1. Field Crops and Natural Resources; The Institute of Plant Sciences, Agricultural Research Organization, Gilat Research Center; M.P. Negev 85280 Israel
2. The Levi Eshkol School of Agriculture, The Hebrew Univ. of Jerusalem; Rehovot 7610001 Israel
3. Dep. of Geography and Environmental Development; Ben-Gurion Univ. of the Negev; Beer-Sheva 84105 Israel