A 1985 U.S. Department of Agriculture (USDA) Economic Research Service economic survey showed that the states in the Midwest United States (Illinois, Indiana, Iowa, Ohio, Minnesota, Michigan, Missouri, and Wisconsin) had by that year approximately 12.5 million hectares that contained subsurface drainage systems (USDA Economic Research Service, 1987). Cropland constituted by far the large majority of this acreage. The same economic survey estimated the 1985 on-farm replacement cost for these cropland subsurface drainage systems to be $18 billion. Today, this subsurface drainage infrastructure would be worth $35 billion based on a 1986 to 2008 average yearly consumer price index inflation rate of 3 percent, and this total does not include the extensive amount of drainage pipe that has been installed in the past 20 years. The magnitude of the area involved along with infrastructure costs indicate how crucial subsurface drainage is to the Midwest U.S. farm economy, without which, excess soil water could not be removed, in turn making current levels of crop production impossible to achieve.

Figure 29.1 is a schematic illustrating drainage pipe placement within the soil profile typical of agricultural fields in Ohio. Prior to the 1960s, agricultural drainage pipe was constructed primarily of clay tile and was then superseded by corrugated plastic tubing (CPT), which is today the material still used in drainage pipe fabrication (Schwab et al., 1981). The drainage pipe diameter is most commonly 10 cm. The pipe is emplaced at the bottom of a trench, which is then backfilled. The trench is typically 0.3 to 0.5 m wide with its bottom depth ranging between 0.5 and 1 m. Modern drain line installation equipment often produces a trench that is wider at the bottom than the top. The water table can be either above or below the drainage pipe depending on the amount of recent rainfall and the mode of operation for the subsurface drainage system (uncontrolled drainage, controlled drainage, or subirrigation). The surface tilled zone is commonly less than 0.3 m, assuming a no-till management strategy has not been adopted.

Increasing the efficiency of soil water removal on farmland that already contains a functioning subsurface drainage system often requires reducing the average spacing distance between drain lines. This is typically accomplished by installing new drain lines between the older ones. By keeping the older drain lines intact, less new drainage pipe is needed, thereby substantially reducing costs to farmers. However, before this approach can be attempted, the older drain lines need to be located.

Finding buried drainage pipe is not an easy task, especially for drain lines installed more than a generation ago. Often, records have been lost, and the only outward appearance of the subsurface drainage system is a single pipe outlet extending into a water conveyance channel. From this, little can be deduced about the network pattern used in drainage pipe placement. Without records that show precise locations, finding a drain line with heavy trenching equipment causes pipe damage requiring costly repairs, and the alternative f|GURE 29.1 The dramage p^e posMon withm of using a handheld tile probe rod is extremely the soil profile-tedious at best. Zucker and Brown (1998) indicate that satellite or airborne remote sensing technologies show some promise, but these methods are only applicable during certain times of the year and under limited site conditions.

Consequently, there is definitely a need to find better ways of effectively and efficiently locating buried agricultural drainage pipe. Ground-penetrating radar (GPR) may provide the solution to this problem. Promising results have been achieved using GPR to find buried plastic and metal utility pipelines (Hayakawa and Kawanaka, 1998; LaFaleche et al., 1991; Wensink et al., 1991). However, to date, investigation of GPR drainage pipe detection capabilities has been limited, especially for the Midwest U.S. Chow and Rees (1989) demonstrated the use of GPR to locate subsurface agricultural drainage pipes in the Maritime Provinces of Canada, and Boniak et al. (2002) showed that GPR could be employed to find drainage pipe beneath golf course greens. Allred et al. (2004, 2005) evaluated GPR drainage pipe detection within agricultural field settings typical of the Midwest U.S. The results obtained by Allred et al. (2004, 2005) regarding impacts of antenna frequency, soil hydologic conditions, pipe construction material, and drain line orientation on GPR drainage pipe detection are summarized within this case history along with an assessment of overall GPR drainage pipe detection effectiveness based on data collected at fourteen test plots throughout Ohio.

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