7: Runoff and Water Erosion
- Page ID
- 38738
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Having considered the process of infiltration in the prior chapter, we are now ready to move to the next processes in the soil water balance-the interconnected processes of runoff and water erosion (Fig. 7-1). An audio overview of this chapter is available here (link).
At the outset, it may be helpful to consider the value question: is runoff a good thing or a bad thing? The answer depends on the context. For example, runoff from cropland is typically undesirable because it results in reduced water availability for the crop, erosion of fertile topsoil, and pollution of surface water bodies with sediment, phosphorus, and other contaminants (Fig. 7-2). In contrast, runoff from uncultivated or impervious areas can sometimes be beneficial when captured and stored for later use. Such water harvesting strategies may be key solutions to meeting critical water challenges in developing regions such as sub-Saharan Africa [1] and even in major metropolitan areas like Sydney, Australia, located in arid or semi-arid regions of developed nations [2]. Runoff is also one of the main sources of water to surface water bodies, so any soil or water management practices that influence runoff are likely to influence surface water quantity and quality.
- 7.1: Overview of runoff and water erosion processes
- In either case, runoff does not begin immediately upon the generation of the first excess water. The soil surface is never perfectly smooth, and the soil roughness results in a finite surface storage capacity, which is the volume of excess water per unit area that can be retained on the surface. Runoff can only begin once the surface storage capacity is exceeded.
- 7.2: Runoff and erosion models
- Over the years, scientists and engineers have developed many models for predicting runoff and erosion, and these models range from relatively simple empirical models to highly complex mechanistic models. One of the most widely used empirical models for predicting runoff is the Curve Number Method, which was developed by the USDA Soil Conservation Service (now the Natural Resources Conservation Service, NRCS) in the 1950’s [5].
- 7.3: Problem set
- Click the “Build Channels” button. Wait until the Status message reads “Displaying Channel Map...Success”. Click the button labeled “Build Subcatchments”. Wait until the Status message reads “Fetching Summary...Success”. From the Summary box, record the total area of the selected watershed in acres.
- 7.4: References
- Dile, Y.T., et al., The role of water harvesting to achieve sustainable agricultural intensification and resilience against water related shocks in sub-Saharan Africa. Agriculture, Ecosystems & Environment, 2013. 181: p. 69-79. Rahman, A., J. Keane, and M.A. Imteaz, Rainwater harvesting in Greater Sydney: Water savings, reliability and economic benefits. Resources, Conservation and Recycling, 2012. 61: p. 16-21.