Water cycle in nature
As shown in figure 1- 1, under the action of solar thermal energy and gravity, water continuously evaporates and transpires from the sea surface, river and lake surfaces, rock and soil surfaces and plant leaves, becomes steam and enters the atmosphere. With the migration of airflow, water vapor condenses again and drops under suitable conditions (such as rain and snow). Part of the water falling to the surface evaporates in situ and enters the atmosphere; Part of it flows along the surface, becoming surface runoff and flowing into rivers, lakes and seas; The other part seeps into the ground and becomes groundwater. In the process of runoff, some groundwater re-evaporates and re-enters the atmosphere in the form of steam, and some groundwater is discharged into rivers, lakes and oceans again. This process of evaporation, precipitation and runoff continues all over the world, forming an extremely complex water cycle in nature.
Figure 1- 1 Schematic diagram of hydrological cycle (according to Wang et al., 1995)
According to the different ways of water cycle, water cycle can be divided into large cycle and small cycle. On a global scale, water evaporates from the ocean surface, and the rising water vapor moves over the land with the airflow, condenses into precipitation and falls to the land surface, and finally flows into the ocean in the form of surface or underground runoff. This cycle between land and sea is called the Great Cycle. The water vapor evaporated from the ocean surface falls into the ocean in the form of precipitation; Or the water evaporated from rivers, lakes, soil surfaces and plant leaves on land falls back to the mainland surface in the form of precipitation. This phenomenon occurs in some areas and is called small circulation. Small circulation is controlled by local meteorological factors, while large circulation is controlled by global climate. Therefore, adjusting the conditions of small circulation and strengthening the frequency and intensity of regional small circulation can improve the local arid climate. However, the change of macro-cycle conditions is still beyond the reach of human beings.