Modeling of dissolved iron production and transport of the Amur River basin

Takeo Onishi
Research Institute for Humanity and Nature, 457-4 Motoyama, Kamigaomo, Kita-ku, Kyoto, 603-8047, Japan


Recent research showed that iron limits phytoplankton growth in the Western Subarctic Pacific and that significant part of it originally comes from the Amur River. It is also revealed that the crucial processes of producing dissolved iron are reductive release of ferrous iron and formation of iron complex with organic compound in wetland. However, drastic conversion of wetland into cultivated land in China part during the last half of 20th century may have great impact on iron production. To assess the land cover conversion impact on dissolved iron production in terrestrial area, a numerical model which simulates dissolved iron production and transport in the Amur River is developed.
A model which consists of two modules was constructed. One for dealing with the physical process that calculates runoff (TOP-RUNOFF), and the other for dissolved iron production process (TOP-FE). Accuracy of TOP-RUNOFF without any calibration is fairly good except for catchments affected by anthropogenic impact such as dam. TOP-FE was formulated as a function of water content which is the prognostic variable of TOP-RUNOFF, saturation duration time which represents the degree of redox condition, and topographic index a/tanb. Calculated values were validated by using dissolved iron data observed at several points. The result showed that the model can predict annual dissolved iron flux. However, inter-annual variability of dissolved iron flux was much larger than that of simulated dissolved iron flux.
The main reason of this discrepancy was supposed to be attributed to no consideration of flooding process especially in flood plains. Incorporating flooding process into the model actually improved the accuracy in predicting inter-annual variability of dissolved iron flux. Thus, soil saturation not only by ponding but also by flooding was proven to be important in dissolved iron production.

Keywords: dissolved iron, redox process, TOPMODEL, flooding, wetland