Soil Erosion and Conservation

of crop-productivity loss. Conservationists refer to this process as soil loss, referring to the net loss of soil over only the portion of the field that experiences net loss over the long-term. Areas of soil loss end where net deposition begins. Off-site concerns, on the other hand, are associated with the sediment that leaves the field, which we term here sediment yield. In this case, we are not necessarily concerned with the soil loss, or for that matter the amount of sediment deposited prior to leaving the field, although estimation of both of these may be used to estimate sediment yields. Ideally, a model will compute soil loss, deposition and sediment yield, and thus have the capability to address both on-site and off-site issues. Data variability and model uncertainty are two related and important issues associated with the application of erosion models. Data from soil-erosion plots contain a large amount of unexplained variability, which is an important consideration for using erosion data to evaluate soil-erosion models, as well as for interpreting erosion data. This variability is due both to natural causes and measurement errors. When comparing measured rates of erosion to predicted values, a portion of the difference between the two will be due to model error, but a portion will also be due to unexplained variance of the measured sample value from the representative, mean value for a particular treatment. Knowledge of variability in soil-erosion data, however, is somewhat limited, although recent studies have enlightened us to some degree. Only one experimental erosion study to date has been conducted with a sufficient number of replicated erosion plots to allow an in-depth analysis of variability. Wendt et al. (1986) measured soil erosion rates on 40 cultivated, fallow, experimental plots located

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