Improvement of BEST (Beerkan Estimation of Soil Transfer parameters) method for soil hydraulic characterization

Interpreting and modeling soil hydrological processes require the determination of the soil hydraulic characteristic curves, i.e. the relationships between volumetric soil water content, pressure head, and hydraulic conductivity. Using traditional methods to determine these properties is 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) procedure, was developed by Lassabatère et al. (2006) to simplify soil hydraulic characterization. BEST considers certain analytic formulae for hydraulic characteristic curves and estimates their shape parameters, which are texture dependent, from particle-size analysis by physical-empirical pedotransfer functions. Structure dependent scale parameters are estimated by a three-dimensional field infiltration experiment at zero pressure head, using the two-term transient infiltration equation 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. The signs of a promising ability of the BEST procedure to yield a reasonably reliable soil hydraulic characterization can be found in the existing literature but there is still work to do. In fact, several problems yet arise with the BEST method, including: (1) the need to carry out many calculations to analyze a single run, which may demand a lot of time; (2) the need to analyze the transient phase of the infiltration process, which may be uncertain for different reasons; (3) the absence of an extensive assessment of the BEST predictions against independent measurements, i.e. with soil data collected by other experimental methods; and (4) the possible sensitivity of the data to soil disturbance and air entrapment during repeated water application, according to the BEST experimental procedure. The main objective of the present thesis was to study and improve the BEST method in order to understand or give a solution to all the former problems and consequently to contribute towards its widespread application throughout the world. With this aim, improvements to BEST method were proposed in terms of analysis of the collected data, estimation of hydrodynamic parameters and automation of the experimental procedure. In particular, a workbook to easily and rapidly analyze databases including several BEST runs, an alternative algorithm to analyze the Beerkan infiltration data and a compact infiltrometer to automate data collection with open source technology were developed. The proposed workbook is a practically useful contribution to an expeditious, intensive soil hydraulic characterization. The alternative algorithm can be considered a promising alternative procedure to analyze the Beerkan infiltration data. Finally, the cheap and automated infiltrometer constitutes a very cost effective alternative to previous proposed equipment. Moreover, BEST was tested in different soils and compared with several alternative field and laboratory methodologies highlighting the pros and cons that characterize the method and allowing to design BEST as a promising, easy, robust, and inexpensive way of characterizing soil hydraulic behavior. The main result of these studies was that BEST yields physically possible scale parameters of the soil characteristic curves in most of the replicated infiltration runs. Moreover, the water retention model used by BEST reproduced satisfactorily the laboratory data. Possible saturated soil hydraulic conductivity values were also obtained. The dependence of the measured hydrodynamic parameters on the experimental procedure used in BEST was also studied with the objective to improve our ability to interpret the field data and the linked hydrological processes. These studies led to the main conclusion that the choice of the procedure should vary with the intended use of the data. If the objective of the field campaign is to obtain data usable to explain surface runoff generation phenomena during intense rainfall events, for example, the most appropriate choice among the tested ones should be a perturbative run, to mimic relatively prolonged rainfall effects on the soil surface. A less perturbative run is more appropriate to determine the saturated hydraulic conductivity of a soil that is not directly impacted by rainfall, due for example to the presence of a mulching on the soil surface. Finally, a simplified method based on a Beerkan infiltration run to determine the saturated soil hydraulic conductivity by only a transient infiltration process was developed. This method is a good candidate method for intensive field campaign with a practically sustainable experimental effort.