Estimating soil water-holding capacities by linking the Food and Agriculture Organization soil map of the world with global pedon databases and continuous pedotransfer functions

Spatial soil water-holding capacities were estimated for the Food and Agric ulture Organization (FAO) digital Soil Map of the World (SMW) by employing continuous pedotransfer functions (PTF) within global pedon databases and l inking these results to the SMW. The procedure first estimated representati ve soil properties for the FAO soil units by statistical analyses and taxot ransfer depth algorithms [Food and Agriculture Organization (FAO), 1996]. T he representative soil properties estimated for two layers of depths (0-30 and 30-100 cm) included particle-size distribution, dominant soil texture, organic carbon content, coarse fragments, bulk density, and porosity. After representative soil properties for the FAO soil units were estimated, thes e values were substituted into three different pedotransfer functions (PTF) models by Rawls et al. [1982], Saxton et al. [1986], and Batjes [1996a]. T he Saxton PTF model was finally selected to calculate available water conte nt because it only required particle-size distribution data and results clo sely agreed with the Rawls and Batjes PTF models that used both particle-si ze distribution and organic matter data. Soil water-holding capacities were then estimated by multiplying the available water content by the soil laye r thickness and integrating over an effective crop toot depth of 1 m or les s (i.e., encountered shallow impermeable layers) and another soil depth dat a layer of 2.5 m or less.