Volumetric water content is the fraction or percent of water in the total soil volume. When water infiltrates into the soil from rain or irrigation, the pore spaces are nearly filled with water. During and immediately after a rain or irrigation the greatest movement of water occurs in the soil. Afterward, water movement continues due to gravity and capillary forces. Capillary forces are also important for retaining water in soil pores.
Capillary force is illustrated by how far water rises in tubes of various diameters.
If the capillary tubes are placed with one end in a pan of water, the water would rise into each tube. The height of the water in each tube would depend on the diameter of the tube. The smaller the tube, the higher the rise. The surface tension of the water itself and the diameter of the tube cause the water to rise. The water must be under negative pressure to rise.