Changing sediment yield as an indicator of improved soil management practices in southern Brazil

Abstract

Catchment-level soil and water conservation programmes have been widely employed in Brazil. An important component of these programmes is the implementation of water and sediment monitoring projects to evaluate the impact of changes in soil management on water resources. In general, results from monitoring projects have been inconclusive, due to a series of difficulties associated with data collection and limited timeframes. This study presents results from a hydrosedimentological monitoring project undertaken in a small (1.19 km2) rural catchment in Southern Brazil before and after the introduction of conservation tillage practices implemented by the RS-RURAL — Program Against Rural Poverty. These practices, including use of winter cover crops and minimum tillage tobacco cultivation, were gradually adopted by local farmers. Data on precipitation, runoff volume, maximum flow and sediment yield were assembled for representative storm events occurring between May 2002 and March 2006, and analyzed to identify changes in the storm runoff and storm-period sediment response of the study catchment. The results provide evidence of statistically significant reductions in storm runoff, maximum flow rate and sediment yield after implementation of the conservation practices. Sediment sources were also investigated using the fingerprinting technique and this work demonstrated a statistically significant reduction over the study period in the proportion of the sediment contributed by fields (62% to 54%) and unmetalled roads (36% to 24%). This reduction was offset by an increased contribution of sediment from channel sources (2% to 22%). The increased proportion of sediment mobilized from the channel is in part a function of the reduced contribution from the two other sources, but it also reflects the reduction in sediment inputs to the channel from the fields and unmetalled roads, which results in an increase in the energy available for channel scour. Results emphasize the complexity of the relationship between sediment yield, the effects of climactic variability, and changes in land use and management.