1. Associate Member, ASCE
2. Grad. Student, Dept. of Soils and Irrig., Lab. for Agronomy and the Environment (DGA-CSIC), Servicio de Investigación Agroalimentaria, DGA, P.O. Box 727, E-50080 Zaragoza, Spain. E-mail: nery@eead. csic.es
3. Res., Dept. of Genetics and Plant Production, Lab. for Agronomy and the Environment (DGA-CSIC), Estación experimental de Aula-Dei, CSIC, P.O. Box 202, E-50080 Zaragoza, Spain. E-mail: playan@eead. csic.es