Crust Formation and Clay Migration Effects on Infiltration Rate

A rainfall simulator study was conducted in the laboratory to investigate the effects of a cationic polymer (CP‐14), soil column thickness, water quality, and initial soil‐water content on infiltration rate (IR) and clay migration through soil. The water quality variable was imposed by synthesizing waters typical of canal water (CW) and well (WW) water from the San Joaquin Valley in California. The main difference between the waters was the electrical conductivity, which was 0.05 and 0.70 dS m⁻¹ for CW and WW, respectively. The IR increased with increasing polymer concentration, and the highest incremental effect was between 0 and 5 mg L⁻¹ polymer. The effluent clay concentrations were very small with WW but high with CW. The effluent clay concentrations tended to decreased with increased polymer concentration. The IRs increased, whereas the effluent clay concentrations decreased, with increased woil column thickness. There was, however, appreciable migration of clay beyond the 30‐cm soil thickness. The IR was slightly higher and the effluent clay concentration slightly lower in initially prewet as compared to air dry soil. If clay retention in columns clogged pores, the IR would be expected to decrease as soil column thickness increases. The results were opposite, suggesting that other mechanisms were involved. The formation of a crust at the soil surface having low hydraulic conductivity would be consistent with the observed results. Increased polymer concentrations were associated with increasing hydraulic conductivity of the crust, but had no consistent effect on the hydraulic conductivity of the underlying layer. Thus, the main treatment effect was on crust formation. Scanning electron micrographs showed loose, bare particles on the soil surface underlain by a dense compacted layer with or without polymer treatments. Below the dense layer the structure was more open. Therefore, under our experimental conditions, crust formation appears to be a more significant factor affecting IR than clogging of pores by clay migration. The relatively small difference in IR between CW and WW could be larger in the field where long‐term soil clogging could be more effective.