DEVELOPMENT OF A WORLD DATA SET OF SOIL-WATER RETENTION PROPERTIES USING PEDOTRANSFER RULES

The World Inventory of Soil Emission Potentials (WISE) database is use d to compile a standardized and spatially explicit data set of soil wa ter retention properties. WISE holds 4353 globally distributed profile s considered to be representative of the soil units shown on a 1/2 deg rees latitude by 1/2 degrees longitude version of the corrected and di gitized 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) a vailable water capacity (AWC) from measured silt, clay and organic mat ter content. All regressions for theta(h) are significant (0.88 < r(2) < 0.94; P < 0.001). The predictive capability of the regression for t he calculation of AWC, however, was relatively low and this PTF system atically underestimated AWC for the independent data set. An alternati ve approach, which uses pedotransfer rules (PTRs) and functional group ing for estimating AWC from FAO-Unesco soil unit type, horizon textura l class and organic matter class, was developed and tested for its pre dictive capability. For the independent data set, the PTR-derived AWC values showed a better correlation (r(2) = 0.80) with measured AWC val ues than was the case for the PIT-derived AWC values. In addition to t his, the median of the relative difference between predicted and measu red AWC values was smaller (-3%) than for the PTF-predicted AWC values (-13%). Nonetheless, both methods showed a fairly large scatter betwe en the predicted and measured AWC values. The PTRs were used for the e stimation of profile available AWC to a depth of 100 cm, except for sh allow Lithosols, Rankers and Rendzinas. Median AWC to a depth of 1 m i s 42 mm for coarse-textured Arenosols, 80 mm for strongly weathered Fe rralsols, 130 mm for Vertisols, 187 mm for Andosols, and 480 mm for Hi stosols. Correction factors were introduced to account for effects of coarse fragments and presence of groundwater at shallow depth. The ''c orrected'' 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 (Kinema n, 1993). The resolution of the spatial data set is considered appropr iate for water balance studies in global assessments of crop productio n potentials, soil vulnerability to pollution, and soil gaseous emissi ons.