The transmissive critical zone: understanding the karst hydrology-biogeochemical interface for sustainable management

Lead Research Organisation: University of Aberdeen
Department Name: Geography and Environment


The Earth's surface (soil and plants), and the rock underneath interact, linked by rainwater flowing through the soil into the rock. The soil imparts a chemical signature to the water, sometimes bad leading to loss of water quality. This signature is mediated by movement through the rock, and then, when underground water re-emerges, in streams and rivers by bacterial activity. As such, how this outer layer of planet Earth functions is 'critical' to key needs of mankind - how much water we have available and its quality; how well the soil functions as a result of water draining through it. The study of how these layers interact is thus called 'critical zone' research.
Our research programme uses such 'critical zone' research in an environment where the local residents face significant environmental challenges - in rural China, an area of rapid growth and where many live under the poverty line. This is a joint research programme between UK and China. We will focus on two of these challenges: water availability and quality, and how movement of water in the critical zone influences surface vegetation. Crucial to this research is that the underlying rock is mostly limestone. Limestone is easily dissolved and water can move very quickly through the subsurface. So soils may dry sooner (as the subsurface beneath is freely-draining) and there is limited water storage on the surface and underground. Limestone is widely distributed world-wide, but particularly in China and so study here is relevant to many world-wide.
The people living in the catchment generally live-off-the-land. It provides their water and food - a phenomenon known as the ecosystem providing services. Where the slopes are not too steep, the land surface is heavily-cultivated. This in turn presents problems e.g., the water quality is poor, with dangerously high-level of nitrate (a chemical that is found in fertiliser); clearance of vegetation exposes rock, limiting how land may be used. Further challenging to local residents is that the climate is changing. How rain is delivered to the catchment has been changing such that water is not available as before. Thus there have also been water shortages, and this led to crop failure and so loss of food.
Land use change is important in shaping these ecosystem services, but climate change may be one of the most significant threats the residents will face; science must help them prepare for facing these threats with successful outcomes. Our research will generate models of how the critical zone functions currently and from these we can then investigate how the critical zone functioning may adapt to different environmental drivers. There is a large body of scientific modelling outside this project that has identified how the climate may change. Thus, we can draw on this to run the models we will develop of the critical zone functioning, not only under land use change, but also under future climate scenarios.
All this research will contribute to understanding where this catchment critical zone is most sensitive to future threats. However, it is important that this understanding reaches the people who need to use it. So the final activity we will undertake comes under the umbrella of 'knowledge exchange' - sharing our findings with those who need this research, and adjusting our understanding based on knowledge they too have. Thus our last, but not least, activity is working with those who live in the landscape and those who manage it, to help them identify how their activities can cause the least harm and offer the most protection to their ecosystem services.
Our collaboration with Chinese colleagues is therefore crucial. We bring new skills to the project (e.g. new hydrological modelling skills) that they will benefit from. Additionally as catchment management practices will be quite different across UK-China, they will learn about other good practice to help improve their environment and remove residents from poverty

Planned Impact

The following will benefit from this joint research:
1. Those living in and managing the 'research' catchment (and wider karst systems) will benefit from a better understanding of the critical zone system resilience, and threats to, its ecosystem services. This knowledge will allow them to think about how best to manage their environment and will lead to improvements in their quality of life, ensuring the fundamental needs (access to water of appropriate quality) and how to manage water resources (to also ensure sustainable soils for food provisioning), are underpinned by a useful knowledge-base.
2. The catchment managers, and those responsible for innovation, that will visit comparable UK organisations will benefit from a deeper understanding of best practise.
3. This joint research will be of benefit to NSFC, raising their profile in the UK and amongst other critical zone scientists. The skill and information exchange that will occur during this research with Chinese colleagues, ultimately demonstrating to the international scientific community, that we value sensitive environments internationally, and particularly international co-operation in research, will help consolidate each country's position as a future key research partner and particularly the Chinese National Science Foundation as a partner of choice for future co-funded research.
4. Through publication and conference activity, the SUERC AMS (NERC-recognised Facility) will receive publicity in China for excellence in novel 14C-dating approaches. They will benefit through enhanced international standing and resultant funded research collaboration.
5. The wider public, and local communities hosting the research, will benefit during the research activity through research team communication activity that meets their passion for and excites them to understand the natural world more deeply. In turn if this encourages greater interest in STEM subjects, the relevant country science base will benefit.


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Description We have now collected extensive stable isotope data from the Houzhai karst catchment over the wet seasons of 2016 and 2017. This includes hourly precipitation, groundwater and streamflow data during many storm events and at different catchment scales. These sites were chose following a joint China/UK field visit in February 2016. Analysis and modelling of these data is now underway; with one paper in review (for a Special Issue of Hydrological Processes on "Water in the Critical Zone" and another in preparation.
As data collection has progressed we have been developing new modelling approaches with data previously collected by our Chinese partners. The first coupled flow and tracer modules in a conceptual model to be able to integrate the hydrological and hydrochemical response of karst systems at different spatial scales. (this is reported in Zhang, Z. Chen, X. and Soulsby, C. (2017) Catchment-scale conceptual modelling of water and solute transport in the dual flow system of the karst critical zone. Hydrological Processes. DOI: 10.1002/hyp.11268). The second has been using the response of stream hydrographs and relating this to landscape properties (this work is reported in the paper: Zhang, R. Chen X. Zhang, Z. Soulsby C. and Gao M. (2018) How can streamflow and climate-landscape data be used to estimate baseflow mean response time? Journal of Hydrology. doi.10.1016/j.jhydrol.2017.12.070).
Exploitation Route through further result dissemination and in particular through the other WPs in the project which are dedicated to impact and knowledge exchange.
Sectors Environment

Title A new coupled tracer-flow model (see Zhang, Z. Chen, X. and Soulsby, C. (2017)) 
Description A new coupled tracer-flow model described in the paper by Zhang, Z. Chen, X. and Soulsby, C. (2017) Catchment-scale conceptual modelling of water and solute transport in the dual flow system of the karst critical zone. Hydrological Processes. DOI: 10.1002/hyp.11268 . 
Type Of Material Computer model/algorithm 
Year Produced 2017 
Provided To Others? Yes  
Impact difficult to assess at this early stage, it was just published in 2017