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Vadose zone hydrology: concepts and techniques
Expanded Title:The project on Vadose Zone Hydrology: Spatial and Temporal Influences, Assessment Techniques and Aquifer Susceptibility (K5/2052), follows from previous work outlining the importance of classifying the vadose zone for improved understanding (Dippenaar et al. 2010). Given the wide range of disciplines involved in vadose zone hydrology, detailed investigation of the vadose zone is generally subject to: • ‘Mono-disciplinary’ investigation for specific purposes such as either plant water availability and nutrients, potential groundwater recharge, groundwater vulnerability, seepage concerns for infrastructure development, etcetera, without adequate ability to transfer the knowledge to other applications • Empirical or modelling assumptions based on measurable surface water and/ or groundwater input data that compromise the integrity of any subsequent modelling. An approach involving numerous disciplines has numerous problems. Symbols and subscripts denoting parameters are not universal and are simplified here to include concepts from civil engineering, hydrogeology, chemistry, physics, soil sciences and other disciplines. Where possible, the symbols used hereafter represent those encountered in majority of the texts (e.g. η or n for porosity instead of Φ; d for particle size diameter instead of 2r, D or Φ) and those particularly pertaining the hydrogeology in favour of other disciplines (e.g. θ for moisture content instead of w or m; or preference over soil mechanics where the symbols are often interchanged, as in k for permeability and K for hydraulic conductivity or q for specific discharge versus Q for flux). The symbols from the governing equations have been changed accordingly. All units – unless specifically noted otherwise – have also been converted to SI units rather than commonly used units. Permeability, for instance, is noted in square meters and not square centimetre, darcy or any other equivalent. Definitions differ per discipline. This becomes important in a multi-disciplinary field of study as confusion and misinterpretation become concerns. Hereafter, definitions apply mainly to the hydrogeological discipline where, for instance, soil refers to a mixture of solids and pore space containing essentially air or water (as opposed to a strength-based distinction from rock in engineering, or a medium subject to a long formation and inclusive or organic matter in soil sciences). Where overlapping terminology exists, all relevant and important terms have been defined. Those pertaining to hydrogeology have, however, been used throughout the text (e.g. specific discharge instead of seepage velocity; pedocrete instead of laterite). Investigation techniques are addressed at a later stage. Techniques, methods and approaches vary between disciplines and are based on the need for investigation. It is aimed in this study to address the most important techniques, emphasising the intermediate vadose zone and subsurface processes rather than soil moisture measurements, and to incorporate these for the use of other disciplines to avoid unnecessary repetition and to find agreement between disciplines. One critical example of this is the quantification of readily available parameters such as hydraulic conductivity or permeability that, by definition, apply to saturated conditions and have to be corrected based on the moisture content or tension. Additional detailed emphasis is placed on the proper description of earth materials, deduction of the shallow flow processes and the development of the conceptual model. Although different disciplines should continue to exist within their own respective fields of expertise, unnecessary duplication and lack of cross-disciplinary understanding should be addressed. Consolidation of diverse aspects from such different disciplines will be beneficial to a holistic understanding of the vadose zone, as well as on its importance in a vast number of applications. Clarity regarding definitions is also important, notably given the duplicate concepts of, for instance, recharge (to groundwater table or that which is potential), confinement (of aquifers or stream channels) and infiltration (including or excluding the term percolation) Without standardisation or infringing on the very important attributes of the individual specialist disciplines, a need exists for more open dialogue between (1) various earth scientists (geologists, engineering geologists, hydrogeologists, soil scientists, pedologists, geomorphologists, hydrologists), and (2) between earth scientists and other decision makers (social scientists, engineers, town planners, economists, legal specialists, managers). Earth scientists need to translate their findings for use by a broad audience in which the results can be interpreted to a variety of applications and by a variety of specialists. As part of this, a standard multi-faceted Vadose Zone Assessment Protocol (VZAP; §9) has been deduced which outlines stages of investigation. Competency of specialists and decision-making are continuously addressed. Although vadose zone hydrology is in itself a specialised interdisciplinary field, the applications rely on the requirements of the individual specialists involved in its study. Better dialogue between specialists is the solution rather than a single uniform discipline, and the contributions of all interested disciplines improve the quality of investigations. Certain aspects identified in recent literature accentuate gaps in knowledge pertaining to hydrogeology and vadose zone hydrology. Some of these include, for instance, the matter of variably saturated fracture flow (Berkowitz 2002), the empirical approaches rather than field quantification, scale and heterogeneity issues (Miller and Gray 2002), and lack of cross-disciplinary understanding (Dippenaar 2012). Although it is impossible to solve issues in vadose zone hydrology given the broad scope of this project, attempts have been made to address the most important of these concerns.
Date Published:10/03/2014
Document Type:Research Report
Document Subjects:Water Resource Management/IWRM - Planning and development, Water Resource Management/IWRM - Catchment Management, Water Resource Management/IWRM - Hydrogeology, R & D - Methodology
Document Keywords:Engineering, Environment, Ground Water, Guidelines, Hydrology, Pollution control, Water Quality
Document Format:Report
Document File Type:pdf
Research Report Type:Technical
WRC Report No:TT 584/13
ISBN No:978-1-4312-0507-3
Authors:Dippenaar MA; van Rooy JL
Project Leader:Dippenaar MA
Project No:K5/2052
Originator:WRC
Organizations:University of Pretoria
Document Size:11 168 KB
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