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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/hess-2019-182
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2019-182
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Review article 29 Apr 2019

Review article | 29 Apr 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Error in hydraulic head and gradient time-series measurements: a quantitative appraisal

Gabriel C. Rau1,2, Vincent E. A. Post3, Margaret A. Shanafield4, Torsten Krekeler3, Eddie W. Banks4, and Philipp Blum1 Gabriel C. Rau et al.
  • 1Karlsruhe Institute of Technology (KIT), Institute of Applied Geosciences (AGW), Karlsruhe, Germany
  • 2Connected Waters Initiative Research Centre (CWI), The University of New South Wales (UNSW), Sydney, Australia
  • 3Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany
  • 4National Centre for Groundwater Research and Training (NCGRT) and College of Science and Engineering, Flinders University, Adelaide, Australia

Abstract. Hydraulic head and gradient measurements underpin practically all investigations in hydro(geo)logy. There is sufficient information in the literature to suggest that head measurement errors may be so large that flow directions can not be inferred reliably, and that their magnitude can have as great an effect on the uncertainty of flow rates as the hydraulic conductivity. Yet, educational text books contain limited content regarding measurement techniques and studies rarely report on measurement errors. The objective of our study is to review currently-accepted standard operating procedures in hydrological research and to determine the smallest head gradients that can be resolved. To this aim, we first systematically investigate the systematic and random measurements errors involved in collecting time series information on hydraulic head at a given location: (1) geospatial position, (2) point of head, (3) depth to water, and (4) water level time series. Then, by propagating the random errors, we find that with current standard practice, horizontal head gradients < 10−4 are resolvable at distances ⪆ 170 m. Further, it takes extraordinary effort to measure hydraulic head gradients < 10−3 over distances < 10 m. In reality, accuracy will be worse than our theoretical estimates because of the many possible systematic errors. Regional flow on a scale of kilometres or more can be inferred with current best-practice methods, but processes such as vertical flow within an aquifer cannot be determined until more accurate and precise measurement methods are developed. Finally, we offer a concise set of recommendations for water level, hydraulic head and gradient time series measurements. We anticipate that our work contributes to progressing the quality of head time series data in the hydro(geo)logical sciences, and provides a starting point for the development of universal measurement protocols for water level data collection.

Gabriel C. Rau et al.
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Gabriel C. Rau et al.
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Short summary
The flow of water is often inferred from water levels and gradients whose measurements are considered trivial despite the many steps and complexity of the instruments involved. We systematically review the four measurement steps required and summarise the systematic errors. To determine the accuracy with which flow can be resolved we quantify and propagate the random errors. Our results illustrate the limitations of current practice and provide concise recommendations to improve data quality.
The flow of water is often inferred from water levels and gradients whose measurements are...
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