Fundamental studies on farm land conservation in Okinawa III : Experimental study on soil erosion by simulated rainfall (Department of Agricultural Engineering)

本実験は, 落下距離5mの高さから, 人工降雨を降らし, 沖縄の代表的な3種の異なった土壌について, それぞれの侵食性を調べるために行なった。人工降雨による雨滴径の分布, 終速度は自然降雨との間にいくらかの差異はあったが, ある程度初期の目的に達したと思われる。降雨装置, 雨滴径の形成方法などを改善し, 今後も引き続き研究を重ねていきたい。主な結果を要約すると, つぎのとおりである。(1)各供試土の雨滴に対する分散性は, ニービ土壌, 国頭マージ土壌, ジャーガル土壌の順に高い。(2)ニービ土壌においては, 10分間降雨量と, 流亡土量, 流去水量との相関関係は極めて高く, 国頭マージ土壌はいずれも関係が薄いといえる。(3)ニービ土壌とジャーガル土壌について10分間降雨量(I_)と流亡土量(E), および流去水量(Ru)との関係は, 指数式E=aI^b_Ru=aI^b_が適用されよう。A study on soil erosion was made by means of simulated rainfall. Three differnt types of soil, all of which were distributed in OKINAWA district, were used as the test soil. They were called KUNIGAMI MAJI-soil (plot 1), JAGARU-soil (plot 2), NIIBI-soil (plot 3) respectively, and the soil grading was as follows : [table] The plots which were 0.335 square meters were set with a slope of 7 degrees. The simulated rain was dropped from a place 5 meters high above the surface of test soil for 10 minutes. Although the drop size distribution and terminal velocity of simulated rain were different from those of natural rain and the amount of rainfall was larger than that of natural rainfall, it may be of some clue to know the relation between soil loss and rainfall intensity on the natural farmland. The results of this experiment are shown as follows : 1. The detachiabillity of test soil is high in the order of NIIBI-soil, KUNIGAMI MAJI-soil, and JAGARU-soil. 2. The soil loss quantity by rainfall for NIIBI-soil and JAGARU-soil increases with the incrcease of rainfall intensity. But the soil loss quantity of KUNIGAMI MAJI-soil seems not to be affected much by rainfall intensity. 3. The relation between the soil loss quantity and the rainfall intensity for JAGARU-soil and NIIBI-soil will be expressed by an equation E=aI^b when they were plotted on a log-log graph. The value of a, b and the corelation coefficient between E and I are as following : plot 2 E=0.073 I^ r=0.728 plot 3 E=0.196 I^ r=0.894 where E : Soil loss g/0.335m^2 I : Rainfall intensity mm/10min. r : corelation coefficient. 4. The relation beteen the quantity of runoff and the rainfall intensity for the test soil will be the same as 3 above