Abstract
Pedotransfer functions (PTF) were developed to estimate the parameters (α, n, θr and θs) of the van Genuchten model (1980) to describe soil water retention curves. The data came from various sources, mainly from studies conducted by universities in Northeast Brazil, by the Brazilian Agricultural Research Corporation (Embrapa) and by a corporation for the development of the São Francisco and Parnaíba river basins (Codevasf), totaling 786 retention curves, which were divided into two data sets: 85 % for the development of PTFs, and 15 % for testing and validation, considered independent data. Aside from the development of general PTFs for all soils together, specific PTFs were developed for the soil classes Ultisols, Oxisols, Entisols, and Alfisols by multiple regression techniques, using a stepwise procedure (forward and backward) to select the best predictors. Two types of PTFs were developed: the first included all predictors (soil density, proportions of sand, silt, clay, and organic matter), and the second only the proportions of sand, silt and clay. The evaluation of adequacy of the PTFs was based on the correlation coefficient (R) and Willmott index (d). To evaluate the PTF for the moisture content at specific pressure heads, we used the root mean square error (RMSE). The PTF-predicted retention curve is relatively poor, except for the residual water content. The inclusion of organic matter as a PTF predictor improved the prediction of parameter a of van Genuchten. The performance of soil-class-specific PTFs was not better than of the general PTF. Except for the water content of saturated soil estimated by particle size distribution, the tested models for water content prediction at specific pressure heads proved satisfactory. Predictions of water content at pressure heads more negative than -0.6 m, using a PTF considering particle size distribution, are only slightly lower than those obtained by PTFs including bulk density and organic matter content.
Highlights
The use of simulation models in agricultural sciences has increased significantly over the last decades
Considering the importance of extending the use of hydrological and agronomic models to tropical regions, in this paper we developed and validated Pedotransfer functions (PTF) for the prediction of water retention characteristics from sand, silt, clay, organic matter content and bulk density data for soils from northeastern Brazil
In most soils used for PTF construction in this study the sand content was high, a common trait in the Northeast of Brazil, as can be seen in the texture triangles (Figure 4)
Summary
The use of simulation models in agricultural sciences has increased significantly over the last decades. One of the major bottlenecks hampering model application is the lack of input data. In the case of water and solute balance modelling, data of soil hydraulic properties is the most relevant information (van Diepen et al, 1991; Pachepsky & Rawls, 1999). The direct determination of hydraulic conductivity and water retention characteristics is time-consuming and depends on expensive laboratory equipment (Wösten & van Genuchten, 1988). Indirect methods have been developed, e.g., pedotransfer functions - PTFs (Minasny, 2000; Cornelis et al, 2001; Rawls & Pachepsky, 2002; Tomasella et al, 2003) that correlate available information such as grain size distribution and organic matter content (OM) with soil hydraulic properties. Indirect methods have been developed, e.g., pedotransfer functions - PTFs (Minasny, 2000; Cornelis et al, 2001; Rawls & Pachepsky, 2002; Tomasella et al, 2003) that correlate available information such as grain size distribution and organic matter content (OM) with soil hydraulic properties. Vaz et al (2005) validated the Arya & Paris (1981)
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