A numerical model (sediment trap efficiency for small ponds—STEP) is developed to simulate sediment deposition in small ponds (i.e. <1 ha) and to calculate the sediment trap efficiency (STE). The algorithms are kept simple to allow the model to simulate larger time periods (i.e. several years). Eight runs with an experimental pond were executed to test the model. The STEP model produces reasonable predictions of STE as well as the shape and magnitude of the effluent sediment concentration graph. The model efficiency of STEP for the prediction of STE equals 0·38 and the root mean square error equals 4·7%. Similar models, such as DEPOSITS and CSTRS, were inefficient in predicting the experimental results. The STEP model was used to simulate the long-term (33 years) STE of small retention ponds in central Belgium using 10-min rainfall data. For a typical pond (1000 m2) with a catchment area of 25 ha, annual STE can vary from 58 to 100%, with a long-term STE of only 68%. Copyright © 2001 John Wiley & Sons, Ltd.