Surface interpolation and, more particularly isarlthm interpolation are common procedures in the Earth Sciences and increasingly common ones in most other disciplines. Data for such Interpolation are increasingly being stored in a computer-accessible form as a prerequisite to a computer-derived solution. This has resulted in the development of a large assortment of computer software to perform the interpolation. The scientist is thus faced with a predicament of which is the optimum method of isarlthm interpolation for his data- This thesis describes an empirical evaluation of the performances of a comprehensive selection of software in relation to data characteristics. Much of the software is available in the academic environment or was developed by the author. The evaluation utilises detailed, photogrammetrically derived digital terrain data which are related to some currently used data classification systems. Additionally, several techniques, mostly derived from geomorphometry, are used to examine the data in more depth, in an attempt to establish reliable descriptors of their data characteristics. These descriptors are designed to be used in a multi-disciplinary environment. The interpolation process is essentially classified into three stages for evaluation. Random-to-grid algorithms are evaluated by considering the interpolated grid in relation to an observed grid. Grid-to-isarithm and random-to-isarithm interpolation are evaluated by considering the interpolated Isarithms with observed isarithms. In both cases, geometric errors and the morphological trueness of the interpolation product are considered using graphical and numerical parameters. These statements of accuracy are correlated with the data characteristics to establish the main factors influencing isarithm interpolation.

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A case study of the seasonal variations in indicator bacteria populations found in water supply wells in a small southern African village has yielded some disturbing findings. This village is typical of the region but has also benefited from the installation of a an experimental, higher- yielding type of wells, a 'collector' or Rannay-type well. In addition to improved yields, it had been assumed that these types of wells would also yield better quality water because of their superior construction. A two- year monitoring programme was carried out to try and understand the seasonal fluctuations in bacteriological quality terms of the hydrological controls. The study has confirmed that water quality (as determined by E.coli population density) in traditional, hand dug well is very variable. Bacterial population density fluctuates throughout the year but rarely drops to zero. This is also true for the collector well, demonstrating that even screened boreholes are pulling water from throughout the regolith, where soil- and ground-water fluxes ensure thorough mixing, and not preferentially from the fissured hardrock as usually supposed. Descriptions of the field techniques employed and resulting data on microbiological E.coli populations, selected hydrochemical determinands and basic hydrological parameters are presented. Analysis and interpretation of the data, together with hypotheses to account for the variability, form a substantial part of the thesis. The implications of the findings on health and general socio-economic development issues, including long-term water supply in similar rural areas, are discussed.

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