Atmospheric water resource and precipitation conversion

Abstract

The concept of water resources is largely limited only to water in rivers, lakes, underground aquifers, and other sources that can be used by humans directly––blue It does not normally include water that directly evaporates from the land surface and transpires from vegetation––green Generally speaking, the sum of blue water and green water is equivalent to total precipitation. Considering the global land surface, as much as about 65% of the total precipitation is converted into green water, and only the remaining 35% is converted into blue water. The concept of green water clearly extends the scope of research and management of water resources to the total amount of land precipitation, but the source of terrestrial water resources has not been intensively studied. In terms of a complete water cycle, water resources should refer to the renewal/replenishment of the fluxes of fresh water in different processes in the hydrosphere. The storage of water in the atmosphere is relatively small, but the huge flux of water transport in the atmosphere is the fundamental source of terrestrial water resources. In the contexts of existing water shortages, future water demands (including for ecological protection), and global climate change impacts on water resources, accurate quantitative studies on the formation of terrestrial water resources from the atmosphere are becoming increasingly important. Such studies require a clear definition of the concept of atmospheric water resource and particular focus on its precipitation conversion. Such research can indeed enhance our theoretical knowledge about the functions of water resources systems and further broaden our view of traditional water resources management. Motivated by this, the present paper attempts to improve our understanding of atmospheric water resources. This is done through: (1) a review of the concept of atmospheric water resource; (2) proposing a quantitative estimation method for atmospheric water resource; and (3) quantifying the precipitation conversion characteristics of atmospheric water resources. For a given period of time, we define the atmospheric water resource as the accumulated atmospheric water matter with potential to precipitate, and call it as Therefore, in a sense, white water is also a renewable/replenishable flux, whose volume is generally considered over a longer period (e.g., per year). However, since the horizontal transportation of water matter in the atmosphere spreads over the entire sphere (as opposed to surface water that is bounded in one-dimensional river channels), the dimension of white water at one point should be in L2T-1. With this, for any given point, the quotient of precipitation rate over white water flux is defined as the white water precipitation efficiency ( WPE ) (dimension: L-1). Therefore, the WPE refers to the proportion of precipitation converted from white water while transported in unit distance. To calculate the quantity of atmospheric water matter that has the potential to precipitate, a background value should be subtracted from the measured vertically integrated water vapor (IWV). In this study, the clear-sky IWV of an ordinal day in a year is used as the background value. With the above, the concept of white water is also extended to surface river basins. In this, the white water flux entering a river basin is defined in the L3T-1 dimension, and the corresponding white water precipitation ratio ( WPR , dimensionless) is obtained by integrating the WPE over each transport path and then calculating the weighted average over all the paths. Therefore, the WPR is essentially the proportion of total precipitation in the basin that is converted from the incoming white water. Finally, the reanalysis data are used to analyze the distribution of white water and the pattern of WPE within the range 5°~50°N and 75°~135°E region. A comparative analysis of the atmospheric-terrestrial water resources characteristics in the Yangtze River, the Yellow River, and the Pearl River basins, as well as in the Sanjiangyuan region is also conducted. The results show that the WPE is a fair measure of the conversion efficiency from white water to precipitation, and the WPR in a river basin is useful for quantitative analysis of the atmospheric-terrestrial coupling relationship.