Downstream circulation: all circulating water enters from the two end walls or the upper parts of the two side walls of the swimming pool and returns from the bottom of the pool. The backwater at the bottom can be used in conjunction with the drain to meet the requirements of uniform water distribution. However, the bottom of the pond is easy to deposit dirt, so the backwater position should be determined in design to prevent short flow. This circulation method is widely used in China and the effect is satisfactory.

Countercurrent circulation: all the circulating water is sent into the pool from the bottom of the pool and overflows from the upper edges around or on both sides of the swimming pool. The water inlets are evenly arranged at the bottom of the pool along the lane markings, and the water is evenly distributed, and there is no scale on the bottom of the pool, which is beneficial to timely cleaning the dirt on the surface of the pool water; This is the circulation mode recommended by FINA.

Water cycle refers to the mode that water changes from different parts of the earth to other parts of the earth by absorbing energy from the sun. For example, water on the ground is evaporated by the sun into water vapor in the air.

The existing forms of water on the earth include solid, liquid and gas. Most of the water on the earth exists in the atmosphere, ground, underground, lakes, rivers and oceans.

Water moves from one place to another through some physical actions, such as evaporation, precipitation, infiltration, surface flow and underground flow. For example, water flowing from rivers to oceans.

Water is the constituent substance of all living things, the essential substance of life metabolism and the important resource of human production activities.

The water on the earth is distributed in oceans, lakes, swamps, rivers, glaciers, snow-capped mountains, as well as in the atmosphere, biology, soil and strata. The total water volume is about 1.4× 10 13m? 96.5% of them are in the ocean, accounting for about 70% of the total area of the earth. Water in land, atmosphere and living things accounts for only a small part.