1 - Wetland Reservoir Subirrigation System (WRSIS)
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Defiance County WRSIS site schematic
The control plots with conventional drainage treatments are not shown.
Construction of the Defiance County, Ohiosite occurred during June 1995, principally with volunteer contractors and donated material during an Ohio Land Improvement Contractors Association field exhibition. This location contains two 1.4 ha (3.5 acre) subirrigated fields and 8.1 ha (20 acres) of cropland with various conventional drainage treatments. Runoff and subsurface drainage are funneled into a 0.12 ha (0.30 acre) wetland having a storage capacity of 700 m3 (185,000 gal) (fig.3). A 2.4 m (8 ft) wide bench at an elevation coinciding with the permanent pool position was excavated along one side of the Defiance County wetland in spring 1999, providing additional wildlife/vegetation habitat and better water treatment capability. After detention within the wetland, water is routed through an adjacent concrete sump, containing two 0.75 kW (1 hp) submersible pumps, to a 0.16 ha (0.39 acre) reservoir having 2950 m3 (780,000 gal) of storage. Water held in the reservoir is used for subirrigation of corn and soybeans during periods when rainfall alone is not sufficient to meet crop demand. Although both subirrigated fields are down-gradient from the reservoir, a 0.37 kW (0.5 hp) submersible pump located in a concrete sump next to the reservoir is used to enhance flow rate.
Subsurface drain pipes at all three WRSIS sites were installed at a nominal depth of 0.76 to 0.91 m (2.5 to 3 ft) beneath the surface. Half of the 2.8 subirrigated hectares (7 acres) at the Defiance County site contain 10 cm (4 in.) diameter corrugated plastic tubing (CPT) drain line spaced 2.4 m (8 ft) apart, and the other half has 10 cm (4 in.) diameter CPT drains spaced 4.9 m (16 ft) apart (fig. 3). The site is mostly covered by Paulding clay (mesic Typic Haplaquepts) with some Roselms silty clay (mesic Aeric Ochraqualfs) also present. From particle size analysis of samples taken at the surface, percent sand was 0% to 11%, silt ranged from 34% to 50%, and the amount of clay was between 48% and 66%.
Saturated horizontal hydraulic conductivity values measured within the soil profile (0 to 0.8 m [0 to 2.8 ft]) with a velocity permeameter ranged from 7 x 10-6 cm/s (0.01 in./hour) to 2 x 10-5 cm/s (0.03 in./hour). These are dense, very low permeability clayey materials which hinder water transfer from the drain pipe to the soil during subirrigation, in turn making it difficult to maintain the target range of water table depths (25 cm [10 in.] at the drain and 46 - 51 cm [18 - 20 in.] midway between drains). In comparison to the 4.9 m (16 ft) spacing, the 2.4 m (8 ft) spacing is better adapted for consistently keeping ground water levels within the desired range, but expected crop yield increases may not be enough to offset the cost of having to install twice the amount of drain pipe. Initially, two hydraulic control structures, one for each subirrigated field, were installed to regulate ground water levels. A wet area within the northwest corner of the west subirrigated field required installation of an additional hydraulic control structure in September of 1999. By doing this, the west field is now divided into two separate zones for water table management (fig. 3). Capital costs for WRSIS construction at the Defiance County site totaled $44,700 U.S.
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