Until the 1970s, global hydrology had to rely on the collection of in-situ observational data and aggregation under condition when electronic computers were not sufficiently available. In the 1980s, earth observation data from artificial satellites began to observe clouds, precipitation, and later surface soil moisture content, and from the beginning of the 21st century, total terrestrial water storage, including groundwater and ice sheets. Along with this, data assimilation systems merging simulated forecasts by numerical models of the atmosphere and in-situ and satellite observations have been developed, and the information on global hydrologic cycles, at least regarding the atmosphere, has become available.
When we applied the atmospheric water balance method to the four-dimensional data assimilation (4DDA) data and compared it with the discharge of major rivers around the world, we found that the seasonal variation was captured well, although the quantitative accuracy was not sufficient. Seasonal variations in total terrestrial water storage are very large in the Amazon Basin and cannot be explained by changes in soil moisture alone. It was suggested that the changes in river water stored in the river channel contributed greatly, but it was necessary to wait until later GRACE observation data were available to obtain conclusive evidence.
In addition, when the atmospheric water balance method is applied, negative runoff is calculated in some regions and seasons, and at first it was thought to be an error in data and the data processing, and an ad hoc correction method was attempted. However, even from the composite of in-situ discharge data, some areas were found where the downstream river discharge was smaller than the upstream, and negative runoff should be estimated. Then, it became apparent that the negative runoff should be mainly due to anthropogenic water withdrawals and consumption, it is necessary to consider human activities in research targeting the actual water cycle, and such interventions can be detected even on a global scale.
Then, starting with storing in and releasing from reservoirs, an integrated water cycle and water resources model that considers human activities such as water withdrawals from rivers and groundwater, irrigation for farmlands, and long-distance water transport through canals has been developed and used. Although such a model was initially for a global scale, it can be applied for local simulation of hydrologic cycles in the Anthropocene considering water supply and sewerage systems and contribute to supporting scientific evidence-based decision makings.
Improvements in observational and computational capabilities alone did not support the development of global hydrology. In addition to the numerical model itself, it should be acknowledged to the development and sharing of critical global data such as topography, land use and land cover, and cropland distribution equipped for irrigation that are essential for proper simulation of the model. Global hydrology is a community-supported discipline and the gift of grassroots solidarity among researchers around the world.
