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Design Considerations 

There are three WRSIS demonstration sites now in operation that are located in the northwest Ohio portion of the Maumee River Basin, one each in Defiance, Fulton, and Van Wert Counties (fig. 2). All have been in operation long enough to experience seven to eight complete growing seasons. Key advantages for the Fulton and Van Wert County locations were that they already had some of the needed WRSIS infrastructure installed, including storage reservoirs and a functioning subsurface drainage system.

Map showing WRSIS test site locations.

In fields where subirrigation is planned, since the water table is to be maintained at a substantially higher level than with conventional subsurface drainage, it is often necessary to design the drain spacing using a high drainage coefficient of 38 to 51 mm (1.5 to 2 in.) per day in order to remove water from the soil quickly enough during heavy or prolonged rainfall events. The consequence of using a high drainage coefficient for design purposes is a smaller spacing between drain lines for subirrigated fields, typically 33% to 50% less than what is used in fields having only subsurface drainage. In addition to allowing faster water removal while in drainage mode, a smaller spacing is also important in providing a more uniform water distribution in the soil during subirrigation. For reasons just discussed, new drain lines at both the Fulton and Van Wert County sites were placed between the old ones already present and then integrated into the pre-existing subsurface drainage system. Control plot(s) having subsurface pipe for drainage only were included at each site for comparison with crop yields obtained through subirrigation.

            Subirrigation requirements were established through model simulations with the computer program, DRAINMOD. From this, the size of the storage reservoir at the Defiance County site was determined based on the irrigation water needed for crops in eight out of every ten years. The existing reservoirs at the other two locations did not meet optimal storage requirements, however, other water sources were available which could be used for subirrigation, including a ground water well at the Van Wert County site and a local stream at the Fulton County site.

            The wetland at the Van Wert County site was constructed initially to hold the 2 year, 24 hour storm event runoff and subsurface drainage from all 20 ha (50 acres) of the encompassing watershed. In northwest Ohio, a storm event of this magnitude provides approximately 66 mm (2.6 in.) of rainfall. During summer of 2003, the wetland at the Van Wert County site was re-engineering into a single wetland/reservoir complex to provide greater water storage capacity. The designed wetland storage capacities at the other two sites were somewhat less than that needed to totally capture a 2 year, 24 hour magnitude rainfall event. Should the need arise, all three locations were built with the capability to allow direct offsite release of water from either the wetland or storage reservoir. Although similar in concept, design details differ among the three WRSIS locations.

The problem and solution in regard to water table management of low elevation areas within an agricultural field.

Although similar in concept, design details differ among the three WRSIS locations. Soil conditions, topography, in-place infrastructure, etc., will all impact the final site design. Specific characteristics for each site are provided in the following sub-sections.

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