Soil Erosion and
Sedimentation
Mark A. Nearing, L. D. Norton, and Xunchang Zhang
CONTENTS
2.1 Introduction
2.1.1 Terminology
2.1.2 Models
2.2 Soil Erosion Processes
2.2.1 Conceptualization of Rill and Interrill Erosion Processes
2.2.2 Rill Erosion
2.2.3 Interrill Erosion
2.2.4 Sediment Transport
2.2.5 Eroded Sediment Size Fractions and Sediment Enrichment
2.3 Soil Erosion Models
2.3.1 Early Attempts to Predict Erosion by Water
2.3.2 The Universal Soil Loss Equation (USLE)
2.3.3 The Sediment Continuity Equation
2.3.4 Forms of the Sediment Continuity Equation
2.3.5 The Sediment Feedback Relationship for Rill Detachment
2.3.6 Detachment of Soil in Rills
2.3.7 Modeling Interrill Erosion
2.3.8 Modeling Sediment Transport
2.3.9 Modeling Sediment Deposition
2.3.10 Modeling Eroded Sediment-Size
Fractions and Sediment Enrichment
2.4 Cropping and Management Effects on Erosion
2.4.1 Effects of Surface Cover on Rill Erosion
2.4.2 Effects of Soil Consolidation and Tillage on Rill Erosion
2.4.3 Buried Residue Effects on Rill Erosion
2.4.4 Canopy and Ground Cover Influences on Interrill Detachment
References
2
© 2001 by CRC Press LLC

2.1 INTRODUCTION
Soil erosion includes the processes of detachment of soil particles from the soil mass
and the subsequent transport and deposition of those sediment particles on land sur-
faces. Erosion is the source of 99% of the total suspended solid loads in waterways
in the United States1 and undoubtedly around the world. Somewhat over half of the
approximately 5 billion tons of soil eroded every year in the United States reaches
small streams. This sediment has a tremendous societal cost associated with it in
terms of stream degradation, disturbance to wildlife habitat, and direct costs for
dredging, levees, and reservoir storage losses. Sediment is also an important vehicle
for the transport of soil-bound chemical contaminants from nonpoint source areas to
waterways. According to the USDA,1 soil erosion is the source of 80% of the total
phosphorus and 73% of the total Kjeldahl nitrogen in the waterways of the U.S.
Sediment also carries agricultural pesticides. Solutions to nonpoint source pollution
problems invariably must address the problem of erosion and sediment control. The
purpose of this chapter is to discuss the basic processes of soil erosion as it occurs in
upland areas. Most of the discussion is focused on rill and interrill erosion. Erosion
modeling concepts are presented as a vehicle for discussing our current understand-
ing of soil erosion by water, and some process-based soil erosion models are dis-
cussed and contrasted in some detail.
2.1.1 TERMINOLOGY
It is useful here to define some basic terms commonly used in formulating concepts
relating to soil erosion. The term soil detachment implies a process description: the
removal of one or many soil particles as a function of some driving force (erosivity)
such as raindrop impact or shear stresses of flowing water or wind. For purposes of
clarity we distinguish between the terms soil and sediment. Soil is considered, for
modeling purposes, to be material that is in place at the beginning of an erosion event.
If the soil material is detached during an event, it is considered to be sediment. The
terms sediment transport and deposition also imply process descriptions. Transport
of sediment may be in terms of transport downslope by small-channel flow or it may
refer to movement of soil particles across interrill areas via very shallow sheet flow
or raindrop splash mechanisms.
The exact meaning of the term deposition has received considerable discussion
in erosion literature. In the framework of an empirical erosion model, it is clear that
deposition refers to the time-averaged amount of sediment (detached soil) that does
not leave the boundaries of the area of interest. We refer to this as total deposition.
In process-based models, the use of the term is dependent on how the process of
deposition is represented in the source/sink term of the continuity equation and is
related to the concept of transport capacity. In certain models, the deposition term
represents a net movement of sediment to the bed from the flow, whereas, in other
models, deposition is considered to be an instantaneous and continuous process that
occurs at all points on the hillslope, including those portions that experience a net
flux of sediment to the flow from the bed. This process will be discussed in more
detail below.
© 2001 by CRC Press LLC