Climate Variability and Drain Spacing In fl uence on Drainage Water Management System Opera on

43 A is an important water management practice in the midwestern United States to remove excess water from poorly drained areas and to provide traffi cable conditions for agricultural production. About 30% of the total land area and 50% of the crop land in Indiana is subsurface drained (Economic Research Service, 1987). Th ere are growing concerns about the potential negative environmental impacts of subsurface drainage on the water quality of receiving water bodies and the hydrology of watersheds. Subsurface drains act as conduits for the movement of NO3 and other nutrients and rapidly deliver them to surface water. Among several ways to reduce nonpointsource pollution from subsurface drains, DWM, also known as controlled drainage, has been shown to reduce drainage outfl ow and NO3 loading during certain times of the year (Evans et al., 1995; Lalonde et al., 1996; Fausey, 2004). In addition, DWM may increase crop yields in some years due to increased water conservation (Wesström and Messing, 2007). A DWM system consists of installing a water table control structure at the outlet of the subsurface drainage system, with which the outlet elevation can be managed at diff erent times of the year (Frankenberger et al., 2006). In the North American Corn Belt, annual drain fl ows vary widely, ranging between 0 and 40% of precipitation (Kladivko et al., 2001; Ruark et al., 2008). The drain flow volume is lower in drier years and also tends to be a lower percentage of total precipitation in these years (Kladivko et al., 2004). Th e horizontal spacing between parallel drains exerts a fundamental control on the drainage volume. Kladivko et al. (2004) found in a fi eld study based in southeast Indiana that both drain fl ow and NO3–N losses are greater per unit area for narrower drain spacings. Similarly, annual drain fl ow increased from 12 to 15 to 21% of annual precipitation as the drain spacing decreased from 20 to 10 to 5 m. Wiskow and van der Ploeg (2003) demonstrated with a two-dimensional drainage model that drain discharge is inversely and nonlinearly related to drain spacing across a range of spacings from 5 to 50 m. Nitrate concentrations apparently do not vary signifi cantly with drain spacing (Kladivko et al., 2004), so it would seem that the greatest potential for drainage reduction, and therefore NO3 load reduction, through DWM is on sites with narrower drain spacings. Several fi eld and modeling studies have demonstrated the potential of DWM for reductions in drain fl ow and thereby the NO3 load (Kalita and Kanwar, 1993; Drury et al., 1996; Lalonde Climate Variability and Drain Spacing Infl uence on Drainage Water Management System Opera on