Comprimento crítico de declive relacionado à erosão hídrica, em três tipos e doses de resíduos em duas direções de semeadura direta

Abstract

(4) RESUMO Nos preparos conservacionistas de solo, a eficácia dos resíduos culturais na redução da erosão hídrica diminui se os resíduos forem transportados pela enxurrada e, ou, se esta sulcar o solo. A esse fenômeno denomina-se falha dos resíduos, caracterizando a existência de comprimento crítico de declive, ou seja, estabelece a distância no terreno a partir da qual a enxurrada adquire tensão cisalhante suficiente para superar a resistência do resíduo ao transporte e, ou, a resistência do solo ao sulcamento. O objetivo deste trabalho foi averiguar a existência de comprimentos críticos de declive em duas direções de semeadura direta, sob diferentes tipos e doses de resíduos culturais. O trabalho foi desenvolvido em campo, entre maio de 2009 e maio de 2011, sobre um Nitossolo Bruno aluminoférrico húmico, em São José do Cerrito (SC), usando unidades experimentais com dimensões de 3,5 x 11 m, com declividade média de 0,144 m m -1 . Foram testadas as formas de execução de semeadura no sentido transversal (em contorno) e paralelo ao declive (na pendente), sobre os resíduos culturais de milho (Zea mays), trigo (Triticum aestivum) e soja (Glycine max), nas doses de 50 e 100 % dos resíduos produzidos. Foram realizados testes de chuva simulada, com o uso de simulador de chuva de braços rotativos, na intensidade de chuva planejada de 65 mm h -1 e com duração de 90 min cada um -tempo esse necessário para se obter a taxa constante de enxurrada. Ao final dos 90 min de chuva, sem desligar o aparelho simulador de chuva, foram aplicados níveis crescentes de fluxo extra de água limpa na cabeceira das parcelas, simulando a enxurrada provinda de declives mais longos. Para identificação da falha dos resíduos,

Water erosion in no-tillage monoculture and intercropped systems along contour lines

“…The variation between water losses (Table 3) was also influenced by soil water content before rainfall application (Table 2), in agreement with results from other studies (Cogo, 1981;Panachucki et al, 2011; Barbosa et al, 2012). The high runoff intensity in some treatments and, especially, at the end of the crop cycles (rainfall test 4), were related to the critical rainfall events.…”

“…The variation between rainfall tests was also influenced by the soil spatial variation although all treatments were included in the same soil texture class. These variations were also reported in experiments carried out by Barbosa et al (2012), working with a typical clayey Alfisol. It is therefore important to remember that this variation in soil water content before the simulated rainfall application can influence the study variables, especially water loss by runoff.…”

“…The high runoff intensity in some treatments and, especially, at the end of the crop cycles (rainfall test 4), were related to the critical rainfall events. This indicates the need for mechanical structures to control runoff under these conditions, as suggested by Barbosa et al (2012). In other words, the problem of water erosion is directly related to the occurence of excess rainfall, which is also possible in areas under no tillage.…”

“…Between tests, this period ranged was from 12 to 45 min, in the average of treatments, while between treatments, the variation was from 26 to 31 min, in the average of the tests. This can be at least partially explained by the effect of the previous soil water content, influencing similarly the time until onset of runoff, according to Luciano et al (2009) and Barbosa et al (2012).…”

“…This type of relationship between soil and water loss is normal, especially in soil management systems without mechanical soil tillage and where no plant residues were removed by the runoff. This occurs because, as the volume of water in the runoff increases, the runoff velocity also increases and consequently so does the breakdown energy and sediment transport (Bertol et al, 1997;Barbosa et al, 2012). The slope of the curve is determined by the soil type, amount and type of crop residue on the soil surface, type and form of crop management, characteristics of rainfall and soil water content prior to rainfall (Cogo, 1981;Luciano et al, 2009;Barbosa et al, 2012), among other factors.…”