A comparison using the caesium-137 technique of the relative importance of cultivation and overland flow on soil erosion in a steep semi-tropical sub-catchment

Abstract

The spatial pattern of net soil loss on 6 downslope transects in a small semi-tropical sub-catchment was measured in 1990—91 using the resident caesium-137 deficit technique. The sub-catchment consisted of 2 opposing hillslopes which shed water to an intermittent stream in the valley bottom of the sub-catchment. There were 3 transects on each of the opposing hillslopes, and measurement indicated net soil loss from all 6 transects. Furthermore, the spatial pattern of caesium-137 deficit did not indicate the accumulation of soil expected due to the slope decrease toward the bottom of the valley. Possible explanations of this finding could be the effect of periodic flooding of the intermittent valley stream, or seepage-accelerated erosion.

Pineapple cultivation in the sub-catchment since 1950 included intensive cultivation at 4-year intervals by downslope-moving rotary hoe. The paper develops a theoretical prediction of the spatial pattern of net soil loss expected due to such cultivation, as well as the expected pattern of soil loss due to overland flow on the hillslopes. The spatial patterns of soil loss due to these 2 different soil erosion mechanisms were then compared with the pattern of net soil loss indicated by caesium-137 depletion to provide an assessment of their likely relative importance in contributing to soil loss. In the upper part of each hillslope, this comparison of spatial trends did not allow the dominant cause of soil erosion to be distinguished. Both the model of erosion due to cultivation and that due to hillside overland flow predicted soil accumulation in the lower valley sides where slope decreased. Neither model represented the net loss of such accumulated soil indicated by caesium-137 deficit, and this loss possibly occurred during periodically observed flooding of the valley floor, or due to surface burial with caesium-137 depleted subsoil.