The return to Barrax – Las Tiesas

In the midst of heatwave conditions, even for La Mancha in August, a small team from the National Physical Laboratory (NPL) returned to Barrax to conduct a second set of reflectance measurements. Our aim was straightforward: collect surface reflectance (specifically hemispherical-conical reflectance factor – HCRF) data to internationally recognised fiducial reference measurement (FRM) standards, coincide with the satellite overpasses of Sentinel-2A, the twin Sentinel-3 pair orbiting in tandem, PROBA-V and Landsat 8 – all within a measurement window of 90 minutes. So the date was set for Thursday the 2nd August 2018.

On Wednesday morning, the day before the overpass, we found ourselves in the familiar surroundings of the Barrax – Las Tiasas experimental farm, greeted by clear blue skies. We set to work assessing the current crop conditions and formulating plans for the next day. The plan was to take two sets of measurements within the overpass window – one over the low reflectance alfalfa field (from the pea family) and one from high reflectance bare soil.

At the height of summer, most fields in the vicinity were not cultivated, except where the rotating hydration system was activated (hence the circular fields). The image below is the Sentinel 2 image on the day. Although it would appear at a glance that most of the area was in a ‘bare soil’ phase, in reality the closest fields to the green alfalfa area contained crop or other debris from previous farming cycles, so discounted as test sites. After some walking around two sites were selected, all we had to do was wait for the satellites!

The location of our two measurement areas marked out in red (click to expand).

Plots, of dimension 200 m by 200 m, were paced out using GPS, and measurement locations within these plots marked with flags. Every preparation was made to save time in the critical period of the following day – a schedule was scrupulously prepared to account for every precious minute of the multi-overpass window.

As Thursday dawned, the team set out provisioned with copious water supplies. At each point, the spectroradiometer was normalised to the reference Spectralon panel (tuning the sensor configuration: integration time, etc.) with two panel reflectance measurements made either side of four reflectances measurements. With bare soil readings completed, we rapidly shifted operations to the alfalfa field and took a further set of measurements there, carrying the heavy calibration panel and other equipment through the dense field all the while watching out for the (well-concealed) irrigation channels that criss-crossed the field. At the time of the overpass it was easy to imagine the satellites zooming over ahead, though of course invisible to us in the bright skies. A couple of hours spent shunting all measurement and calibration equipment around the fields in 40 degree temperatures was sufficient to complete our practical work, in time for a late lunch of refreshing gazpacho soup at the local roadhouse cafe.

Measurements were taken at the alfafa and bare soil sites by Niall Origo, using the portable Analytical Spectral Devices (ASD) FieldSpec 4 instrument. The Spectralon panel is shown in the foreground (left with cover on; right off).

We are now in the process of analysing the results, and, furthermore, applying FRM techniques to these data.

Thank you to Dr José González Piqueras and Alonso Garrido, of the Universidad de Castilla La Mancha (UCLM), for generously assisting with logistics and storage of equipment. Their department does a great deal of work on both Earth Observation and long-term in-situ monitoring at this field site – check out their website (in Spanish – click small flag for the English version). We’d also like to thank ‘super-sub’ Dr Paul Green, of NPL, for stepping up to the plate to very ably assist with transport and field assistant duties.

Second validation campaign conducted by FRM4VEG scientists at Wytham Woods

Beatriz Fuster Ochando of EOLAB measures the fraction of absorbed photosynthetically active radiation.

Following the Las Tiesas campaign earlier this summer, scientists from the National Physical Laboratory (NPL), University of Southampton and EOLAB travelled to Wytham Woods on 3rd July for the second campaign of the FRM4VEG project.  The team was also joined by Dr. Mukunda Dev Behera, an Associate Professor visiting from the Indian Institute of Technology (IIT).

Located near Oxford, England, Wytham Woods is a prime example of semi-natural woodland, and has been extensively monitored over the past 75 years.  The site consists of a variety of hardwood and softwood species, including ash, beech, hazel, oak and sycamore.

Harry Morris and Julio Pastor-Guzman of the University of Southampton measure canopy chlorophyll content.

On the first day of the campaign, airborne hyperspectral data were acquired by the Natural Environment Research Council (NERC) Airborne Research Facility (ARF).  During the flight, a range of measurements were carried out on the ground to enable the effects of the atmosphere to be accounted for.  The highly detailed data provided by the instruments on-board the aircraft will be used to evaluate a range of upscaling approaches.

Throughout the rest of the week, the team focussed their efforts on measurements of the fraction of absorbed photosynthetically active radiation and canopy chlorophyll content – key parameters for describing the health of vegetation.  The sampling strategy, which was based on that adopted at Las Tiesas, was modified to enable measurements of both the forest understory and overstory.

Sentinel-2B MSI true colour composite acquired on 29th June (site shown in red). Contains modified Copernicus Sentinel data 2018.

Over the course of the campaign, a total of 42 elementary sampling units were characterised, although the logistical issues associated with sampling leaves in forest environments meant that measurements of canopy chlorophyll content were restricted to 25 of these.  Cloud-free conditions were experienced during overpasses of Sentinel-2 and -3, meaning the data collected during the campaign can be fully exploited for product validation.

A further campaign is set to take place later in the summer, in which measurements of surface reflectance will be conducted.  These measurements will make use of an unmanned aerial vehicle (UAV) to access the top of the canopy.

FRM4VEG scientists travel to Barrax – Las Tiesas experimental farm for validation campaign

Luke Brown, Julio Pastor-Guzman, and Harry Morris of the University of Southampton measure canopy chlorophyll content in an alfalfa field.

Scientists from the National Physical Laboratory (NPL), University of Southampton, EOLAB, and the European Space Agency (ESA) travelled to the Barrax – Las Tiesas experimental farm on 1st June, in preparation for a week-long campaign.  The campaign, the first of the FRM4VEG project, aimed to provide data to support validation of Copernicus products from Sentinel-2, -3 and PROBA-V.

Located near Barrax in Castilla-La-Mancha, Spain, the site has been the previous focus of multiple ESA-funded campaigns, and was selected due to its typically clear skies, flat terrain, and well-managed crops.

After a successful campaign kick-off meeting hosted by the University of Castilla-La-Mancha (UCLM) in Albacete, the teams set out to begin data collection on the morning of 2nd June, led by Dr. Fernando Camacho de Coca of EOLAB, who has extensive experience working at the site.

Harry Morris (University of Southampton), Niall Origo (NPL), Dr. José González Piqueras (UCLM), Dr. Fernando Camacho de Coca (EOLAB), Lorena de la Madrid Descalzo (EOLAB), Dr. Valentina Boccia (ESA), Luke Brown (University of Southampton), and Dr. James Ryder (NPL).

Throughout the week, the teams from EOLAB and the University of Southampton carried out measurements of biophysical variables, focusing on two key parameters that describe the health of vegetation: the fraction of absorbed photosynthetically active radiation, and canopy chlorophyll content.  Meanwhile, the team from NPL concentrated their efforts on measurements of surface reflectance, the fundamental quantity upon which satellite-derived biophysical variables are based.

Measurements of the biophysical variables were conducted in elementary sampling units of 20 m by 20 m, enabling them to be directly related to data from Sentinel-2’s Multispectral Instrument (MSI).  By the end of the campaign, 52 elementary sampling units had been characterised, covering 7 different crop types (alfalfa, garlic, rapeseed, spring onion, sunflower, poppy and wheat), in addition to a number of bare soil areas.

Sentinel-2A MSI true colour composite acquired on 13th June (site shown in red).  Contains modified Copernicus Sentinel data 2018.

Despite persistent cloud cover throughout the campaign, conditions improved the following week, during which cloud-free Sentinel-2 and -3 overpasses were made.  Given the stability in vegetation conditions at the site, the data acquired during these overpasses will prove useful, both for upscaling and product validation.  Nevertheless, the cloud cover hampered the collection of surface reflectance measurements, which require stable illumination and coincidence with the satellite overpass.  As a result, a follow-up campaign is set to take place in July.

 

Campaign readiness review

In preparation for the validation campaigns at Barrax – Las Tiesas in June and Wytham Woods in July, the campaign readiness review was successfully completed on 23rd May 2018.  In addition to  logistical details, the readiness of all campaign instrumentation was confirmed.