Author(s): Kozue Yuge, Mitsumasa Anan, Toru Sakata
Keywords: Shortage of irrigation water; Groundwater storage; Agricultural water demand; Evapotranspiration; Salty wind damage;
Abstract: In subtropical islands of southwest of Japan, agriculture is main industry taking advantage of warm climate. On the other hand, there is a serious issue of shortage of water resource in these islands because there are very few natural rivers. As agricultural water depends on the groundwater in these islands and they have suffered from chronic shortage of the agricultural water, subsurface dams have been constructed in some islands. The agricultural productivity increased because of agricultural water supply from the subsurface dam and agricultural water demand is recently increasing because profitable crops have been introduced. In addition, agricultural water demand for prevention of salty wind damage has been grown. It is important to quantify the available groundwater resource and optimize the groundwater usage for agriculture.
The aim of this study is to evaluate the groundwater condition and agricultural water demand in the irrigated farming area of a subtropical island with the subsurface dam. The study site was Kikai Island (N28˚ 19', E129 ˚ 56') which is located in southwest of Japan. Main industry of Kikai Island is agriculture and staple crop is sugar cane. As there is no natural river in this island, agricultural water depends exclusively on the groundwater. A subsurface dam was constructed to enhance the groundwater usage in 2003 and agricultural productivity have increased.
The field investigation was performed to clarify geology and groundwater level in the study site. Agricultural water management was investigated and water consumption in agricultural fields was evaluated. Compared with the water supply and water consumption in agricultural fields, we evaluated whether the groundwater amount was adequate for agricultural water demand in the study site. A scenario analysis was performed, assuming expected growth of agricultural productivity in future. The results revealed that the additional water resource should be developed or it is necessary to enhance the agricultural water management for increase of agricultural yield in the study site.