Doug has filled a long tray with an even layer of sand. Doug then uses two bricks to prop up one side of the tray. Next, Doug uses a watering can to pour water onto the high side of the tray. what natural process is he making

Percolation of water through the soil.

Explanation:

When a tray containing a mixture of dough and sand is propped on one end of the tray with bricks then there is a slope from one end of the tray to the other end of the tray.

When water is poured on the higher end of the tray then the it tries to flow from the higher altitude to the lower altitude of the tray by the process of percolation through the granular spaces created due to the mixing of sand in the dough. Sand being granular in structure is easily permeable to water and passes water easily through its tiny pores.

Similar is the process of percolation of water through the layers of soil on the earth which also contains granular particles in them.