O espaçamento dos terraços em culturas anuais, determinado em função das perdas por erosão

portugais. Neste trabalho é apresentada uma tabela para espaçamento de terraços, baseada nos dados experimentais de perdas por erosão obtidos em nossos solos. Utilizando os dados das determinações de perdas por erosão obtidos nos principais tipos de solo do Estado, numa média de 10 anos de observações, em talhões de diferentes comprimentos de rampa e diferentes graus de declive, com culturas anuais, determinou-se uma equação que permite calcular as perdas médias de terra para os diferentes graus de declive e comprimentos de rampa. Considerando não só as perdas de terra e água como também o teor de terra na enxurrada, foi determinada a posição média relativa para esses três fatores, considerados conjuntamente. Os dados assim conseguidos foram estudados com relação ao grau de declive e ao comprimento de rampa, e a equação resultante foi denominada de "índice de erosão". Como o terraceamento se baseia, essencialmente, no efeito do grau de declive e do comprimento de rampa, com a equação do "índice de erosão" foi organizada uma tabela para espaçamentos de terraços.

anglais. The purpose of this study is to present a terrace spacing table that may be helpful to others undertaking soil conservation extension service. The determination of soil and water losses in the São Paulo State began, about 10 years ago, with the runoff catching tanks installed on the chief soil types of the State, at some of the experiment stations of the Instituto Agronômico. Making use of the soil loss and runoff data obtained at the slope length plots and several degree slope plots planted with annual crops, an equation was determined which permits to evaluate the average of total soil loss in runoff as influenced by the degree and length of land slope. Considering the soil and water losses, and the calculated percent amount of soil in runoff, the relative average of these three factors was determined. By this way the data were studied in relation to the degree and the length of land slope and the resulting equation was named "índice de Erosão" (Erosion Index). As the terracing is based, essentially, on the effect of the degree and the slope length, with the erosion index equation a table for terrace spacing was organized. On the basis of this study the following conclusions were drawn: a) doubling the degree of slope increases the total soil loss in runoff 2.26 times and the erosion index 1.57 times; b) doubling the horizontal length of slope increases the total soil loss in runoff 3.09 times and the erosion index 1.83 times; c) the relative averages, with respect to soil loss and runoff, for sandy soil and olayish soil are larger in about 30% and 20%, respectively, when compared with the diabase soil ("terra roxa"); d) for practical purposes the erosion index is represented by the equation: E = 1,03 D0,66 C D'87. in which E is the erosion index, D is the degree of slope, and C is the length of slope. The equation is based updn a limited amount of data which were not developed for the purpose of this study, therefore, refinements will be necessary; e) the following formulas for terrace spacing were developed: EH = 45,18/D0,42 and EV = 0,4518 K D0,58 in which EH is the horizontal interval between terraces, K is the soil type coefficient, D is the degree of land slope and EV is the vertical interval between terraces.