Meta-analysis as a statistical tool for evaluating the hydrologic effects of water table management

Controlled drainage is a water table management practice used to reduce drainage volumes and environmental impact of subsurface drainage. Meta-analysis was conducted with fifty-three controlled drainage volume reduction results selected from twenty papers published between 1979 and 2008 to study the underlining factors influencing controlled drainage responses. The observations showed a wide variation in effectiveness of controlled drainage across different soils, crops and locations: drainage volumes reductions from -8% to 95% have been reported in literature. To investigate the potential causes for this wide variation, we performed a meta-analysis to aggregate the controlled drainage results using the log-response ratio effect size. The results of the meta-analysis showed that the effectiveness of controlled drainage depends on a combination factors: soil texture, crop type, and varies by hardiness zones. A chronologic, cumulative metaanalysis of fifty-three controlled drainage studies demonstrated that controlled drainage is effective and with a mean effect of 47%. A categorical meta-analysis suggested that soil types, crop types and differences in seasonality affect the effectiveness of controlled drainage to reduce drainage volumes.