Assessment of erosion rate in a rainfed agricultural area on the Northwestern coast of Egypt using 137Cs measurements

In the present work, the use of the fallout radionuclide 137Cs as a tracer for estimating soil redistribution rates over the past 60 years was tested in Egypt at an agricultural field with a smooth slope (0–10%) located in Ras El-Hekma on the Northwestern coast. The average annual soil erosion rate was derived from 137Cs inventories (Bq m−2) and by using the conversion Diffusion and Migration Model (DMM). The activity concentrations of 137Cs with uncertainty ranging between 7.5 and 20.9% were measured using HPGe gamma spectrometry. Approximately exponential shape for the distribution of 137Cs activity concentration was observed within the upper 20 cm of the soil profile associated with the reference site while for the study sites, the 137Cs depth distribution profiles indicate generally an increase of activity reaching a maximum peak of 137Cs usually at a depth of 10–15 cm followed by an abrupt exponential decrease of 137Cs activity concentration with depth. The measured average reference inventory for this area was 697 Bq m−2 with a coefficient of variation of 23%. This value is close to the estimated reference inventory using the conversion model, which was 744 Bq m−2. Transect sampling strategy was adopted at the study area where three parallel transects were chosen based on the common slope. The estimated average annual erosion rate for the three transects was found to be 7.5 t ha−1 y−1, and sediment delivery ratio was approximately 98%, which shows the moderate vulnerability of the Ras El-Hekma area to water erosion.