This paper presents the simulation results of soil water of wheat field by SWAP. The average and highest values of soil water
potential decrease as the increase of the amount of
irrigation, and they were much lower than the water potential at which the growth of crop was prohibited. Whether in clay sandy soil formatted by irrigation with Yellow River water, or in desert sandy soil, wheat was not threatened by drought while the soil moisture was kept in 40% of field capacity. Because of the large amount of irrigation and bad water holding capacity,
irrigating water infiltrated downward deep soil layer by 60%~90% in which crop could not uptake the soil water. The infiltrating amount was increased as the increase of irrigating amount. Therefore, in arid desert area, the irrigating standard could be reduced to lower than 6 490 m\+3\5hm\+\{-2\} with surface irrigating method. Due to low soil fertility, the increase of fertilizing amount with organic matter and chemical fertilizers could hardly lower soil water potential, and enhance the use rate of water. The soil water potential changed very significantly in the soil layer at 0~40 cm deep, but it did not change underneath 40 cm of soil layer.