Managing Semi-Arid Watersheds: Watershed Basics - Soils and Their Effects on the Water Cycle

Evaporation, transpiration, surface and ground water movement in the water cycle are all influenced by physical soil characteristics such as texture, or particle size, and structure, or the arrangement of soil particles. Water can move quickly through sandy textured soil, but the ability of a sandy soil to store water is low. On the other hand, water moves slowly into and through a soil containing a high clay content. However, a clay soil has a relatively high water storage capacity.

The forces that drive the water cycle system include solar energy, gravity, and the physics of water content and moisture gradients. Soil that is saturated means that all the pore space between the soil particles is filled with water, and gravity is the dominant force that controls saturated flow. In a non-saturated soil, the physical properties of the soil are the dominant factor controlling water movement. Pore sizes and shapes determine quantity and speed of water movement. In simple terms, water will move from areas of higher moisture content to areas of lower moisture content until equilibrium is reached. In addition, the size of the pore space affects the amount of water that is available to be removed by plants, through transpiration, and by the sun, through evaporation. The smaller the pore size, the more energy it takes to remove the water.

Soil has an effect on the water cycle by influencing the rate at which water evaporates from or moves into and through the soil to saturated zones or aquifers. When it rains on a sandy soil or beach at the ocean, the rain quickly moves into the sandy soil surface and through the soil layer. Because the sandy soil can absorb the rain, it also quickly dries out after the rain stops. However, when it rains on a clay soil, the rain often remains on the soil surface and starts to run off because it is unable to move quickly into and through the clay soil.

Soil can also have an effect on the quality of the water that flows both on the surface and below ground. Surface water that becomes runoff will have a sediment load that is related to the texture of the soil it has come from, or over, and the speed of the runoff, which is determined by the slope of the land. Soil has the ability to filter and, to a limited extent, renovate water as it percolates toward the water table. This may reduce the impact of poor quality surface water on ground water quality.