Estimation of Water Retention Characteristics of Shrink-Swell Soils using Pedotransfer Functions

The shrink-swell soils under orchards (irrigated) over a long period develop unique hydraulic characteristics. As a case study, water retention characteristics of sweet orange growing soils were analyzed to define their analytical behaviour so that simulations can be made for projecting changes. Soil water retained (θ) at seven varied matric potentials (h) namely 33, 50, 75, 100, 400, 800, 1500 kPa was measured in the laboratory and seven water retention functions proposed by different researchers were fitted to the laboratory measured data. It was also observed that the h-θ relationship proposed by van Genuchten (VG) fitted better for these soils. Pedotransfer function (PTF) ‘Rosetta’ (computer code) was used to predict VG parameters from basic soil data. The parameters estimate suggested that the soils were unique in hydraulic behaviour. It was observed that the residual soil-water content Θr and saturated soil-water content Θs were lower in magnitude as compared to the class values. The estimates of VG parameters were used to predict soil-water retention at varied suction pressure(s). Observed and predicted values of soil-water retention were compared to evaluate the performance of Rosetta. The coefficient of determination (R² 0.88) values suggested that it could predict soil-water retention in lower suction range with relatively greater accuracy. Comparatively better predictions were obtained with basic data on texture, bulk density and field capacity as an input.