Environmental assessment of soil erosion in Inabanga watershed (Bohol, Philippines)

Abstract

Analysis of soil redistribution patterns in watershed areas provides information for understanding soil erosion and deposition for implementing management practices to improve agricultural land conditions and reduce sediment loads in river systems. This paper has demonstrated the use of Cesium-137 (¹³⁷Cs) in estimating retrospective medium-term soil erosion rates in selected cassava cultivated areas in Pilar, a sub-watershed within the Inabanga watershed. To estimate erosion and deposition rates and elucidate the factors affecting the soil redistribution, samples were collected from a total of ninety-eight grid intersections representative of the local land use and slope gradients in the presence or absence of soil conservation practices. A proportional model was used to deduce soil redistribution rate estimates from ¹³⁷Cs inventories measured from individual soil samples. Soil measurements of ¹³⁷Cs activity-generated soil erosion rate gave values of 13.15 t ha⁻¹ year⁻¹ in cultivated areas practicing conservation measures and 22.23 t ha⁻¹ year⁻¹ in those with typical upland plantation, with the former corresponding to slight erosion case and the latter to moderate erosion case. The obtained values have provided an overview of the pattern of soil redistribution in the watershed area and reflect the impact of strategic soil erosion management being applied. Interestingly, in light of recent severe meteorological events befalling the region, e.g., super typhoon Haiyan and the 7.3 magnitude earthquake, the data obtained may serve as benchmark values for ¹³⁷Cs activities against which changes in the soil movement and soil redistribution pattern along the watershed areas can be evaluated.

Keywords

Agriculture / Cesium-137 radioactivity / Environment / Sediments / Soil redistribution / Typhoon / Water resources

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