Abstract

Starting in 1968, a series of droughts hit the Sahel region till early 80s and approximately 100,000 people died due to food shortages and disease. Charney (1975) first hypothesized the positive vegetation-rainfall feedback for the desertification and drought in the Sahel.

However, at the time, information on global energy and water cycles that drive the climate was extremely limited. Therefore, GEWEX was planned under WCRP, and an international study on water and energy cycles was comprehensively carried out using field observations, Earth observations from space, and objective analysis data using four-dimensional data assimilation, which was just beginning at the time.

Under GEWEX, an international research project, Global Soil Wetness Project, was carried out to calculate the energy and water balances at the land surface based on observed meteorological forcings, such as precipitation, solar radiation, wind, temperature, humidity, using LSMs (land surface models) (Dirmeyer et al., 2006), and the global water cycle was estimated, leading to a modern schematic diagram on it (Oki and Kanae, 2006).

On the other hand, when the global distribution of precipitation-evapotranspiration was calculated using the atmospheric water balance method, it showed a good correspondence with river runoff on an annual average, as expected from the land surface water balance, but there were some areas in the world where water vapor is “welling up” from land, mainly in semi-arid areas, i.e., the amount of evapotranspiration is greater than the amount of precipitation. Ground observation data for rivers also revealed areas where the flow rate upstream is greater than the flow rate downstream. This shows that in the Anthropocene, where human activity is affecting the global environment, it is necessary to simulate the real water cycle, which is strongly influenced by human activities such as storage, discharge, and withdrawal, rather than pristine nature without human activity, even on a global scale. Accordingly, hydrological cycle and water resource models that incorporate human activities such as storage and discharge in reservoirs, water conveyance through canals, groundwater pumping, and irrigation of farmland have been developed, and are being used for evaluating global water supply and demand and for assessing the impact of climate change.

The emergence of global hydrology has made it possible to build early warning systems for water-related disasters such as floods, droughts, storm surges, and landslides, and to predict future impacts considering climate change and social changes, contributing to the promotion of academic research and reducing the risk of water-related disasters in society and enhancing the blessings of water.

This modern development of global hydrology has been made possible thanks to remarkable advances in information and communication technology, which have made it possible to access big data from earth observation satellites and other sources, enable large-scale numerical calculations covering the entire planet, and enable researchers scattered around the world to share knowledge, experience, data, and sometimes even numerical calculation codes. Global hydrology is thus a community science, and an overview of its development will be introduced.