Beijing Is Sinking

Envisat's ASAR sees Tianjin, China's third largest city.  Image courtesy of ESA.

Envisat’s ASAR sees Tianjin, China’s third largest city. Image courtesy of ESA.

Last month a paper was publishing in the Remote Sensing journal demonstrating that the city of Beijing is gradually sinking as a result of subsidence.

The paper ‘Imaging Land Subsidence Induced by Groundwater Extraction in Beijing (China) Using Satellite Radar Interferometry’ by Chen et al described work undertaken by a team of researchers using satellite images from between 2003 and 2011.

The research was undertaken using Interferometric Synthetic Aperture Radar (InSAR), which is a microwave based technique that uses phase measurements from two or more successive satellite SAR images to determine the Earth’s shape and topography and so to measure millimetre-scale changes.

The study used 41 images from Envisat-ASAR acquired from 2003 to 2010 collected in Stripmap mode with VV polarisation, together with 14 images from TerraSAR-X acquired from 2010 to 2011 also in Stripmap mode, but this time using HH polarisation. The images were processed alongside some baseline interferograms and the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model, at 90 m resolution, to produce land subsidence information. The results showed that parts of Beijing, such as the business district of Chaoyang, had sunk by as much as 11 cm within a year. In addition, they estimated that parts of the city had sunk by more than 75cm during the course of the eight year study period.

Land subsidence is often caused by human activities and the researchers identified the main cause to be the extraction of groundwater from beneath the city. Beijing has an enormous appetite for water and is the fifth most water-stressed city in the world according to a 2014 report published by the Nature Conservancy. However, the amount of groundwater extracted was not the only factor with the researchers also seeing relationships with soil type, aquifer type and distance to the pumping wells. In addition, given the number of buildings that have been erected in Beijing in the last fifteen years the additional weight is also likely to be contributing to the problem.

This sinking is not noticeable to the inhabitants of Beijing, but there is concern how this subsidence will impact the transport infrastructure, particularly high speed trains and research is already underway on this area. It is also not just Beijing that’s suffering from subsidence. The research also identified 45 other areas across China as having had or having significant land subsidence. In addition, Mexico City is sinking by between 6 cm and 28 cm per year and the Indonesian capital of Jakarta has dropped by around 8 cm per year; both are due to groundwater extraction. Even London is sinking, although in our case it only 1 – 2 millimetres per year!

Water is a precious resource and one that is necessary for growing populations and economies alike. Research such as this demonstrates why the world needs to get better at water conservation. If we continue as we are, there could be potentially serious consequences for parts of the globe.

Geo-Business 2014!

We’re on a company road trip this week! By the time you read this blog we will both be attending the Geo-Business 2014 event at the Business Design Centre in London – technically my attendance will be dependent on trains as I won’t be arriving until Wednesday morning.

As we highlighted last week we’re at the conference to promote our new service of the virtual water gauge. At the very moment this blog is published to the world, Sam will in the Coastal and Hydrographic Session preparing to give her presentation on Using satellite altimetry to measure water height in estuaries, rivers and lakes – she’s due to talk at 12.50pm in Room A, so you’ve still got time to get there.

Sam’s presentation will look the sites where she’s used satellite based radar altimeters to determine water heights. The first study site was the Congo river, where we had the European/USA Jason-2 mission tracks intersecting the river at several locations, and the requirement was for a long time-series analysis plus the potential for new data collection going forward. Sam also looked at the extensive flooding in the Somerset Levels in south-west UK, from the end of 2013 to early 2014. In this instance, the location was covered by the 2013 launched India-France SARAL/AltiKa mission, and the result showed the depth of flood water being in the 2-4 m range, which seems to agree with personal reports (on the news yesterday morning there was an article where the depth of flood water was described as around 10 foot). Future work involves increasing the data density, and hence confidence in the height estimates, by including the ESA CryoSat-2 mission that was launched in 2010.

There’s a rich conference programme at the event which Sam will be visiting, plus over 80 commercial workshops to continue my professional development in all things geospatial. We’ve also got meetings arranged with a number of exhibitors to discuss how we might be able to work together in the future. If you’re at Geo-Business and want to catch up with us to discuss virtual water gauge or any earth observation issues, get in touch via Twitter at @pixalytics or on LinkedIn. We’d love to see you!

We’ve also decided that this will be a two blog week. We’ll give you an update on how things have gone on the first day of Geo-Business with Sam’s presentation, plus we’ve also got some exciting news to announce on a collaboration we’ve been working on recently. See you tomorrow …

Measuring Water Heights, upcoming presentation at GEO-Business

Freshwater is integral to our survival on earth; whether it’s for drinking, growing food, sanitation or energy production. However, water is also a finite natural resource controlled by the complex and evolving water cycle. Many people know that 97% of the world’s water is salt water, but of the remaining freshwater 70% is locked in ice caps and of what remains only 1% is readily accessible.

The bodies of UN Water and Water.org estimate that 85% of the world’s population live in the driest half of the planet; taking a five-minute shower uses more water than the average person in a developing country uses for an entire day and more people in the world have access to a mobile phone than a toilet. Global demand for water is forecast to increase by 55% in the next 40 years, added to which climate evolution is going to change the distribution and availability of freshwater across the world. Last winter’s weather in the UK demonstrated how important it’s going to be to for us to adapt to new water patterns.

Satellite remote sensing has an important role to play in helping the world monitor and manage this natural resource. From the identification and mapping of water bodies by optical remote sensing, through the monitoring of hydrologic variables (like rainfall, soil moisture and water quality) to real time flood monitoring and disaster relief. Remote sensing applications are offering real value to the world and with launch of Sentinel-1 the European Copernicus data stream has started to come online; this week I’m at the Sentinel-2 for Science Workshop. Sentinel-2 is a high resolution optical mission due to launch in early 2015.

Water height calculation in the Congo using Jason 2

Water height calculation in the Congo using Jason 2

Over the last year I’ve developed a system to determine water heights in estuaries, rivers and lakes using satellite optical and altimetry data. Radar altimeters emit short bursts of microwave energy towards the earth’s surface, and the time delay of the return of those pulses gives a height. It becomes complicated over inland water bodies, especially those that are relatively small (not large inland seas) and varying river banks and general land topography; however there are improved approaches and new data coming on-stream.

Testing my altimetry based height determination has given positive results, when compared to in situ data taken for the Congo; the first study site. By using this approach I was able to provide the customer with water heights without them needing to get data from a water gauge. The other major advantage was the generation of a historical time series for several sites of interest where water gauges had never been installed.

Wednesday next week, 28th May, I will be giving a presentation on my work at the 2014 Geo-Business Conference in London and I’ll give you more details in a future blog. If you’re at Geo-Business, come up and say hello, otherwise come back to the blog for more details.