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

Abstract

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

Future-proofing hydrogeology by revising groundwater monitoring practice

“…Using these sensors, water-level changes inside boreholes are measured and stored at programmable time intervals providing time series of pressure that can be converted to hydraulic heads. For this conversion, manual water level measurements are required to calibrate and check automated measurements (e.g., Rau et al 2019).…”

“…The advent of automated measurement systems, in conjunction with data telemetry, enables increasingly comprehensive and cost-effective collection of groundwater data, even in remote locations (e.g., Rosenberry 1990;Post and von Asmuth 2013;Rau et al 2019). Many countries such as the US Geological Survey (Freeman et al 2004) These examples summarize advances that testify to the giant steps that have been made during the last three decades.…”

“…Practitioners often simply reference to ground level or the top of the borehole casing and neglect the accuracy of this information. Please refer to Rau et al (2019) for a detailed discussion of this issue. & In the trade-off between pressure transducer range (maximum limits) and resolution (smallest resolvable signals), practitioners seem to prefer maximizing range as it allows for greater versatility (less chance of failure due to overpressurization and a wider range of deployments).…”

“…Typically, such data are collected and maintained by different institutions, which again increases interpretation efforts. & Perhaps because of a lack of market push, many standard pressure transducers do not measure to subcentimetre resolution (Rau et al 2019). During procurement, affordability is all too often favoured over performance, even though lowend pressure transducers are known to have operational issues (e.g., resolution, accuracy, clock accuracy, longevity, etc.)…”

“…Monitoring groundwater, for example by measuring heads, underpins virtually all groundwater flow and storage investigations (e.g., Rau et al 2019) including the calibration of groundwater flow models (e.g., Hill and Tiedeman 2007) as well as ground-truth interpretations of large-scale indirect remote sensing or surface geophysical observations (e.g., Alley and Konikow 2015). Furthermore, groundwater monitoring is critical in times of unprecedented environmental change as the past is not necessarily a good predictor for the future.…”