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NEO
Postbus 2176
NL-3800 CD Amersfoort
Tel: + 31 (0)33 463 74 33
Fax: + 31 (0)33 463 74 40
Website: www.neo.nl
Email: info@neo.nl
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LIMITATIONS OF DRYMON
As any validated and tested method DRYMON has limitations. These limitations are related to the global coverage, data continuity and detail.
DRYMON does not perform well in:
- High mountains: there is hardly any soil moisture to monitor or it is covered by snow and ice;
- Deserts: the very low quantities of soil moisture are hidden under shifting sands;
- Rain forests: the soil is hidden under 20-40 meters of vegetation cover and is always so wet, that it is not useful to speak of droughts;
- Arctic zones where there are no well-developed soils, and where soils are frozen (e.g., permafrost).
DRYMON performes less at all these landscape types, but drought is usually not an environmental problem or a bottle-neck in crop production here.
A mask is applied to DRYMON-products where no meaningful information can be obtained. The mentioned problems do not reduce the value of DRYMON: it performs well within the area where 99.9% of agricultural production occur, 95% of the livestock products comes from and 90% of the forest products.
The method is based on the ERS-SCAT and will be used on the METOP-ASCAT. The SCAT- instrument has been flown on ERS-1 (1991-1999) and on ERS-2 (1995-present). The ASCAT is onboard the METOP series of satellites of whim the first was launched October 2006. The satellite DRYMON depends on now, ERS-2, has suffered from a number of incidents since 1999. The incidents with ERS-2 since 1999 cause several interruptions in the data flow up to fall 2002.
In the whole data series since 1991 a number of discontinuities exist in the data, making that the archive does not contain the every 2-3 days observation for all locations on earth. Other wise the archive is a tremendous asset for DRYMON.
The ERS-SCAT provides a signal averaging data over areas of 25 * 25 km (major parts of the image archive are still based on 50 * 50 km grid cells and waiting to be reprocessed.). For ASCAT this minimum area will be 12.5 * 12.5 km. We cannot obtain more spatially detailed information in DRYMON. Of course this is a problem if one wants to monitor soil moisture in a particular field. Alternatively the scale of rainfall events and therefore of soil moisture evolution over time is very much in harmony with the sampling distance of the SCAT. With ASCAT there will be a surplus in this respect.
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