Fig. 1
Models used in Converting FRNs to Erosion /depositional rates_
| Model | Input parameter | FRNs and site used | Advantage | Limitation |
|---|---|---|---|---|
| Proportional model | Plough depth, bulk density | 137Cs (Cultivated) | 1.Requires few input parameters 2.Easy to apply | 1.Oversimplification of 137Cs accumulation in the soil 2.Model does not account for dilution of 137Cs concentrations in the soil |
| Mass Balance Model 1 (MBM1) | Plough depth, bulk density, year of tillage commencement proportional factor, relaxation depth, annual fallout flux required for 137Cs only | 137Cs (Cultivated) | 1. Accounts for progressive reduction in the 137Cs concentration of the soil | 1.Model does not account for removal of freshly deposited 137Cs by erosion during a rainfall event 2.The model assumes that concentration of 137Cs in eroded sediments remains constant through time. Also, the concentration of deposited sediment is assumed to be constant |
| Mass balance model 2 (MBM2) | Tillage depth, tillage constant, proportional factor, tillage relaxation depth, slope length and slope gradient for each section of the transect, annual fallout flux required for 137Cs only | 137Cs (Cultivated) and 210Pb (Cultivated) | 1.Accounts for tillage effects 2.Caters for 137Cs dilution in the plough layer 3.Obtained results are more realistic than from the previous models | More data is required than from the previous models |
| Mass Balance Model 3 (MBM3) | In addition to MB2 parameters Diffusion coefficient, relaxation depth, migration, coefficient, annual fallout flux required for 137Cs only | 137Cs (Cultivated) and 210Pb (Cultivated) | 1.Results from this model are closer to reality 2.It accounts for tillage redistribution in flow direction | The model is data intensive |
| Profile shape model | Profile shape factor, Mass depth of soil | 137Cs (Uncultivated) 7Be (Cultivated) | Simple and easy to use | Does not account for temporal variation of 137Cs |
| Diffusion and migration and model | Diffusion coefficient, Migration rate, /retardation factor | 210Pb (Uncultivated) | 1. Accounts of temporal variation of depth profile resulting from the time-dependent fallout input | Requires more time and data to use |
Summary of radionuclide activity in reference sites_
| Country (Region) | Author | Reference inventory (Bq/m2) | FRN used |
|---|---|---|---|
| Uganda (Kongta) | Ruecker et al., 2008 | 392 | 137Cs |
| (Magada) | 439 | 137Cs | |
| Kenya | Jeff and Shepherd, 2009 | <30 (Below minimum detectable limits) | 137Cs |
| Ethiopia | Denboba, 2005 | 2064 ± 112 | 137Cs |
| (Kefa zone) | |||
| Morocco (Boussouab) | 1450 | 137Cs | |
| Sadiki et al., 2007 | 3305 | 210Pb | |
| Benmansour et al.,2013 | 1445 | 137Cs | |
| 101 | 7Be | ||
| Bukina Faso (Dano and Wahable) | Schmengler, 2011 | 729 ± 14 | 137Cs |
| Ghana | Pennock, 2000 | 925.1 | 137Cs |
| Niger (Southwest) | Chappell et al., 1998 | 2066 ± 125 | 137Cs |
| Zimbabwe | Quine et al., 1993 | 30 | 137Cs |
| Zambia (Kaleya) | Walling et al., 2003 | 2602 | 210Pb |
| (Collins et al, 2001) | 202.0 ± 22.8 | 137Cs | |