Flip-Sides of Soil Moisture

Soil Moisture changes between 19th and 25th August around Houston, Texas due to rainfall from Hurricane Harvey. Courtesy of NASA Earth Observatory image by Joshua Stevens, using soil moisture data courtesy of JPL and the SMAP science team.

Soil moisture is an interesting measurement as it can be used to monitor two diametrically opposed conditions, namely floods and droughts. This was highlighted last week by maps produced from satellite data for the USA and Italy respectively. These caught our attention because soil moisture gets discussed on a daily basis in the office, due to its involvement in a project we’re working on in Uganda.

Soil moisture can have a variety of meanings depending on the context. For this blog we’re using soil moisture to describe the amount of water held in spaces between the soil in the top few centimetres of the ground. Data is collected by radar satellites which measure microwaves reflected or emitted by the Earth’s surface. The intensity of the signal depends on the amount of water in the soil, enabling a soil moisture content to be calculated.

Floods
You can’t have failed to notice the devastating floods that have occurred recently in South Asia – particularly India, Nepal and Bangladesh – and in the USA. The South Asia floods were caused by monsoon rains, whilst the floods in Texas emanated from Hurricane Harvey.

Soil moisture measurements can be used to show the change in soil saturation. NASA Earth Observatory produced the map at the top of the blogs shows the change in soil moisture between the 19th and 25th August around Houston, Texas. The data is based on measurements acquired by the Soil Moisture Active Passive (SMAP) satellite, which uses a radiometer to measure soil moisture in the top 5 centimetres of the ground with a spatial resolution of around 9 km. On the map itself the size of each of the hexagons shows how much the level of soil moisture changed and the colour represents how saturated the soil is.

These readings have identified that soil moisture levels got as high as 60% in the immediate aftermath of the rainfall, partly due to the ferocity of the rain, which prevented the water from seeping down into the soil and so it instead remained at the surface.

Soil moisture in Italy during early August 2017. The data were compiled by ESA’s Soil Moisture CCI project. Data couresy of ESA. Copyright: C3S/ECMWF/TU Wien/VanderSat/EODC/AWST/Soil Moisture CCI

Droughts
By contrast, Italy has been suffering a summer of drought and hot days. This year parts of the country have not seen rain for months and the temperature has regularly topped one hundred degrees Fahrenheit – Rome, which has seventy percent less rainfall than normal, is planning to reduce water pressure at night for conservation efforts.

This has obviously caused an impact on the ground, and again a soil moisture map has been produced which demonstrates this. This time the data was come from the ESA’s Soil Moisture Climate Change Initiative project using soil moisture data from a variety of satellite instruments. The dataset was developed by the Vienna University of Technology with the Dutch company VanderSat B.V.

The map shows the soil moisture levels in Italy from the early part of last month, with the more red the areas, the lower the soil moisture content.

Summary
Soil moisture is a fascinating measurement that can provide insights into ground conditions whether the rain is falling a little or a lot.

It plays an important role in the development of weather patterns and the production of precipitation, and is crucial to understanding both the water and carbon cycles that impact our weather and climate.

Jason-3 Sets Sail for the Oceanographic Golden Fleece

Artist rendering of Jason-3 satellite over the Amazon. Image Courtesy NASA/JPL-Caltech.

Artist rendering of Jason-3 satellite over the Amazon.
Image Courtesy NASA/JPL-Caltech.

The Jason-3 oceanographic satellite is planned to launch on Sunday 17th January from Vandenberg Air Force Base in California, aboard the Space-X Falcon 9 rocket. Named after the Greek hero Jason, of the Argonauts fame, Jason-3 is actually the fourth in a series of joint US-European missions to measure ocean surface height. The series began with the TOPEX/Poseidon satellite launched in 1992, followed by Jason-1 and Jason-2 which were launched in 2001 and 2008 respectively.

Jason-3 should provide a global map of sea surface height every ten days, which will be invaluable to scientists investigating circulation patterns and climate change.

The primary instrument is the Poseidon-3B radar altimeter, which will measure the time it takes an emitted radar pulse to bounce off the ocean’s surface and return to the satellite’s sensor. Pulses will be emitted at two frequencies: 13.6 GHz in the Ku band and 5.3 GHz in the C band. These bands are used in combination due to atmospheric sensitivity, as the difference between the two frequencies helps to provide estimates of the ionospheric delay caused by the charged particles in the upper atmosphere that can time delay the return.

Once the satellite has received the signal reflected back, it will be able to use its other internal location focussed instruments to provide a highly accurate measurement of sea surface height. Initially the satellite will be able to determine heights to within 3.3cm, although the long-term goal is to reduce this accuracy down to 2.5cm. In addition, the strength and shape of the return signal also allows the determination of wave height and wind speed which are used in ocean models to calculate the speed and direction of ocean currents together the amount and location of heat stored in the ocean.

In addition, Jason-3 carries an Advanced Microwave radiometer (AMR) which measures altimeter signal path delay due to tropospheric water vapour.

The three location focused instruments aboard Jason-3 are:

  • DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) – Uses a ground network of 60 orbitography beacons around the globe to derive the satellite’s speed and therefore allowing it’s precise position in orbit to be determined to within three centimetres.
  • Laser Retroreflector Array (LRA) – An array of mirrors that provide a target for laser tracking measurements from the ground. By analysing the round-trip time of the laser beam, the satellite’s location can be determined.
  • Global Positioning System – Using triangulation from three GPS satellites the satellites exact position can be determined.

The importance of extending the twenty-year time series of sea surface measurements cannot be underestimated, given the huge influence the ocean has on our atmosphere, weather and climate change. For example, increasing our knowledge of the variations in ocean temperature in the Pacific Ocean that result in the El Niño effect – which have caused coral bleaching, droughts, wet weather and movements in the jet stream in 2015, and are expected to continue into this year – will be hugely beneficial.

This type of understanding is what Jason-3 is setting sail to discover.

SMAP ready to map!

Artist's rendering of the Soil Moisture Active Passive satellite.  Image credit: NASA/JPL-Caltech

Artist’s rendering of the Soil Moisture Active Passive satellite.
Image credit: NASA/JPL-Caltech

On the 31st January NASA launched their Soil Moisture Active Passive satellite, generally known by the more pronounceable acronym SMAP, aboard the Delta 2 rocket. It will go into a near polar sun-synchronous orbit at an altitude of 685km.

The SMAP mission will measure the amount of water in the top five centimetres of soil, and whether the ground is frozen or not. These two measurements will be combined to produce global maps of soil moisture to improve understanding of the water, carbon and energy cycles. This data will support applications ranging from weather forecasting, monitoring droughts, flood prediction and crop productivity, as well as providing valuable information to climate science.

The satellite carries two instruments; a passive L-Band radiometer and an active L-Band synthetic aperture radar (SAR). Once in space the satellite will deploy a spinning 6m gold-coated mesh antenna which will measure the backscatter of radar pulses, and the naturally occurring microwave emissions, from off the Earth’s surface. Rotating 14.6 times every minute, the antenna will provide overlapping loops of 1000km giving a wide measurement swath. This means that whilst the satellite itself only has an eight day repeat cycle, SMAP will take global measurements every two to three days.

Interestingly, although antennas have previously been used in large communication satellites, this will be the first time a deployable antenna, and the first time a spinning application, have been used for scientific measurement.

The radiometer has a high soil moisture measurement accuracy, but has a spatial resolution of only 40km; whereas the SAR instrument has much higher spatial resolution of 10km, but with lower soil moisture measurement sensitivity. Combining the passive and active observations will give measurements of soil moisture at 10km, and freeze/thaw ground state at 3km. Whilst SMAP is focussed on provided on mapping Earth’s non-water surface, it’s also anticipated to provide valuable data on ocean salinity.

SMAP will provide data about soil moisture content across the world, the variability of which is not currently well understood. However, it’s vital to understanding both the water and carbon cycles that impact our weather and climate.