Saturation Excess Overland Flow

Saturation Excess Overland Flow occurs when the soil becomes saturated, and any additional precipitation or irrigation causes runoff. This type of runoff is the main mechanism behind VSA hydrology. To learn more about saturation excess overland flow, mechanisms for occurrence, and areas prone to saturation, read on.

Check out our Saturation Excess Overland Flow animated slideshow

Animated Examples of Saturation Excess Overland Flow:

Mechanisms for Occurrence

In many regions runoff is most commonly generated on relatively small portions of the landscape that are susceptible to becoming completely saturated. Once the soils in these areas saturate to the surface, any additional rainfall that falls (irrespective of intensity) becomes overland flow. This process is termed "saturation-excess overland flow". Therefore the propensity of an area to produce runoff is largely independent of rainfall intensity. Instead, the total rainfall amount and landscape factors such as soil depth (i.e., available water storage capacity), upland-watershed area, and local topography are the important factors determining whether or not a particular area in a watershed will generate runoff. Moreover, as rainfall continues the saturated area grows in extent, increasing the area generating runoff (hence the term variable source area, VSA). This is in contrast with Hortonian infiltration-excess runoff generation, which depends on soil type (infiltration rate vs. rain intensity) but is independent of position in the landscape, with the runoff generating area always the same. Examples of regions where VSA hydrology is significant include the Northeast and the Pacific Northwest as well as forested mountain areas in the US.

Sources: DPSA and Return Flow

Saturation-excess overland flow comes from two distinguishable sources. Rain falling on already-saturated soil has no option but to run off - this case is termed direct precipitation on saturated areas (DPSA). The other source, termed return flow, occurs if the rate of interflow entering a saturated area from upslope exceeds the capacity for interflow to leave the area by flowing downhill through the soil. The excess interflow thus "returns" to the surface as runoff, hence the term. Whereas DPSA runoff only occurs during and just after a rainfall event, return flow seepage can continue as long as an interflow excess exists.

Areas Prone to Saturation

Areas prone to saturation have either a high ground water table or hard pan (fragipan) at shallow depth. Interflow results in the formation of saturated areas at the bottom of slopes, usually in concave areas, or quickly resurfaces in seeps and ditches (see photos of this). Some hydrological processes observed in Upstate New York, primarily the Catskills region, can serve to further elucidate mechanisms resulting in saturation. Soils in the Catskills are generally permeable (relative to rainfall intensity) and underlain by a shallow, low-permeable, restrictive layer, typically bedrock or fragipan. Rainwater easily permeates the soil and, by-and-large, runs laterally as interflow on top of the restrictive layer down-slope. Many researchers have observed evidence of the accumulation of interflow water at the bottom of a slope in the Catskills during wet periods in the form of increased moisture content at hill bottoms relative to the steep parts of hills (Frankenberger et al. 1999, Ogden and Watts 2000, Mehta et al. 2002). Some common locations where saturation occurs are areas where the soil above the restricting layer is shallow, in places where the downhill topographic slope decreases such as the toe-slope of a hill, or in topographically converging areas (Figure 1). All three incidences are locations in the landscape where the Darcy flow capacity, or interflow capacity, is reduced either by a decrease in hydraulic transmissivity or hydraulic gradient. When interflow capacity is sufficiently restricted the soil will saturate. During periods of enhanced rainfall, interflow will be higher and often expand the extent of saturation around saturation-prone areas; conversely, dry periods will decrease interflow and extent of saturation. This is illustrated for wet and dry seasons in Figure 2 (from Frankenberger, 1996).

Figure 1: Incidence of Saturation-excess hydrology: 1) Shallow soil; 2) convergence area; and 3) decreasing downhill slope.

Figure 2: Seasonal changes of saturated areas (variable source areas) in a watershed in New York State. Color denotes areas more prone to saturation.

Variable Source Area Hydrology

Extension of Saturation Excess Concept

Variable source area hydrology (VSA) is an extension of the saturation excess concept, recognizing that the extent of saturated areas in a watershed will expand and contract, i.e. vary temporally. The variation in the extent of saturated areas has been studied over a range of temporal scales ranging from storm duration, hours and days, to seasons (figure 2). Click here to view a saturated area that varies over the time period of several days, during which a storm occurred. View an animated clip of a watershed over several years time (Note: This is a large .mpeg file). The red areas are saturated; notice how they change in size over the course of a year.

VSA Basics

The concept of VSAs thus goes beyond drawing the water/soil interface boundary at the soil surface and making the simple assumption that water either infiltrates or runs off. The VSA concept includes the soil reservoir in a spatial landscape context to determine locations where the soil reservoir becomes saturated and begins to generate runoff. The soil reservoir methodology utilizes the bedrock, impermeable soil layers (relative to above layers), and/or the depth to the water table as the underlying hydrologic boundary instead of the soil surface. Thus, both hydrologic and soil water (i.e., porous media) concepts are combined to evaluate potential runoff areas in the landscape.

Questions or comments may be directed to mtw5@cornell.edu.