Hydrogeology Journal (1998) 6 : 77–93 Q Springer-Verlag Received, June 1997 Revised, December 1997 Accepted, December 1997 Isaac J. Winograd (Y) U.S. Geological Survey, 432 National Center, Reston, VA 20192, USA Fax: c1-703-648-5274 e-mail: ijwinogr@usgs.gov Alan C. Riggs U.S. Geological Survey, MS 413, Denver Federal Center, Denver, CO 80225, USA Tyler B. Coplen U.S. Geological Survey, 431 National Center, Reston, VA 20192, USA The relative contributions of summer and cool-season precipitation to groundwater recharge, Spring Mountains, Nevada, USA Isaac J. Winograd 7 Alan C. Riggs Tyler B. Coplen Abstract A comparison of the stable-isotope signa- tures of spring waters, snow, snowmelt, summer (July thru September) rain, and cool season (October thru June) rain indicates that the high-intensity, short-dura- tion summer convective storms, which contribute ap- proximately a third of the annual precipitation to the Spring Mountains, provide only a small fraction (per- haps 10%) of the recharge to this major upland in southern Nevada, USA. Late spring snowmelt is the principal means of recharging the fractured Paleozoic- age carbonate rocks comprising the central and highest portion of the Spring Mountains. Daily discharge meas- urements at Peak Spring Canyon Creek during the pe- riod 1978–94 show that snowpacks were greatly en- hanced during El Nin ˜ o events. Résumé La comparaison des signatures isotopiques stables des eaux de sources, de neige, de fonte de neige, des pluies d’été (juillet à septembre) et de saison froide (octobre à juin) montre que les précipitations convecti- ves d’été de forte intensité et de courte durée, appor- tant un tiers des précipitations annuelles reçues par les Monts Spring, ne participent que pour une faible part (10%) à la recharge de cette importante zone d’altitude du sud du Nevada (États-Unis). La fonte tardive de la neige au printemps constitue l’essentiel de la recharge des roches carbonatées fracturées d’âge paléozoïque formant la partie centrale et la plus haute des Monts Spring. Les données journalières de débit sur la rivière du canyon de Peak Spring, entre 1978 et 1994, mon- trent que les hauteurs de neige ont été plus élevées pendant les événements El Nin ˜ o. Resumen La comparación entre las marcas isotópicas de aguas de manantiales, nieve, deshielo, lluvias de ver- ano (julio a septiembre) y resto de lluvias (octubre a junio) indican que las tormentas de verano, de corta duración y gran intensidad, las cuales suponen alrede- dor de un tercio de la precipitación total anual en las Spring Mountains, proporcionan sólo una fracción pe- quen ˜ a (alrededor del 10%) de la recarga en esta zona al sur de Nevada (EE.UU.). El deshielo de finales de la primavera es la principal fuente de recarga de las rocas carbonatadas fracturadas de edad Paleozoica que for- man las partes central y superior de las Spring Moun- tains. Las medidas de descarga diarias en el Desfiladero de Peak Spring Canyon durante 1978–94 muestran que los espesores de nieve aumentaron coincidiendo con los fenómenos de El Nin ˜ o. Key words USA 7 stable isotopes 7 groundwater recharge/water budget 7 hydrochemistry Introduction In a pioneering paper, Eugene Simpson, David Thorud, and Irving Friedman (1972) were the first to demon- strate that the isotopic signatures of winter and summer precipitation in southern Arizona (USA) were differ- ent, and that the differences can be used to determine the seasonality of groundwater recharge. They ob- served that winter precipitation from Pacific Ocean frontal systems was approximately 20 per mill depleted in deuterium compared with summer convective preci- pitation, derived from the Gulf of Mexico. (For ex- pressing isotopic results, we use “per mill” herein, as recommended by ISO 1992.) A comparison of these precipitation data with the deuterium content of groundwater led Simpson et al. (1972) to conclude that winter precipitation is the dominant source of recharge to the Tucson groundwater basin. The innovative study by Simpson et al. (1972) prompted the work described herein for the Spring Mountains in southern Nevada (USA).