Exercise 2. Runoff Risk Assessment

You are to determine the runoff risk for a 40 acre wintering site in central Alberta.

The site characteristics are:
  • The soil is an Orthic Black Chernozem developed on fine textured parent material.
  • The site is located on the upper ridge of a low relief valley within a confined floodplain.
  • The bedding site is directly above the 10% slope.
  • The length of slope from the bedding site to the creek is approximately 150 feet.

Cattle water directly out of the creek when on grass, but in the winter months rely on developed springs that flow overland in 2 adjoining gullies. The cattle have uncontrolled access to the spring water during the winter months. The area generally receives 6 inches (150 mm) of water equivalent snowfall from November to the end of March plus 3 inches (75 mm) of rainfall in April and May.

The surrounding area is native pasture that is grazed only in the spring during calving. There is an ample rest period with 60% carryover of current annual growth of native grasses present. Adjacent to the creek there are abundant willows and other healthy riparian vegetation. Due to the surrounding vegetation, runoff flow is dispersed and slow moving. Although the site is close to surface water within a confined floodplain, flooding of the wintering site rarely occurs (1 in 25 year flood).

No berms or diversions are in place to stop runoff, or prevent runon, and the runoff usually flows over frozen ground.

50 head of cows are wintered here for 5 months. Bedding is provided every other day and the cattle swath graze 30 acres of mixed barley and oats within the overall 40 acre wintering site.

Rate the following categories based on the data you have just read. Place a check in the appropriate box. When you have completed the 13 categories, provide a summarized total at the end of this exercise.

Site characteristic Risk Factors

1. The slope of a site determines runoff mobility and velocity. The greater the degree of slope, the more velocity runoff has, lessening the ability of plants and other buffers to slow down, filter, disperse and utilize the nutrients. The increased velocity creates erosion problems as well, as faster moving runoff has a greater potential to move soil particles and animal manure. Slope is measured as rise over run. For instance, a slope of 4% means a difference in elevation of 4 ft in a 100 ft length; or 4 m in a 100 m length.
Slope where site is located Risk to Surface Water Your Site
Flat Ground (less than 2% slope). Low  
Slight to moderate slope (2-15% slope). Moderate  
Steep slope (over 15%). High  

 

2. Slope length affects runoff volume. Longer slopes offer a greater surface area to collect precipitation, increasing potential runoff volume and nutrient movement. Increasing slope length also allows the runoff velocity to increase unless buffers, berms or natural breaks are present to retard flow.
Length of Slope on which site occurs Risk to Surface Water Your Site
Less than 15 m (50 feet) Low  
15 to 120 m (50 to 400 feet) Moderate  
Greater than 120 m (400 feet) High  

 

3. Precipitation, either in the form of snow or rain, is the major contributor to the amount of runoff that will occur at a site.
Precipitation (Rain and snow water equivalent occurring
during the confinement period plus 2 months after)
Risk to Surface Water Your Site
Less than 125 mm (5 in) Low  
From 125 to 200 mm (5 to 8 in) Moderate  
Over 200 mm (8 in) High  

 

4. Vegetative filters or buffers include areas such as grassed waterways shrubs and tree rows used to slow runoff. This occurs through a process of settling, filtration, absorption and infiltration with a subsequent uptake of nutrients by the vegetation.
Ground Cover during time of runoff or nutrient application Risk to Surface Water Your site
No exposed soil or site surrounded with trees and shrubs, or 50% or greater residue of current annual growth of grasses. Low  
Less than 50% current annual growth cover of grass or residue; few trees or shrubs but not surrounding the site. Moderate  
Negligible cover and dominated by exposed soil High  

 

5. Soil texture affects the infiltration of water into the soil profile. Sandy soils have a larger particle size with greater spaces between particles, allowing water an easier movement through the porous soil. Clay has a smaller particle size, allowing itself for greater compaction which restricts downward movement of water. Therefore, runoff is high from clayey and/or frozen soils.
Soil characteristics at time of runoff or nutrient application Risk to Surface Water Your Site
Well drained (coarse textured or sandy) soils Low  
Moderately drained (medium textured or loamy) soils Moderate  
Frozen (Impervious) or slowly drained (fine textured or clayey) soils High  

 

6. Flooding of a site can displace nutrients and transfer them to other areas where water contamination can occur. Low-lying areas are at a higher risk because of the potential for water to overflow the streambanks and wash through areas of high nutrient concentration. Sites where flooding occurs often should be avoided.
Flood Hazard Risk to Surface Water Your Site
Sites that never flood to those with a low flood risk (approx. a 1 in 25 year flood) Low  
Moderate flood risk (less than 1 in 10 years) Moderate  
High flood risk (less than 1 in 5 years) High  

 

7.Runoff flow affects the level at which runoff can cause erosion and the ability of plants to utilize nutrients. An evenly dispersed, slower moving flow will result in less erosion and a more uniform dispersal of nutrients. A channeled, higher velocity runoff will result in a more concentrated transfer of nutrients and soil particles from the site that will cause contamination.
Type of Runoff Flow Risk to Surface Water Your Site
Dispersed flow (uniform or sheet-flow) Low  
Moderately concentrated flow (sheet-flow to rills) Moderate  
Channelled flow (rills to gullies) High  

 

8. The greater the distance from the site to surface water, the greater and more even is the distribution of nutrients. This also allows for more efficient use of vegetation to utilize the nutrients.
Distance from Site to Surface Water Risk to Surface Water Your Site
Greater than 60 meters/200 feet Low  
30-60meters/100-200feet Moderate  
Less than 30 meters/100feet High  

 

9.I Internal soil characteristics play a role in groundwater protection. Frozen or fine textured soils prevent or reduce the risk of contaminants in runoff from reaching groundwater. If runoff occurs on sandy sites, the contaminants are at a high risk of affecting the groundwater because runoff can easily filter through the more porous soil.
Internal Soil Drainage (affecting groundwater) Risk to Ground Water Your Site
Frozen or Impervious (clay textured) Low  
Poorly to moderately drained (fine to medium textured) soils Moderate  
Well drained (coarse textured) soils High  

 

Management Risk Factors

1. The number of livestock in an area is an important indication of the expected level of manure and resultant nutrient concentration. This has to be taken into consideration with the length of the feeding or bedding period, at the site, to get an accurate estimate of nutrient production.
A = Animal Unit Density
(number of head at site/area of confinement (acres))
B = Length of time at site
(months)
C* = A x B
Density factor
     
* Take the density factor calculated and match it to the numbers listed below.
Density factor Risk to Surface Water Your Site
0 – 15 Low  
15 - 60 Moderate  
> 60 High  

 

2. The diversion of upslope water (runon) prevents clean water from becoming contaminated through flow over a nutrient rich site such as a feedlot or manure storage area. Water diversion also reduces the volume of runoff that a manager must deal with. Diverting water also can increase production through improved cropland management or by improving animal health and sanitation.
Diversion of Clean Water Risk to Surface Water Your Site
No off lot water allowed or total diversion by berms or ditches Low  
Partial diversion Moderate  
No diversion of upslope water High  

 

3. Type of water access plays a critical role in the quality of the water body and the health of riparian areas (the sensitive area directly bordering a water body). Uncontrolled access to riparian areas result in high levels of nutrients being released both directly and indirectly. The physical action of livestock trampling and removing the riparian vegetation decreases the sites ability to prevent soil erosion and slow runoff. In addition, there can be direct entry of faeces and urine into the water by the livestock.
Type of Water Access Risk to Surface Water Your Site
Off stream water or dugout Low  
Controlled access to surface water Moderate  
Uncontrolled access to surface water High  

 

4. Runoff from animal feeding/bedding areas or manure storage/composting sites may have a high nutrient and organic matter content, which may pose a pollution threat. Contaminated runoff depletes the oxygen supply in water bodies, thus killing fish. Excess nutrients accelerate plant and algae growth. Directing runoff to storage ponds or catch basins, where the runoff can be stored until field application, protects surface water bodies.
Runoff Control Risk to Surface Water Your Site
Catch basin with calculated volume Low  
Berms or dikes to provide partial retention Moderate  
Use of vegetative buffers; or untilled cropland Moderate  
No runoff control High  

 

SUMMARY:
Add up the number of lows, moderates and highs to evaluate the relative runoff risk factors.

Total number of:

Low  
Moderate  
High  

A dominance of low runoff risk shows that runoff is not a problem and a change in management is not required. Moderate runoff risk factors present a risk to the surrounding water bodies. Alterations could be implemented to decrease the environmental risk from your operation. A high degree of runoff risk confirms a definite risk to surface water bodies.

Areas that show a high risk factor should be focused on to prevent further damage. Factors such as slope, precipitation, and soil characteristics cannot be controlled at the management level. Therefore, large scale changes, such as the movement of the wintering site may be required.