Deriving point and parametric pedotransfer functions of some gypsiferous soils

Abstract

Parametric description of the soil water retention curve as well as the hydraulic conductivity curve is needed for modelling water movement and solute transport in the vadose zone. The objective of this study was to derive pedotransfer functions (PTFs) to predict the water retention curve and the van Genuchten and the van Genuchten–Mualem parameters of some gypsiferous soils. Consequently, 185 gypsiferous soil samples were collected and their physical properties were measured. The particle size distribution was determined in 2 steps: (i) with gypsum, by covering the particles with barium sulphate; (ii) without gypsum, using the hydrometry method. The easily obtainable variables were grouped as (1) particle size distribution, bulk density, and gypsum content; and (2) bulk density, gypsum content, geometric mean, and geometric standard deviation of the particle diameter. Stepwise multiple linear regression method was used to derive the PTFs. Two types of parametric and point functions were derived using these variables. The first group of variables predicted water retention and the van Genuchten and van Genuchten–Mualem parameters better than the second group. The gypsum content appeared to be the second dominant parameter for predicting water retention at 0, −330, −1000, −3000, −5000, and −15 000 cm. The derived PTFs were compared with the Rosetta database as independent dataset. The validity test indicated that in order to predict the hydraulic properties of gypsiferous soils the derived PTFs are more accurate than what can be obtained from the Rosetta database. Removal of gypsum increased the water retention at pressure heads of 0, –100, –330, –1000, –3000, –5000, and –15 000 cm (P < 0.01). The results also indicated that hydraulic parameters were different for the same soil with and without gypsum.