1. Key Lab. for Agriculture Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education Northwest A&F Univ. Yangling Shaanxi Province 712100 PR China
2. Soil Physics and Land Management Group Wageningen Univ. & Research PO Box 47, Droevendaalsesteeg 3 Wageningen 6700 AA the Netherlands
3. Hydrology and Quantitative Water Management Group Wageningen Univ. & Research PO Box 47, Droevendaalsesteeg 3a Wageningen 6700 AA the Netherlands