Change in Water Availability in Territories of River Basins Located in Different Regions of the World due to Possible Climate Changes

The Soil Water–Atmosphere–Plants (SWAP) land surface model was developed at the Water Problems Institute of the Russian Academy of Sciences. It is used to obtain scenario predictions of the change in the climatic values of plant water availability (the index of water availability averaged over the climatic period of ~30 years), as well as the changes in the variability in the annual values of the water availability of vegetation upon a possible climate change in the 21st century, for several large river basins of the world that are characterized by different natural conditions. In this respect, the ratio of actual to potential annual transpiration serves as an indicator of the water availability in the territory. The objects of geographical study include 11 river basins; they are explored within the framework of the ISI-MIP international project and are located on different continents and in different environments: the Rhine, Tagus, Ganges, Lena, Upper Yellow, Upper Yangtze, Niger, Mackenzie, Upper Mississippi, Upper Amazon, and Darling. The meteorological characteristics obtained with the use of five atmosphere-ocean general circulation models for four RCP scenarios of climate change are used as forcing data in calculations with the SWAP model. It is shown that only five of the 11 selected river basins are characterized by a lack of moisture: the basins of the Tagus, Ganges, Niger, Upper Mississippi, and Darling. The basin territories of the other rivers have a humid or semihumid character of moistening that will persist, even with climate change, until the end of the 21st century. For the five indicated basins, the change in their water availability due to possible climate change is analyzed in more detail. The changes in both the average (climatic) value of their annual water availability and the degree of water availability variation for certain years are studied based on the construction of an integral function of the water availability distribution for the climatic periods considered in this work (a historical period and prognostic periods). It is shown for the studied arid and semiarid basins that a small degree of their water availability either will stay at the present level or will decrease even more by the end of the 21st century. Thus, the tendency of possible climate changes in the 21st century in arid and semiarid regions will further aggravate a serious problem of food security, which involves the agricultural industry and the related water industry. This problem will require an accelerated transition of the interaction of humans with Nature to the principles of green farming, which is based on nature-based technologies for the use of natural resources that rationalize the consumption of water, soil, energy, and other resources of our planet.