Water balance in the agricultural watershed of Kumamoto groundwater region

Agricultural fields in the middle Shira River basin play an important role as a source of groundwater recharge in the Kumamoto Urban Region of west Japan; however, the water balance between the agricultural water and river water is unclear. This study was conducted to investigate water balance in the fields by measuring the stream flow of agricultural water channels, which draw water from the Shira River.

The measurement of the agricultural irrigation was started on Dec. 2007, and the preliminary annual cycle of water balance was obtained. The total agricultural intake during summer was as high as the flow observed in the middle of the Shira River. Even during winter, the amount of agricultural water taken was about 30% of the water flow in the main river. The daily recharge rate was approximately 166.9 mm/day in the paddy rice fields during summer, and about 30 mm/day in the upland fields during winter. The annual recharge over the targeted area exceeded 15,000 mm and was greater than 27,000 mm in the paddy fields.

In the year of 2009, a spell of low rainfall was occurred from March to June. Especially in the first half of June, the beginning of rainy season, the total rainfall was lower than 30% of normal year. However, the agricultural irrigation in this season was 5-10% lower than that of the same season in 2008, because of the preparation for the planting of paddy rice, soy bean etc. in this season and because of the high infiltration of the surface soil as mentioned above. After agricultural intake, the stream flow in the middle of Shira River decreased as low as 10% of annual mean value during in the dry condition in June.