Introduction

In order to ensure economic and industrial activities at the Arctic territory of Siberia it is necessary to develop new technologies of a monitoring and forecasting of state of soil covers within extensive areas of Arctic tundra in the conditions of permafrost degradation influenced by the global climate changes. The sparse network of meteorological stations that measure the soil moisture is insufficient (in matter of spatial resolution) to be used by climatologists at the such large territory.

Soil moisture

Soil moisture – dimensionless quantity (usually described as a percentage), the amount of water in the soil. Can be defined as ratio of the mass of water to the mass of soil. In 2010 the World Meteorological Organization (WMO) added the soil moisture to the list of 50 Essential Climate Variables that need to be monitored systematically.

Many studies have shown that soil moisture influences the relation between the Earth's surface and the atmosphere. Soil moisture controls the amount of water that seeps into the ground and evaporates from the ground. Thereby, we can say that the soil moisture has a significant impact on the weather and the climate.

Soil moisture measurement

Traditional measurements of soil moisture is carried out using special sensors, instruments (hygrometers). This approach provides high measurement accuracy, but do not suit to be used over large inaccessible areas. For those areas it is more convenient to use remote sensing methods – mostly from the space.

The disadvantage of using Earth remote sensing data is a low spatial resolution (radiometers that are suitable for retrieving the soil moisture have a spatial resolution of about 40 km) and the fact that the soil moisture is not measured directly – it is calculated using a measured brightness temperature and a physical model of soil.

Soil moisture satellites

At the present time there is few satellites that were especially developed to observe the soil moisture. Among them we can mention:

• SMOS (Soil Moisture and Ocean Salinity) satellite with the radiometer MIRAS (Microwave Imaging Radiometer with Aperture Synthesis) on board. SMOS was designed by ESA (European Space Agency) and launched in November of 2009. It was designed to measure the soil moisture and the ocean salinity – the key climate variables, for the first time observed from the space. The spatial resolution is 43 km at the frequency of 1.42 GHz.
• GCOM-W1 (Global Change Observation Mission 1st – Water) was launched by the JAXA (Japan Aerospace Exploration Agency) in May of 2012. Satellite is equipped by passive microwave radiometer AMSR-2 (Advanced Microwave Scanning Radiometer). GCOM-W1 provides data with the spatial resolution from 5 km to 50 km (25 km for soil moisture data). AMSR-2 is an improved version of the radiometers AMSR-E and AMSR, mounted on the ADEOS-2 and Aqua satellites. The antenna of the radiometer that obtains data over a 1450 km swath enables the AMSR-2 to acquire data with more than 99% coverage of the Earth every 2 days.
• SMAP (Soil Moisture Active Passive) launched by NASA (National Aeronautics and Space Administration) in January of 2015 is equipped by passive microwave radiometer working at frequency of 1.4GHz (L-band). The radiometer was designed to provide data of spatial resolution 40 km. The active radar, also mounted on the satellite, was designed to increase the spatial resolution up to 3 km. Unfortunately the radar stopped transmitting in July 2015 and it was concluded that the radar is likely not recoverable in September 2015.

Retrieving soil moisture from the brightness temperature observations

Remote sensing methods of retrieving soil moisture and temperature and the state of freezing / thawing soil have been development for many years in the Laboratory of Radiophysics of the Earth Remote Sensing, Kirensky Institute of Physics, SB RAS.

Applying of the specific soil dielectric model based on the laboratory measurements of the organic-rich soil samples earlier collected at the test site on the Yamal Peninsula has enabled to get a very good agreement with the data obtained in situ during the expedition at the test site on the Yamal Peninsula.