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Strategies of regulated irrigation and fertilization of farmland in semi-arid regions of the Loess Plateau are analyzed in this paper. Theory and approach of niche-fitness are introduced into the research on field management of water use. Niche fitness of crop species describes the closeness degree between optimum niche point and actual resource state. Mathematical model is constructed to evaluate the adaptive extent of crop to different water-fertilizer conditions and the influence on yield. There are four water treatments in the research, in which rich water (RW), moderate water (MW) and low water (LW) indicate the irrigations of 520, 420 and 260 mm, respectively, while natural water (NW) the irrigation of precipitation (416.7mm). Four treatments of fertilizers (i.e. high, moderate, low and non-fertilization) are disposed in RW, MW and LW while three types of plastic film coverings (i.e. 30, 60 days and non-cover) in NW. Field experiments with 15 dispositions repeated three times and there are 45 fields of 4 m x 4 m. Contrast to the result under LW, larger leaf area index (LAI) appear in RW, MW and NW, in which LAI of fertilization is even larger than that under non-fertilization, and the same result goes to the LAI influenced by plastic film covering. The mean values of niche-fitness of spring wheat under LW, NW, MW and RW are 0.4797, 0.5594, 0.5617 and 0.6508, and the grain yields are 1616, 2337, 2479 and 3190 kg ha(-1), respectively. These results show that the value of niche-fitness and the yield of spring wheat will increase with the irrigation and the plastic film covering. The yield is significantly linear-correlated with niche-fitness. In addition, yield increase more significantly in MW and RW than in LW. The values of water use efficiency (WUE) calculated from grain yields of RW, MW, NW and LW are 6.504, 6.196, 5.609 and 6.217 kg ha(-1) mm(-1), in which the maximum WUE (8.660 kg ha(-1) mm(-1)) and the maximum grain yield (4416.5 kg ha(-1)) are supplementing and in accordance with each other under rich water and high fertilizer, which shows rich water and high fertilizer as the best water-fertilizer condition. The above results might be helpful and support the farmland water-fertilizer management in semi-arid regions. (C) 2004 Elsevier B.V. All rights reserved.