Investigating rainfall duration effects on transport of chemicals from soil to surface runoff on a loess slope under artificial rainfall conditions

The release and transport of soil chemicals in water erosion conditions are important for the local environment, soil and water resources conservation. According to the artificial rainfall experiments with a constant rainfall intensity of 90 mm/h and different rainfall duration (30, 60, 90, 120 and 150 min), the traits of soil PO43-, K+, and Br- release and transport from soil to surface runoff on the loess slope were analysed, and a model describing the chemical concentration change in surface runoff under soil erosion conditions was developed. The runoff coefficient quickly increased in 15 min or so, and then it was stable in the range of 0.60-0.85. The sediment intensity decreased in 30 min and soon increased after severe sheet erosion occurred on the slope. The concentration curve of Br- in surface runoff can be divided into two stages, quickly decreasing in the initial 30 min after the surface runoff occurred, and then stable. The concentration curve of PO43- and K+ in surface runoff can be divided into three stages, quickly decreasing like Br- was decreasing, then stable, and increasing after severe sheet erosion began. Compared with the exponential function, the power function was found more suitable for fitting the change in chemicals in runoff with unsaturated soil; while neither of them could well fit the PO43- and K+ concentration change after severe erosion occurred. The transport of chemicals under complex soil erosion conditions seems to be a dynamic release process between surface runoff and sediment. Based on the convection-dispersion mechanism and desorption kinetics, the polynomial model under soil erosion conditions was created. For adsorbed PO43- and K+, it is more suitable to simulate that process than the power function, while it is not so good for mobile Br-.