[A-HW18] Material transportation and cycling at the land-sea interface: from headwaters to the ocean

[AHW18-P11] Sediment budget and discharge at a small pond of an agricultural land in a subtropical island

*Wanna Zaw¹, Shin-ichi Onodera¹, Mitsuyo Saito¹, Hide Omae², Yoshiko Iizumi³, Yang Cao¹ (1.Graduate School of advanced science and Engineering, Hiroshima University., 2.Tropical Agriculture Research Front, Japan International Research Center for Agricultural Sciences , 3.Nihon University)

Keywords:sediment recycle, sediment deposition, turbidity

To confirm deposition process in a small pond and the availability of its sediment recycle to agricultural land in a subtropical 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, Japan. Our monitoring was started from February 2017 after removing all sediment in a pond in January, and up to November 2018. Observation of sediment deposition was conducted with collecting the samples at 5 sites in June 2017, January 2018 and June 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 approximately nothing from June 2017 to June 2018. Based on the monitoring of stream runoff and turbidity, one big storm event in April 2017 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 by using this nutrient and that was deposited later. This soft layer is suggested to be effective as organic fertilizer, it is expected to apply to agricultural lands.

A part of this research was commissioned by JIRCAS and supported by JSPS(18H04151), respectively.