1. Hydrologic Cycle
watershed
outlet
Key terms with the hydrologic cycle, including the watershed. Also referred to as the water cycle, illustrates the sources, pathways, and sinks of water on, in, or above the surface of Earth. Definitions of terms are included in the watershed hydrology literacy document.
Modified from Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG). No copyright is claimed for this image.
modified from Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG).

2. Critical Terms in the Hydrologic Cycle
Precipitation—the amount of rain, snow, or hail that falls on the watershed, usually expressed in inches or centimeters of water. In a hydrologic budget or model, precipitation is the input.
Runoff—the portion of rainfall that occurs as streamflow from a given watershed. Expressed in inches or centimeters of water, runoff is can be visualized as the depth to which a drainage area would be covered if streamflow were uniformly distributed over it.
Infiltration–the process by which rainfall is absorbed by and transmitted through soil. The infiltration capacity of a soil, the rate at which water is infiltrated, is dependent on a number of factors, including soil moisture, soil texture, land cover, and soil health.
Evaporation and Transpiration—often combined and represented as evapotranspiration or ET. It represents water that returns to the atmosphere after it evaporates from land or water surfaces or from within soils or is transpired by plants during photosynthesis.
Key terms with the hydrologic cycle, including the watershed. The definitions for the terms involved directly or indirectly in the water balance equation are provided. Definitions of terms are included in the watershed hydrology literacy document.
Modified from Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG). No copyright is claimed for this image.
modified from Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG).

3. Hydrologic Cycle
watershed
outlet
What is the value of this representation or model of the hydrologic cycle? What is its purpose? Strengths? Weaknesses?
Strengths: simplified, easy to understand, a teaching tool, global view of the main processes and pathways for water movement
Weaknesses: not very detailed, may or may not be applicable locally, not very functional except as a learning tool
Modified from Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG). No copyright is claimed for this image.
modified from Stream Corridor Restoration: Principles, Processes, and Practices, 10/98, by the Federal Interagency Stream Restoration Working Group (FISRWG).

4. Watershed Components Subwatershed and Subwatershed Divide Watershed
Subwatershed Outlet
Watershed
Divide
A watershed is defined by the area that collects to a specific point along a stream, called the outlet. So watersheds can be any size and consist of smaller and smaller subwatersheds. Components of a watershed and subwatersheds include divides and outlets.
The runoff data used in this unit module are measured from a stream gage at the outlet of the Rock Creek watershed.
Watershed Outlet

5. Watershed as a System Output Storage (or Stock) Input Output WatershedET
Output
Watershed
Divide
Watershed
Precipitation
Runoff
Storage (or Stock)
Soil Moisture or Groundwater
Input
Output
The hydrologic cycle of a watershed can also be considered from a systems perspective. In a watershed system, we consider inputs to and outputs from the system, flow within the system, and characteristics of the system that affect flow. In the watershed system, input is through precipitation and output is by runoff and evapotranspiration. The watershed acts to regulate the amount of input converted to output through storage of soil moisture and/or groundwater. The amount of water that is stored in the system, either as soil moisture or groundwater, is controlled by characteristics of the watershed, some natural like slope and soil, others controlled by humans like land use and cover and soil erosion.

6. Infiltration Capacity
Watershed System ET
Watershed Divide
Watershed
Soil
Erosion
Precipitation -
Runoff -
Vegetation
Density
Infiltration Capacity +
Conceptual model of a watershed system, including input, outputs, storage, and selected characteristics that impact flow within the system. System characteristics that impact flow include soil erosion, infiltration capacity of soils, and vegetation density. The variable relationships are also shown as positive or direct (+) or negative or inverse (-). They are described as follows:
Vegetation Density-Soil Erosion: The relation between vegetation density and soil erosion is negative or inverse—the greater the vegetation density is, the less soil erosion occurs. And vice versa, the less the vegetation density is, the more soil erosion occurs.
Soil Erosion-Infiltration Capacity: The relation between soil erosion and infiltration capacity is negative or inverse, with greater soil erosion, infiltration capacity of the soil is lowered. And vice versa, with less soil erosion, infiltration capacity is higher.
Infiltration Capacity-Vegetation Density: The relation between infiltration capacity and vegetation density is positive or direct, with greater infiltration capacity, vegetation density is generally higher. And vice versa, with lower infiltration capacity, vegetation density is less.
An important attribute of systems are feedback loops that develop between interrelated system characteristics or variables. Feedback loops are referred to as positive or negative feedback loops, depending on whether the feedback occurring when the system is disturbed tends to be reinforced (positive) or dampened (negative). The terms positive and negative do not imply any value judgements as to the desirability of the feedback.
For example, in this slide, the feedback loop is positive. A disturbance that increased soil erosion would tend to decrease infiltration capacity that would stress and decrease vegetation density, and that would tend to promote additional soil erosion. Within the watershed, water storage as soil moisture would decrease because less water is infiltrated, and at the watershed outlet, output as runoff is increased. Alternatively, if soil erosion is slowed, infiltration capacity is increased, promoting vegetative growth and vegetation density and further protecting soils from erosion. As a result, more water is stored in the watershed as soil moisture and watershed output as runoff is less. Both represent positive feedback, but the former instance would be viewed in a negative way or an environmentally unsustainable way. And increased runoff from the watershed increases the potential for downstream flooding.
In what ways might we disturb the system in a harmful or unsustainable way? In what ways might we help the system or mitigate a negative impact? The utility of systems thinking is especially evident here as the variables with which we’ve chosen to describe our system are involved in potentially solving the problem. For example, to slow or eliminate soil erosion, we might increase vegetation density at critical times of erosion (e.g., plant a cover crop over the winter months) or promote infiltration capacity (e.g., install a rain garden at critical points in the watershed). Both actions would decrease runoff from the watershed.
Soil Moisture or Groundwater
Input
Storage (or Stock)
Output

7. Water Balance Equation
P = Q + ET + ΔS
where,
P = precipitation (in),
Q = runoff (in),
ET = evaporation (in), and
ΔS = the change in storage.
From the systems perspective, watershed hydrology can be considered quantitatively using a water balance equation. It is represented by the flow of water in and out of a system by precipitation and runoff, respectively. The difference between input and output required to balance the equation is evaporated from the surface, transpired by plants, or stored in lakes, streams, or soils in the watershed. The unit of study in which input and output are measured is the watershed.
What is the value of this representation or model of the hydrologic cycle? What is its purpose? Strengths? Weaknesses?
Strengths: simplified, easy to understand, inputs are measured locally (e.g., the daily weather report, especially reference to depths of rainfall in the viewer region following a rainfall event), outputs are more evident (e.g., flooding) or significant (e.g., water supply), quantitative using simple math (addition and subtraction), specific to a watershed, functional (i.e., if streamflow from a watershed is the basis for a public water supply or for irrigation, the amount available as output cannot exceed input and is in fact much less), illustrates variability (e.g., seasonal, year-to-year)
Weaknesses: requires precipitation and streamflow data to compute
Water Balance - represents the flow of water in and out of a system. It is represented by the equation P = Q + ET + ΔS, where P is precipitation (in), Q is runoff (in), ET is evaporation (in), and ΔS is the change in storage.

8. Streamflow is measured at the outlet of a watershed and illustrated as a streamflow hydrograph. This would be the output from which runoff is calculated.
Streamflow is a combination of stormflow and baseflow and is represented as a streamflow hydrograph. Baseflow is derived from groundwater, water that has infiltrated in the soil and percolated more deeply and become groundwater. Stormflow is the direct surface contribution from runoff resulting from the rainfall event.
If runoff is high, the streamflow hydrograph, specifically stormflow, reflects this. With high runoff, the hydrograph is large (i.e., the area under the curve is high) and peak discharge is high. This can be the beginning of a discussion about ecosystem services in the watershed. This slide can be moved or duplicated near the end of the presentation as a segue to the assessment.
Other Definitions
Streamflow - general term used to describe water flow within a stream channel. It can refer to stage, a relative depth of water at a gaging station, or discharge (defined below). Streamflow is composed of both stormflow and baseflow.
Discharge - volume of water, or streamflow, that passes a given location in a channel within a given period of time. Discharge is expressed in cubic feet per second (cfs) or cubic meters per second (cms). Runoff is calculated from discharge.
Streamflow Hydrograph - literally a graph of water data related to streamflow, either stage or discharge, over time. The shape of a hydrograph (e.g., peakedness or flashiness) is dependent on watershed properties. Key characteristics of a streamflow hydrograph are illustrated in Figure 3.
Baseflow – the groundwater contribution to streamflow. It is a significant part of the streamflow hydrograph in wet climates. If present, it is generally more regular or constant than stormflow.
Stormflow – the stormwater contribution to streamflow. Stormflow occurs in response to a rainfall event and represents the direct runoff in response to the event. It generally increases with development in a watershed.
Peak Discharge – the point on the hydrograph where discharge is greatest. In small watersheds, it generally occurs immediately following the end of rainfall. Peak discharge depends on the direct runoff and the time it takes for the runoff to concentrate in the stream. It tends to increase with development in the watershed.
Time to Peak Discharge - time from the start of rainfall to peak discharge. It tends to decrease with development in the watershed.

9. Rock Creek at Tiffin OH USGS Gaging Station 04197170
Runoff data from the Rock Creek watershed in Ohio is used in this module. Location, basic watershed characteristics, and a link to the real-time streamflow data from the U.S. Geological Survey are included here.
Rock Creek at Tiffin OH
USGS Gaging Station
Lat 41°06'49", long 83°10'06“
Seneca County, OH
Hydrologic Unit
Drainage area 34.6 mi2
Realtime Stage and Discharge Data:
Rock Creek at Tiffin Ohio
U.S. Geological Survey station data from

10. Aerial image of the Rock Creek watershed, illustrating land use
Aerial image of the Rock Creek watershed, illustrating land use. Note that most of the land cover is associated with agriculture. Urban land use dominates the area around the outlet. Tiffin, Ohio, is the community at the outlet and downstream of it. Instructors can navigate to the watershed using the .kmz file at the Rock Creek Watershed link.
Questions to guide discussion and/or review of the hydrologic cycle and water balance:
Where might precipitation fall in the map area during a given rainfall event? Where does it flow to in general? In what direction? To where? What area defines the water balance?
How is precipitation, or the input to this watershed system, measured? Where? Rainfall is generally measured at a rain gage and reported as a depth, but it is really a volume when considered over the area (depth x area) of the watershed.
What are the potential areas or reservoirs in water is stored? Ask them to visualize a rainfall event? What happens to rainfall, most of it in fact, after rainfall ends? Using prompts like "Why (or when) do we water our lawns?" can get these conversations going depending on the level of the course.
When and where does evaporation occur? Transpiration?
Where is output, or runoff, measured in the watershed? Where does it come from? The stream network? But where does streamflow come from? Watershed output is the measured streamflow at the outlet, generally as a volume or volume per time (i.e., streamflow discharge or total discharge). It is converted to a depth, referred to as runoff when distributed over the entire watershed area (volume/area).

11. Land cover in 2011 in the Rock Creek watershed
Land cover in 2011 in the Rock Creek watershed. Land cover was taken from the National Land Cover Database. It is dominated by cultivated crops, hay/pasture, and deciduous forest. Though not natural land cover, this type of land use maintains a pervious surface and infiltration of water.
This should be used in conjunction with the previous slide. It better defines land cover. The discussion of runoff, evaporation, and transpiration may be more focused by this slide.

12. Classification Description
2011 National Land Cover Database Classification Descriptions
Class
Value
Classification Description
Water 11
Open Water - areas of open water, generally with less than 25% cover of vegetation or soil. 12
Perennial Ice/Snow - areas characterized by a perennial cover of ice and/or snow, generally greater than 25% of total cover.
Developed 21
Developed, Open Space - areas with a mixture of some constructed materials, but mostly vegetation in the form of lawn grasses. Impervious surfaces account for less than 20% of total cover. These areas most commonly include large-lot single-family housing units, parks, golf courses, and vegetation planted in developed settings for recreation, erosion control, or aesthetic purposes. 22
Developed, Low Intensity - areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 20% to 49% percent of total cover. These areas most commonly include single-family housing units. 23
Developed, Medium Intensity – areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 50% to 79% of the total cover. These areas most commonly include single-family housing units. 24
Developed High Intensity -highly developed areas where people reside or work in high numbers. Examples include apartment complexes, row houses and commercial/industrial. Impervious surfaces account for 80% to 100% of the total cover.
Barren 31
Barren Land (Rock/Sand/Clay) - areas of bedrock, desert pavement, scarps, talus, slides, volcanic material, glacial debris, sand dunes, strip mines, gravel pits and other accumulations of earthen material. Generally, vegetation accounts for less than 15% of total cover.
Forest 41
Deciduous Forest - areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75% of the tree species shed foliage simultaneously in response to seasonal change. 42
Evergreen Forest - areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75% of the tree species maintain their leaves all year. Canopy is never without green foliage. 43
Mixed Forest - areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. Neither deciduous nor evergreen species are greater than 75% of total tree cover.
Class, value, and descriptions of different land covers from the 2011 National Land Cover Database (NLCD). The classification system is modified from the Anderson Land Cover Classification System. Full documentation of the NLCD can be found at
Rock Creek watershed is dominated by
Cultivated Crops—areas used for the production of annual crops, such as corn, soybeans, vegetables, tobacco, and cotton, and also perennial woody crops such as orchards and vineyards. Crop vegetation accounts for greater than 20% of total vegetation. This class also includes all land being actively tilled.
then
Pasture/Hay—areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops, typically on a perennial cycle. Pasture/hay vegetation accounts for greater than 20% of total vegetation.
Deciduous Forest—areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75% of the tree species shed foliage simultaneously in response to seasonal change.
adapted from legends and descriptions for the NLCD available from

13. Classification Description (cont.)
2011 National Land Cover Database Classification Descriptions
(cont.)
Class
Value
Classification Description (cont.)
Shrubland 51
Dwarf Scrub - Alaska only areas dominated by shrubs less than 20 centimeters tall with shrub canopy typically greater than 20% of total vegetation. This type is often co-associated with grasses, sedges, herbs, and non-vascular vegetation. 52
Shrub/Scrub - areas dominated by shrubs; less than 5 meters tall with shrub canopy typically greater than 20% of total vegetation. This class includes true shrubs, young trees in an early successional stage or trees stunted from environmental conditions.
Herbaceous 71
Grassland/Herbaceous - areas dominated by gramanoid or herbaceous vegetation, generally greater than 80% of total vegetation. These areas are not subject to intensive management such as tilling, but can be utilized for grazing. 72
Sedge/Herbaceous - Alaska only areas dominated by sedges and forbs, generally greater than 80% of total vegetation. This type can occur with significant other grasses or other grass like plants, and includes sedge tundra, and sedge tussock tundra. 73
Lichens - Alaska only areas dominated by fruticose or foliose lichens generally greater than 80% of total vegetation. 74
Moss - Alaska only areas dominated by mosses, generally greater than 80% of total vegetation.
Planted/
Cultivated 81
Pasture/Hay – areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops, typically on a perennial cycle. Pasture/hay vegetation accounts for greater than 20% of total vegetation. 82
Cultivated Crops – areas used for the production of annual crops, such as corn, soybeans, vegetables, tobacco, and cotton, and also perennial woody crops such as orchards and vineyards. Crop vegetation accounts for greater than 20% of total vegetation. This class also includes all land being actively tilled.
Wetlands 90
Woody Wetlands - areas where forest or shrubland vegetation accounts for greater than 20% of vegetative cover and the soil or substrate is periodically saturated with or covered with water. 91
Emergent Herbaceous Wetlands - Areas where perennial herbaceous vegetation accounts for greater than 80% of vegetative cover and the soil or substrate is periodically saturated with or covered with water.
Continuation of NLCD classification system. The classification system is modified from the Anderson Land Cover Classification System. Full documentation of the NLCD can be found at
Rock Creek watershed is dominated by
Cultivated Crops—areas used for the production of annual crops, such as corn, soybeans, vegetables, tobacco, and cotton, and also perennial woody crops such as orchards and vineyards. Crop vegetation accounts for greater than 20% of total vegetation. This class also includes all land being actively tilled.
then
Pasture/Hay—areas of grasses, legumes, or grass-legume mixtures planted for livestock grazing or the production of seed or hay crops, typically on a perennial cycle. Pasture/hay vegetation accounts for greater than 20% of total vegetation.
Deciduous Forest—areas dominated by trees generally greater than 5 meters tall, and greater than 20% of total vegetation cover. More than 75% of the tree species shed foliage simultaneously in response to seasonal change.
adapted from legends and descriptions for the NLCD available from

14. Rainfall-runoff data from Rock Creek watershed
Rainfall-runoff data from Rock Creek watershed. Runoff increases in proportion to, or directly with, rainfall. The solid diamonds represent annual totals for rainfall and runoff from the years 1984–2014. The open diamond represents the average annual rainfall and runoff for the same time period. A link to the data and an empty plot are available as a word document in the Description and Teaching Materials section. A link to a spreadsheet version of the data and the plot above is available in the Teaching Notes and Tips section.
Several guiding questions should be used to introduce the plot:
General to Scientific Data
How is scientific data plotted? By convention, the independent variable is plotted on the X-axis and the dependent variable is plotted on the Y-axis. Which is the independent variable? Which is the dependent variable?
Specific to Rainfall-Runoff Data and the Water Balance Equation
If runoff equaled rainfall, what would the relations look like on an X-Y scatterplot? Does runoff equal rainfall? Does output equal input? Go back and forth between input and rainfall and output and runoff. Use them interchangeably in your presentation and discussion.
If not, is there a consistent ratio between runoff and rainfall? Students can calculate this for several data pairs or visualize this on the plot.
The discussion should end with a sense of why this is not the case. Where is the missing water? And in what proportion or range of proportions?

15. Rainfall = Runoff or 1:1 Ratio of Rainfall to Runoff
Q if Rainfall (P) = Runoff (Q)
Ave ET + ΔS
Depending on the instructor, this slide can be used or not. It is the same as the previous slide, rainfall-runoff data from Rock Creek watershed—but the annotation provides a graphical explanation to a couple of the questions from the previous slide.
If runoff equaled rainfall, what would the relations look like on an X-Y scatterplot? Does runoff equal rainfall? Does output equal input? It would scatter about the line representing a 1:1 relation between rainfall and runoff.
If not, is there a consistent ratio between runoff and rainfall? Students can calculate this for several data pairs or visualize this on the plot.
The discussion should end with a sense of why this is not the case. Where is the missing water? And in what proportion or range of proportions?
Slides like this (or drawings on the board) are invaluable in that they show the power of annotation in creating a more effective or useful slide.
Ave Q
Ave P

16. Water Balance Equation for Rock Creek Watershed
P = Q + ET + ΔS
where,
P = precipitation (in),
Q = runoff (in),
ET = evaporation (in), and
ΔS = the change in storage.
End with a review of the water balance equation. You can do this at the board or via this slide and the next. Water Balance—represents the flow of water in and out of a system. It is represented by the equation P = Q + ET + ΔS, where P is precipitation (in), Q is runoff (in), ET is evaporation (in), and ΔS is the change in storage.
Ask students what terms can be quantified from the Rock Creek watershed data. This can be done for any given year. It can be done for an average over the years of record as well. The next slide shows the equation rearranged with data for 2014 for Rock Creek watershed.
For the Rock Creek watershed, the average for the period of record would be
P = Q + ET + ΔS
37.37 in = in + ET + ΔS
The remainder, to balance the equation, is in, the amount of water that is stored within the water as soil moisture and eventually leaves the watershed through evaporation and transpiration.

17. Water Balance Equation for Rock Creek Watershed
P = Q + ET + ΔS
P – Q = ET + ΔS
where,
P = precipitation (in),
Q = runoff (in),
ET = evaporation (in), and
ΔS = the change in storage.
The water balance equation rearranged with 2014 rainfall-runoff data for Rock Creek watershed.
For the Rock Creek watershed, the average for the period of record would be
P = Q + ET + ΔS
37.37 in = in + ET + ΔS or
37.37 in in = ET + ΔS = in
The remainder, to balance the equation, is in, the amount of water that is stored within the water as soil moisture and eventually leaves the watershed through evaporation, transpiration, or deeper percolation to groundwater.
for in – in = in
Average ? ______ _______ = _______
( )

18. Water Balance Equation for Rock Creek Watershed
P = Q + ET + ΔS
P – Q = ET + ΔS
where,
P = precipitation (in),
Q = runoff (in),
ET = evaporation (in), and
ΔS = the change in storage.
This slide may or may not be used by the instructor. It shows the water balance equation and data from 2014, but also provides some verbiage for how this data may sound if used in a written description or explanation.
for in – in = in
Percentage of rainfall occurring as runoff = in/34.51 in x 100 = 32 %
68% of rainfall is infiltrated and temporarily stored as soil moisture, some of which evaporates from the surface or soil, some of which is transpired by plants, and some of which percolates deeper to groundwater.