Abstract
The specific yield (Sy) is the gain or loss of water associated with a corresponding amount of water table rise or fall. Rain may be intercepted by the canopy, refill the soil pore system, taken up by plant roots or drained. Transpired soil water may be replenished from a shallow water table, so water table loss is indirectly tied to evapotranspiration (ET). The purpose of this study was to use continuous data for the different components of the water cycle to backโcalculate Sy for both wetting and drying conditions. Two time periods were considered for a toeslope site in an Iowa corn (Zea mays L.) field: growing season from 22 June to 10 July 2007, and postโgrowing season 13โ30 October 2007. The field was instrumented with eddy covariance instrumentation for ET estimates, tipping bucket raingage for precipitation, automated wellโdepth recorder for water table depth and water content reflectometers (CS616s) for soil water content. During drying in the growing season, mean Sy was 0ยท19, but drying Sy could not be determined postโgrowing season because of no net upward water movement at this time. During wetting, estimates of Sy were improved when soil water increase was subtracted from total rainfall; then the mean wetting Sy was 0ยท052 versus 0ยท31โ0ยท46 without this correction. Large differences between seasonal and postโgrowing season data reflected the seasonal strong upward gradient created by soil dried from water uptake. Postโgrowing season data was dominated by drainage, but seasonal root water uptake intercepted drainage water. Published in 2010 by John Wiley & Sons, Ltd.