Interception and Depression Storage

 Interception

Interception is the removal of water that wets and adheres to plant foliage, buildings, and other objects above ground surface. This water is subsequently removed from the surface through evaporation. Interception can be as high as 2 mm during a single rainfall event, but typically removes about 0.5 mm during a single rainfall/storm event. The quantity of water removed through interception is usually not significant for an isolated storm, but, when added over a period of time, it can be significant. It is thought that as much as 25 percent of the total annual precipitation for certain heavily forested areas of the Pacific Northwest of the United States is lost through interception during the course of a year. Interception is the process by which water is captured on vegetation (leaves, bark, grasses, crops, etc.) during a precipitation event. Intercepted precipitation is not available for runoff or infiltration, but instead is returned to the atmosphere through evaporation. Interception losses generally occur during the first part of a precipitation event and the interception loss rate trends toward zero rather quickly (Fig. 11.1).

Interception losses are described by the following equation

Li = S + KEt

Where,

Li is the total volume of water intercepted.

S is the interception storage.

K is the ratio of the surface area of the leaves to the area of the entire canopy.

E is the rate of evaporation during the precipitation event, and

t is time.

As the Horton equation suggests, the total interception is dependent on the storm duration, as longer duration storms allow more evaporation from the canopy during the storm event. The intensity of the storm also plays a role in canopy interception however, there is debate as to whether intensity increases or decreases interception storage in canopy. There are many other factors that influence interception potential. Interception varies widely by season as deciduous trees lose much of their canopy storage potential during winter months.

Fig. 11.1.Interception Rate versusTime.(Source: Singh, 1994)

There are three Main Components of Interception:

1. Interception Loss

2. Throughfall

3. Stemflow

1. Interception loss: The water that is retained by vegetation surfaces that is later evaporated into the atmosphere, or absorbed by the plant.Interception loss prevents water from reaching the ground surface and is regarded as a primary water loss.

 2. Throughfall: The water which falls through spaces in the vegetation canopy, or which drips from the leaves, twigs and stems and falls to the ground.

 3. Stemflow: The water which trickles along the stems and branches and down the main stem or trunk to the ground surface.

Factors Affecting Interception

Interception Storage: The ability of vegetation surfaces to collect and retain Precipitation, capacity will be highest at the onset of rainfall when the vegetation is dry, when water is held by surface tension.

 Evaporation: Even when the interception storage capacity is exceeded water may be lost by evaporation off leaf surfaces, which increases in windy conditions, though the interception storage capacity may be reduced with increased windspeed.

Duration of Rainfall: Influences interception by determining the balance between reduced storage of water on vegetation surfaces and increased evaporative loss over time.Total interception losses increase with duration of rainfall (but only gradually), though the relative importance of interception decreases with time.The importance of interception decreases with time, due to duration of rainfall and changes in the interception storage capacity.

Rainfall Frequency: The highest levels of interception loss occur when the leaves are dry and interception storage is large, so the frequency of re-wetting is more significant than the duration and amount of rainfall.

Precipitation Type: The contrast between rain and snow.

Snow clings to leaves and branches more, but interception loss is limited due to low temperatures and evaporation rates. May be a contrast between coniferous and deciduous trees

 Type and Morphology of the Vegetation Cover: Different vegetation types have:

· Different interception storage capacities

· Different aerodynamic roughness characteristics

· Different rates of evaporation from their surfaces.

Interception losses are generally greater from trees than other types of vegetation (grasses and agricultural crops)due to the greater aerodynamic roughness of trees in promoting increased evaporation in wet conditionsor to their higher interception capacities (in some cases) especially when wetted and dried frequently. Interception losses are greater from coniferous forests than from deciduous woodlands.

Depression Storage

Depression storage is the term applied to water that is lost because it becomes trapped in the numerous small depressions that are characteristic of any natural surface. When water temporarily accumulates in a low point with no possibility for escape as runoff, the accumulation is referred to as depression storage. The amount of water that is lost due to depression storage varies greatly with the land use. A paved surface will not detain as much water as a recently furrowed field. The relative importance of depression storage in determining the runoff from a given storm depends on the amount and intensity of precipitation in the storm. Typical values for depression storage range from 1 to 8 mm (0.04 to 0.3 in) with some values as high as 15 mm (0.6 in) per event As with evaporation and transpiration, depression storage is generally not directly calculated in highway design.

If the soil surface has a low infiltration capacity and low hydraulic conductivity, and if the topography allows for surface storage, then water may be stored at the surface in small pools or depressions. These water-filled depressions, called vernal pools, are often seasonal features that form because of perched water tables. These depression storage areas may become hydrologically connected during high water conditions and develop a flow network to deliver water to streams or other surface water bodies. Depression storage refers to small low points in undulating terrain that can store precipitation that otherwise would become runoff. The precipitation stored in these depressions is then either removed through infiltration into the ground or by evaporation. Depression storage exists on both pervious and impervious surfaces.

 The volume of water in depression storage at any time during a precipitation event can be approximated as (Linsley 1982):

 (11.2)

Where,

V is the volume of water in depression storage.

Sd is the maximum storage capacity of the depression.

Pe is the rainfall excess, and

k is a constant equal to 1/Sd.

 Factors Affecting Depression Storage

(1) Nature of terrain

(2) Slope

(3) Type of soil surface

(4) Land use

(5) Antecedent rainfall

(6) Time

 Empirical Estimates of Depression Storage (for storms)

Sand                     0.20 inches

Loam                    0.15 inches

Clay                     0.10 inches

Impervious areas  0.062 inches

Pervious urban     0.25 inches