the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A rainfall-tracking travel time distribution model to quantify mixing and storage release preference in a large shallow lake by two-year stable isotopic data
Abstract. Lake Taihu is the largest eutrophic lake in China that is shallow and connected to a dense river network. Severe eutrophication is frequently observed in Lake Taihu due to excess pollutant loadings. Understanding water cycle dynamics is essential for investigating this problem. The travel time distribution (TTD), residence time distribution (RTD) and storage selection (SAS) function describing how water is stored, mixed and released in the lake provide fundamental information on water cycle dynamics. In this study, a rainfall mixing model is established and coupled with the age master equation model to estimate the time-variant TTDs and RTDs of rainwater, river water and all water in Lake Taihu based on the two-year high-resolution isotopic data. In the rainfall mixing model, a novel rainfall mixing factor is introduced to quantify the mixing of rainwater with older lake water. The estimated range of travel time varies between 2–4 months and 7–9 months, depending on lake volume. Lake Taihu shows an inverse storage effect, i.e., the release preference shifts toward younger water when lake volume is large. The results of rainfall mixing model reveal distinct patterns and control factors between the TTDs of rainwater and river water, with rainwater contributing to 15 %–25 % of outflow. Then, the SAS functions of rainwater and river water are analyzed, which are controlled by different source zones and flow patterns. Temporal variation of spatial distribution of deuterium isotope composition illustrates storage release preference is controlled by the variation of horizontal flow paths and velocities in the lake.
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RC1: 'Comment on egusphere-2024-1513', Anonymous Referee #1, 11 Jul 2024
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Review of “A rainfall-tracking travel time distribution model to quantify mixing and storage release preference in a large shallow lake by two-year stable isotopic data” by Mao et al submitted to Hydrology and Earth System Sciences
In my opinion, the manuscript will be a great addition to the journal and be of interest to its readers. Mao et al targets the variations in time and in space of residence time and travel time at Lake Taihu, China, by processing and analyzing measurements of deuterium in the lake. Temporal scales in lakes have been correlated to water quality issues, therefore knowing how long a water parcel remains in the lake depending on the meteorological conditions will provide a clearer picture on how to better manage water quality in Taihu Lake. For example, their results show that depending on the inflow of water (through rivers and rain) the travel time can decrease by half.
However, I recommend the manuscript to be brought back to the authors for revisions for the following reasons:- There have been publications mentioning Lake Taihu’s residence time. I am thinking for example of Xu et al 2009 (https://doi.org/10.4319/lo.2010.55.1.0420), Xu et al 2015 (https://doi.org/10.1021/es503744q) and Paerl et al 2014 (https://doi.org/10.1371/journal.pone.0113123). Interestingly the value of residence time is not consistent among these publications, but they remain below the 1 year mark. I think the manuscript would benefit by including a comparison and a possible explanation of the discrepancy between the publications.
- There are portions of paragraph in Section 5 that appear contradictory to the results described in the abstract. For example, at the end of Section 5.2.1, lines 477-478, Mao et al states that “older water parcels may have greater chances leaving the system than younger water parcels”. However, in the abstract it is stated that “the release preference shifts toward younger water when lake volume is large”. A reorganization of the subsections would help to make things clearer.
- In the introduction, lines 51-52, the authors mention that only Smith et al. 2018 applied the travel time distribution theory to a lake. Unless the authors were implicitly focusing on the usage of isotopic compound, I disagree with the authors. Temporal scales have been addressed in previous work in lakes. For example, Rueda and Cohen, 2005 (https://doi.org/10.4319/lo.2005.50.5.1638) looked at variation in space and time of residence time in an embayment of Lake Ontario, Canada.
- The authors mention the spatio-temporal variations of the time scales are controlled by horizontal velocities on several occasions (in the abstract and in the conclusion), they did not discuss past work identifying the flow field at Lake Taihu. The manuscript would benefit from including comparisons with past work on average flow field in Lake Taihu Liu et al. 2018 (doi:10.3390/w10060792) shows for example a rather complex average flow field, which would definitely hinder the flow of water from inflow to outflow.
I noticed a couple of typos in the text:
- Figure 2: There is no sampling point 2. I assume it is supposed to be where sampling point 32 is.
- Line 304: If I understand properly, s_rain(t,tau) is the volume of rain water aged tau, and not s(t,tau).
- Line 499: please replace “board range” with “broad range”.
- Line 505: please replace “Lake” with “lake”.
Citation: https://doi.org/10.5194/egusphere-2024-1513-RC1 -
AC1: 'Reply on RC1', Rong Mao, 15 Jul 2024
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Please find the attached document for our point-by-point replies.
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