Monitoring soil moisture from space

by Sancha Lancaster (Bluemonday) and Julia Figa (EUMETSAT), Wolfgang Wagner (TU Wien)

ASCAT data from Metop showed that soil moisture levels at that time were low, particularly in in Spain and Portugal (Figure 1).

The ASCAT (Advanced Scatterometer) instrument’s primary function is to provide ocean surface wind measurements over the world's oceans using radar. But data from ASCAT can also be used to give a picture of how wet or how dry land is.

ASCAT is a real aperture radar which transmits an electromagnetic pulse and measures the energy scattered back from the moist or dry land surface and the rough or smooth ocean surface (known as backscatter).

In partnership with the SAF on Support to Operational Hydrology and Water Management (H SAF), EUMETSAT uses the backscatter data from ASCAT to produce an estimate of the surface soil moisture.

The algorithms and processing software used here, have been carefully derived and tuned at the Institute for Photogrammetry and Remote Sensing (IPF) at the Vienna University of Technology, to provide accurate ASCAT soil moisture estimates.

The ASCAT backscatter is first normalized to a common incidence angle (40°) using a radar backscatter model. The obtained radar backscatter coefficient is a function of the soil moisture: low values correspond to low soil moisture, high values are associated with high soil moisture.

Radar backscatter values are scaled between 0% (dry soil) and 100% (wet soil, saturated with water). The obtained relative soil moisture estimates represent the moisture in the topmost five centimetres of the soil. These data are used in hydrological models that feed into flood forecasting.

In regions with snow, forest, as well as in areas where the soil is frozen, the backscatter estimate doesn’t reflect water content and these data are discarded. Over concrete, the measurements are also not representative of moisture, so the cities show up as ‘noise’ in the data.

IPF has visualised the data on a Google map application , to allow for easy comparison over previous time periods. By combining surface measurements over time it is possible to estimate soil moisture down to one metre — the soil water index. Lots of deep blue on the IPF visualised data means every satellite pass over the period has detected soil moisture, dark brown means the opposite.

Droughts

Spain and Portugal

Comparison of the datasets over the previous months showed that Spain and Portugal had been very dry.

The anomaly data in the graphics below (the difference between actual and average estimates) showed these regions were drier than usual.

In April, both countries asked the European Union to study the option of providing help for farmers to diminish the impact of the drought affecting both countries.

In March, the dry soils and wind conditions combined to generate of numerous fires in northern Spain and Portugal, a unusual situation for March.

The civil alerts issued were more typical for August than March.

• Spain had only 14% its normal rainfall over the 12 months and a record-breaking dry winter, the driest for 70 years.
• According to the Portuguese Drought Monitoring Centre of the Institute of Meteorology, more than half of the mainland was in a situation of extreme drought (53%) and the remaining portion (47%) in severe drought.

England

According to the Met Office, below average rainfall amounts for the previous two years also led the Environment Agency declaring a drought in parts of England. As the country prepared for the coming Olympics, held primarily in South East England, authorities introduced emergency measures.

Why are we in drought? — Met Office Blog

However, as areas with significant amount of concrete produce too much backscatter ‘noise’, the dry areas of the England can’t been seen as clearly as those of Spain and Portugal. Nevertheless, comparisons over a longer time period highlight a dry trend over the southern parts.

Floods

Soil moisture controls the partitioning of rainfall into runoff and infiltration and, therefore, has an important effect on the runoff response of catchments.

Surface runoff is the water flow that occurs when the soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources, flows over the land – often causing flooding.

For hydrological forecasting the ASCAT time series is useful to show areas where the ground is already saturated and where even normal amounts of rain could lead to flooding.

The ASCAT soil moisture service first became operational end of 2008 and EUMETSAT and the H SAF keep working together to improve the products and facilitate their use.

Fires

Dry soils and winds combined to create the perfect conditions for more than 150 wildfires in northern Spain and Portugal (seen as coloured dots in Figure 5), an unusual situation for March.