## 4 Discussion  
At Site 1, GPR profiles (Figure 8) showed soils to the west of the white dashed line in Figure 7a to have a relatively homogeneous GPR response, while soils to the east of the white dashed line exhibited a far more homogeneous GPR response. Prior research [5] has shown that fine-textured silty/clayey soils tend to have a homogeneous appearance in GPR profiles, while coarse-textured sandy soils tend to have a more heterogeneous, or cluttered, appearance in GPR profiles. This finding could indicate for Site 1 that there were generally fine-textured silty/clayey soils west of the white dashed line in Figure 7a and generally coarse-textured sandy soils east of the white dashed line. Textural analysis of soil cores obtained across Site 1 will be required in order to confirm this supposition. When GPR is integrated with RTK/GNSS, spiral or serpentine GPR transects, or spiral/serpentine segments of GPR transects, can be employed where needed to determine drain line directional trends. Figure 9: Interpreted drain line patterns (blue lines) overlaid on GPR measurement transects (yellow lines) and potential drainage pipe locations (red dots): (**a**) Site 1; (**b**) Site 2; and (**c**) Site 3. A thin black line within a thicker blue line indicates a main collector pipe. Initial linear GPR transects at Site 1 clearly indicated, just by themselves, based on DVL potential pipe locations flagged in the field, the drain line directional trends; therefore, spiral/serpentine GPR transects or spiral/serpentine segments of GPR transects were not needed to establish drain line patterns at this site. While not required at Site 1 to determine drain line patterns, spiral or serpentine GPR transects (or spiral/serpentine segments of a GPR transects) did provide valuable insight on drain line directional trends at Sites 2 and 3. Figure 10 provides clear examples from Sites 2 and 3 as to how spiral/serpentine GPR transect segments were employed to determine the direction of an agricultural field drain line. It is readily apparent that the drain line patterns at Sites 2 and 3 would have been almost impossible to discern without the integration of GPR with RTK/GNSS and the use of spiral/serpentine GPR transects or spiral/serpentine segments of GPR transects. Field data collection using GPR integrated with RTK/GNSS to map subsurface drainage was quite efficient, requiring only four to six hours at each of the three sites. This study shows that a dense grid of GPR transects is not needed to determine drainage pipe patterns. Actually, all that is required in a particular agricultural field to find drain lines and determine their directional trends are a few select GPR transects (usually with incorporated spiral/serpentine segments). The efficiency of GPR-RTK/GNSS for subsurface drainage mapping could be improved further over what was obtained in this investigation by using an all-terrain vehicle (ATV) pulling a sled to mount all the GPR and RTK/GNSS equipment instead of mounting the equipment on the push cart shown in Figure 3a,c. The GPR-RTK/GNSS drainage pipe mapping approach described is applicable for mapping subsurface drainage systems in agricultural fields up to tens of hectares in size. With suitable soil/hydroologic conditions and widespread adoption of this approach over large areas, it might be possible to gain insight on drainage practice intensity at small watershed scales. As indicated, the types of soils present can be important. Soils having very high clay contents oftentimes, but not  
Figure 10: Spiral/serpentine segments of a GPR transect can be employed to determine drain line presence and direction: (**a**) spiral transect segment example from Site 2 (main collector pipe); (**b**) serpentine transect segment example from Site 3. Drainage pipe reflection hyperbolas interpreted with EKKO Project 5 (red dots on GPR profile insets) were mapped (red dots) along with transect segments paths (yellow lines with directional arrowheads).