Development of a world data set of soil water retention properties using pedotransfer rules

The World Inventory of Soil Emission Potentials (WISE) database is used to compile a standardized and spatially explicit data set of soil water retention properties. WISE holds 4353 globally distributed profiles considered to be representative of the soil units shown on a 1/2 degrees latitude by 1/2 degrees longitude version of the corrected and digitized 1:5 M FAO-Unesco Soil Map of the World. Pedotransfer functions (PTFs) are presented for the prediction of (a) volumetric soil-water content (theta) at 9 pre-selected soil-water potentials (h), and (b) available water capacity (AWC) from measured silt, clay and organic matter content. All regressions for theta(h) are significant (0.88 < r(2) < 0.94; P < 0.001). The predictive capability of the regression for the calculation of AWC, however, was relatively low and this PTF systematically underestimated AWC for the independent data set. An alternative approach, which uses pedotransfer rules (PTRs) and functional grouping for estimating AWC from FAO-Unesco soil unit type, horizon textural class and organic matter class, was developed and tested for its predictive capability. For the independent data set, the PTR-derived AWC values showed a better correlation (r(2) = 0.80) with measured AWC values than was the case for the PIT-derived AWC values. In addition to this, the median of the relative difference between predicted and measured AWC values was smaller (-3%) than for the PTF-predicted AWC values (-13%). Nonetheless, both methods showed a fairly large scatter between the predicted and measured AWC values. The PTRs were used for the estimation of profile available AWC to a depth of 100 cm, except for shallow Lithosols, Rankers and Rendzinas. Median AWC to a depth of 1 m is 42 mm for coarse-textured Arenosols, 80 mm for strongly weathered Ferralsols, 130 mm for Vertisols, 187 mm for Andosols, and 480 mm for Histosols. Correction factors were introduced to account for effects of coarse fragments and presence of groundwater at shallow depth. The ''corrected'' values were used to generate a 1/2 degrees latitude by 1/2 degrees longitude world data set of AWC properties. This raster image file uses the general format of the Global Ecosystems Database (Kineman, 1993). The resolution of the spatial data set is considered appropriate for water balance studies in global assessments of crop production potentials, soil vulnerability to pollution, and soil gaseous emissions.