Abstract: Field observations for ten years were carried out in a catchment basin-basic ecological unit to investigate the natures of water equilibrium. This unit had been selected as the representative ecosystem of subtropical hilly land of South China. Obtained data include the annual variation and distribution of the rainfalls, evaporation and run off, and irrigation requirement in the ecosystem. The results showed that the amount of rainfalls was abundant and able to maintain water equilibrium in the ecosystem. However, the distribution of rainfall varied largely in a year. During the rain season(late March to June),the amount of rainfalls was much greater than evaporation(including plant consumption), which resulted in water logging.In contrast,during the period from late July to late September, the amount of rainfalls was smaller than half of evaporatioin. Over this period, water deficiency dominated the water ecology of the system and resulted in dry environment. With such water environment, crop productions in the subtropical hilly regions suffer seriously from both impacts of water logging and drought.Based on the water characteristics in the subtropical hilly ecosystems, the main effort for arising crop production is to reduce harms from both of water logging and drought. The countermeasures are to strenghen the construction of farmland(engineering and agronomical method) and to improve water management, in order to control waterlogging and best use water resource.