A Pedotransfer Resource Database (PTFRDB) for tropical soils: test with the water balance of WaNuLCAS

Dynamic models of the soil-plant-atmosphere continuum can help to clarify relationships between land use and water resources, provided that they are correctly parameterized. The WaNuLCAS model for Water, Nutrient, and Light Capture in Agroforestry Systems was developed for such tasks in tropical conditions. However, the field measurements required for full model parameterization are laborious, costly and time consuming, so shortcuts are desirable. Pedo transfer functions (PTF) have been developed to predict the main soil physical relationships (T-h-K) from the measured percentages of clay, silt, organic matter content, and bulk density. However, existing equations are largely based on temperate soils and as they are empirical, rather than based on first principles, these PTF’s may gave erroneous, or even completely absurd predictions when used outside the range of soils from whose data they were derived. Even so, complete textural data are not easily obtained, and as a further step, these data themselves may be derived from soil classification data and manually assessed texture classes. A new database, PTFRDB, of input parameters for PTF’s was collected and summarized from 8915 data available worldwide, with good representation of tropical soils. When the resultant estimates are used as basis for T-h-K relationships, the results of the PTFRDB appeared close to those derived from field measurements. The largest deviations occurred on vertisols and mollisols, where bulk density and soil organic matter content diverged. When further tested in the WaNuLCAS model for a 10-year simulation of a simple agroforestry system, differences between soil types were simulated consistently, although on various soil types one or more of the processes of the water balance appeared to vary with the way input parameters were derived. For crop yields, tree wood production and cumulative water balance terms, however, these impacts were small, with the largest consistent difference in the partitioning between surface runoff and vertical drainage on the vertisol.

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