Testing different approaches to characterize Burundian soils by the BEST procedure
Research Highlights
► Simple methods have to be used to hydraulically characterize soils in Burundi. ► Different approaches to apply the BEST procedure were tested for some Burundian soils. ► BEST is expected to reliably characterize sampled soils. ► Estimating the time validity of the infiltration equation does not seem crucial. ► A few particle size distribution data are enough to apply BEST in fine-textured soils.
Introduction
Studying soil hydrological processes requires the determination of soil hydraulic properties. Several methods have been developed to determine the hydraulic characteristic curves of the soil, i.e. the relationships between volumetric soil water content, θ (L3 L−3), soil water pressure head, h (L), and soil hydraulic conductivity, K (L T−1), both in the laboratory and the field. However, determining these properties using traditional methods is both expensive and time consuming. Haverkamp et al. (1996) pioneered a specific method for soil hydraulic characterization known as the “Beerkan method”. An improved version of this methodology, called the Beerkan Estimation of Soil Transfer parameters (BEST), was developed by Lassabatère et al. (2006). BEST considers certain analytic formulae for hydraulic characteristic curves (Brooks and Corey, 1964, Burdine, 1953, van Genuchten, 1980) and estimates their shape parameters, which are texture dependent, from simple particle size analysis by physical–empirical pedotransfer functions (PTFs). Structure dependent scale parameters are estimated from a three-dimensional field infiltration experiment at zero pressure head, using the two-term infiltration equation developed by Haverkamp et al. (1994).
BEST is very attractive for practical use since it substantially facilitates the hydraulic characterization of unsaturated soils, and it is gaining popularity in soil science (Gonzalez-Sosa et al., 2010, Lassabatère et al., 2007, Lassabatère et al., 2010, Mubarak et al., 2009, Mubarak et al., 2010, Xu et al., 2009, Yilmaz et al., 2010). However, few studies have been conducted to assess the real potential of the procedure. Moreover, simplifying the application approach of the BEST procedure may allow a larger use of the methodology, including areas where soil hydraulic characterization is difficult or even impossible due to the lack of laboratories and skilled personnel. Minasny and McBratney (2007) proposed simple methods to predict shape parameters of the water retention and hydraulic conductivity curves, considering that sand and clay content or the USDA soil textural class can be the only available data. Also Bagarello et al. (2009) suggested that a reduced experimental information on the soil particle size distribution may be used to estimate shape parameters. These alternative methods have practical interest but they have not been tested with field data.
A potential attraction of the BEST procedure is that it allows a field evaluation of the soil physical quality, which is a subject that increasingly receives attention. According to Reynolds et al., 2007, Topp et al., 1997, an agricultural soil with a good physical quality has the ability to store and transmit water, air, nutrients and agrochemicals in ways which promote both maximum crop performance and minimum environmental degradation. Therefore, evaluating soil physical quality for an area of interest is an important diagnostic tool and may help in the arrangement of effective development programs for agriculture. These objectives are particularly important in developing areas of the world, where increasing human well-being depends, among many things, on an improved agriculture that does not compromise the environment quality.
Burundi is a country in Central Africa that has a great agricultural potential given its favorable surface water availability and climate.According to Bationo et al., 2006, Eswaran et al., 1997, Oxisol are the most dominant Soil Taxonomy order in Burundi. The hydraulic properties of Burundian soils are largely unknown and these properties are rarely measured directly, due to the scarcity of resources for experimental soil research (Bagarello et al., 2007, Bagarello et al., 2009). The BEST procedure appears to be simple from a methodological point of view, and seems to be potentially suitable to characterize Burundian soils. The Beerkan method has already been applied in other tropical African countries (Galle et al., 2001).
An alternative, reasonably simple means to estimate hydraulic properties of Burundian soils could be using exclusively PTFs. Most of the available PTFs for estimating soil water retention, developed in temperate regions, appear to be inadequate in tropical areas, due to chemical and physical differences between temperate and tropical soils (Tomasella and Hodnett, 2004). However, several PTFs were specifically developed for use in tropical soils (e.g., Lal, 1979, Pidgeon, 1972, Tomasella and Hodnett, 1998, Tomasella et al., 2000, Tomasella et al., 2003). To our knowledge, less work has been carried out to develop PTFs for predicting saturated hydraulic conductivity, Ks, of tropical soils, and the few existing studies suggest a poor performance of temperate PTFs in predicting Ks of tropical soils (Mbagwu, 1995, Sobieraj et al., 2001). Moreover, PTFs should not be used to predict point values of Ks due to the differences between measurement scales of Ks and the soil data used as predictors (Timlin et al., 2004). The BEST procedure, in which infiltration is measured locally in the field, could potentially yield a more representative estimate of in-situ Ks compared to PTFs.
Testing soil hydraulic characterization methods on Burundian soils has both a local and a general interest since Burundi is rarely considered in soil investigations and Burundian soils are not widely represented in international soil databases — if represented at all.
The general objective of this investigation was to test the BEST procedure with different approaches to collect and analyze input data for hydraulic characterization and physical quality evaluation of some Burundian soils. The specific objectives were to: i) determine the fitting ability of the soil particle size distribution and infiltration models used in the procedure; ii) evaluate the physical quality of the sampled soils; iii) test an alternative analysis of the infiltration data; and iv) test simplified methods to estimate water retention shape parameters.
Section snippets
The BEST procedure for soil hydraulic characterization
The BEST procedure for soil hydraulic characterization (Lassabatère et al., 2006) may be applied using either cumulative infiltration or infiltration rates. The former version is shortly described here because it was found to perform better than the latter one by the original authors (Lassabatère et al., 2006). BEST focuses specifically on the van Genuchten (1980) relationship for the water retention curve with the Burdine (1953) condition and the Brooks and Corey (1964) relationship for
Soil physical quality indicators
Soil physical quality indicators are soil parameters allowing to quantify the level or degree of physical quality of the soil (Topp et al., 1997). In agricultural soils, for example, indicators quantify, directly or indirectly, the soil's ability to store and to provide crop-essential water, air and nutrients (e.g., Reynolds et al., 2007). Several indicators and associated optimal ranges or critical limits have been suggested to evaluate soil physical quality (e.g., Reynolds et al., 2002,
Materials and methods
The BEST procedure of soil hydraulic characterization was applied in two selected areas of Burundi (Fig. 1). The area of Kinyami (2° 54′ 30″ S, 29° 49′ 06″ E) is located in the agroecological zone of Buyogoma. The mean annual rainfall in the region ranges between 1156 and 1215 mm (www.climateofburundi.altervista.org) with a mean annual temperature of 18.6–21.9 °C. Savanna with acacia trees is the prevailing land cover, whereas crops for human subsistence cover 26% of the region. Both annuals
The original BEST procedure
A total of 85 infiltration runs, or 88% of the 97 runs, yielded valid hg and Ks results with the BEST/OR procedure; all five replicated infiltration runs yielded valid hg and Ks results for 14 sites. Invalid results (tk > tmax, negative Ks) were obtained at a single sampling point for two sites, and at three and four sampling points, respectively, for two other sites. Valid results were obtained at all (four) sampling points for the NY-00-01 site. On the other hand, invalid results were obtained
Conclusions
The BEST procedure was applied for hydraulic characterization and physical quality evaluation of some Burundian soils. Clay was the most dominant soil texture class, since it was dominant for 14 of the 19 sampled sites. Silty clay, silty clay loam, clay loam and loam were other represented classes.
On average, the fitting ability of both the particle size distribution (PSD) model (mean relative error, Me(Er) = 2.0%) and the cumulative infiltration transient model (Me(Er) = 2.3%) was good according
Acknowledgments
This study was partially supported by a grant from the Università degli Studi di Palermo, Italy (progetto CORI, 2008). V.B., M.I., G.P., and A.S. contributed to set up the research. S.D.P. carried out the experimental work in Burundi. All authors contributed to analyze the results and write the paper. We wish to thank the Reviewers for their constructive comments.
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