Two-way coupling between an LSM and an IAM for assessing the future global water stress

In recent years, water shortages have become increasingly apparent, and it is hoped that by predicting where and to what extent water shortages will occur in the future and reflecting this in water use planning, we can contribute to the sustainable development of society. Future changes in water resources are calculated using an Earth System Model (ESM) and a Land Surface Model (LSM) within the ESM, and economic and social changes are calculated using an Integrated Assessment Model (IAM). We have therefore achieved a realistic water balance model between the environment and the economy/society in land areas by two-way coupling between an ESM/LSM and an IAM. We used ILS [1] as the LSM and GCAM [2] as the IAM. When passing the amount of water resources from ILS to GCAM, we did not pass the raw values but the deviation. The constant values that GCAM originally had were rewritten by the values modified using the deviation. The deviation was calculated by comparing the average values for the past 10 years and the average values for the past 5 years from the time steps when the data exchange occurred, and evaluating the ratio of how much the 5-year average was larger/smaller than the 10-year average. The coupling from GCAM to ILS was achieved by passing the land use changes downscaled using Demeter [3]. When the definition of land use types differed between Demeter and ILS, the type of Demeter’s output was converted to that of the input of ILS by carrying out the method developed in [4]. Using this coupled system, we present the results of annual and seasonal evaluations of water stress from 2020 to 2100. In addition, the assessment results of the population exposed to high water stress are also presented.

 

[1] Nitta, Tomoko, et al. "Development of integrated land simulator." Progress in Earth and Planetary Science7.1 (2020): 1-14.

[2] Calvin, Katherine, et al. "GCAM v5. 1: representing the linkages between energy, water, land, climate, and economic systems." Geoscientific Model Development12.2 (2019): 677-698.

[3] Vernon, Chris R., et al. "Demeter–a land use and land cover change disaggregation model." Journal of Open Research Software 6.PNNL-SA-131044 (2018).

[4] Fushio, Keigo, et al. “Water stress assessment by coupled simulation of Integrated Land Simulator (ILS) and Integrated Assessment Model (IAM).”, SEISAN KENKYU75.2 (2023): 135-140.