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6 <br />• clay particles swell as they become wetter, reducing the size of pores <br />(Dunne and Leopold, 1978, p. 166). <br />For other soils, the formation of a crust on the soil surface <br />caused by direct raindrop impact may be primarily responsible for the <br />decrease in infiltration rates during rainfall. Experimental evidence <br />by Morin and Benyamini (1977) on the infiltration capacities of five <br />different soils, some bare surface and some covered with a straw mulch, <br />showed the straw mulched soil retained its high infiltration capacity <br />compared to that of the bare soil even though the moisture content and <br />the depth of water penetration under the mulch were greater. The mulched <br /> soil showed no decrease in infiltration capacity while the infiltration <br /> rates of the bare sails could be predicted by a Horton type equation, <br /> f = fc + (fa_fc)e-kt~ where f is infiltration rate, fc is final infil- <br />• tration capacity, fo is initial infiltration rate, t is time, and k is <br /> a decay rate coefficient for the particular soil. <br /> Earlier experimental work by McIntyre (1958) found the soil crust <br /> to consist of two distinct parts; a skin seal 0.1 mm thick over the <br /> surface, and beneath it a washed in zone with a slightly higher hydraulic <br /> conductivity than the surface skin but much lower than the soil beneath <br /> it. He described two mechanisms which seemed important in the formation <br /> of the crust while rain is falling. First, wet soil aggregates are <br /> broken down by raindrop impact and the fine soil materials are washed <br /> into the surface pores which reduces their volume and creates the <br /> lower part of the crust, the washed in region. Second, after breakdown <br /> of the surface aggregates, continued raindrop impact causes compaction <br /> of the surface, producing the skin seal. Unless the skin sear is <br />• <br />