DOUBLE CROPPING WHEAT AND CORN IN A SUBHUMID REGION OF CHINA

Double cropping wheat (Triticum aestivum L.) and com (Zea mays L.) is common in sub-humid regions of north China. This study was conducted t o evaluate the system without irrigation. During the 5-year study peri od, annual rainfall averaged 638 mm with 387 mm occurring in July, Aug ust and September. Water stress often occurred from mid-April to the e nd of the wheat growing season in early June, and sometimes extended t o the early part of the com season that begins immediately after wheat harvest and ends in mid-September. Wheat and com grain yields increas ed as a linear function of evapotranspiration. Average grain yield was 5.2 Mg ha-1 for wheat and 5.1 Mg ha-1 for com, with water-use efficie ncies, expressed as grain yield per unit of water consumed, of 1.48 an d 1.94 kg m-3, respectively. Average precipitation values for the 5-ye ar study period closely parallelled long-term averages, and a probabil ity analysis indicates that yields obtained in the study may be repres entative of what can be expected over a long period. Therefore, double cropping wheat and com in this region of China appears highly feasibl e and results in more than 95% of the precipitation being utilized by evapotranspiration during the growing seasons. Soil water content fluc tuates periodically during the year. Phase lags and amplitude decrease with soil depth. One-half of the water utilized by wheat, 175 mm, was furnished by rainfall during the growing season, and the other half w as obtained from soil water stored at the time of seeding in mid-Octob er. In contrast, soil water increased during the com growing season in which rainfall was 384 mm, and evapotranspiration was 263 mm.