Redefinition of new Soil Physical Groups
Analysis AWC based on CERU32 compared to current AWC in CGMS
There are differences in new AWC estimates according to CERU32 and the estimates used in the current CGMS since 1998. The differences can be traced back through the more refined set of pedotransfer rules, which have been applied to the STUs, and from the new soil parameter values in the soil database. For the analysis of these differences the STUs have been grouped by FAO85 soil name into 222 groups, for which AWC values were known in both the old MARSCGMS and the new CERU32. Of these groups, 65 have AWC values in CERU32 of more than 10 mm/m above the MARS-CGMS values, and 85 groups have AWC values, which are more than 10 mm/m lower, and for 73 groups the difference is less than 10 mm/m.
Overall, the AWC value has increased. The largest differences are due to the recognition that organic soils have very high AWC values. This affects the five groups of Histosols (soil name O), where the AWC has more than doubled from about 170 mm/m in MARS-CGMS to 439 according to the CERU32 program.
The second largest effect is that in subsoils of secondary chalk and loess, the sponge effect of the subsoil, leading to capillary rise of moisture during the growing season, has been translated into a much higher AWC value (an additional amount of about 100 mm of water). This effect concerns some 10 groups of soil units.
A next factor leading to higher AWCs is the lower packing density of the subsoil in soils derived from calcaric and volcanic material or having very thick humous layers. Typical soils for this effect are Rendzina (soil code E0), Andosols (T), Phaeozems (H), Chernozems (C) and plaggen soils (p).
The factor that leads to lower AWC estimates in CERU32 is the higher packing density in the subsoil. This affects the Panosols (soil code W), many soils with stagnic properties (soil subunit code s), and gleyic (g) and vertic (v) properties, and the non-humic and non-mollic subgroups of alluvial soils (F, Fluvisols) and Gleysols (G). The effect plays more in the heavy (fine) textures than in the light (coarse) texture classes.
The updating of the texture codes notably for the soils units on the FAO soil map of the world has caused some shifts in AWC values. This concerns the soil units Solonetz (S), Xerosols (X), Yermosols (Y) and Solonchaks (Z).
When comparing the two AWC-maps on a country basis, the CERU32-AWC map (see Figure 5-3) shows a larger differentiation within most countries. In the northern regions of Europe the CERU32-AWC is higher than the current CGMS-AWC (see Figure 5-4), especially in Scotland, Sweden, North Finland, the Northern half of Russia and Belarus the shift in AWC values is considerable. In contrast, in Norway and southern Finland the CERU32 AWC values are lower. Countries with clearly higher AWC values on the AWC-CERU32 map are the Netherlands, Belgium, Spain and Portugal. In central Europe this is the case for Poland, the Czech and Slovak Republics, and Hungary. Clearly much lower AWC values are mapped for Southern Russia and Turkey. All the other countries the average level of AWC values is more or less the same on the two maps.
