The importance of sediment characteristics and trap efficiency in assessing sediment yield using retention ponds

Retention ponds are a valuable tool for assessing sediment yield (SY) over large areas. SY is normally expressed in t ha⁻¹ a⁻¹ or t km⁻¹ a⁻¹ and therefore measured sediment volumes in ponds need to be converted to masses using representative values for the dry bulk density (dBD) and corrected for trap efficiency (TE). Mean dBD varied from 1 to 1.35 t m⁻³ for eight retention ponds in central Belgium. Within the Hammeveld retention pond, dBD ranged from 1.14 to 1.58 t m⁻³ with a mean value of 1.34 t m⁻³. Variations in dBD are mainly controlled by texture and the length of dry periods between rain events. Values of TE were calculated for this pond by comparing soil losses due to water erosion in the catchment with sedimentation rates in the pond. TE varied between 58% in spring-summer and 96% in autumn-winter because of different hydrological conditions.