By: Amy Kaleita-Forbes, Agriculture and Biosystems Engineering, Iowa State University
Soil and water conservation has long been recognized as a critical need for Iowa, the country, and the world. Two of the most important soil and water conservation ‘tools’ are terraces and grass waterways. In the past, challenges were associated with installing and with crop management around these structures. Today, technology is improving the ease of installation and efficiency of management associated with these soil and water con- servation tools. Especially in hilly terrain, terraces and grassed waterways are sometimes the unsung heroes of soil and water conservation. As part of an overall conservation plan, these structures can reduce in-field erosion and sediment (and relatedly phosphorus) loads to receiving water bodies. The effectiveness of these struc- tures has been long recognized, but today, advances in surveying and siting, enabled by Geographic Positioning Systems (GPS), LiDAR-based digital elevation maps (DEMs), and Geographic Information Systems (GIS), are streamlining the layout and construction of these elements.
Terraces for soil erosion are embankments across a hillside that effectively break a long slope into a series of shorter slopes. Those shorter slopes mean that runoff doesn’t flow unimpeded down the hill, building up erosive momentum as it goes. Instead, the runoff comes to a stop (or makes a hard turn) at the terrace, slowing in velocity and depositing eroded sediment it may have picked up along the way.
Grassed waterways are also a way to protect vulnerable parts of a field. In places where runoff begins to con- centrate and moves at high velocities, installation of a grassed waterway protects the soil below from erosion, and provides a safe way for runoff to travel the rest of its path through the field. In many cases, terraces are used in combination with grassed waterways; the terraces are gently sloped to direct runoff to a grassed waterway which safely conveys the excess surface water downhill.
Both are effective at reducing erosion and sediment movement from the field, but the extent to which they do so depends on the characteristics of the field as well as the placement of those structures. Because both are about managing fast-moving runoff, effectiveness drops as the field slope decreases. In hilly terrain, a grassed waterway can reduce sediment delivery by as much as 80 or 90% or more by reducing gulley formation where water is flowing in these channels (eg. Chow and Rees, 1999; Fiener and Auerswald, 2003).
Because effectiveness is so closely linked to landscape and placement, technology that helps document and analyze the topography is useful in planning terraces and grassed waterways. Advances in terrain analysis are improving our ability to predetermine where ter- races and grassed waterways may have the most impact, and to properly site and lay out such systems.
For example, Guo and Maas (2012) describe a technology enabled approach for designing a terrace system. They used elevation data captured from a tractor’s onboard GPS guidance system. In GIS, they developed a routine that designed the terrace layout, and synched the resulting design to Google Earth for easy review by the farmer. The layout was also imported into the tractor’s guidance system so that the farmer could easily stake out the terrace system in the field as the first step in construction. Using readily available DEM data, either from onboard GPS systems or from LiDAR (now available for the whole state of Iowa) decreases the time and cost associated with an onsite survey that is other- wise required for both planning and construction. In another example, Pike et al. (2009) analyzed GPS-derived DEM data to map eroded channels in a field, and confirmed that such mappings could then be used to site grassed waterways.
Even though grassed waterways and most terraces require taking a small percentage of the field out of production, these conservation structures may be more appealing to farmers in cool, wet climates (like Iowa) because they do not result in the slight yield drop sometimes seen in no till systems. There are also ways that technology helps farmers manage these systems. For example, section control for spraying and planting, that is, turning on and off sections of spray nozzles or planter bins individually, allows for more precise management of the edges of where the waterway meets the crop. This enables a more complicated layout that better matches the field topography, and makes the waterway management less inconvenient for farmers.
Zhou, X., M. Al-Kaisi, and M.J. Helmers. 2009. Cost effectiveness of conservation practices in control- ling water erosion in Iowa. Soil and Tillage Re- search 106(1): 71-78.
Fiener, P., and K. Auerswald. 2003. Effectiveness of grassed waterways in reducing runoff and sedi- ment delivery from agricultural watersheds. Jour- nal of Environmental Quality 32: 927–936.
Pike, A.C., T.G. Mueller, A. Shorgendorfer, S.A. Shearer, and A.D. Karathanasis. 2009. Erosion in- dex derived from terrain attributes using logistic regression and neural networks. Agronomy Jour- nal 101(5): 1068-1079.
Guo, W. and S.J. Maas. 2012. Terrace layout design utilizing geographic information system and au- tomated guidance system. Applied Engineering in Agriculture 28(1): 31:38.
Chow, T.L, and H.W. Rees. 1999. Effectiveness of ter- races/grassed waterway systems for soil and water conservation: A field evaluation. Journal of Soil and Water Conservation 54(3): 577-583.