1. Associate Member, ASCE
2. Res., Soils and Irrig. Unit, Servicio de Investigación Agroalimentaria, Government of Aragon (DGA), P.O. Box 727, 50080 Zaragoza, Spain. E-mail: jfaci@aragob.es
3. Student, Dept. Genetics and Plant Production, Estación Experimental de Aula Dei, Superior Scientific Research Council (CSIC), P.O. Box 202, 50080 Zaragoza, Spain. E-mail: rsalvadore@yahoo.com
4. Res., Dept. Genetics and Plant Production, Estación Experimental de Aula Dei, Superior Scientific Research Council (CSIC), P.O. Box 202, 50080 Zaragoza, Spain. E-mail: playan@eead.csic.es
5. Res., Inst. of Production Engrg., Fed. Res. Ctr. of Agr., Bundesallee 50, 38116 Braunschweig, Germany. E-mail: sourell@kepler.dv.fal.de