FAQs - DRAINAGE & SALINITY

Q: How does capillarity affect soil water holding capacity?
A: One important characteristic of soil is its ability to hold water against the force of gravity and supply a portion of that water to plants. Much of this capacity is related to the number and size of pores and channels distributed throughout a soil. Some water can be held so tightly on polar surfaces in the soil that many atmospheres of pressure are required to force this water out. Plant roots must out-compete the forces that hold water in soil to survive, especially as more and more water is removed from the soil. However, much of this water would not even be in the soil in the first place without capillarity. All rainfall would drain rapidly from the soil and not be available for long-term use by many organisms nor would it be available for plant root uptake days or weeks after rainfall events. Most of the water available for plants in soil is that water categorized as capillary water.*

 

Q: What role does soil play in water movement?
A: When water reaches the land surface as precipitation, it can seep downward through pores between soil particles. Soil is made up of tightly packed particles of many shapes and sizes. A high porosity soil has the ability to hold large amounts of water due to the presence of many pore spaces. If the pores are well connected and allow water to flow easily, the soil is permeable. The size and shape of clay particles along with the arrangement of the pores between these particles result in clay soils being relatively impermeable and resistant to infiltration. Sands and gravels allow more rapid infiltration due to their high permeability. The initial water content of the soil is also important. In general, water infiltrates drier soils more quickly than wet soils. The intensity of a storm, or the length of time during which precipitation occurs, can also influence infiltration. If rain or snowmelt reaches the soil surface faster than it can seep through the pores, then the water pools at the surface, and may run downhill to the nearest stream channel. This limitation on the soil's capacity to allow infiltration is one of the reasons why short, high intensity storms produce more flooding than do lighter rains over a longer period of time.*