Terracing as a ‘Best management practice'
for controlling Erosion and Protecting water Quality
Rolland Z. Wheaton and Edwin J. Monke, Agricultural Engineering Department, Purdue University
Indiana farmers have long recognized the importance of controlling soil erosion and the benefits of applying conservation measures. For years, the basic motivation behind their conservation efforts was “to maintain the soils resource in a highly productive state.”
Although that’s reason enough to use sound land management practices, today farmers have an additional ‘motive’—compliance with the national Clean Water Act (Public Law 92-500), which seeks to eliminate the pollution of streams, rivers and lakes by any source, including runoff from agricultural land.
The law specifies that control of nonpoint-source pollution is to be accomplished primarily by application of ‘best management practices.’ BMPs are those conservation measures and/or land management techniques deemed most effective in preventing pollution by runoff or seepage from a given field or land area into watercourses.
One BMP applicable to some of the cropland of Indiana is terracing. This publication seeks to answer the commonly-asked questions about terraces, including: what are the different types?, how do they work?, where are they most effective (or ineffective)?, what does it take to construct and maintain them?, and where does one get help with design, installation and financing? From the information presented, hopefully farmers can better assess the potential value of this BMP to their particular situations.
What is terracing?
Terracing is a soil conservation practice applied to prevent rainfall runoff on sloping land from accumulating and causing serious erosion. Terraces consist of ridges and channels constructed across-the-slope.
What types of terraces are there, and how do they differ?
To some people, the word ‘terrace’ brings to mind bench terraces that are used in various mountainous regions of the world. More often, one pictures contour terraces, with their point rows and grassed waterway outlets, that follow the lay-of-the-land (Figure 1).
Today, however, the type most common on agricultural land in the U.S. are parallel terraces, so named because they are constructed parallel to each other and, where possible, to the direction of field operations (Figure 2). Parallel terraces eliminate the production losses associated with point rows and minimize interference to farming operations when spaced at multiple widths of planting and harvesting equipment.
Some terraces are constructed with steep backslopes that are kept in grass. Most, however, are broad-based having gently sloped ridges that are cultivated as a part of the field.
In semi-arid regions (e.g., western Kansas and Nebraska), contour terraces may have level channels to collect and hold the water until it seeps into the soil. In humid regions like Indiana, most terraces will have graded channels that convey the water to an outlet.
Terraces that are constructed in parallel and discharging runoff through subsurface drains are known as parallel tile outlet terraces. With PTO terraces, water that accumulates behind a terrace ridge is discharged through a surface inlet into a subsurface drain. The surface inlet, called a riser, has a restricted section to control the discharge rate, causing some of the runoff to be temporarily stored (Figure 3). This storage period is long enough for sediment to settle out of the water, but not so long as to damage the crop.
COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA