ESTIMATING SOIL WATER-HOLDING CAPACITIES BY LINKING THE FAO SOIL MAP OF THE WORLD WITH GLOBAL SOIL PROFILE DATABASES AND PEDO-TRANSFER FUNCTIONS

Authors: Reynolds, Curt; Jackson, Thomas; Rawls, Walter

Submitted to: American Geophysical Union
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Spatial soil water-holding capacities were estimated from the Food and Agriculture Organization (FAO) digital Soil Map of the World (SMW) by employing continuous pedo-transfer functions (PTF). First, the following representative soil properties were estimated for two-layers of depths (0, 30, and 30-100 cm): particle-size distribution, dominant soil texture, organic carbon content, coarse fragments, bulk density, and porosity. The available water content was then calculated by utilizing the Saxton, et al., (1986) PTF model which is a function of particle-size distribution data. The soil water-holding capacities were then estimated by multiplying the estimated available water content with the effective crop root depth of one meter or less (i.e., encountered shallow impermeable layers). All global soil property images are available on a CD-ROM for easy introduction into spatial water balance models. These raster images have the same 5-minute spatial resolution of the original SMW to preserve the integrity of the original data.

Technical Abstract: Spatial soil water-holding capacities were estimated for the Food and Agriculture Organization (FAO) digital Soil Map of the World (SMW) by employing continuous pedo-transfer functions (PTF) within global pedon databases and linking these results to the SMW. The procedure first estimated representative soil properties for the FAO soil units by statistical analyses and taxo-transfer depth algorithms (FAO, 1996). The 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 fragment, bulk density, and porosity. After representative soil properties for the FAO soil units were estimated, these values were substituted into three different PTF models by Rawls, et al., (1982), Saxton, et al., (1986), and Batjes (1996). The Saxton PTF model was finally selected to calculate available water content because it only required particle-size distribution data, and results closely agreed with the Rawls and Batjes PTF models that used both particle-size distribution and organic matter data. Soil water-holding capacities were then estimated by multiplying the available water content by the soil layer thickness and integrating over an effective crop root depth of one meter or less (i.e., encountered shallow impermeable layers). All soil property images are available on a CD-ROM for easy introduction into spatial water balance models. These raster images have the same five-minute spatial resolution of the original SMW to preserve the integrity of the original data.