the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Functional data analysis to quantify and investigate controls on and changes in baseflow seasonality
Kathryn A. Leeming
John P. Bloomfield
Gemma Coxon
Yanchen Zheng
Abstract. Baseflow is the delayed component of streamflow from subsurface storage and is critical for sustaining ecological flows and ensuring water resource security. Understanding controls on and changes in baseflow, including the seasonality of baseflow, is therefore an important task. Baseflow seasonality has typically been investigated using pre-defined hydrological seasons. Instead, here, we investigate baseflow seasonality using data-led approaches that identify and cluster average annual baseflow hydrographs that exhibit early-, mid-, or late-seasonality. We apply a novel functional data analysis (FDA) approach and examine temporal changes in the timing of seasonal peaks in annual standardised baseflow hydrographs for 671 catchments across Great Britain (GB). We use data from the CAMELS-GB dataset for the period 1976 to 2015 split into two twenty-year time blocks (1976–1995 and 1996–2015). Functional clustering enables groups of catchments with similar distributions between time blocks to be identified. Changes in baseflow seasonality with time are investigated by identifying and characterising catchments that move between functional clusters and time blocks, while analysis of the timing of baseflow peaks provides additional temporal resolution to the early-, mid-, and late-season discretisation generated by the functional clustering. The analysis shows that baseflow seasonality has a spatio-temporally coherent structure across GB and catchment characteristics are a first order control on the form of seasonal baseflow clusters. Changes in climate are inferred to be the first order control on changes in baseflow seasonality between the two time blocks. A change to earlier seasonal baseflow in snow-melt influenced catchments in upland northern GB is associated with systematic warming across the two time blocks, and a move to earlier (later) baseflow seasonality across lowland southern, central and eastern (western, north-western and northern) catchments in GB is associated with earlier (later) seasonality in effective rainfall (defined as precipitation minus potential evapotranspiration). These changes in baseflow seasonality in non-snow-melt influenced catchments are consistent with the proposition that, in temperate environments, climate warming leading to vegetation phenology-mediated changes in evapotranspiration may be modifying the timing of hydrological cycles.
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Kathryn A. Leeming et al.
Status: open (until 24 Oct 2023)
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RC1: 'Comment on hess-2023-202', Anonymous Referee #1, 15 Sep 2023
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General comments:
The authors of the manuscript entitled "Functional data analysis to quantify and investigate controls on and changes in baseflow seasonality" aims at applying a novel functional data analysis (FDA) approach and examine temporal changes in the timing of seasonal peaks in annual standardized baseflow hydrographs for 671 catchments across Great Britain (GB). It is an important topic about watershed management plans. Unfortunately, I find that the manuscript does not break new ground nor offer insightful new methodology to the field of water resources. In general, the manuscript does not meet the high standard of HESS journal due to many flaws as follow (Main concerns). I cannot support this version of manuscript.
Main concerns:
1. As the authors state baseflow is an important component of runoff during periods of low flow and drought, and it is critical to water security in the basin. However, how to accurately simulate baseflow has been a challenge in hydrology. The authors only mention very briefly in the manuscript the use of digital filtering to calculate baseflow, which is not very reasonable. Although the Lyne-Hollick digital filtering method is superior to the parameter less separation simplicity, the parameter selection as well as the number of filters need to be considered. Finally, the accuracy of the simulated values of the baseflow needs to be verified, and subsequent analyses should be performed after these tasks are completed to ensure the reliability of the research results.
2. The manuscript focuses on analyzing the seasonality of baseflow and splits the hydrologic data from all stations into two windows. This approach is too subjective, as hydrological processes are affected to different degrees in each basin due to human activities and climate change, and in some cases, the timing of hydrological surges may even occur. Therefore, it is recommended to analyze the abrupt changes in baseflow in each basin before performing temporal and spatial analyses.
3. The manuscript contains insufficient research and more analysis needs to be added subsequently. For example, the authors mention that there is some consistency or correlation between changes in effective rainfall and baseflow seasonality, and a quantitative result is needed. On the other hand, the effects of other factors on baseflow seasonality, such as TWS and NDVI, need to be analyzed.
Citation: https://doi.org/10.5194/hess-2023-202-RC1 -
RC2: 'Comment on hess-2023-202', Anonymous Referee #2, 20 Sep 2023
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This manuscript introduced a functional data analysis (FDA) approach to examine how the timing of seasonal peaks in annual standardized baseflow changes. It is applied to >600 catchments across Great Britain (GB). Understanding changes in (base)flow conditions is relevant, but before the paper is publishable several concerns need to be addressed (in my opinion).
- The split of data from all stations into two windows is very arbitrary and may overlook some of the actual changes occurring (but are associated with other timings).
- It is unclear how the chosen method of baseflow would affect the results.
- The association of vegetation-related (NDVI) causes is rather handwavy and qualitative in nature.
- Color scales of Figures 1b and 2b could have a more informative range to highlight catchment differences better.
- Could maps of figure 6 be enlarged so markers overlap less).
If these points are meaningfully addressed the paper seems publishable.
Citation: https://doi.org/10.5194/hess-2023-202-RC2
Kathryn A. Leeming et al.
Kathryn A. Leeming et al.
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