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In　order　to　make　clear　the　mechanism　of　surface　runoff,the　authors　carried ou七experiments　of　surface　rmoff　induced　by　heavy　rainfa1I　on　the　man-made　slope in　the　rainfaユ1simula亡or　of　the　Na士iom1Research　Center　for　Disaster　Prevention. The　experimen亡a1plo亡forms　a　s1ight1y　concave　slope　and　its　area　is102m2.It　bad b㏄n1eft　ou士doors　for士wo　years.Roo亡s　of　grasses　in　summer　and　freezing　in　winter had　made七he　surface　soi1porous.The　range　of七he　intensity　of　rainfa11produced by士his　simu1ator　is20_1OO　mm/hour　which　covers　almos士士he　who1e　range　of　heavy rainfal1in　Japan.　surface　runo丘,which　includes　a　short-Period　interiow　in　this repor士,apPears　in　every　simulated　s七〇工m.