Curve numbers for long-term no-till corn and agricultural practices with high watershed infiltration

The Curve Number (CN) method is an engineering and land management tool for estimating surface runoff from rainstorms. We investigated CN under continuous longterm no-till corn (Zea mays L.; watershed WS191) and compared it with other potentially high infiltration agricultural practices using data from three experimental watersheds (average area = 0.74 ha [1.83 ac]) at the North Appalachian Experimental Watershed (NAEW) near Coshocton, Ohio. Practices compared were wheat (Triticum aestivum L.) and first- and second-year meadow as part of a 4-year corn–wheat–meadow–meadow rotation (CWMM), continuous meadow, and continuous no-till corn, mulch-tilled corn, and conventionally tilled corn (i.e., moldboard plow). Double-mass curves of runoff for no-till and permanent meadow documented the significant and immediate reduction in runoff from the no-till management. Runoff was generated at a rate of 4.9 mm y–1 (0.19 in yr–1) during the no-till period compared with permanent pasture at 44 mm y–1 (1.73 in yr–1) (no-till was 11% of the meadow rate). The asymptotic CN (CNi) for the 48-year record of continuous no-till corn practice at WS191 decreased substantially to 66.3 (–24.3 CN units) from the period of conventional corn in rotation on the same site. The Natural Resources Conservation Service handbook value for no-till CN was 16 CN units larger at 82. The larger than expected reduction in CN was probably due the presence of a thriving earthworm population and numerous earthworm-created macropores that enhance infiltration. The CN for the mulchtilled corn culture treatment (74.6) was lower than conventional corn (90.6 largest) by 16 CN units. CN was surprisingly high (77.4) for a watershed in continuous meadow, which was similar to mulch-tilled corn. The wheat years in rotation also had some of the smallest CN values (63.7), suggesting a smaller CN for a practice similar to that used in organic agriculture today. Continuous no-till corn had a slightly larger CN (66.3) than meadow (61.9) in the CWMM rotation, suggesting that simple grass establishment can be as effective as longterm no-till to reduce runoff. CN of approximately 60 was the lowest found. The results are applicable for areas of similar terrain, management practices, geology, soils, subsoil permeability, and climate of the NAEW watersheds. Differences between handbook and measured CN suggest that local values are more representative than handbook values.