Impact of El Nino/Southern Oscillation on Low-Flows in South Georgia, USA

Article excerpt

We investigated the impact of El Nino and La Nina climate patterns on the low flows of seven streams in south Georgia. Four stations were located in heavily irrigated areas of southwestern Georgia, whereas three other stations were located in areas of southeastern Georgia with virtually no agriculture. El Nino (La Nina) climate patterns were defined when the 5-month averaged sea surface temperature anomalies recorded by the Japanese Meteorological Association (JMA) are greater than +0.5[degrees]C (lower than 0.5[degrees]C) for at least six consecutive months, including the October to December period. With the standard JMA approach, an episode lasts from October to the following September. We also investigated the impacts of a modification to this classical method. In the modified method, an ENSO phase only lasts as long as the temperature conditions ([+ or -] 0.5[degrees]C) are met. For the three southeastern stations, low flows recorded during La Nina (El Nino) episodes were usually lower (larger) than the low flows recorded during Neutral episodes from November to April, with differences significant at a level of at least 90 percent from February to April (January to March) only. The four southwestern stations responded similarly, but the significance of the differences was usually lower than for the southeastern stations. The modified classification method tended to give more significant results than the standard method.

Investigamos los efectos de los patrones climaticos de El Nino y La Nina en los niveles bajos de siete rios en el sur de Georgia. Se localizaron cuatro estaciones en zonas altamente irrigadas del suroeste de Georgia, mientras que se localizaron otras tres estaciones en zonas del sureste de Georgia donde la agricultura es practicamente inexistente. Los patrones climaticos de El Nino (La Nina) fueron definidos cuando las anomalias en temperatura de la superficie del mar promediadas en un espacio de 5 meses, registrados por la Asociacion Meteorologica de Japon (JMA), son superiores a 0.5[grados]C (inferior a -0.5[degrees]C) durante al menos seis meses consecutivos, incluyendo el periodo de octubre a diciembre. Con el enfoque estandar de JMA, un episodio dura desde octubre hasta el siguiente mes de septiembre. Tambien investigamos los impactos de una modificacion a este metodo clasico. En el metodo modificado, una fase de ENSO solo dura mientras las condiciones de temperatura ([+ or ] 0.5[grados]C) se cumplen. Para las tres estaciones del sureste, los bajos niveles registrados durante los episodios de La Nina (El Nino) fueron generalmente mas bajos (mas extensos) que los bajos niveles registrados durante los episodios neutrales desde noviembre hasta abril, con diferencias significativas a un nivel de al menos el 90 por ciento solamente desde febrero hasta abril (enero a marzo). Las cuatro estaciones del suroeste respondieron de manera similar, pero el grado de diferencia fue generalmente inferior a la de las estaciones del sureste. El metodo de clasificacion modificado demostro resultados rods significativos que el metodo estandar.

KEY WORDS: ENSO, stream flows, low flows, drought, Georgia

INTRODUCTION

Southern Georgia, USA is primarily an agricultural region, which since the middle of the 1970s relies increasingly on irrigation. For example, in the lower Flint River Basin (southwestern Georgia), irrigation accounts for up to 90 percent of water consumption during the growing season (April-September). Moreover, southern Georgia streams host a number of mussels and fish species recognized as endangered by the U.S. Fish and Wildlife Service. Long periods of low-flows, as exacerbated by a decrease of precipitation, could be detrimental to the reproduction and survival of those species. Recent droughts prompted the Georgia General Assembly to pass the Flint River Drought Protection Act. When the Director of the Georgia Environmental Protection Division of the Department of Natural Resources declares that a drought exists or is expected in the Lower Flint River Basin on March 1st, a voluntary auction process is initiated, for which users can relinquish with remuneration their irrigation rights to the State for that given growing year. …