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2.10 Uncertainties

Insignificant trends in the assessment are mostly due to the relatively short time series in the dataset. The minimum length of measurement series to be used for the calculation of the GLT indicator set at 5 years within a ten-year time window. This 5-year minimum is based on the minimum range of timeseries requested by the WFD CIS document on trends analysis of groundwater pollutants (EC, 2009). However, basing the indicator calculation on less than 10 years of the evaluation period creates uncertainties in the indicator results and may give the false impression that no groundwater level trends occur. It is recommended to improve the dataset and increase the length of the time series at the monitoring locations as much as possible.

In addition, the relatively simple approach of the Groundwater Level Trend indicator compared to other indicators may bring some uncertainties:

  • GLT indicator is more sensitive to seasonal variability than the Standardized Groundwater trend indicator. However, because of the aggregation at the annual scale, the effects of seasonal variability in the indicator calculation are minimized.
  • Like other groundwater quantity indicators, the GLT indicator focusses on one aspect of the groundwater. The GLT indicator does not explain the cause of the increase or decrease of the groundwater levels. Groundwater levels and the derived GLT indicator can be strongly influenced by location. Groundwater level is mainly influenced by local and site-specific recharge processes and by regional to site-specific saturated flow processes that are not simply spatially correlated with the long-term climate change impacts where data coverage is not sufficient. Consequently, any interpretation or analysis of the resulting GLT needs to reflect an appreciation of the hydrogeological context of the observation monitoring sites. A set of indicators might be needed for undertaking integrated groundwater assessments and explaining the reasons for changing in groundwater levels.

Previous comments

  • scheidand (Andreas Scheidleder) 22 Oct 2021 14:06:35

    Comment from AT

    1st Para: Please do not use the CIS guidance 18 as a reference giving recommendations on the minimum criteria for GW quantity trend assessments. The 10/5 years are solely relevant for GW quality data!!! Better propose and apply your own criteria about the time series.

    1st bullet: it would be good if you explain the difference between the GLT and the Standardized Groundwater trend indicator in the chapter 2.4 methodology and to explain why you have chosen the GLT approach.
    2nd bullet: please specify at the end of the first sentence which one aspect is focused at.


    It would be also worth mentioning that it is not clear whether the selected sites are representative in terms of areal distribution or whatsoever.
    Furthermore, in Alpine or mountainous areas groundwater levels are not relevant (and often not measurable) but spring discharges.

    Furthermore, if GWBs are confined and under pressure, the measurement uncertainty of pressure heads is more than 0.01m.

    • zalllnih (Nihat Zal) 26 Nov 2021 15:59:28

      Thank you for your comment. The reference to the CIS guidance is removed.

      In the methodology section of the “supporting information”, more information is provided about the review of groundwater indicators that was made before developing this GLT indicator.

      Information about the spatial representativity is added to this section about “accuracy and uncertainties”: “No additional information or metadata is available for the groundwater monitoring stations reported in the WISE SoE. Data that provide the specifics of the measurement location (e.g. filter depth), the geohydrological setting (mountainous, coastal, river plain), the type of aquifer (confined, semi-confined, unconfined), the aquifer properties (e.g. transmissivity of sedimentary, karstic, hard-rock aquifers), as well as information related to climate and land use are required to interpret the behaviour of the groundwater level changes and trends. Moreover, without such information the representativeness of the available monitoring stations in terms of spatial distribution cannot be determined.” 

      With respect to the comment about the confined GWB: this is true. We have improved the classes of the GLT indicator. Please refer to responses to the comments above.

      Comment from AT

      1st Para: Please do not use the CIS guidance 18 as a reference giving recommendations on the minimum criteria for GW quantity trend assessments. The 10/5 years are solely relevant for GW quality data!!! Better propose and apply your own criteria about the time series.

      1st bullet: it would be good if you explain the difference between the GLT and the Standardized Groundwater trend indicator in the chapter 2.4 methodology and to explain why you have chosen the GLT approach.
      2nd bullet: please specify at the end of the first sentence which one aspect is focused at.


      It would be also worth mentioning that it is not clear whether the selected sites are representative in terms of areal distribution or whatsoever.
      Furthermore, in Alpine or mountainous areas groundwater levels are not relevant (and often not measurable) but spring discharges.

      Furthermore, if GWBs are confined and under pressure, the measurement uncertainty of pressure heads is more than 0.01m.

       

  • bednamal (Malgorzata Bednarek) 22 Oct 2021 16:45:17

    POLAND

    See comments referring to the methodology approach incl. incomparability of the data.

    • zalllnih (Nihat Zal) 26 Nov 2021 15:59:47

      Thank you for your comment. Please refer to response above.

      POLAND

      See comments referring to the methodology approach incl. incomparability of the data.

       

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