the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Sep 2024
Imprints of Increases in Evapotranspiration on Decreases in Streamflow during dry Periods, a large-sample Analysis in Germany
Abstract. Decreases in streamflow (Q) during dry periods have the potential to negatively affect river ecosystems and human societies, and understanding their causes is crucial to anticipate them. The contribution of increases in catchment actual evapotranspiration (E) to decreases in Q during dry periods remains poorly quantified. To address this gap, we performed a data-based analysis for 363 small (< 1000 km2) catchments without substantial water management influences in Germany over 1970–2019. We quantified trends in the magnitude of summer low flows, i.e., the minimum 7-day Q during summer months (7dQmin, JJA). We attributed these trends to their main potential predictors (namely, long-term variations in E, summer precipitation, P, as well as spring and winter P as proxies for storage). Furthermore, we assessed potential changes in the annual P-Q relationship during a multi-year drought in the early 1990s, and investigated whether these changes were related with trends and anomalies in E and P. Summer low flows decreased significantly in 31 % of the catchments, with a median trend of -3.7 % decade-1 across all catchments. Increases in E were a relevant driver of these decreases particularly in relatively drier Eastern catchments (contribution to long-term dynamics of 7dQmin, JJA of 35 % based on multiple linear regression, and correlation coefficient between trends in 7dQmin, JJA and in E of -0.74). Changes in the P-Q relationship occurred in 26 % of the catchments that experienced a multi-year drought between 1989 and 1993, with lower Q than expected from the relationship before the drought. These changes occurred in catchments with concurrent strong increases in E (median trend of 6.1 % decade-1). Our findings point to the importance of increases in E, especially in dry catchments, when assessing potential future decreases in Q during dry periods for water management and climate adaptation strategies.
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RC1: 'Comment on egusphere-2024-2678', Anonymous Referee #1, 26 Sep 2024
This paper analysed the summer low flows by using the datasets from 363 small catchments. The relationships among evapotranspiration (E), precipitation (P) and streamflow (Q), as well as the storage (S) were quantified. Results showed that summer low flows decreased significantly, and increased E played the main driver in the eastern catchments. In addition, the P-Q relationship changed in 26% of catchments between 1989 and 1993. Generally, the structure of the paper is clear and well-organized, however, there are some concerns.
Specific comments:
- The coefficients of a and b in equation (2) should be vary with wind direction and elevation of gauges. Does authors calculate them for each gauges? If so, please provide the analysis which are the main uncertainty for precipitation datasets.
- It is quite difficult to understand the equation (4), where the dynamics of storage was approximated by P_mam and P_djf. Please provide more explanations. In addition, since baseflow is 0.66 in this area, which means the soil moisture and groundwater both plays important roles in runoff variation. But it seems they are not taken into account in the analysis.
- For multiple linear regression in predicting summer low flows, the authors showed the R2 in four clusters which exhibited good performance in Table S1. However, there is spatial variation among different gauges so the coefficients vary at each gauge, does the regression in the gauge scale follow the same trend with cluster?
- Could authors discuss why the r= -0.49 in P_djf for the easter cluster in lines 258?
- For 363 catchments, there is only 15 catchment with negative Cp-q rel values which distributed sparsely in Fig.8(a). I am curious about the possibility caused by data process uncertainty.
- For Fig.5 and Table S1, it showed that P_jja played a major contribution to Q changes in Pre-Alphine and South-Central cluster, while E played much more contribution in Eastern and Northern area. Does it relate to elevation changes?
Citation: https://doi.org/10.5194/egusphere-2024-2678-RC1 - AC1: 'Reply on RC1', Giulia Bruno, 10 Dec 2024
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RC2: 'Comment on egusphere-2024-2678', Anonymous Referee #2, 02 Oct 2024
Bruno et al. analyzed the decreasing trend of low flow in 363 small catchments in Germany. They also attributed the decrease to the increase of ET. They further unraveled that the change of P-Q relationship during drought produces lower flow which is generally due to the increased ET. They conducted this work based on observations of P and Q, empirical expression of subsurface storage, and ET derived based on water balance and statistical analysis. I think studying the decrease of low flow and trying to find the major drivers is very important in the climate change background. The data and analysis are generally reliable, the structure and the writing are good. However, there the following concerns which need clarification from authors for further review.
- I was more or less confused by the overall idea of the authors. If you wanted to check if the decrease of the flow is caused by increased ET, and you also have calculated the water balance, why not do a straightforward analysis of the overall change of P, ET, Q, and S. Then it is easy to get if the decrease of flow is mainly driven by ET. Then you can do the analysis in your manuscript as a follow-up. Otherwise, I feel the conclusions are even not that convincing as, for example, the decrease of low flow might be just because of the shift of the timing of streamflow.
- You mentioned you used Kirchner’s approach to calculate Sdyn which needs that S is the main control of Q generation. I am wondering if the 363 catchments you used meet this requirement and where is your analysis for this?
- So, how do you quantify the uncertainties in E you derived from water balance as I am not sure the uncertainties in Sdyn.
- Also, I have to say, for the catchments with areas ranging from 50-150km2, the lateral groundwater flow is significant which has been discussed in Ying Fan’s paper ‘Are catchments leaky?’ and also quantified in our research (not published yet). Therefore, Equation 3 might be problematic.
- Line 161, PDIF and PMAM are used as proxies of storage processes. How and why they e used as proxies?
- All conclusions occur in less than 30% of the catchments, so how do you think about the generality of the study?
- I am wondering why ET is increasing? The land cover change or the temperature increasing? Authors listed a lot of attributes of the catchments in table 1 but are limited used in the analysis.
- Have you cited the paper talking about the similar thing? Tran et al. (2023), Frontiers in Water.
I suggest returning this manuscript to authors for major revision.
Citation: https://doi.org/10.5194/egusphere-2024-2678-RC2 - AC2: 'Reply on RC2', Giulia Bruno, 10 Dec 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-2678', Anonymous Referee #1, 26 Sep 2024
This paper analysed the summer low flows by using the datasets from 363 small catchments. The relationships among evapotranspiration (E), precipitation (P) and streamflow (Q), as well as the storage (S) were quantified. Results showed that summer low flows decreased significantly, and increased E played the main driver in the eastern catchments. In addition, the P-Q relationship changed in 26% of catchments between 1989 and 1993. Generally, the structure of the paper is clear and well-organized, however, there are some concerns.
Specific comments:
- The coefficients of a and b in equation (2) should be vary with wind direction and elevation of gauges. Does authors calculate them for each gauges? If so, please provide the analysis which are the main uncertainty for precipitation datasets.
- It is quite difficult to understand the equation (4), where the dynamics of storage was approximated by P_mam and P_djf. Please provide more explanations. In addition, since baseflow is 0.66 in this area, which means the soil moisture and groundwater both plays important roles in runoff variation. But it seems they are not taken into account in the analysis.
- For multiple linear regression in predicting summer low flows, the authors showed the R2 in four clusters which exhibited good performance in Table S1. However, there is spatial variation among different gauges so the coefficients vary at each gauge, does the regression in the gauge scale follow the same trend with cluster?
- Could authors discuss why the r= -0.49 in P_djf for the easter cluster in lines 258?
- For 363 catchments, there is only 15 catchment with negative Cp-q rel values which distributed sparsely in Fig.8(a). I am curious about the possibility caused by data process uncertainty.
- For Fig.5 and Table S1, it showed that P_jja played a major contribution to Q changes in Pre-Alphine and South-Central cluster, while E played much more contribution in Eastern and Northern area. Does it relate to elevation changes?
Citation: https://doi.org/10.5194/egusphere-2024-2678-RC1 - AC1: 'Reply on RC1', Giulia Bruno, 10 Dec 2024
-
RC2: 'Comment on egusphere-2024-2678', Anonymous Referee #2, 02 Oct 2024
Bruno et al. analyzed the decreasing trend of low flow in 363 small catchments in Germany. They also attributed the decrease to the increase of ET. They further unraveled that the change of P-Q relationship during drought produces lower flow which is generally due to the increased ET. They conducted this work based on observations of P and Q, empirical expression of subsurface storage, and ET derived based on water balance and statistical analysis. I think studying the decrease of low flow and trying to find the major drivers is very important in the climate change background. The data and analysis are generally reliable, the structure and the writing are good. However, there the following concerns which need clarification from authors for further review.
- I was more or less confused by the overall idea of the authors. If you wanted to check if the decrease of the flow is caused by increased ET, and you also have calculated the water balance, why not do a straightforward analysis of the overall change of P, ET, Q, and S. Then it is easy to get if the decrease of flow is mainly driven by ET. Then you can do the analysis in your manuscript as a follow-up. Otherwise, I feel the conclusions are even not that convincing as, for example, the decrease of low flow might be just because of the shift of the timing of streamflow.
- You mentioned you used Kirchner’s approach to calculate Sdyn which needs that S is the main control of Q generation. I am wondering if the 363 catchments you used meet this requirement and where is your analysis for this?
- So, how do you quantify the uncertainties in E you derived from water balance as I am not sure the uncertainties in Sdyn.
- Also, I have to say, for the catchments with areas ranging from 50-150km2, the lateral groundwater flow is significant which has been discussed in Ying Fan’s paper ‘Are catchments leaky?’ and also quantified in our research (not published yet). Therefore, Equation 3 might be problematic.
- Line 161, PDIF and PMAM are used as proxies of storage processes. How and why they e used as proxies?
- All conclusions occur in less than 30% of the catchments, so how do you think about the generality of the study?
- I am wondering why ET is increasing? The land cover change or the temperature increasing? Authors listed a lot of attributes of the catchments in table 1 but are limited used in the analysis.
- Have you cited the paper talking about the similar thing? Tran et al. (2023), Frontiers in Water.
I suggest returning this manuscript to authors for major revision.
Citation: https://doi.org/10.5194/egusphere-2024-2678-RC2 - AC2: 'Reply on RC2', Giulia Bruno, 10 Dec 2024
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