Hydrologic Cycle

By Special Contributing Author: Dennis Jackson, Hydrologist

The hydrologic cycle describes the transport of water through the environment. Water moves as precipitation from the atmosphere onto the earth's surface. Once precipitation reaches the earth's surface, it can follow any one of the several pathways listed below on its journey back to the atmosphere:

• Interception by Vegetation
• Infiltration into the Soil
  • Replenish Soil Moisture
    • Evaporation from Soil Surface
    • Transpiration of Vegetation
    • Subsurface Storm Runoff to Stream Network
    • Saturation Overland Flow to Stream Network
  • Percolate to Ground Water Zone
• Hortonian Overland Flow to Stream Network
• Evapotranspiration from the Land Surface, Streams, Lakes, and the Ocean

The hydrologic cycle. Image courtesy of FISRWG 1998.

Precipitation

In San Mateo and Santa Cruz Counties, precipitation occurs primarily as rainfall. Large winter storms provide most of the annual rainfall. Light snowfall may also occur in the winter. The presence of snow on the ground can increase the runoff from the next rainfall event. There are monthly precipitation records for the City of Santa Cruz extending back to 1905. When the annual water year (October 1 through September 30) precipitation for the City of Santa Cruz is graphed, it shows that the average annual precipitation is 29.71 inches, and ranges from 10.20 inches to 61.29 inches. See the Climate section for more details about precipitation in San Mateo and Santa Cruz Counties.

The annual precipitation, based on the annual water year of October 1 through September 30, for the City of Santa Cruz from 1905 through 2000 ranges from 10.20 to 61.29 inches, and averages 29.71 inches.

Fog drip may also have a strong influence on water budget of forest areas near the coast during dry summer months. By increasing humidity, fog decreases the evaporation rate from soil and stream surfaces, as well as the transpiration rate from vegetation.

Interception

A portion of the precipitation is temporarily stored on the surface of vegetation and is eventually returned to the atmosphere by evaporation. The amount of precipitation returned to the atmosphere in this way is known as interception loss. The presence of impervious surfaces such as buildings and roads also produce interception losses.

Typical interception losses are in the range of about 20 percent of the total precipitation. The total amount of interception loss during a storm depends mostly on the intensity and total amount of precipitation. The nature of the intercepting surface is the next most important variable in determining interception losses. For example, deciduous trees have no foliage available in the winter whereas coniferous forests have a more constant amount of foliage through the year.

Annual interception tends to be higher for coniferous forests than for deciduous forests. The needles of conifers can hold more water than the broad leaves of deciduous trees. In addition, conifers tend to intercept a higher proportion of light precipitation events than deciduous trees. The forests of Santa Cruz and San Mateo Counties are predominately coniferous. Logging and conversion of forest land to urban-residential are the primary land use activities that may alter the amount of interception losses from an area.

Infiltration

Infiltration is the process of water entering the soil. Soils consist of particles of sand, silt, clay, organic matter, and the spaces or pores between the particles. After precipitation reaches the ground surface it enters the larger soil pores by the force of gravity and enters the smaller pores by capillary action.

The diameter of the larger pores determines the maximum rate water can enter the pore by gravity. The surface of the soil particles surrounding the pore space offer resistance in the form of friction to the gravity flow of water through the pore space. The relative surface area of the pore increases as the diameter of the pore decreases, so gravity flow resistance is inversely proportional to pore diameter – the smaller the pore diameter, the greater the resistance. The force of gravity only moves water vertically downward.

Capillary attraction moves water through small diameter pores. Capillary attraction can exceed the force of gravity, so flow through small diameter pores can be horizontal or even upward. The surface of the particles surrounding the small diameter pores also offers resistance to flow induced by capillary action. In fine textured soils the resistance to flow is large, so there is little movement of water.

The process of infiltration can continue only if there is room available for additional water at the soil surface. The available volume for additional water in the soil depends on the porosity of the soil and the rate at which previously infiltrated water can move away from the surface through the soil. The maximum rate that water can enter a soil in a given condition is the infiltration capacity. If the arrival of the water at the soil surface is less than the infiltration capacity, all of the water will infiltrate. If the water arrives at the soil surface at a rate that exceeds the infiltration capacity, ponding begins and is followed by runoff over the ground surface. This runoff is called Horton overland flow.

The rate of infiltration is affected by soil characteristics including ease of entry, storage capacity, and transmission rate through the soil. The soil texture and structure, vegetation types and cover, water content of the soil, soil temperature, and rainfall intensity all play a role in dictating infiltration rate and capacity. For example, coarse-grained sandy soils have large spaces between each grain and allow water to infiltrate quickly. Vegetation creates more porous soils by both protecting the soil from pounding rainfall, which can close natural gaps between soil particles, and loosening soil through root action.

Infiltration capacity rapidly declines during the early part of a storm and then tends towards an approximately constant value after a couple of hours. Previously infiltrated water fills the available storage spaces and reduces the capillary forces drawing water into the pores. Clay particles in the soil may swell as they become wet and thereby reduce the size of the pores. In areas where the ground is not protected by a layer of forest litter, raindrops can detach soil particles from the surface and wash fine particles into surface pores where they can impede the infiltration process.

Once water has infiltrated the soil it remains in the soil, percolates down to the ground water table, or becomes part of the subsurface runoff process.

Soil Moisture

Soil is said to be saturated when all of its pores are filled with water. The soil's saturation moisture capacity is equal to its porosity, measured as a percentage of the soil volume. As gravity drains water out of the soil profile, the large soil pores empty. When no more gravity drainage occurs, the soil is said to be at field capacity. A saturated soil will reach field capacity after draining for one or more days.

After gravity drainage stops, soil moisture is further reduced by direct evaporation from the soil surface and by the transpiration of plants. The total water loss from the soil moisture reservoir due to evaporation and transpiration is referred to as evapotranspiration (ET).

Vegetation that has an unlimited supply of water to its roots loses water to the atmosphere at a rate called the potential evapotranspiration rate. If the water supply is limited, then the actual ET is less than the potential ET. Actual ET depends on the weather, soils, and the type of vegetation.

Runoff Processes

Only a portion of the precipitation on a region becomes runoff. Rantz (1974) produced a contour map of annual average runoff for the San Francisco Bay region for the period 1931 through 1970. Streamflow and precipitation data presented in the Rantz report show that the average annual runoff for 16 stream gages in the Santa Cruz Mountains was 31 percent of average annual precipitation. The remaining 69 percent of the precipitation was transpired, diverted, or entered deeper ground water systems.

Precipitation is routed down a hillslope to the stream channel network along one of the following four pathways:

• Horton overland flow
• Ground water flow
• Subsurface stormflow
• Saturation overland flow

Horton overland flow occurs when the rate of water reaching the ground surface is greater than the infiltration capacity. When rainfall exceeds the infiltration capacity, the excess water begins to fill small depressions on the surface. The water stored in the depressions is called depression storage and either evaporates or eventually infiltrates into the soil. Depression storage ranges from about 0.2 inches on steep hills to 2.0 inches on furrowed fields. After the depressions have been filled, water begins to flow across the land surface and the process is called overland flow. The amount of water flowing over the surface of the hillside is called surface detention. The velocity of overland flow ranges from about 10 to 500 meters/hour (0.108 to 4.8 inches per second). Overland flow usually occurs only on a small portion of a watershed.

The amount of streamflow that occurs during the dry period between storms is called baseflow. Baseflow is ground water that has moved into the stream channel. The ground water table is higher than the stream surface in areas where baseflow is entering the stream channel. The elevation difference between the surface of the stream and the ground water table is what forces the ground water to move into the channel. The water table near a stream at the base of a hill will rise during a storm. The rising water table steepens the ground water surface, which causes the flow into the stream channel to increase. The runoff produced by this process is subsurface stormflow.

Subsurface stormflow also results when a layer with low permeability underlies the soil on a hillslope. Drainage from the soil is impeded by the layer and water accumulates above it. The accumulated water flows downhill along the interface through the lower portion of the more permeable soil and enters the stream channel. Subsurface stormflow moves on the order of a few meters to tens of meters per day.

Under some conditions, such as a layer with low permeability near the surface or a shallow ground water table, a large rainstorm will cause the water table to rise to the ground surface. In this situation, subsurface flow can return to the surface and flow overland into the channel network. This type of runoff process is called return flow. The term return flow is also used to describe irrigation water that seeps through the ground to the channel network and augments dry weather stream flows.

Areas where the ground water table has risen to the land surface are impervious to rainfall and are saturated. The rain falling on these saturated areas flows overland to the channel network since it cannot infiltrate. The overland flow generated by rain falling on saturated areas, together with the return flow, is called saturation overland flow. Saturation overland flow moves rapidly into the stream network.

Storm runoff processes in the Santa Cruz Mountains are probably dominated by subsurface stormflow. But there may be some areas where saturation overland flow occurs. Hortonian overland flow would be expected to occur only on impervious surfaces such as roads, exposed bedrock, and areas with soil of low permeability.

Evapotranspiration

Water is returned to the atmosphere through the processes of evaporation and transpiration, or evapotranspiration. Evaporation is the phase change that water undergoes when it becomes a gas. Transpiration is the evaporation that occurs on the surfaces of photosynthesizing leaves. During the summer months, riparian vegetation can withdraw significant amounts of water from streams to support photosynthesis, respiration, and transpiration.

References

Dunne, T., and L.B. Leopold. 1978. Water in Environmental Planning. San Francisco, CA: W. H. Freeman and Company.

Federal Interagency Stream Restoration Working Group (FISRWG). 1998. "Stream Corridor Restoration: Principles, Processes, and Practices." Federal Interagency Stream Restoration Working Group (FISRWG). GPO Item No. 0120-A; SuDocs No. A 57.6/2:EN 3/PT.653. ISBN-0-934213-59-3. View on-line document.

Rantz, S.E. 1974. Mean Annual Runoff in the San Francisco Bay Region, California, 1931 -70. USGS Map MF-613.

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