Soil Erosion Studies on Steep Slopes of Humid-Tropic Philippines

Abstract

An in-situ water-induced soil erosion study of two years duration was conducted at a low slope of about 10%, and also on steep slopes of about 50%, 60% and 70% at the experimental site located at a soil and water research area contiguous to the Department of Agricultural Engineering and Applied Mathematics of the Visayas State College of Agriculture (ViSCA). ViSCA is located in the town of Baybay at the island of Leyte in humid tropical Philippines (latitude of 10 degrees 44' north and longitude of 124 degrees 48' east). The study has the following aims, namely: 1) to investigate the effect of local farming practices on soil erosion, particularly at steep slopes; 2) to study the hydrology and soil erosion processes in steep slope conditions in humid tropical climate; and 3) to apply and determine the limitations of a physical process-oriented sediment transport model for water-induced soil erosion model (Griffith University Erosion System Template, GUEST) to a very permeable soil under very high rainfall conditions. In the context of the steep slope experiments at ViSCA: a) across the slope planting of corn is equally effective in controlling soil loss as hedged farming alone or in combination with peanut intercrop; b) across the slope planting of corn in hedged runoff plots is as effective in controlling soil loss as when combined with peanut intercrop; c) farmer’s practice of planting a rowcrop like corn along the slope (providing ready-made downslope pathways for runoff water thus, encouraging flow-driven soil erosion) produces significant soil loss, often larger than from a bare runoff plot; d) hedgerows significantly reduced sediment concentration provided they remain intact; e) "failure" in hedgerows, often results in massive soil loss, being even greater than that in a comparable bare runoff plot. (Failure is manifested by runoff breaching the litter barrier at the base of the hedgerow in a localized position, leading to concentrated flow and extension of rills upslope from base of hedgerow) of hedged runoff plot (even though contour planted) and f) application of stubble in combination with across slope planting in hedged runoff plots (with or without a closely growing intercrop) provided both maximum surface cover (aerial and contact) and protection against soil loss in steep slopes. These conclusions are likely to apply to crops similar to corn, with an intercrop similar to peanut in morphology and root system. These conclusions might also be expected to apply to low slopes, though the effect of such protection might not be as pronounced as in steep slope runoff plots. Soil erosion mechanisms in steep slopes are dominated mainly by the runoff-driven processes of entrainment and re-entrainment. Average sediment concentration c and total soil loss M are shown to be functions of positive coefficient runoff related regressors and a parameter that measures rills formed and permanently imprinted on soil surface, and of negative coefficient regressors related to surface contact cover, at least for the steep slopes considered in this study. The relatively high streampower associated with overland flow on steep slopes suggests that overland flow dominates over rainfall impact as a source of soil sediment concentration. This was confirmed by the low value of the measured ratio of soil sediment concentration shed by net-rainfall detachment trays to that lost from the bare runoff plots. On the effect of surface contact cover on average sediment concentration c, the following findings were made, namely: a) surface contact cover of only about 20% to 30% is sufficient to reduce c to low levels, assuming the cover is well distributed; b) an "exponential-decay" form of relationship existed between surface contact cover and the average sediment concentration from the non-bare soil without normalizing with respect to sediment concentration from a reference bare soil runoff plot; and c) such nonlinear relationship is indicative of the dominating runoff processes of entrainment and re-entrainment compared to the linear form of relationships if rainfall –driven processes of detachment and re-detachment dominate. In runoff dominated soil erosion processes, surface contact cover (including both living and dead vegetative cover close enough to the soil surface to impede overland flow) is more significant in curtailing soil erosion than aerial surface cover. ... [continued].