Pedotransfer functions to determine water conducting macroporosity in South African soils

Macropores play an important role in the rapid transport of water, solutes and pollutants through the soil. Transport through these pores (>0.5 mm) is dominated by gravitational forces (i.e. matrix forces have low impact) resulting in flow rates orders of magnitude higher than rates that would be predicted, posing problems for modelling and understanding water and solute transport through soils. This study aimed to quantify the water conducting macroporosity (WCM) in a range of soils in South Africa and to develop three pedotransfer functions (PTFs) able to predict WCM. Saturated (K_s) and unsaturated (K₃₀) conductivities were measured in situ on 120 soil profiles using double ring and tension infiltrometers methods. Differences between K_s and K₃₀ in conjunction with Poiseuille's law and the capillary rise equation were used to calculate WCM. The first two multiple regression functions made use of all available soil properties influencing WCM using a ‘best model’ and ‘backward’ analysis approach respectively. The third model used only easily observable soil properties to predict the WCM. The functions were validated using a double-cross method. Results are encouraging with R² values of 0.78, 0.74 and 0.69 for functions 1, 2 and 3 respectively.