黄河流域のトウモロコシ畑における必要灌漑水深の"動的圃場容水量"に基づく評価

An irrigation experiment was conducted at an experimental cornfield established in the Yellow River basin, Inner Mongolia, China, in order to investigate the potential for a saving of irrigation water in this region. The root zone was divided into two layers; the first layer 40 cm thick and the second layer 60 cm thick, and the irrigation requirement, Wa (mm), was assumed to be expressed asWa=(max[W1FC - W1, 0]+ max[W2FC - W2, 0]) (1+ α)where Wi is the amount of water contained in the ith layer (mm)(i=1,2), WiFC is the "dynamic field capacity" of the layer (mm), and α indicates a leaching part for salinity control and uniform distribution of water in the root zone. The dynamic field capacity WiFC is defined as the value of Wi when a balance of forces is made on an element of the water at the bottom surface of the ith layer, and is evaluated using the two-layer BBH model. This critical point is not static but dynamic in nature and depends on the depth of water table and the potential water extract by plant roots.The mean value of α for the experimental cornfield was evaluated to be 0.75 from the irrigation practiced as usual in 2004. It is apparent that this value was too large to be optimum and over-irrigation occurred, because the groundwater level rose up to within the root zone just after the irrigation and the lower part of the zone was almost saturated for a long time.