The literature in hydrological science on how long a rain drop takes to pass through the soil, may be taken up by trees, end up back in the atmosphere via evaporation, or recharge the groundwater and streams is ever growing (see graph below).
In our accepted manuscript on "The demographics of water: A review of water ages in the critical zone", we provide an overview on the current developments and open questions in this vibrant field dealing with water ages to improve the understanding of flow paths. This manuscript evolved from a workshop on “Water Ages in the Hydrological Cycle” held in October 2017 in the Black Forest funded by the Wassernetzwerk Baden-Württemberg. Back then, we discussed for about three days our experiences, new thoughts and challenges in estimating water ages in the terrestrial water cycle.
Our interdisciplinary group of scientists brought together various aspects that appear to be relevant for a better understanding and would be worth looking into in the future. This review manuscript is the result of these discussions at the workshop and via emails afterwards. It has been a great experience to work with that many bright scientists and put different views based on different backgrounds together. The collaborative and supportive atmosphere within the group of co-authors were extremely motivating and a great example for the success of interdisciplinary work. Thanks to the Wassernetzwerk Baden-Württemberg and the DFG for the financial support to work on this manuscript. You can download the accepted manuscript here.
Annual numbers of publications (orange line) with either "Travel times" or "Transit times" or "Water age" or "Residence times") in their title or keywords published in the journals “Water Resources Research”, “Journal of Hydrology” or “Hydrological Processes”. Horizontal lines indicate the average values over the indicated decade. Numbers of publication are also given in relation to the total number of publications in the considered journals (blue line, given in %). (Figure is not part of the review manuscript.)
Study to explore the "Mechanisms of consistently disconnected soil water pools over (pore)space and time" in discussion for Hydrology and Earth System Sciences
We present in HESSD an extensive stable isotope data set gathered by the Surface Hydrology and Erosion group at IDAEA-CSIC, Barcelona. Our study sheds light on the hypothesis of “ecohydrological separation” that was stated in the seminal paper by Brooks et al. (2010) almost ten years ago. Their publication initiated many stable isotope (2H and 18O) studies in ecohydrology, tree physiology, and vadose zone hydrology. However, the mechanism of how two subsurface water pools of different isotopic compositions evolve and persist are not yet understood.
This lack of understanding is partly due to insufficient sampling frequencies of isotope data in the field. We overcome this limitation by a unique sampling design gathering isotope data of mobile and bulk soil water, rainfall, groundwater, and stream water in a fortnightly frequency over 8 months. We further extended our study by four years using long-term stable isotope and soil moisture data to reveal that the seasonal dry down of the soil and the wetting up of the soil pores with isotopically distinct rainfall can explain the disjunct isotopic compositions of water that is either stored in the soil or routed quickly to the groundwater and streams.
Our findings provide the scientific basis to refute the often-made assumption in environmental modelling that water is well mixed in the subsurface. We further provide suggestions on how to implement our findings in future modelling frame works, by accounting for pore scale variability of water transport.
During my visit at the Center for Agricultural Resources Research in Shijiazhuang, Hebei last summer, I got involved in the work by master student, Meijia Zhu, who gathered an impressive hydrochemical data set in the Xiong’an New Area. I learned that this area will provide new housing for the ever growing region around the Chinese capital. However, the North China Plain, where these developments take place is characterized by a dramatic drop in the groundwater levels over the last decades due to its over consumption; mainly for agricultural use. Due to the water scarcity related to the groundwater mining, rivers, are running dry. To counter this development water gets regularly transferred from other basins (see Figure 4). Though, often, the river flow is dominated by sewage water from urban areas.
The work lead by Meijia Zhu now shows that this sewage water is not only affecting the water quality of Lake Baiyangdian, largest freshwater lake, but it also aggravates the underlaying groundwater body. We see, that the Chloride and electrical conductivity increases along the groundwater underlying studied river that flows into the Lake Baiyangdian (see Figure 6). We used end-member mixing analysis to show the ratio groundwater recharge sourced from either rainfall, lake water, or river water (see Figure 9).
The study shows clearly how the groundwater body is not only threatened by depletion, but that the combination of over use and sewage disposal into rivers affects the groundwater challenging the water supply of the new developing Xiong’an New Area.
Figure 9. d18O values versus Cl- concentrations of the groundwater and end-members. The mean values of Cl□ concentration and stable isotopes of the Fu River water in seasons without the influence of the precipitation and transferred water (samples collected in January 2018 and March 2018) were taken as values of one end-member that reflects sewage influence.
The manuscript to this study is openly available here.
Zhu M, Wang S, Kong X, Zheng W, Feng W, Zhang X, Yuan R, Song X, Sprenger M (2019): Interaction of Surface Water and Groundwater Influenced by Groundwater Over-Extraction, Waste Water Discharge and Water Transfer in Xiong'an New Area, China, Water, 11(3), 539, doi: 10.3390/w11030539.
EGU session on "Stable isotopes to study water and nutrient dynamics in the soil-plant-atmosphere continuum"
We are organizing again a session on "Stable isotopes to study water and nutrient dynamics in the soil-plant-atmosphere continuum" for the European Geoscience Union General Assembly. It will be the third year that we invite abstracts to this topic and we are looking forward to hear about the recent developments in the field of stable isotope applications in soil plant interactions. The session was well-received in the last years with a full room during the interesting talks in our oral session and many inspiring discussions at the posters. We hope to attract once again many scientists and welcome abstract submission until January 10, 2019. https://meetingorganizer.copernicus.org/EGU2019/session/31635
Study on inferring storage, mixing and fluxes in the critical zone of northern environments based in stable isotopes of soil water accepted for a Special Issue on "Water in the Critical Zone" in Hydrological Processes
We compiled stable isotope data (2H and 18O) of bulk soil water at five long-term experimental catchments across a hydro-meteorological gradient in the northern latitudes (Figure 1). The comparison of the extensive data set, covering different landscape units at each catchment, showed that vegetation, topography and elevation affect the isotopic composition over time and soil depth. Soil water beneath conifers is more enriched in heavy isotopes than beneath heather or oak vegetation. Sampling sites closer to the stream are generally less variable in their stable isotopic composition than sites at gentle hillslopes. Isotopic fractionation due to soil evaporation was observed mainly in the top 10 to 30 cm and was mainly controlled by the precipitation amount while temperature (as proxy for evaporation) was less important and soil moisture did not influence the isotopic fractionation, as revealed by multiple linear regression.
The study will be published (here) in a Special Issue on "Water in the Critical Zone" in Hydrological Processes. The work was funded by the ERC grant VeWa and you can find a presentation by Doerthe Tetzlaff about that project here.
The manuscript "Measuring and Modeling of Stable Isotopes of Mobile and Bulks Soil Water" now available as Open Access in Vadose Zone Journal
The final version on the manuscript on "Measuring and Modeling Stable Isotopes of Mobile and Bulk Soil Water" is now online available and can be downloaded here.
Manuscript on water ages in the critical zone currently in discussion for Hydrology and Earth System Sciences
A study on water ages and travel times in the critical zone is now out in Hydrology and Earth System Sciences Discussion. We used in this study the SWIS model that was tested at different sites in the northern latitudes in a previous investigation. We tracked the infiltrated water through the soil profiles and in the evaporation, transpiration and recharge fluxes. This way, we could derive travel times (which show how long the water takes to leave the soil via evaporation, transpiration or recharge), and median water ages (to estimate the median age of water in soil storage or the evaporation, transpiration and recharge fluxes). Our results showed for each study site, that water ages of soil storage, evaporation, transpiration and recharge were inversely related to the storage volume of the critical zone: water ages generally decreased exponentially with increasing soil water storage. These findings on the 1-D soil profile support the "inverse storage effect" as recently discussed for the catchment and hillslope scales. You can download the manuscript here.
Oral and poster session on "Stable isotopes to study water dynamics in the soil-plant-atmosphere continuum" during EGU 2018
The EGU session HS10.5/BG2.1/SSS13.40 on "Stable isotopes to study water dynamics in the soil-plant-atmosphere continuum" is scheduled for Friday, 13 April 2018 with oral presentations in the morning (08:30–10:00 in Room 2.15) and the poster presentations in the afternoon (17:30–19:00). Make sure you don't miss the interesting talks and posters covering field studies, methodological developments and modeling applications in the context of stable isotope hydrology to foster process understanding in the soil-plant-atmosphere continuum.
New paper out in Vadose Zone Journal on the differences between stable isotopic signal in mobile and bulk soil water
We compare in our latest study soil water isotope data from suction cup lysimeter, that are limited to sample the mobile water (MW), with soil water isotope data sampled with the direct water-vapor analysis, that samples the bulk soil water (BW). We present for six landscape units at three VeWa sites that the BW isotopic compositions shows a kinetic fractionation, which is indicative for soil evaporation, but MW does not. We suggest that the relative volume of MW to BW is relevant for explaining these isotopic differences, since MW volumes are usually relatively low during periods of high evaporation. We additionally use the numerical 1-D flow model SWIS (Soil Water Isotope Simulator) to simulate the hydrometric and isotopic dynamics at the studied sites. The simulations accounting for a fast and slow flow supported the conceptualization of two soil pore domains (MW and BW) with isotopic exchange via vapor exchange. Please see the manuscript here.
Very interesting discussion on the topic of water ages in the hydrological cycle during last weeks workshop funded by the Wassernetzwerk Baden-Württemberg. The interdisciplinary organizing committee (Prof. Dr. Markus Weiler , Chair of Hydrology, University of Freiburg; Prof. Dr. Werner Aeschbach , Institute of Environmental Physics, Heidelberg University; Prof. Dr. Christiane Werner , Chair of Ecosystem Physiology, University of Freiburg; PD Dr. Christiane Stumpp , Helmholtz Zentrum München and University of Freiburg) brought together early career and senior scientists working on the different compartments of the hydrological cycle. This way, the workshop fostered exchange between the disciplines to get a better understanding of how the water flow in the unsaturated and saturated zone is affected by vegetation and atmosphere. The workshop ended with a field visit of a long-term experimental forest site close to Freiburg (Conventwald).