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4. Surface drainage
Surface drainage is a cheaper option to prevent surface water ponding and controlling runoff without causing erosion. In this system, excess water from the soil surface flows over the naturally or artificially sloping ground toward shallow ditches and grassed waterways.
It is best to install surface drainage prior to installing a subsurface drainage system. However, surface drainage can be installed after subsurface drainage installation, but caution is advised to keep the minimum 2-ft cover above the drain pipe during land grading. Two types of surface drainage systems are described below: Parallel ditch and targeted.
4.1. Parallel ditch surface drainage
One type of surface drainage is called a parallel ditch system that requires parallel shallow ditches or grassed waterways to remove surface water. This system is generally suitable in relatively flat fields with less than 0.5% slope. In this system, construction equipment is needed to make the shallow graded ditches or grassed waterways.
The parallel ditch system has two layouts. For the first layout, perform land grading to direct surface water into shallow ditches or grassed waterways spaced wide apart (Figure 2). For the second layout, perform land smoothing to direct water in the downslope direction, when there is an existing continuous slope in the field. The second layout is the cheaper option because of using the field’s existing continuous slope. In the parallel ditch system, you need enough topsoil for land-grading or land-smoothing. In both layouts, the shallow ditches empty into a drainage ditch.
For both layouts, the ditches run perpendicular to the slope, and the spacing between the ditches can be as large as 1500 ft, depending on the natural or artificial land grading (Philips 1963). The ditches do not need to be equally spaced across the field. It is best if the ditch spacing matches a certain number of passes by the farm equipment. On heavy clay soil, the maximum ditch length should be about 800 ft with a minimum grade of 0.1%. The ditches should have a minimum height of 8 inches and allow passage of farm equipment. Planting, harvesting, and field operations should be performed perpendicular to the shallow ditches.
4.2. Targeted surface drainage
One type of surface drainage is called a targeted system (Figure 3). In this system, surface drainage targets the wet spots or depressions. This is suitable if the farmer does not want to pay for land-grading or land-smoothing. In this system, shallow ditches meander from one wet spot to another, so water moves away from the wet spots and into the drainage ditch. Minor depressions are filled with soil.
