日本地球惑星科学連合2018年大会

講演情報

[EE] ポスター発表

セッション記号 A (大気水圏科学) » A-HW 水文・陸水・地下水学・水環境

[A-HW20] 流域の物質輸送と栄養塩循環-人間活動および気候変動の影響-

2018年5月21日(月) 15:30 〜 17:00 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:齋藤 光代(岡山大学大学院環境生命科学研究科)、小野寺 真一(広島大学大学院総合科学研究科)、細野 高啓(熊本大学大学院先導機構、共同)、Adina Paytan(University of California Santa Cruz)

[AHW20-P27] Estimation of erosion rate and deposition process of a small pond in an agricultural catchment, subtropical small island

*小野寺 真一1齋藤 光代2大前 英3飯泉 佳子4曹 楊1友澤 裕介1 (1.広島大学大学院総合科学研究科、2.岡山大学、3.国際農林水産業研究センター熱帯・島嶼研究拠点、4.国際農林水産業研究センター)

キーワード:浸食速度、堆積過程、亜熱帯池

In tropical and subtropical region, the rainfall intensity is strong and soil erosion in an agricultural land is one of big environmental issues. Especially, the impact of large sediment discharge on the coral reef is serious around small islands, such as Okinawa and Philippines. In case of Okinawa Prefecture, to reduce the sediment discharge and conserve corals, many sediment trap ponds are constructed in a downstream area of agricultural lands. However, sediment deposition rate, variation of trap effect, and component of sediment in a pond have not been confirmed.

To estimate of erosion rate and deposition process of a small pond in an agricultural catchment of a subtropical small island, we examine to observe the sediment deposition in a small pond and monitor water level and chlorophyll a in a pond and runoff and turbidity of an inflow stream in a small catchment with the area of 300m2 in Ishigaki Island, Okinawa Prefecture, Subtropical region. Our monitoring was started from February 2017 after removing all sediment in a pond in January, and up to April 2018. Observation of sediment deposition was conducted with collecting the samples at 5 sites in June 2017 and January 2018. Inflow and outflow water samples were collected every one month.

The sediment deposition was extremely large from January to May 2017, the average erosion rate in all catchment area was estimated to be 6mm. On the other hand, the sediment deposition was nothing from June 2017 to January 2018. Based on the monitoring of stream runoff and turbidity, one big storm event in April with strong intensity mainly contributes to sediment discharge and overflow of the pond occurred. On the other hand, the later period had also big events. This difference is suggested to be the difference of land cover by crops and availability of sediment trap in this pond. In addition, the sediment component mainly was composed of surface soft layer, subsurface sandy layer, and bottom gravel layer. The average thickness was 2cm in surface layer, 100cm in mid sandy layer, and 10cm in gravel layer, respectively. Especially, the surface soft layer had high nutrient content. The dissolved nutrient of inflow stream water and pond water were high concentration. But the 10% of nutrient in the inflow decreased in pond. This means the plankton was produced, using this nutrient and that was deposited later. The soft layer is suggested to be effect of organic fertilizer, it is expected to apply to agricultural lands.