|Freshwater Resources in Arid Lands (UNU, 1997, 94 p.)|
|6: Global warming and groundwater resources in arid lands|
In the middle-latitude zone, the natural vegetation changes from forests to grasslands near an isohyet of 500 mm/year, and from grasslands to deserts near that of 250 mm/year. The amount of recharge to the groundwater is the difference between the annual precipitation and the annual evaporation, provided that no surface run-off occurs. In the case of a dry climate with precipitation less than 500 mm/year and where the potential evaporation far exceeds the annual precipitation, as in drylands, the amount of groundwater recharge is heavily dependent on the actual evaporation lost from the land surface. Deforestation and desertification result in a decrease of local precipitation due to positive feedback effects, as discussed above. On the other hand, it is quite difficult, if not impossible, to increase the amount of local precipitation anthropogenically.
For the sustainable use of drylands, it is desirable to develop such methods that induce positive feedback effects to increase the amount of available water. The amount of actual evaporation lost from the soil surface is an issue of human intervention in drylands. Dry farming is a technique to maximize the input from meteoric water into the soil, and at the same time to minimize the evaporation loss from the soil surface. Deep cultivation of soil before the rainy or snowy season is a technique to introduce rain or snow-melt water into deep soil layers. The evaporation loss of soil moisture could be decreased by forming a loose surface layer in which capillary continuity of soil pores is discontinued by disturbing or crushing the surface soil.
Yamanaka et al. (1994) conducted an experiment that contributed to the increased understanding of the dynamic behaviour of water vapour in the soil layer and the role of a surface dry layer (SDL) on soil evaporation. When the SDL is formed during the evaporation process at the bare soil surface, the water vapour in the SDL plays an important role by connecting the liquid water in the soil and the water vapour in the atmosphere. The thickness of the SDL is about 4-5 cm, and the lower boundary of the SDL coincides with a surface where soil water vaporizes. The SDL acts as a strong barrier against the transport of the water vapour in the soil evaporated from the evaporation surface below the SDL.
Since the experimental result mentioned above is obtained for the diurnal change in water vapour concentration within a standard sand layer only, its applicability is of limited nature. Future progress in research on the dynamic behaviour of transport processes of heat and water near the soil surface, including the dynamic behaviour on a much longer time-scale of seasons and years, and for different soil types, may contribute to the proper management of soil and groundwater in arid lands.
Groundwater, soil water, river water, lake water, and mountain glaciers are linked through the regional hydrological cycle. However, in such hydrological characteristics as the residence time, water storage, water quality, and recharge and discharge processes, they are quite different from each other. For the proper use of groundwater in arid lands, the conjunctive use of waters with different hydrological characteristics is necessary. The science of hydrology may contribute to an increased understanding of hydrological processes and hydrological characteristics of natural waters, especially for groundwater in arid lands.