Simulating the Tone River eastward diversion project in Japan carried out 4 centuries ago

Abstract. The Tone River is the largest river in Japan, flowing from the Kanto Plain westward to the Pacific Ocean. The river originally flowed southward, entering Tokyo Bay, but the Tone River eastward diversion project (TREDP) in the 17th century and many later projects changed the flow route to that of today. The gradual process of eastward diversion has been extensively studied from the historical viewpoint, revealing that the initial project in the 17th century was principally intended to establish a stable navigation route. However, no scholars have yet proven this hypothesis via hydrological modeling. We used the H08 global hydrological model to reconstruct historical flow direction maps at a 60 arcsec spatial resolution with a 1 d temporal resolution. We hypothesized that the historical claims could be numerically verified using a relatively simple simulation. First, we confirmed that our modeling framework reasonably reproduced the present river flows by adding two present-day bifurcation functions. Next, using the reconstructed historical maps, we quantified low flows (20th percentile) in the 17th century and confirmed that the Tone River diversion aided navigation because it connected areas that increased low flows. Finally, the validity of our historical simulation was proven by contrasting the distribution of simulated low flow rates with the flows at the historical river ports that lie furthest upstream. We show that it is possible to bridge two different disciplines, history, and numerical hydrological modeling to obtain a better understanding of human–water interactions. One limitation is that we only reconstructed historical land maps in the present study; the meteorological forcing inputs employed were identical to those of the 20th century. The historical inputs are not known.