DiHPS - A Distributed Heat Pulse Sensor Network for the quantification of subsurface heat and water fluxes

We propose the development of a novel, distributed soil moisture and temperature monitoring network that will present a step change from currently available monitoring technologies, which tend to be based on data collection at isolated points in time and space, to real-time, spatially-distributed data collection that enable effective and responsive decision making to deal with rapidly increasing demands in a changing climate.

The proposed system will be based on combining Distributed Temperature Sensor (DTS) technology with an active heat source. The so-called Active DTS (A-DTS) allows observing flow-related heating and cooling patterns along the actively heated cable. It is this Distributed Heat Pulse, A-DTS technology that we will use in the development of the proposed Sensor network - DiHPS. While broader applications in heat and water flux assessments will be possible with DiHPS, in this project we will focus on demonstrating its suitability to quantify the moisture content and thermal properties of the soil.

Description of the spatial and temporal distribution/ dynamics of these soil parameters is important for many agricultural, engineering and meteorological applications. Soil moisture, for example, is a key state variable in controlling land-atmosphere interactions and an early indicator of changes in the hydrological system, e.g. associated with daily evapotranspiration or event-based recharge cycles or with extreme events (i.e., droughts or floods). It is widely used in agriculture (e.g., irrigation management), forestry (e.g. plantation water demand estimations), meteorology (e.g. local and regional weather forecasts), water resources management (e.g. estimation of groundwater recharge) as well as a state indicator variable in drought / flood early warning systems. Soil thermal properties influence the partitioning of energy within the ground and at the ground surface, and are related to soil temperature and the movement of heat and water within the ground and their transfer across the ground surface. For these reasons, soil physicists, crop scientists and micrometeorologists study thermal properties, and they are also important in engineering applications, (e.g., determining the electric current rating of buried cables, ground heat exchanger design).

We will build on earlier work which has demonstrated the principal capacity of A-DTS to measure soil moisture under controlled conditions in a lysimeter facility. We will expand this technology from a single point application to a distributed, real time sensor network and a coiled, vertical A-DTS profiler for measuring soil moisture content and thermal properties at high spatial and temporal resolution. The network and profiler will be tested at TRL4 by installing it at our test site at the Birmingham Institute for Forestry Research (BIFoR). The results will be compared with in-situ soil moisture content and thermal property data from Frequency-Domain Reflectometry (FDR) soil moisture probes and thermal needle probe measurements, respectively. To achieve real-time, autonomous system operation, a set of heating strategies will be tested and initial threshold (trigger values) will be defined. These will define at what change in soil temperature, as observed by continuous temperature observation in DTS passive mode, initiation/cessation of the DTS active mode will be triggered.

A key component of DiHPS is the real-time control for triggering the change from active to passive observation mode. This will require the development of a set of algorithms, based on inverse modelling and asymptotic approaches, which can process the raw DTS data in real-time to provide the required outputs. No existing DTS-application attempts to provide temperature or soil moisture data temperature in such detail and in real-time. If successful, we anticipate a step change in the way DTS is employed, e.g. in early warning systems or for providing detailed process understanding.

Technical constraints to measure and monitor soil moisture and subsurface heat and water fluxes at adequate spatial and temporal scales result in critical limitations of the predictive capacity of hydrological and atmospheric models and hence hinder the reliable assessment of available water resources.

This project will develop Active Distributed Temperature Sensor network technologies for unprecedented high-frequency/resolution monitoring of soil moisture, subsurface temperature and thermal properties in real time. This development will improve the understanding of non-linear responses of soil moisture and sub-surface heat and water fluxes to climate and land use changes. The project will therefore benefit a wide range of academic researchers (hydrologists, ecologists, and atmospheric scientist, engineers, regulators, practitioners and policy makers.

The potential for monitoring soil moisture, water and heat fluxes in real time using the DiHPS system will lead to an improved quantification of water resources and support a move towards adaptive resource management in agriculture and forestry. Climate/management-induced shifts in the water balance towards drought or flood conditions or excessive drying/wetting of the soils will be easier detectable (and at an earlier point in time) with the improved methodology than currently feasible, and therefore can be dealt with at an earlier stage. Furthermore, DiHPS' capability to capture fine-scale variability in soil moisture over relatively large areas will also support the validation and enhance the usefulness of large-scale remote sensing products, such as from ESA's SMOS, Sentinel-1 or ASCAT sensors or from SMAP (NASA). Hence, DiHPS has the potential to directly impact national and European policy on the use of the natural resources, and directly links to NERC programmes such as Living with Environmental Change.

We developed an Impact Strategy (see Pathways to Impact) that is based on engagement with main stakeholders (see Letters of Support) and potential users of our technology and the process understanding derived by its application (Forestry Research, European Space Agency, CEH-COSMOS, BIFoR - Birmingham Institute for Forestry Research, Norburry Park Estate and our project partners SILIXA Ltd.) in the fields of forestry and agriculture, environmental planning and regulation, hydrological modelling and forecasting, water supply, thermal (ground) engineering, as well as provision of distributed fibre-optic sensing solutions.

To best facilitate the uptake of the projects research the project established an Advisory Board (meeting in month 1, 6 and 12 with month 4 + 8 progress reports in between) consisting of practitioners and industry partners has been formed. Board members have already contributed actively to the design of the project and will play a key role in the facilitation of the projects outreach and impact strategies. The research will be disseminated to the scientific community by peer-review paper publications and attendance at relevant scientific conferences. To create immediate impact beyond the scientific community, project outcomes, summary reports and newsletters as well as data and tools will be communicated through the interactive project website via links to social media and wiki-functionality. Other communication pathways will include linkages to stakeholder and NERC web portals, public media appearances and the regular (at least 3-monthly) publication of general-interest articles, podcasts and vodcasts online and distributed to all stakeholders. A practitioners workshop will be organised with the support of the SILIXA Ltd. project partners to provide training on the real-time monitoring of soil moisture with A-DTS to demonstrate the capacity of A-DTS technology to relevant stakeholders and users of DiHPS. (For out detailed impact strategy please see the Pathways to Impact document).