In the section on "what is soil" we saw that water plays a role in the formation of soil. Let's move on to consider how water is stored and moves through the soil.
Most water arrives at the soil surface as precipition, that is rainfall or snow melting at the surface. We know that soils contain pore spaces between the soil particles, and that the size of these pore spaces is determined by the particle size distribution of the soil. In big pore spaces gravity is the most important force acting on the water and it causes the water to move vertically downwards through the soil; this movement of water is termed infiltration. Water will continue to drain downwards, and possibly into whatever substrate lies beneath the soil, unless it reaches a barrier, which could be a band of impermeable rock or soil, or a zone in which the soil is already saturated.
Water that reaches such a barrier cannot drain vertically so will either accumulate to create, or extend, a saturated zone, or find a lateral pathway and move sideways through the soil. Continuing rainfall may cause the saturated zone to rise so far that it reaches the surface and causes puddles to form. The forming of puddles is usually termed surface ponding. In soils with poor drainage this surface ponding through saturation can be a long-lasting phenomenon.
In the above description surface ponding has been caused by the soil becoming saturated and the saturated zone reaching the surface. However, it can also occur because the intensity of the precipitation is so great that it exceeds the infiltration capacity of the soil. This type of surface ponding is usually short lived, as the water can fairly rapidly infiltrate into the available pore spaces.
The infiltration capacity of a soil clearly depends on the properties of the soil, but can also be modified by the weather. Extended hot and dry weather can lead to some soils forming a crust, which can limit infiltration, while other soils may crack, which can allow water to quickly by-pass normal infiltration paths and quickly enter the subsurface. In conditions of extreme cold, soils can freeze and greatly reduce or stop infiltration.
In additional to gravity, there are other forces that control the movement and storage of water in the soil. Capillary forces are the result of surface tension and works to hold water in the pore spaces between soil particles. These forces increase as the pore space decreases, and at the scale of a soil pore, can overcome the force of gravity, holding the water suspended in the soil, and preventing water from draining. There is also a small amount of water that is bound tightly to the soil particles and cannot be shifted by either gravity or surface tension forces; this is known as hygroscopic water.
One final process affecting soil moisture is evaporation. Close to the surface, if there is sufficient available energy, water can be converted to water vapour and lost to the atmosphere. Where there is vegetation then water is also transferred to the atmosphere through transpiration, the process by which plants draw up water through their roots and loose water through the pores (stomata) in their leaves. Wheras evaporation from the soil can only take place near the soil surface, transpiration can draw water from deep in the soil. The term evapotranspiration is given to the combination of evaporation and transpiration to represent all transfers of water from the soil to the atmosphere. The rate of evapotranspiration clearly depends on many factors and will vary with the time of day, season, water availability, weather, vegetation type and soil type.
These various processes combine to determine the soil moisture status of the soil, over the range from dry to saturated. Within this range are two notable points. Field capacity is the water content when the water has fully drained under gravity so that the remaining water is held as capillary and hygroscopic water. The wilting point, or permanent wilting point, marks the moisture content below which plants can longer access the stored water or will not recover even when soil moisture increaseses.
Finally, a key concept relating to the water stored in a soil is the water table. Simply put, the water table represents the top of the saturated zone, and this moves vertically up and down as water infiltrates and drains through the soil. However, this isn't strictly accurate as the water table is actually the surface at which the water pressure equals atmospheric pressure. Beneath the water table water pressure is greater than atmosperic pressure. Above the water table the soil can still be at saturation with the water being held by capillary forces; in this zone the water pressure is less than atmospheric pressure. The zone of saturation resulting from capillary forces can extend to a considerable depth in fine (clay) soils. It should also be noted that water tables can be both large and small scale features. Local zones of saturation can form in response to small variations in soil properties (e.g. small lenses of impermeable soils contained in otherwise well drained soils), and can be 'perched' above a deeper water table.
We are now able to put all of this together and say something about how soil moisture varies through time. When precipitation exceeds the combined losses of water from evapotranspiration and drainage, soil moisture increases. In the UK this is what typically begins to happen in the autumn. During the winter months evapotranspiration is very low, and we expect soil moisture to increase, perhaps to the extent that the soil becomes saturated and suface ponding occurs. During the spring, temperatures rise, plants begin to grow and evapotranspiration increases so that soil moisture begins to decrease. This continues during the summer months with further decreases in soil moisture, perhaps reaching the wilting point in particularly dry years.
This annual cycle is what we expect across the UK, but is highly dependent on both climate and soil type. In some places it will only be in exceptionally dry years that soil moisture falls below field capacity. At the other extreme soil moisture may not return to field capacity during a dry autumn and winter.
It is only by measurement that we can quantify this variability through time and across the UK.
Previous: What is soil?
Next: Why measure soil moisture?