| The sustainable agriculture in Red Soil area of China is restricted severely by the seasonal drought and water shortage. During 1999-2000, the study was performed in the Ecological Experimental Station of Red Soil, Academia Sinica. Peanut or rice were planted in the large-scale weighing Lysimeter, pit plots, and standard runoff plots, where high, moderate, and low soil moisture levels were designed as 3 treatments, assembling with some instruments such as time domain reflectometry, portable photosynthesis system, dew point microvoltmeter, and porometer. Combining the "five waters" transition viewpoint and SPAC theory, the water balance, water transition, and crop productivity were studied. From the results, some scientific basis and practical techniques were provided for using water resource effectively, regulating farmland water rationally, and improving crop productivity.The characteristics of water balance and its components were expounded systematically both in years and in crop growing seasons. The precipitation was concentrated from March to July, earlier than the evapotranspiration peak-day, which was from July to September. The farmland downward water drainage was produced mainly during April to June, formed as deep soil layer water storage, which was difficult for crop root to absorb. And the hazard to crop, caused by the seasonal water shortage during August to October, was then intensified by it. The peanut physiological water-costing process presented as an unsymmetrical parabola, i.e., water amount consumed by peanut in seedling and pod-filling stages was less than that of in blooming and pod-setting stages. More than 400 mm water was demanded in the peanut full growth stage. More than 580 mm water was required by early rice after transplanting, and the total precipitation during the period was sufficient for the demand. The intensive water-saving pattern "drain only without irrigation" in early rice was then proposed. Later rice needed more than 470 mm water after transplanting, and the rainfall was far below the demand, so irrigation was crucial.It was approved that crop water use efficiency (WUE) was increased and the preferable yield was obtained in the moderate soil moisture. Under lasting low soil moisture, the peanut plant was short and small, tillers were reduced, leaf area and leaf area index were decreased, leaves were more easy to fall off and premature senility was showed in the late growth stage, but fine roots were increased. Under moderate soil water content, the peanut root/shoot ratio and kernel/pod ratio were increased, and high yieldcould be obtained if irrigation was performed during the water critical period. But the peanut yield was decreased by the large size in low soil moisture. The rice yield and WUE both in leaf and yield level were increased in moderate soil water content, for spikelets per panicle and filled spikelets were increased in the early rice, and spikelets per panicle were increased in the later rice.It was proved that the water flow resistances in SPAC existed largely as node resistances, and the resistance between soil and root was the major factor that determines the water status in crop. The water potentials in peanut plant were higher than that of in rice. The water potential differences were existed mainly in the interface of soil-root, stem-leaf, and leaf-air. And the resistances in plant were presented mostly as soil-root and stem-leaf node resistances. The water flow resistance in leaf-atmosphere interface account for more than 90 percent of the total resistance; and the resistance in plant was 20 to 50 times larger than soil. The water flow drive force was augmented with the increase of SPAC water flux, which was induced by the radiation.The problem of simulating the feedback and coupling relation between crop and soil water movement was solved in the study. After analyzing the disadvantage of some soil water movement models, a computer model was established to simulate soil water movement and crop growth. The complicated interrelatio...
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