Hydraulic properties of rain impact surface seals on three clay soils - influence of raindrop impact frequency and rainfall intensity during steady state

This paper investigates the influence of rainfall intensity, and its components drop size and impact frequency, on steady state flow through sealed soils. Infiltration rates are often observed to increase with greater rainfall intensity. There appears to be no consensus in the literature on the reasons for this behaviour, making it difficult to incorporate in infiltration models. Seals were formed on 2 swelling Black Vertosols and a contrasting non-swelling Brown Sodosol, under simulated rainfall treatments with a range of rainfall intensities, drop sizes, and drop size distributions. For Vertosols, aggregate size composition of the seal, sub-seal moisture content, and suction were similar across rain treatments and between soils. Steady infiltration rate and conductivity increased 3-fold as rainfall intensity increased from 30 to 120 mm/h for Vertosols. The component of intensity responsible for this trend was isolated, with a strong positive linear relationship found between conductivity and raindrop impact frequency (R-2 = 0.87). This trend was attributed to increased water entry by hydraulic penetration of drops through pores in the seal as the frequency of drop impacts increased, on more porous Vertosol seals. In contrast, rain intensity and drop impact frequency had little effect on steady infiltration rates and conductivity for the Sodosol. Abundant fine particles in the seal decreased porosity, apparently leaving few pores of large enough diameter to allow hydraulic penetration. While hydraulic penetration was suppressed, other seal properties for the Sodosol were influenced by rainfall characteristics. Larger, more energetic drops produced seals with lower moisture content and higher subseal suction than seals formed under other treatments. Results of this study indicate that water entry through sealed surfaces may consist of an actively driven component associated with hydraulic penetration of water through larger pores (jetting), as observed elsewhere in early stages of surface sealing. Water entry by this mechanism continues through to steady state on soils where significant numbers of larger pores remain open at the soil surface. This mechanism would act alone, or in addition to other mechanisms such as erosive stripping or microrilling of seals, to explain widely observed increases in infiltration rates with increasing rainfall intensity.