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1. Methodological approach to the computation of the indicator

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scheidand (Andreas Scheidleder) 22 Oct 2021 14:07:30

Comment from AT

The EU CIS Working Group Groundwater is going to elaborate some guidance on the trend assessment for GW levels. It could be a good idea to join efforts and elaborate a methodology together?

If not, it would be good if you come forward with own proposals on minimum requirements for the time window and the minimum number of considered years, gaps etc because the recommendations in CIS guidance 18 (EC, 2009) are not valid for GW quantity

Please describe also the methods you have considered and why you have chosen the GLT approach. Out of a sudden in chapter 2.10 you are mentioning pros and cons compared to other methods.
- zalllnih (Nihat Zal) 26 Nov 2021 16:02:19
  
  Thank you for your comments.
  
  The indicator was developed with the intention to make best use of the capacity of the available groundwater level datasetfor a European overview on the GWL change. Developing a scientific standard for the length of groundwater level monioting series was not the scope. However, for the development of the EEA groundwater level trend indicator, an overview of existing groundwater quantity indicators and literature reporting on multi-year groundwater level trend analysis was made. None of these existing groundwater approaches was fully suited due to insufficient data availability in the WISE SoE.
  
  More information about the considered methods and literature review, is added to the methodology section in “supporting information”.
  
  Comment from AT
  
  The EU CIS Working Group Groundwater is going to elaborate some guidance on the trend assessment for GW levels. It could be a good idea to join efforts and elaborate a methodology together?
  
  If not, it would be good if you come forward with own proposals on minimum requirements for the time window and the minimum number of considered years, gaps etc because the recommendations in CIS guidance 18 (EC, 2009) are not valid for GW quantity
  
  Please describe also the methods you have considered and why you have chosen the GLT approach. Out of a sudden in chapter 2.10 you are mentioning pros and cons compared to other methods.
bednamal (Malgorzata Bednarek) 22 Oct 2021 16:50:12

POLAND

1. Heterogeneity of input data for time series analysis – time sequences of different lengths and time intervals should not be compared, as proposed in the submitted work. In the situation of a nonlinear phenomenon, which are in natural conditions fluctuations of groundwater, differences in the length of the analyzed sequences may lead to erroneous conclusions.

2. Lack of harmonisation of input data units from different countries. In case of Poland, data refer to the depth to water table, but they seem to be treated as water levels (opposite). The above means that any conclusions about the ascending trend of the water table in recent years are wrong and need to be recalculated.

3. Failure to include R2 values in the presented linear regression analysis. In the interpretation of linear regression results, not only the p-value itself is generally taken into account, but also R2 – this is in accordance with "Aqueduct 3.0: Updated Decision. Relevant Global Water Risk Indicators", which is referred to by the authors of the "draft", but there is no such approach in the "draft".
- zalllnih (Nihat Zal) 26 Nov 2021 16:03:26
  Thank you for your comments.
  1. We are aware of that heterogeneity and limitations of the groundwater level data series in the WISE database, and the uncertainties this causes for the presented Groundwater level indicator. The indicator was developed with the intention to make best use of the capacity of the available groundwater level dataset. We have improved the indicator sheet at this point in several ways:
  - In the first section of the indicator assessment we have stated more clearly the limitations and the overall approach.
  - We added the time period of data availability for each country in the note section of Figure 1a and figure 1b.
  - In the “supporting information” section the limitations and capacity of the available groundwater level data are described in more detail.
  1. The required units of the data are described in the WISE data dictionary. For groundwater levels this is groundwater heads (m) with respect to sea level. During revision, the data from the Polish monitoring stations were removed from indicator assessment and the indicator sheet. When the data for these monitoring station are uploaded again in the unit described by the WISE data dictionary, they can be taken up in the assessment of the groundwater level trends again.
  1. The R2 is not evaluated because it does not provide information about the significance of the groundwater level trends assessed in the developed approach. It could provide information about the level of temporal variability of the groundwater level trends. However, interpreting and aggregating such information for a range of individual groundwater level monitoring stations would lead to ambiguous conclusions. In the Aqueduct 3.0 approach, the Declining Groundwater Table Declining Trend (GTDT) is based on gridded global model results that are aggregated to the scale of large groundwater bodies before trend calculation is applied. In this apporach, the R2 values show a strong correlation with the p-values (low p-values with high R2, and vise versa).
  POLAND
  
  1. Heterogeneity of input data for time series analysis – time sequences of different lengths and time intervals should not be compared, as proposed in the submitted work. In the situation of a nonlinear phenomenon, which are in natural conditions fluctuations of groundwater, differences in the length of the analyzed sequences may lead to erroneous conclusions.
  
  2. Lack of harmonisation of input data units from different countries. In case of Poland, data refer to the depth to water table, but they seem to be treated as water levels (opposite). The above means that any conclusions about the ascending trend of the water table in recent years are wrong and need to be recalculated.
  
  3. Failure to include R2 values in the presented linear regression analysis. In the interpretation of linear regression results, not only the p-value itself is generally taken into account, but also R2 – this is in accordance with "Aqueduct 3.0: Updated Decision. Relevant Global Water Risk Indicators", which is referred to by the authors of the "draft", but there is no such approach in the "draft".

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