Unsaturated flow of different quality irrigation waters through clay, clay loam and silt loam soils and its dependence on soil and solution parameters

Abstract

The effect of total electrolyte concentration (TEC) and sodium adsorption ratio (SAR) of irrigation water on unsaturated hydraulic properties, viz. unsaturated hydraulic conductivity, wetting front advancement and soil-water diffusivity of clay (Typic Haplustert), clay loam (Vertic Haplustept) and silt loam (Lithic Haplorthent) soil was studied in a laboratory experiment. Twenty-four types of irrigation waters, encompassing four TEC levels, viz. 5, 10, 20 and 50 me l −1 and six SAR levels, viz. 2.5, 5, 10, 15, 20 and 30 m mol 1/2 l −1/2 were synthesised to equilibrate the soil samples using pure chloride salts of calcium, magnesium and sodium at Mg:Ca=1:2. Unsaturated hydraulic conductivity ( K θ ), was determined both directly (by ‘inflow–outflow’ vertical infiltration method for water potentials of −5 and −10 kPa) and indirectly (from diffusivity functions and soil-water retention curves) in two different water potential ranges. Wetting front advancement and diffusivity ( D θ ) were determined by the horizontal infiltration method. Water quality had larger impacts on the unsaturated hydraulic conductivities of the silt loam soil than on the clay and clay loam soils. The effects of TEC and SAR were both large for the silt loam soil whereas the effects of SAR were predominant for the clay and clay loam soils. At a water potential of −5 kPa (50 cm suction), the hydraulic conductivity of clay and clay loam soils was approximately 10 times smaller than at saturation under different quality waters. However, the hydraulic conductivity of silt loam soil was approximately 10 and 100 times smaller than at saturation for low SAR (up to 10 m mol 1/2 l −1/2) and high SAR (15 m mol 1/2 l −1/2 and above) waters, respectively. Influence of irrigation water quality on K θ increased with decreasing water content. SAR of water and exchangeable sodium percentage (ESP) of soils influenced K θ significantly. Besides SAR, TEC also influenced K θ of silt loam significantly. D θ increased with increase in TEC and/or decrease in SAR of water. The clay soil was found to be the most sensitive soil to unsaturated flow above SAR 15 m mol 1/2 l −1/2. At higher and lower water contents, the dependence of D θ on SAR of water and ESP of soil was found to be highly significant. However, in silt loam soil it was significantly dependent on TEC of water also.