How paired is paired? Comparing nitrate concentrations in three iowa drainage districts

Quantifying the effectiveness of perceived best management practices (BMPs) at the field and landscape-scale is difficult, so paired watershed studies are used to detect water quality improvements. We evaluated concentrations of NO-N discharged from three tiled Iowa watersheds during a 4-yr period to assess their suitability for a paired watershed approach. Our objectives were to evaluate similarities in physical characteristics, concentration patterns, and correlation among the three paired sites and perform a minimum detectable change (MDC) analysis on paired site configurations. The study results demonstrate that concentration variability within and between sample sites affected correlation among the paired basins, even though the physical characteristics of the basins are quite similar. Restricting comparisons to the active tile drainage period (March-July) improved correlations. The lack of a suitable correlation will impair the ability to detect changes expected to result from BMP implementation. The MDC for NO-N concentration change detection varied from 6.9 to 12.9% and averaged 8% for the best control-treatment pair. To ensure that conservation resources are being used effectively, implemented BMPs should focus on practices capable of achieving at least this magnitude of change. These practices may include reduced fertilizer applications, adoption of cover crops, and land use change.