Soil moisture

Observations and measurements of soil moisture are of great interest because they make it possible to devise models of hydrological conditions on Earth, i.e. the way in which water moves and is distributed around the Earth. Quite simply, this provides us with greater insight into global climate conditions.

Role of soil moisture in global water budget

Moisture in the soil only accounts for a small percentage of the global water budget, but nonetheless plays an important role in this.

For example, soil moisture is an important factor in determining where and how well different plants grow. And the amount of water plants have at their disposal also affects photosynthesis, and thus the ability of plants to absorb CO2 from the atmosphere. Soil moisture also plays an important role in predicting weather and climate conditions, and the lack of knowledge about soil moisture is currently the main wild card in the climate models used by scientists to predict future climate change.

As existing measurements of soil moisture are incomplete, scientists have to make an educated guess about the amount of water when devising climate models of hydrological conditions on Earth. In order for scientists to make a more qualified estimate more data are needed.

Measuring soil moisture

Soil moisture can be measured by satellite using two different techniques; an active technique whereby a radar system measures a reflected radar wave, or a passive technique whereby a radiometer system measures the naturally emitted radiation using microwave frequencies. The radar technique is based on the principle that the reflection of radar waves from the Earth’s surface depends, among other things, on the degree of soil moisture. By measuring the reflected energy of the radar wave it is possible to estimate the soil moisture. The radar technique is suitable for detailed measurements of soil moisture at ground level, whereas the radiometer technique is better suited for measurements of large areas.

Quite simply, a radiometer is a highly sensitive radio receiver that can register and distinguish the extremely minute amounts of electromagnetic energy that all forms of matter with a temperature above absolute zero emit when their molecules interact with each other. Studies show that different matter emits different amounts of energy, and for ground surfaces this amount of energy will depend on the soil moisture.

SMOS satellite

In 2009 ESA launched the SMOS satellite (Soil Moisture and Ocean Salinity) with a microwave radiometer on board for measuring soil moisture and ocean salinity. In addition to measuring the desired Earth parameters, the satellite is experimental, combining technological and scientific research. In recent years DTU Space has played a pivotal role in both these areas. Most recently the Institute has been closely involved in extensive experiments to verify and refine the models that link satellite measurements to physical conditions on Earth. In the coming year, these experiments will be expanded to include actual calibration of the satellite and its measurements.

Using an instrument that simulates measurements from space, DTU Space has completed missions in such different places as Norway, Finland, France, Spain and Australia.


SMOS: Measuring Soil Moisture & Ocean Salinity (Illustration: ESA).



Niels Skou
Professor Emeritus
DTU Space
+45 45 25 37 68