The water content of soil can vary from oven dry (zero water) to saturation (a flooded condition). After a field is saturated, or flood irrigated, it will soon drain down to Field Capacity, the maximum amount of water that the individual grains of soil can attract and retain. Similarly, a sponge under water is saturated but when taken out of water, soon drains to sponge Field Capacity.
After proper irrigation, a soil is at full Field Capacity and turfgrass root hairs now draw off water that adheres to to the soil grains. As the water thins out, the turfgrass root hairs have an increasingly more difficult time attracting water. The cohesive attraction of water to the soil grain is greater than the root hairs' ability to extract that water. This point of turfgrass starvation, or about 10% water, is known as the permanent Wilt Point. "Available Water" is that which is actually used by the grass. In natural soil, Available Water is only about 15% of soil volume. The Rain Trap System stores water at about 3 times that amount, or full saturation level.