Current use of soil data in CGMS
For the second level of the MCYFS where the CGMS is running to produce decadal crop growth indicators soil data is needed as a static input variable.
In the current CGMS the Soil Geographical Data Base of Europe (SGDBE) at 1:1,000,000 version 3.1 (INRA, 1995; Le Bas, 1996; Jones and Buckley, 1996) is used. The MAGHREB countries, Turkey, Russia, Moldova, Ukraine and Belarus are not covered in this version of the SGDBE implemented in the CGMS. For these countries, data from the Digitised World Soil Map at 1:5,000,000 (FAO, 1995) are used.
The soil data is used to determine soil input variables to the agrometeorological model giving information about the soil type’s geographical location and the soil properties which are needed to simulate the crop growth during the year. The main soil properties are soil depth aiming at defining the potential rooting depth, and water retention properties giving through soil physical groups. The description of the soil characteristics for the crop simulation model in CGMS only relies on these two parameters. They fully describe the soil for simulation purposes. Each STU is attributed to a soil physical group defining the available water capacity (AWC). The AWC is a static soil characteristic and gives the amount of water between field capacity (wet soil) and wilting point (no water available for plants anymore) per unit length rooting depth. Multiplication of AWC and rooting depth gives the maximum available water which a soil can supply to a plant. It should be noted that the rain fed crop yields of the CGMS are more sensitive to the rooting depth than to the soil physical group (van der Goot, 1998).
Further, the soil map is used as a ‘land use probability map’ to define which crops have to be included in the simulation for a given soil unit. Ideally, this decision would be taken on the basis of actual land-use information, but a European-wide detailed classification of land cover is not available. Hence, within the CGMS the decision is simply based on the soil suitability of the different crops: if at least part of the soil mapping unit (one or more STU’s) is deemed appropriate then the simulation will be performed. The result of this strategy is that the yield figures produced by the CGMS are assessed for suitable soils only.
Suitable soils are determined per crop group on the basis of crop growth limiting properties of these soils. The limiting soil properties are for instance slope, texture, agricultural limiting phase, rooting depth, drainage, salinity and alkalinity. The slope, texture and phase data can be obtained directly from the soil database, while others like rooting depth, drainage conditions, salinity and alkalinity have to be derived from basic soil properties using pedotransfer rules.