Field Evaluation of a Subsurface Irrigation Drainage System in North Central South Dakota

Combination subsurface irrigation/drainage systems have the capability to remove excess water during planting and fall harvesting seasons and provide water to the root zone during summer growing seasons. The dual-purpose systems have been used to control the water table depth. in humid regions where drainage is the primary objective and irrigation is viewed as being of secondary importance. However, the performance of such systems in a semi-arid environment. where subirrigation may be the primary source of water during the growing season has not been investigated. A combination subsurface irrigation/drainage system installed at the James Valley Research and Extension Center near Redfield, SD provided the basis for this research. The system was installed with 9 and 18 m (30 and 60 ft) drain line spacings and operated using a static water table management practice and a fluctuating water table management practice. Water table elevations and corn grain yields were recorded during the 1986 and 1987 growing seasons. The water management simulation mode 1, DRAINMOD, which was developed to simulate the performance of subsurface irrigation/drainage systems in a humid environment, was successfully modified for application in a semi-arid environment. Model simulations were conducted for seven plots during 1986 and 1987. Standard error and average deviation were used to describe the variability between observed and model estimated water table depths. Standard errors ranged from 9.7 to 22.6 cm (4.0 to 8.9 in) and average deviations ranged from 7.6 to 18.5 cm (3.0 to 7.3 in). Model simulations represented 14 site-years of data with a standard error of 15.6 cm (6.1 in) and an average deviation of 12.8 cm (5.0 in) resulting from 450 paired observations. Based on the results of this study it was concluded that DRAINMOD can be used to evaluate the effect of subirrigation/drainage system design on water table depths in an irrigation environment. A statistical analysis of corn grain yields for the 9-m (30 ft) drain linespacing indicated no difference between the static and the fluctuating water table management practices for the two years considered. An average 0.6 t/ha (10 bu/ac) yield depression was observed from 1986 to 1987 and a 1.2 t/ha (20 bu/ac) yield depression was observed at the midway position in 1987. Additionally, a 2.8 t/ha (45 bu/ac) yield depression from 1986 to 1987 was observed for the 18-m (60 ft) drain line spacing. Yield depressions were determined to be the result of increased climatic demand and lower than normal precipitation in 1987