Abstract
Abstract. Hydraulic head and gradient measurements underpin practically all investigations in hydrogeology. There is sufficient information in the literature to suggest that head measurement errors can impede the reliable detection of flow directions and significantly increase the uncertainty of groundwater flow rate calculations. Yet educational textbooks 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 measurement 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 hydrogeological sciences and provides a starting point for the development of universal measurement protocols for water level data collection.
Highlights
Water level and hydraulic head time series are critical for understanding water flow-related processes and properties in both surface and subsurface aquatic environments
Reliable water level measurements are at the core of every hydrogeological investigation, and the measurement error determines which processes or properties can be resolved
By propagating the random errors, we find that with current standard practice, horizontal head gradients < 10−4 are only resolvable at distances
Summary
Water level and hydraulic head time series are critical for understanding water flow-related processes and properties in both surface and subsurface aquatic environments. Measurements of hydraulic head are used to determine groundwater flow, estimate aquifer properties, and investigate aquifer processes such as the response to pumping or groundwater recharge (e.g., Freeze and Cherry, 1979; Domenico and Schwartz, 1997). Rau et al.: Hydraulic head errors water level time series common practice This has been a major advancement in our capability to study hydrological processes, but the proper use of automated sensors means that researchers need to have a good understanding of instrument technology and operating procedures. Modern hydrologists and hydrogeologists require a broad skill set, which is typically too extensive to be comprehensively covered in standard textbooks and water-related educational programmes
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