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

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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