Effects of multiyear precipitation variations on watershed runoff and sediment yield

Astudy was conducted on the Fort Cobb reservoir watershed (787 km2 [304 mi2]) in central Oklahoma to investigate impacts of persistent multiyear precipitation variations on watershed runoff and sediment yield. Precipitation variations lasting five or more years with predominantly above or below average annual precipitation are commonly found in long-term precipitation records, and are referred to as wet or dry periods. Evaluation of the 1940 to 2005 annual precipitation record for central Oklahoma identified three dry periods and one wet period. Watershed runoff data were available for 1940 to 2005, and sediment yield was estimated based on a suspended sediment-discharge rating curve developed with runoff and suspended sediment measurements made in 2004 to 2005. As such, estimated sediment yield is the yield that would likely have existed if the 2004 to 2005 land use, agronomic practices, and conservation measures were in place for the entire 1940 to 2005 period. The study showed that runoff and estimated sediment yield were sensitive to wet and dry periods. For the case at hand, a mean annual precipitation difference between wet and dry periods of 33% led to a 100% difference in runoff, which in turn led to a 183% difference in estimated sediment …