SURFACE SEAL DEVELOPMENT UNDER SIMULATED RAINFALL ON AN ACTIVELY ERODING SURFACE

Surface seals developed under controlled conditions were examined micr omorphologically to obtain detailed information on the causal processe s. Five tests were carried out using a crusting-prone binary soil in a 5 m long laboratory flume at an average slope of 5-degrees under simu lated rainfall of 32 mm h-1 for 1 hour. Three types of surface seal we re identified in thin section. Disruptional seals were 0.3-0.8 mm thic k surface layers formed on interrills by raindrop impact due to (i) ra pid destruction of surface aggregates by direct drop impact; (ii) rear rangement of disrupted fragments and textural separates by splash tran sport, and (iii) compaction and flattening of surface material by cont inued drop impact. Sedimentational seals developed under rill and inte rrill flow with their structure being dynamically dependent upon local flow conditions. They were typically > 1 mm, often with a complex str ucture of 2 or more sedimentational layers. Afterflow seals were extre mely thin (< 50 mum) ''skin'' features with strong continuous orientat ion of clay particles, formed by fine particle deposition after rainfa ll cessation and not by raindrop impact mechanisms as sometimes though t.