A watershed, drainage basin, or catchment is an area of land from which surface runoff (and subsurface recharge, and groundwater usually) flows to a common discharge outlet.  The main functions of a watershed are:

• The collection and redistribution of water
• Providing a diversity of soils, vegetation, and landscape features to support life functions of insects, animals and humans. A watershed's development and basic features or morphology characteristics result from climate and geologic interactions over long time periods.

Similar to the concepts discussed in Competency Area 2: Soil Hydrology, the components of the hydrologic cycle and the water budget for the soil profile can be applied to watersheds. Some differences in these concepts need to be considered, however, because of the larger scale and spatial variations that occur in watersheds, especially as they become larger.

In watersheds, the spatial distribution of elevations, soils, geology, vegetation, and land use interact with the hydrologic cycle, causing variations with how water collects, drains, and is redistributed within soil associations and throughout the landscape.

In the Soil Survey, distinctive patterns of soils, elevation relief, and drainage are commonly grouped by their association. For example, the following figure shows a typical relationship of soils and underlying geologic material in the Hudson-Rhinebeck-Manlius general soil association map unit (From Soil Survey of Saratoga County, New York).

image source: NRCCA Soil and Water Management Study Guide

This general soil association is further described in the survey as gently sloping to hilly with scattered rock outcrops in the Manlius part of the unit. Large watersheds commonly consist of a composite of these general soil associations, creating further complexity in watershed hydrology.

The following terms are thus described in the context of watersheds.

• Precipitation – often varies across a watershed, and the spatial variation tends to be even greater in watersheds with significant topographic relief because the cooler air at higher elevations has a lower saturation vapor pressure and the humidity condenses easily. The amount of precipitation also varies more with convective type storms as compared to broadly occurring frontal storms.
• Storms – are classified as frontal, convective, and orographic.
• Frontal storms occur as warm or light air masses meet cold or heavy air masses. This is the dominant type of precipitation that occurs in the Northeast, generally producing the low intensity, long duration rainy days.
• Convective storms occur as a result of air expanding when heated by solar energy, and the lighter air moistened with evapotranspiration rises, cools, and condenses. In the Northeast, this produces the summer thunderstorms, often delivering high intensity but short duration rains.
• Orographic storms occur as wind forces warm, moist air masses to rise over hills and mountains, so although this occurs in conjunction with frontal and convective storm types in high relief watersheds, this type of storm process better explains why watersheds in the western side of the Adirondacks (the Tug Hill Plateau) gets more precipitation than the eastern side (Hudson Valley and Lake Champlain).Some important considerations of watersheds in regards to storm direction are the watershed size, shape, gradient, and aspect. Storms moving slowly over large, elongated, steep gradient watersheds in the opposite direction of the aspect are likely to create flashy increases in stream flow and depth, often leading to flooding conditions in the lower end of the watershed.
• Infiltration and percolation – will vary substantially across a watershed. Generally, infiltration and percolation tend to be higher in soils that are higher in elevation than the other soils in the same association in the watershed, or higher in soils that straddle convex slopes. However, there are numerous exceptions to this because of the variation in soils, soil layering and geologic material.
• Storage (Depression, Detention, Channel, Groundwater, Retention) – refers to the locations in the watershed where surface runoff or excess drainage water is held, at least temporarily.
• Depression storage is the water stored in field surface depressions and therefore not contributing to surface runoff.
• Detention storage refers to the water in excess of the depression storage which is temporarily stored somewhere in the watershed while enroute to streams. Wetlands are a good example of detention storage.
• Channel storage is the water temporarily stored in channels while enroute to an outlet or it also refers to the drainage water that can be stored above the start pumping level in ditches and floodways without flooding adjacent land.
• Groundwater is stored until such time that the water table starts to rise above the stream.
• Retention is the precipitation on an area that does not escape as runoff.

• Vegetation – plays an important role in the amount of retention, both in terms of interception and transpiration. The effect of interception on retention is more significant when precipitation comes in low intensity, small amounts. A tall dense alfalfa canopy can intercept about 0.20 inches of rain, and under the right conditions it can evaporate directly with little of this ever hitting the ground.

image source: http://www.css.cornell.edu/faculty/hmv1/watrshed/budget.htm

• Base Flow – occurs when the stored groundwater raises the water table elevation above that of an adjacent ditch or stream. When this occurs, the hydraulic head of the water table begins to exceed that of the adjacent stream, and the groundwater flows to recharge the stream.
• In a perennial stream (always flowing), base flow is a continuous process of transferring temporarily stored groundwater into surface water.
• For an intermittent stream (one that only flows part of the year), the groundwater is stored until the deep percolation recharge raises the water table, and then the excess groundwater is transferred to the stream causing it to flow.
• Storm Flow – is most often considered to be the direct surface water runoff that flows to and recharges a stream following a precipitation event. Some hydrologists, however, consider storm flow as both the direct surface water runoff plus the increase in base flow (over pre-storm event) that occurs in a stream in response to a rain event.
• The distinction and separation of storm flow and base flow in a stream is very important to stream water quality, as the quality of the storm flow and the base flow is usually quite different. An ephemeral stream is one that flows only during and immediately after a rain event, carrying only surface water runoff with no base flow contribution. This type of stream channel is above the water table at all times. The term concentrated flow path is often used in reference to a channel carrying only storm flow.
• Runoff (Surface, Channel and Subsurface) – varies from point to point in a stream within a watershed. Since these different sources of runoff contribution to a stream can vary, the water quality of a stream can vary substantially from one location to another.
• Runoff tends to accumulate as one approaches the outlet of a watershed, and larger watersheds generally exhibit more runoff. Land use can greatly impact the different components of runoff, and subsequently the water quality of the stream(s) within the watershed. Large areas of impervious surface will increase the surface runoff component by reducing the opportunity for infiltration, percolation, and storage.
• Converting large areas of forest to other land uses often increases surface runoff by changing interception and evapotranspiration, and perhaps infiltration and percolation components. Channel alterations and modifications impact the amount of channel storage.
• Evaporation and Transpiration – varies spatially in watersheds with respect to the extent of open water, types of vegetation, and impervious surfaces. Even more importantly, evaporation and transpiration vary temporally (or seasonally throughout the year), causing major shifts in retention and runoff.

A stream hydrograph is a graphical or tabular representation of the stream flow rate (cubic feet per second) with respect to time. A pollutograph would be a similar graphical or tabular representation of the concentration of pollutants or contaminants with respect to time. When the stream flow rate and pollutant concentrations are graphed or tabulated together with respect to the same time, one can observe how the stream flow and pollutant concentrations vary with respect to each other. By multiplying the stream flow rate times the pollutant concentration, the total load (i.e., pounds) of pollutant delivery is determined, and can be accumulated for some stream flow time period.

image source: http://www.css.cornell.edu/faculty/hmv1/watrshed/Plutogrf.htm

Some characteristic features of a stream hydrograph are its rising limb, peak flow, and falling limb or recession curve. The rising limb portion is where the stream flow rate is increasing. The peak flow represents the maximum flow rate that occurs, and the falling limb or recession curve is the portion showing how the stream flow rate decreases. When the rise and subsequent fall of the stream flow rate happens quickly as a result of a rain event, the stream is referred to as flashy. In contrast, if this rise and fall takes a long time, the stream is sluggish. At the outlet of a small, steep gradient watershed, stream hydrographs tend to be flashy in response to thunderstorms because the runoff (storm flow) concentrates and then dissipates quickly. A watershed with extensive impervious surface or compacted soils will also have a flashy response, compared to a similar watershed that is forested.

Some pollutant concentrations increase quickly with increasing stream flow, and the concentration may peak before the flow rate peak. Suspended sediments, phosphorus, fecal coliforms, pesticides and other typically sediment bound type pollutants (with high soil-water adsorption partitioning coefficients) would be examples of this type of response. This is often referred to as the first-flush of pollutant delivery. The concentration of other dissolved pollutants such as nitrate-nitrogen and chloride salts are less affected with increases in stream flow rate, and may actually decline somewhat during the stream flow rate peak, but then gradually increase during the recession of the flow. The delayed increase in concentration of dissolved pollutants is often a result of the base flow (flushed groundwater) contributions.

image source: http://www.css.cornell.edu/faculty/hmv1/watrshed/Stream.htm

Pollutant delivery requires a pollutant to be 'available' when water is moving over or through the soil and landscape. Soil erosion is an example of soil being available (by dislodgement) when water is flowing. Generally, pollutants that are readily soluble in water and that have a low soil-water adsorption partition coefficient (do not readily attach to soil) can be transported in either surface runoff or in water percolating through the soil. Nitrate-nitrogen is an example of this and can thus easily be leached to groundwater. Pollutants that have high soil-water adsorption partition coefficients (easily attach to soil) are more readily transported in surface runoff, but are typically removed as water infiltrates and percolates through the soil. Phosphorus would be an example of a pollutant with a high adsorption partition coefficient. However, soils with macropores may have a limited ability to absorb pollutants with high adsorption partition coefficients because of the limited interaction of the percolating water with the soil particles. Thus, the importance of nutrient budgets is to minimize nutrient pollutant availability when water is flowing over or through the landscape. It is typically easier to manage nutrients, than to manage excess flowing water. When nutrients are applied in excess of crop nutrient requirements, the opportunity increases for pollutants to be available and transported off-site to other receiving water bodies. The off-site transport and diffuse loss of pollutants from storm water runoff (from agriculture and other sources) is termed nonpoint source pollution.

The total maximum daily load (TMDL) is the maximum amount of a pollutant that a water body can receive and still meet water quality standards. Federal and state regulatory agencies establish the maximum allowable point and nonpoint source loading for designated stream reaches, based on characteristics of stream flow, ability of the stream to process pollutants, and designated uses. Nutrient budgets or developing and implementing comprehensive nutrient management plans (CNMP's) is a methodology for agricultural producers to minimize the off farm loss of excess available nutrients to reduce nutrient pollutant loads to designated receiving streams and waterbodies. With implementation of CNMP's, nutrient losses and nonpoint source pollutant loads can be reduced, and TMDL targets can be achieved.

Precipitation return periods were discussed in CA 1, PO 10. The following figure shows contour lines of the 25-year, 24-hour precipitation amounts for the Northeast (note precipitation amounts in inches on the right hand side of the contour lines). The 25-year, 24-hour precipitation amount varies between 4 to 5 inches for much of the Northeast, and increases to 6 inches for Long Island, NY, and southern areas of Connecticut. Information for many different frequency-duration storms can be obtained via the National Oceanic Atmospheric Administration (NOAA) web site http://www.erh.noaa.gov/er/hq/Tp40s.htm.

The main agricultural point or concentrated sources from livestock-based systems are from barnyards, feedlots, silage storage and milkhouse wastewater discharges. For conventional cash grain, horticultural and vegetable production systems, the point sources arise from fertilizer, pesticide and fuel storage areas. The main non-point sources of contaminants from agricultural operations are sediments (largest on a mass basis), phosphorus, pesticides, pathogens, and nitrate-nitrogen (usually from tile drain discharges).

Aquifers are geologic formations that store groundwater in the saturated pores of these sediment or rock formations, and are sufficiently permeable to transmit economic quantities of water to wells or springs. Aquifers consist of two types, confined and unconfined, as distinguished by differences in their hydraulic behavior.

A confined aquifer has an upper, and perhaps lower natural soil or rock layer boundary that does not transmit water readily, and thus, the stored water is confined within the permeable layer materials. The importance of this hydraulically is the stored water can then develop a hydraulic head pressure that exceeds the level of the upper confining layer. A well that is drilled into this type of aquifer and where the hydraulic head pressure is adequate to raise the water past the upper confining boundary and to the surface is commonly referred to as an artesian or flowing well.

An unconfined aquifer is one where the upper boundary consists of a relatively porous natural material that transmits water readily, and thus it does not confine the stored water, and the water table is free to rise and fall with recharge or withdrawal of water. When water is withdrawn (pumped) from an unconfined aquifer, the water table is drawn down and the water (specific) yield comes from the drainable porosity of the soil or rock material.

The water yield from wells depends on the intrinsic permeability or hydraulic conductivity of the saturated soil or rock material. For soil type aquifer materials consisting of well-sorted gravels, well yields will range from 500 to 1500 gallons per minute (gpm). Well-sorted sands and glacial outwash yield 100 to 500 gpm. Fine sands and mixed silty sand materials generally yield 10 to 100 gpm. For rock materials, well yields are also highly correlated to the extent of rock fractures and consolidation. Wells tapping the large solution channels in the Onondaga limestone formation can yield several thousand gallons per minute. Carbonate formations yield considerably less, more typical of around 200 to 500 gpm. Sandstone formations are still less (around 50 to 200 gpm), and shale formations range from 2 to 50 gpm. Wells yielding only 2 to 10 gpm may be suitable for supplying homes, but are generally not sufficient to support livestock operations. Well yields of 100 gpm or more are generally needed for irrigation.

image source: http://groundwater.oregonstate.edu/groundwater/html/GroundwaterMovement.htm

When wells are pumped, the water level in the casing is lowered (drawn down) relative to the water level in the aquifer. This drawdown causes stored water in the aquifer to flow towards the well. The cone of depression refers to this drawdown, since the water level surface in the aquifer material exhibits a conical shape with increasing distance in all directions away from the well. The bottom of the cone is the drawdown water level in the well. The size of the cone of depression, or the distance away the water level is affected by a pumping well depends on the pumping rate, the specific yield and conductivity of the aquifer material, and the length of time the well is pumped. High pumping rates for long periods of time will lower water levels a long distance away. In unconfined aquifers, the water level is typically affected less than 100 feet away, but can extend for several hundred feet. However, in confined aquifers, the water level can be affected for several hundred to several thousand feet away.

The capture zone of a well refers to the area on the surface under which water drains towards a pumping well. Essentially this area extends up-gradient from the well to the boundary of the groundwater divide (where the groundwater would slope to another direction). The importance of defining the extent of the capture zone is that activities on the land surface that overlie the capture zone may eventually pollute the well. Pollutants that readily leach, and which are transported to the aquifer, will affect the well water quality. Shallow unconfined aquifers are particularly vulnerable to contamination because there is no restricting soil layer above these aquifers to divert leached pollutants.

image source: http://groundwater.oregonstate.edu/groundwater/html/GroundwaterWells.htm

Soil layering and varying subsoil geologic conditions will significantly affect water movement. Layers that are permeable allow water to flow in any direction. However, layers that are impermeable impede water movement, and the flow of water is then redirected through more permeable layers. The depths, separation distance between layers of differing permeability, and the slope of impermeable layers all affect water movement and the direction in which it moves. Different combinations of depths and separation between layers also affect water movement in different ways, and whether water flows primarily to surface or groundwater.

Groundwater is most easily contaminated when the soil is permeable to great depths. The deep unconsolidated permeable materials of alluvial valley-fill common to Central NY and NY's Southern Tier valleys or in the Coastal Plain area of Long Island, NY are examples where leaching of nitrates and soluble pesticides can contaminate the groundwater. In contrast, an impermeable layer that lies just below the surface, such as in soils with fragipans, impedes downward water movement, and excess water is forced to move laterally (interflow). If the soil is sloping and the fragipan is not, the water is forced back to the surface. This will show up as a seep in the landscape, and the runoff from this may cause surface water contamination. These types of soils are common in the Catskills region of New York, where surface runoff contaminated with phosphorus facilitates the eutrophication of reservoirs.

The deep permeable soil deposits facilitate deep percolation and provide areas for groundwater recharge. In contrast, soils with shallow impermeable layers or drainage restrictions recharge surface water.

Hydrologically sensitive areas (HSA's) are areas in the landscape where there is a high hydrologic risk for water movement off-site. Thus, HSA's have a high potential for transporting pollutants off-site.

Examples of HSA's include:

• Impervious surfaces (barnyards, bunker pads).
• Relatively impermeable (crusted or frozen soil, soil with shallow hardpans or fragipans) and erosion prone (low permeability, sloping) soils.
• Saturated soils (poorly drained soils, variable source saturating areas, seeps).
• Flood plains (especially those that flood frequently).
• Flowing surface water areas (perennial and intermittent streams, concentrated flow paths).
• Groundwater recharge areas (highly permeable soils, sinkholes, shallow soil over Karst – a special type of landscape that is formed by the dissolution of soluble rocks, including limestone and dolomite).

The first two examples of HSA's occur as a result of rainfall and processes of infiltration-excess. For nutrient management planning and CAFO permitting purposes, various NRCS standards are available to address these areas, and RUSLE is used to address the erosion prone soil areas. The latter four examples are induced by saturation-excess hydrologic processes, and these areas are addressed with the NRCS nutrient management standard, in conjunction with the P and N Leaching Index risk tools.

Critical management zones are where the application or loading of potential pollutants, and their availability, overlap with the HSA, resulting in the mixing and transport of the pollutants. A common example of a critical management area is when manure is applied in a low lying area of the field that saturates quickly during a rain event, producing runoff that transports the manure off-site.

Wetlands are areas of wet soil (hydric soil) that is inundated or saturated under normal circumstances, and would support a prevalence of hydrophytic vegetation (plants adapted to survive in low soil oxygen environments). Key processes that occur in wetlands, in addition to providing diverse biota and wildlife habitat, are to slow the transport of water and provide surface water storage (unless already filled), resulting in the retention and removal of suspended and many dissolved pollutants. For example, denitrification and reduction of other chemical compounds (pesticides) are a key function. Wetlands are quite effective at removing suspended and particulate materials, such as phosphorus bound to soil particles, but are less effective at retaining dissolved phosphorus.

Riparian buffer zones (an area of land immediately adjacent to water bodies) share many of the same characteristics and functional processes of wetlands, but depending on their position in the landscape are generally less saturated.

Thus, riparian buffers may also serve to infiltrate incoming surface runoff, facilitating the removal of suspended and some dissolved pollutants that readily absorb to soil. When streams flood, water flowing out into the riparian buffer may also be cleansed as sediments, nutrients, and other pollutants are attenuated (reduced and retained) in the buffer zone.

The multiple barriers concept is a strategy applied to minimize the occurrence, availability, and transport of pathogens to surface and groundwater supplies. It consists of four major barriers:

• Imports and bio-security to reduce occurrence of unwanted pathogens
• Animal health and hygiene to minimize the spread and availability of pathogens
• Waste management to contain and further reduce availability
• Land application BMP practices to reduce transport and off-site exports

More information and implementation of the multiple barrier concept is addressed by the New York's USDA NRCS Conservation Practice Standards (http://www.nrcs.usda.gov/programs/watershed/).

As part of the Federal Clean Water Act Section 303(d), EPA requires state environmental agencies to periodically assess and report on the quality of waters in their state, and to identify and develop a list of impaired waters (water not potentially able to meet quality standards of TMDL criteria). In New York, the Department of Environmental Conservation (NYS-DEC) carries out this task, and has developed the New York State Section 303(d) List of Impaired/TMDL waters. The NYS-DEC has developed and recently updated this list with input from the public, which was approved by the EPA in September 2008. See http://www.dec.ny.gov/docs/water_pdf/303dlist08.pdf. The list identifies the impaired water bodies in New York, lists the type of pollutant impairment and probable causes, and the most likely source of the impairment. The list also prioritizes the impaired water bodies which are in further need of TMDL review and development. Other states in the Northeast would also have similar lists for their states. Where agriculture is listed as the cause of the impairment, Federal and State agency programs working with agriculture will give priority in directing resources to participants (Soil and Water Conservation Districts and producers) in those watersheds.

• Watersheds are areas of land used to catch and redistribute water, and maintain ecological diversity and health at the location. The water inputs, storage capacity, and release of water are influenced by the geography, storms, vegetation, and other factors. Watershed issues include runoff (particularly of pollutants) and flooding. Tools like the hydrograph and pollutograph are used to visualize the flow of water or pollutants in these systems.
• Aquifers are water storage areas partially or completely encased underground. The effects of well-drilling and pollution of aquifers is an important issue.
• Hydrologically-sensitive areas are those at risk of contamination, based on their locations and factors like proximity to pollutant sources. Watershed protection is based on a multiple-barrier concept that seeks to reduce pollution through reduction of the pollutant source and careful management of remaining pollutants.

For an animated example of how a watershed works control click on: www.fs.fed.us/.../stream-riparian.shtml