological models. This may facilitate a more realistic representation of the catchment hydrological processes.
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    MicroCT-based bulk density measurement method for soils
    (South African Institution of Civil Engineering, 2019) Le Roux, S. G.; Du Plessis, A.; Clarke, C. E.
    High-resolution micro-computed tomography (microCT) is a method growing rapidly in popularity and has been applied to various soil studies with great success, especially for 3D characterisation of pore spaces or mineral distributions. However, microCT is not typically used for soil bulk density measurements, probably due to relatively simple and fast alternatives. Besides the complex process of image analysis from microCT scans, the method is also limited in resolution, which can result in incorrect total porosity estimation. This is especially true for granular materials, such as soils with small pore spaces between particles. In this work we demonstrate a different, yet very simple methodology for microCT adapted to overcome these limitations by using only volumetric measurements of the samples, and not segmentation of pore spaces or density calibrations. This method allows accurate bulk density determination for soil clods and cores. The method is faster than tradition methods, and it allows for additional analyses, such as surface area, macro-porosity, connected pore network and macro-particle shape analysis. The method is tested and directly compared for the same samples to the traditional waxing Archimedes method, with good correlation. The microCT scans of waxed samples also indicate sources of possible error in the waxing Archimedes method by visualising trapped air and wax penetration into open pore spaces. The method is then applied to cores and local bulk density measurements, and their variability down the cores is demonstrated, which can be very useful in complex soil profiles. The method is robust in varying resolution and image blur as it makes use only of volumetric measurements of the entire sample, not image grey-value calibration or segmentation of pore spaces.
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    Quantifying a sponge : the additional water in restored thicket
    (Academy of Science of South Africa, 2019-05-29) Mills, Anthony J.; De Wet, Ruan
    No abstract available.
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    Assessing the influence of DEM source on derived streamline and catchment boundary accuracy
    (South African Water Research Commission, 2019) Mashimbye, Zama Eric; De Clercq, Willem Petrus; Van Niekerk, Adriaan
    Accurate DEM-derived streamlines and catchment boundaries are essential for hydrological modelling. Due to the popularity of hydrological parameters derived mainly from free DEMs, it is essential to investigate the accuracy of these parameters. This study compared the spatial accuracy of streamlines and catchment boundaries derived from available digital elevation models in South Africa. Two versions of Stellenbosch University DEMs (SUDEM5 and DEMSA2), the second version of the 30 m advanced spaceborne thermal emission and reflection radiometer global digital elevation model (ASTER GDEM2), the 30 and 90 m shuttle radar topography mission (SRTM30 and SRTM90 DEM), and the 90 m Water Research Commission DEM (WRC DEM) were considered. As a reference, a 1 m GEOEYE DEM was generated from GeoEye stereo images. Catchment boundaries and streamlines were extracted from the DEMs using the Arc Hydro module. A reference catchment boundary was generated from the GEOEYE DEM and verified during field visits. Reference streamlines were digitised at a scale of 1:10 000 from the 1 m orthorectified GeoEye images. Visual inspection, as well as quantitative measures such as correctness index, mean absolute error, root mean squares error and figure of merit index were used to validate the results. The study affirmed that high resolution (<30 m) DEMs produce more accurate parameters and that DEM source and resampling techniques also play a role. However, if high resolution DEMs are not available, the 30 m SRTM DEM is recommended as its vertical accuracy was relatively high and the quality of the streamlines and catchment boundary was good. In addition, it was found that the novel Euclidean distance-based MAE and RMSE proposed in this study to compare reference and DEM-extracted raster datasets of different resolutions is a more reliable indicator of geometrical accuracy than the correctness and figure of merit indices.
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    Effects of anabolic and catabolic nutrients on woody plant encroachment after long-term experimental fertilization in a South African savanna
    (Public Library of Science, 2017-06-29) Mills, Anthony J.; Milewski, Antoni V.; Snyman, Dirk; Jordaan, Jorrie J.
    The causes of the worldwide problem of encroachment of woody plants into grassy vegetation are elusive. The effects of soil nutrients on competition between herbaceous and woody plants in various landscapes are particularly poorly understood. A long-term experiment of 60 plots in a South African savanna, comprising annual applications of ammonium sulphate (146±1166 kg ha-1 yr-1) and superphosphate (233±466 kg ha-1 yr-1) over three decades, and subsequent passive protection over another three decades, during which indigenous trees encroached on different plots to extremely variable degrees, provided an opportunity to investigate relationships between soil properties and woody encroachment. All topsoils were analysed for pH, acidity, EC, water-dispersible clay, Na, Mg, K, Ca, P, S, C, N, NH4, NO3, B, Mn, Cu and Zn. Applications of ammonium sulphate (AS), but not superphosphate (SP), greatly constrained tree abundance relative to control plots. Differences between control plots and plots that had received maximal AS application were particularly marked (16.3 ± 5.7 versus 1.2 ± 0.8 trees per plot). Soil properties most affected by AS applications included pH (H2O) (control to maximal AS application: 6.4 ± 0.1 to 5.1 ± 0.2), pH (KCl) (5.5 ± 0.2 to 4.0 ± 0.1), acidity (0.7 ± 0.1 to 2.6 ± 0.3 cmol kg-1), acid saturation (8 ± 2 to 40 ± 5%), Mg (386 ± 25 to 143 ± 15 mg kg-1), Ca (1022 ± 180 to 322 ± 14 mg kg-1), Mn (314 ± 11 to 118 ± 9 mg kg-1), Cu (3.6 ± 0.3 to 2.3 ± 0.2 mg kg-1) and Zn (6.6 ± 0.4 to 3.7 ± 0.4 mg kg-1). Magnesium, B, Mn and Cu were identified using principal component analysis, boundary line analysis and Kruskal-Wallis rank sum tests as the nutrients most likely to be affecting tree abundance. The ratio Mn/Cu was most related to tree abundance across the experiment, supporting the hypothesis that competition between herbaceous and woody plants depends on the availability of anabolic relative to catabolic nutrients. These findings, based on more than six decades of experimentation, may have global significance for the theoretical understanding of changes in vegetation structure and thus the practical control of invasive woody plants.