Evaluation of pedotransfer functions for predicting water retention of soils in Lower Congo (D.R. Congo)

Abstract

The soil water retention curve (SWRC) is important to solve many soil and water management problems related to agriculture, ecology, and environmental issues. However, it is well recognized that its direct measurement is laborious, time-consuming and expensive. An alternative is the estimation of the SWRC by pedotransfer functions (PTFs), which are well documented for temperate soils. Few works, however, have been devoted to PTFs for tropical soils. The main objective of this study was to evaluate the ability of a number of published “point” and “parametric” PTFs to predict water retention of soils in the Lower Congo (the South-Western region of the Democratic Republic of Congo). The “point” PTFs of Oliveira et al. (2002) and Dijkerman (1988) performed best at −33kPa, while those of Arruda et al. (1987) and Pidgeon (1972) were best at −1500kPa. Regarding the parametric PTFs which predicted the Van Genuchten, 1980 parameters, the “tropical” PTFs of Hodnett and Tomasella (2002) and the “temperate” PTFs of Schaap et al. (2001) gave the best results. Preliminary results of this evaluation study suggest that estimates of water content by several existing “temperate” as well as “tropical” PTFs may induce errors in the outputs of watershed models used in various agricultural studies under the humid tropics. Large discrepancies in the derived soil hydraulic data can substantially reduce the quality of the modelling results particularly in regions where soils may have been formed and evolved in similar climatological and pedological conditions as soils from the Lower Congo. We further found that a predictor such as dithionite-citrate-bicarbonateextractable iron (DCB-Fe) has great potential to reduce the uncertainty of PTFs for predicting water retention parameters of tropical soils.