Determining the portion of precipitation that becomes runoff volume is a complicated matter. Precipitation may first fall on a vegetation canopy that intercepts a portion of the precipitation. Surface depressions capture some of the precipitation reaching the ground and allow it to infiltrate. Water that does not infiltrate generally moves over the ground surface to become runoff volume. Once water is in the soil it can move vertically and a portion that infiltrated may return to the atmosphere through evapotranspiration. The weight of consideration given to each of these components depends on the purposes of the hydrologic study. Studies using event simulation methods tend to focus on the initial condition at the beginning of the storm and the portion of the storm volume that becomes runoff. Studies using continuous simulation methods usually focus on infiltration and evapotranspiration in order to estimate monthly or annual runoff volumes.

Interception

Many watersheds have some type of vegetation growing on the land surface. The vegetation in a natural watershed could be grass, shrubs, or forest. Agricultural watersheds could have field crops such as wheat or row crops such as tomatoes. Even urban watersheds often have vegetation with some cities maintaining extensive urban forests. Falling precipitation first impacts the leaves and other surfaces of the vegetation. Some of the precipitation will remain on the plant while the remainder will eventually reach the ground. The portion of precipitation that remains on the plant is called interception.
Precipitation that is intercepted can return to the atmosphere through evaporation. Evaporation is significantly reduced during a precipitation event because the vapor pressure gradient is reduced by the high humidity associated with precipitation. However, after the precipitation event is over, the humidity will usually drop and restore the vapor pressure gradient. This allows evaporation to increase and intercepted precipitation will return to the atmosphere.

The amount of interception is a function of the species of plant and the life stage of the plant. In general, forests have the highest potential for interception with evergreen species collecting more precipitation than deciduous types. Shrubs often have an intermediate about of interception capability with grasses and crops showing the least ability to capture precipitation. Life stage is also important. Young plants are usually smaller and consequently capture less precipitation. Deciduous trees can capture a significant amount of precipitation during summer months when the canopy is full, but collect almost no precipitation in the winter when the leaves have fallen off.
Water that impacts on vegetation and does not remain as intercepted precipitation can reach the ground through two primary routes: throughfall or stemflow.

Throughfall refers to precipitation that initially lands on the vegetation surface, and then falls off the vegetation to reach the ground. The leaves of a particular plant species have a limited capacity for holding water in tension. Water beyond this capacity cannot remain on the vegetation for very long and will eventually fall off the leaf. It is possible for the amount of water that can be held on a leaf to be affected by atmospheric conditions such as windspeed.

Stemflow refers to precipitation that initially lands on the vegetation surface, and then moves along the leaf stems, branches, and trunk to reach the ground. The amount of stemflow observed in a plant species in dependent on the shape of the leaves and branches. Stemflow will be high in plants that have leaves and branches shaped in a way that collects intercepted water and directs it to the trunk. Evergreen trees with needles or plants with irregular branches often have higher throughfall and reduced stemflow.

Surface Depressions

Precipitation can arrive on the ground surface through a variety of pathways. The precipitation lands directly on the ground when there is no vegetation in the watershed, or the precipitation can pass through gaps in the vegetation cover. Precipitation may also arrive on the surface as throughfall or stemflow. The water on the ground will collect in depressions. The capacity of depressions to hold water varies according to the land use. For example, a typical asphalt parking lot has a very small capacity for storing surface water. Conversely, conservation agriculture practices use tillage techniques designed to increase the capture of water in surface depressions.
Water captured in surface depressions can infiltrate into to the soil after precipitation has stopped. The amount of depression storage can control the partitioning of precipitation between infiltration and surface runoff. Watersheds with a small depression storage capacity will capture very little precipitation and infiltration will occur only during storm events. Watersheds with substantial depression storage will capture precipitation and infiltration it during the storm event, and water in depressions at the end of the event will infiltration after the storm has stopped. Water that is not captured in surface depressions will usually flow over the surface as direct runoff.

Infiltration During a Storm

Infiltration is the process of water on the surface of the soil moving down into the soil. Soil is a porous media with a structure composed of a variety of grain sizes with air between the individual grains. In some rare cases, the pores of the soil are completely saturated with water and no air remains. In most cases the soil is unsaturated and only some of the pores contain water. Therefore, simulating the behavior of water in soil is complicated because the exact nature of the pore spaces (their volumetric ratio and connectedness) is not known. Further, while the total amount of water in the soil may be estimated with reasonable accuracy, it is generally unknown exactly where the water is located in the pore spaces. This entire descriptive task is complicated many times over by the spatial variation of soil and pore properties horizontally and vertically throughout the watershed.

Soil as a porous media can be visualized similar to a sponge that cannot deform. The soil grains are analogous to the structure of the sponge and the pore spaces are analogous to the empty space in the sponge. In order to be a porous media, the pore spaces must be small enough for capillary forces to be significant. The magnitude of capillary force is proportional to the radius of the pore space. Most soils have a wide variety of pore space sizes, and the individual spaces are irregularly shaped. Therefore, a measurement of the capillary force can change dramatically from point to point throughout the soil. The only way to development a meaningful measurement is to use an instrument that determines the average value over a volume of several cubic centimeters. Such a measurement is termed the soil water potential and by convention is negative. Units could be kilopascals, millimeters of water, or Bar.

The flow of water in either saturated or unsaturated soil can almost always be described by Darcy's Law. It is a basic relationship that states the vertical flow of water is proportional to the potential gradient. When the spatial coordinate is taken as zero at the soil surface and measured downward, Darcy's Law can be stated in the following form:

1) v=K\frac{d\psi}{dz}

where ν is the flow per unit area, ψ is the matric potential (a negative value), z is the spatial coordinate (measured positive downward), and K is the hydraulic conductivity. If the soil is saturated, then K is the saturated hydraulic conductivity and is a function of the soil properties and the water properties. For unsaturated conditions the conductivity is still a function of soil and water properties, but is additionally a function of the matric potential. As the water content decreases from saturated toward the residual content, the matric potential becomes increasingly negative. The relationship between conductivity and matric potential is nonlinear resulting in the magnitude of the conductivity varying by several orders of magnitude over the possible range of water content. Darcy's Law is at the heart of all physically-based models of infiltration and is also used on many conceptual models.

The water moves in soil through a combination of two basic forces: absorption and gravity. The term absorption is used to describe the process of water on the surface of the soil being drawn into the soil because of a gradient in the matric potential. Recall that the magnitude of the matric potential increases as the soil becomes dryer. The greater the magnitude of the matric potential, the greater the matric potential gradient will become. It is usually the case that infiltration during the first portion of a precipitation event is dominated by absorption, since the soil is often dry when precipitation begins. The matric potential will decrease as the water content of the soil increases, eventually becoming zero at saturation. Therefore, the contribution of absorption to the total infiltration will also decrease as the precipitation event progresses.

The degree to which absorption plays a role in overall infiltration into a soil is determined by the properties of the soil. Soils with small pore spaces (such as clay soils) have a greater matric potential for a given water content than soils with large pore spaces (such as sandy soils). One measure of this property of a soil is the bubbling pressure or air entry potential. Soils with a large bubbling pressure show a great deal of absorption as part of infiltration. Soils with a small bubbling pressure will very quickly transition from absorption effects to gravity effects during a precipitation event.

Water begins moving through soil under the effect of gravity after the soil is saturated. The matric potential becomes zero when the soil is saturated and Darcy's Law predicts water will move at the saturated hydraulic conductivity. However, there is a degree to which gravity effects water flow in soil just as there is a degree to which absorption effects flow. Some portions of the soil will be saturated long before the bulk soil is saturated. A certain collection of soil pores that form a vertical path from the ground surface to the bottom of the soil layer may become saturated. Once those pores become saturated, gravity will drive infiltration through those pores even if surrounding pores are unsaturated and still dominated by the absorption process.

Absorption and gravity effects work together to produce the total infiltration rate. Water is pulled into dry soil by absorption. Absorption is the dominate force driving infiltration as a front of water enters a dry soil at the ground surface and begins moving down toward the bottom of the soil layer. Soil will be saturated some short distance behind the wetting front. Water in the saturated pore spaces, closer to the soil surface, will be dominated by gravity effects. Therefore, the gravity effects supply the water through the saturated pores to the areas along the wetting front where absorption is expanding the saturated soil region.

The combination of absorption and gravity gives a high total infiltration rate at the beginning of a precipitation storm. The total infiltration rate will then decrease as the soil layer becomes saturated. Infiltration in the saturated portions of the soil can only be driven by gravity and lacks the additional component of absorption. Eventually the soil layer becomes completely saturated and the absorption component becomes zero throughout the soil. At this point a steady-state is reached and infiltration is only a function of gravity effects. The overall result is that the high initial rate of infiltration decreases as the soil becomes saturated and eventually becomes constant at the saturated hydraulic conductivity.

Processes Occurring Between Storms

Physical forces act on water in the soil between storm events. The water will move in response to capillary suction forces in a process that redistributes the wetting front. Some of the water will be removed through evapotranspiration. Representation of these processes is critical in continuous simulations. A storm event simulation simply specifies the wetting front and soil moisture states as initial conditions at the beginning of a simulation. A continuous simulation must model these two key processes accurately from the end of one storm event to the beginning of the next storm event. If the simulated state of the soil is incorrect when the second storm begins, then the infiltration and runoff volume for the second storm will not be accurate.

Water does not stop moving in the soil after the precipitation stops. The process of absorption, driven by the matric potential in the unsaturated pore spaces, will redistribute water. During a precipitation event, the wetting front can advance down into the soil relatively quickly due to the combined effects of absorption and gravity. When the precipitation ends, the wetting front no longer has a supply of water coming from the surface. The wetting front will not advance without a supply of water so the water that is in the soil must move in response to matric forces. The matric potential is greater below the wetting front than it is the soil above the wetting front. Therefore, the water will move from the area above the wetting front toward the bottom of the soil layer. Reducing the water content above the wetting front will increase the matric potential in this region of the soil. The matric potential below the wetting front will decrease as water is pulled down from the formerly saturated region above the wetting front. Eventually the matric potential above the wetting front will be equal to the matric potential below the wetting front. At this point it will no longer be possible to identify the wetting front.

Evapotranspiration influences water movement in the soil between storm events by changing the matric potential. Plants extract water from the soil during the day. The water is drawn into the roots, which reduces the water content in the soil surrounding the roots. The decrease in the water content around the roots causes the matric potential to increase. The increase in the matric potential will cause water to move toward the roots from other regions of the soil layer that have a higher water content. Evapotranspiration can make it impossible for the soil matric potential to come to a true steady-state condition. The result is that soil water is dynamic and is often in a constant state of flux.

Data Requirements

The program considers that all land and water in a watershed can be categorized as either:

  • Directly-connected impervious surface
  • Pervious surface

Directly-connected impervious surface in a watershed is that portion of the watershed for which all contributing precipitation runs off, with no infiltration, evaporation, or other volume losses. The infiltration loss methods included in the program include the ability to specify the percentage of the watershed which is impervious. Impervious surface is usually associated with urbanized areas including roads, parking lots, and building roofs. Precipitation on the pervious surfaces is subject to losses.