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

Gabriel C. Rau, Vincent E. A. Post, Margaret A. Shanafield, Torsten Krekeler, Eddie W. Banks, Philipp Blum
2019 Hydrology and Earth System Sciences Discussions  
<p><strong>Abstract.</strong> 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
more » ... chniques 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&amp;thinsp;<&amp;thinsp;10<sup>&amp;minus;4</sup> are resolvable at distances&amp;thinsp;&amp;gtrapprox;&amp;thinsp;170&amp;thinsp;m. Further, it takes extraordinary effort to measure hydraulic head gradients&amp;thinsp;<&amp;thinsp;10<sup>&amp;minus;3</sup> over distances&amp;thinsp;<&amp;thinsp;10&amp;thinsp;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.</p>
doi:10.5194/hess-2019-182 fatcat:7vjvktghejamvacukfgwo2kaua