ARE THE EL-NINO AND LA-NINA PREDICTORS OF THE IOWA-RIVER SEASONAL FLOW

The association between the El Nino/La Nina and seasonal streamflow fo r the Iowa River is investigated. The seasonal Southern Oscillation in dex (SOI) was ranked and the extreme quartiles for each season were se lected to condition the composite analysis of streamflow. The either c oncurrent or lagged association between anomalous SOI index and stream flow was obtained with a composite analysis that windowed a 3-yr perio d. The existence of statistically significant streamflow responses to El Nino and La Nina has been demonstrated for lags ranging from zero t o five seasons. The long lag of the streamflow-SOI association is attr ibuted to 1) the time to establish global and regional circulation con ducive to excess or deficit rainfall in the Midwest and 2) the inertia of anomalous high (low) soil water. Streamflow responses to El Nino a nd La Nina are out of phase. Above normal streamflow is associated wit h El Nino, whereas dry conditions are associated with La Nina. Sensiti vity analysis of the streamflow-SOI association with respect to the ma gnitude of SOI seasonal anomalies suggests that winter SOI < -0.73 yie lds above normal streamflow from fall (three-season lag) to spring (fi ve-season lag), with 70% consistency. Below-normal streamflow during f all is associated with SOI > 0.63 in preceding spring and summer, with 70% and 75% consistency, respectively. Streamflow predictive models c onditioned on SOI anomalies were developed for lead times up to five s easons.