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Abstract

We describe interplays of water and food production over the 70 years (1950-2020) of Japanese watersheds. Here, we focus the changes on the rice transplanting period because it is critical for rice cultivation and requires considerable water for puddling. The transplanting periods have shifted from mid-June in the 1950s to early May in the 2000s. We propose a framework for assessing how the shifts of the transplanting period affected rice production and drought risks based on two process-based models. We conducted the simulation with shifted transplanting dates (or starting date of irrigation) from the current date by one week up to five weeks earlier and later. We then integrate the results of mean crop yield and drought risk for each transplanting date to examine the interrelated nature of crop production and drought risk. We applied the method to two watersheds, contrasting in terms of hydrological regimes. The earlier transplanting periods compared to those in the 1950s generally increased the yield, whereas the changes in the drought risk differed. In the Shinano river, the earlier transplanting date resulted in lower drought risk and the current transplanting period corresponds to the day that minimizes drought risk and maximizes yields. In the Kinu river, the earlier transplanting date resulted in a higher drought risk. Thus, the shift in the transplanting period would have conflicted with the water management agencies. We highlight the importance of the socio-economic (i.e., increase food production) can be a driver of changes in water cycles.