Johnston Creek - Banff Park

Runoff is the water that we see in rivers, streams and creeks. Runoff begins as precipitation - in the form of rain or snow. Precipitation falls over a drainage basin, that collects the water in small channels and tributaries and ultimately directs it to rivers and streams. If the precipitation falls as snowfall, then the water is held in storage over the basin until springtime when the snow melts. Water that is not held in the soil as soil moisture, that does not evaporate from the surface, or that does not seep into the ground to recharge groundwater aquifers eventually becomes surface runoff.

The amount of runoff produced by a storm varies significantly. Much more runoff will be produced from a storm when the soils are wet or saturated, versus the same storm occurring after a prolonged dry period. During dry periods, the amount of water stored in the soil and in low areas and depressions is depleted. Precipitation falling first has to saturate the sails and fill the depressions before significant runoff can be generated. Often this process can be significant enough that very little of the rainfall actually ends up as runoff.

Definitions from the North American Lake Management Society (NALMS):

Runoff

(1) That part of the precipitation, snow melt, or irrigation water that appears in uncontrolled surface streams, rivers, drains or sewers. It is the same as streamflow unaffected by artificial diversions, imports, storage, or other works of man in or on the stream channels. Runoff may be classified according to speed of appearance after rainfall or melting snow as direct runoff or base runoff, and according to source as surface runoff, storm interflow, or ground-water runoff.

(2) The total discharge described in (1), above, during a specified period of time.

(3) Also defined as the depth to which a drainage area would be covered if all of the runoff for a given period of time were uniformly distributed over it.

Runoff, Average Annual

Average of water year runoff in millimeters or dam³ for the total period of record.

Direct Runoff

Runoff, Direct

The runoff entering stream channels most immediately after rainfall or snowmelt. It consists of surface runoff plus interflow and forms the bulk of the hydrograph of a flood. Direct runoff plus base runoff compose the entire flood hydrograph.

Runoff, Ground Water

That part of the runoff, which has passed into the ground, has become ground water, and has been discharged into a stream channel as spring or seepage water. Also referred to as base runoff or base flow.

Runoff Percentage

Runoff expressed as a percentage of the precipitation.

Runoff Plots

Areas of land, usually small, arranged so the portion of rainfall or other precipitation flowing off and perhaps carrying soluble materials and soil may be measured.

Runoff Rate

The volume of water running off in a unit of time from a surface, expressed as millimeters of rainfall per hour, cubic meters per second, or other units.

Runoff, Surface

(1) That part of the runoff, which travels over the soil surface to the nearest stream channel.

(2) That part of the runoff of a drainage basin that has not passed beneath the surface since precipitation. Surface runoff is not the same as direct runoff.

Meteorological factors and the physical characteristics of the basin influence the amount and timing of the runoff from a basin.

Meteorological Factors Affecting Runoff:

1. Type of precipitation (rain, snow, sleet, etc.). Rain will collect and form runoff immediately, while snow will accumulate and runoff when temperatures rise above freezing.
2. Rainfall intensity, amount and duration. Small amounts of precipitation create less runoff than large amounts. Similarly the intensity is directly related to runoff. More intense precipitation events will generate more direct runoff, where as less intense precipitation events will allow more water to enter the ground and become part of the slower subsurface runoff and groundwater.
3. Distribution of rainfall over the drainage basin. If the entire basin is being rained on, there will usually be more runoff than if only a portion of the basin is receiving precipitation.
4. Direction of storm movement. As the storm moves over a basin, the precipitation will be affected by the elevation of the basin. If the storm is moving up the basin, being forced over mountains or other high elevations, more rainfall will be generated as a result of the cooler temperatures and higher elevations. This phenomenon is called the orographic effect.
5. Other meteorological and climatic conditions that affect evapotranspiration like temperature, wind, relative humidity and season. The temperature influences precipitation significantly. Evaporation and evapotranspiration can effectively remove water from the basin before and during the runoff period. Seasonality also is important, as in winter most of the precipitation is solid, and the ground can be frozen. Runoff will not occur until spring. In the summer, dry soils soak up some of the water preventing it from forming runoff. This is called initial abstraction of water.

Physical Characteristics Affecting Runoff:

1. Land use has a significant effect on runoff from an area. For example, an area covered with parking lots will create much more runoff than a forested area. This is because the parking lots do not absorb any water, so almost all of the precipitation is turned immediately into surface runoff. Forests produce less runoff because the trees intercept some of the water, and some will be absorbed by the soil (abstraction).
2. Vegetation and soil type influence the runoff. Different types of vegetation and soils will limit the amount of precipitation that eventually turns into runoff. Sandy soils absorb more precipitation than soils composed of clay. The type and density of the vegetation will have an impact on how much water evaporates and how fast it flows over the surface of the land. Slow moving water has more time to infiltrate into the ground and to evaporate.
3. If there is more drainage area, there is more potential to generate runoff. But the larger a basin is, the better the chances are for runoff to be stored or lost back to the atmosphere through evaporation and evapotranspiration.
4. Basin shape, slope and topography affect how fast or slow the water is collected, concentrated and transported within the basin. Steeper, narrower basins generate more water than flat, broad ones.
5. Elevation and direction of orientation can impact how much precipitation a basin receives. Depending on the weather patterns for an area, differently oriented basins will receive different amounts of precipitation. Elevation also plays a key role in the amount, and type of precipitation a basin receives.
6. Drainage network patterns determine how efficient a basin is at transporting the runoff created on it. Basins with lots of rivers and streams can pass large volumes of water relatively quickly, whereas basins with lots of swamps, marshes and lakes tend to collect and store the runoff within these water bodies reducing the amount of runoff leaving the basin. Both basin types may have created the same total volume of runoff, however in one it is stored in the basin and the other passes the runoff downstream.
7. Basin conditions such as soil moisture, when and how much precipitation has recently fallen, and other factors will affect how much of the precipitation falling onto it is turned into runoff. Basins with wetter soils are called "wet". When the soils dry out they are called "dry". Wet basins usually generate more runoff than dry ones for an equal amount of precipitation.

Runoff varies greatly throughout Alberta: not only on a regional basis, but also from year-to-year in response to climate variability. The map on the right (click to view a full-size pdf) represents the expected average annual runoff yield for a particular location in Alberta. Similar to a precipitation map, it shows which areas are wetter and which are drier. Generally, area with less runoff receive less precipitation and have higher evaporation and evapotranspiration. The second map, below, shows several examples from different watersheds across Alberta, giving an average and range for the amount of water the basins generate in one year. Hydrologists and hydrogeologists study and model these processes to better understand and predict how basins will convert precipitation into surface runoff and groundwater. We need to understand these processes so we can manage the quantity and quality of our water.