Date of this Version
Environmental Modelling: Finding Simplicity in Complexity, Second Edition. Edited by John Wainwright and Mark Mulligan.
22.1 The problem
Accelerated soil erosion induced by human activities is the principal cause of soil degradation across the world. The main culprit behind the problem is agriculture, and at stake is the long-term viability of the agricultural production capacity of the planet. Barring major unknown scientific advances in the future, and if soil erosion and population growth remain unchecked from their current rates, humanity will eventually lose the ability to feed itself. Another significant problem associated with soil erosion is off-site sediment pollution. Costs associated with the delivery of sediment to streams and other water bodies worldwide are huge (e.g. Pimentel, 1995). This chapter will focus on models of soil erosion as they are used for purposes of soil conservation. In particular, we focus here exclusively on soil erosion by water (see also Chapter 15). Models of other agricultural erosion processes, such as wind erosion and tillage erosion, are certainly important, but they will not be addressed here.
Models can be used in conservation work for three primary purposes: (a) to help a land owner or manager choose suitable conservation practices from among alternatives, (b) to make broad-scale erosion surveys in order to understand the scope of the problem over a region and to track changes in erosion over time, and (c) to regulate activities on the land for purposes of conservation compliance.
In selecting or designing an erosion model, a decision must be made as to whether the model is to be used for onsite or offsite concerns, or both. On-site concerns are generally associated with degradation or thinning of the soil profile in the field, which may become a problem 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.