1993 Nutrient / Sediment Loading: Findings  
The term "load" refers to the amount of material (weight of nutrient or sediment) transported within a stream, past a vertical cross section within a specific period  of time.  Load is calculated by multiplying stream flow by substance concentration. The "export co-efficient" or unit area load is defined as the quantity (kg) removed from a specified area (ha) for a given period of time (year).  Rough annual load estimates can be determined in this manner.  

Discussion  
Table 2 shows the 1993 loads for nitrogen, phosphorus, and non-filterable residues (NFR-equivalent nutrient values expressed in lbs per acre).  More than 80% of the surface water generated in 1993 by the Twin Watersheds is attributable to spring runoff (7.5 dam vs. .92 dam for rainfall).  As expected, a major portion of the total nutrient and sediment loads also occurred during spring runoff.  Less than 15% of the total nutrient and sediment load is thought to be from airborne matter deposited in the Twin Watersheds during the winter.  
 
 

Figure 3.  Dissolved and Particulate Nitrogen at West and East Weirs  
of Twin Watersheds for Snowmelt and Runoff in 1992 and 1993.. 

The South Tobacco Creek generated approximately 16,000 kg (2.4 kg N/ha/yr) of nitrogen, 4000 kg (.61 kg P/ha/yr) of phosphorus, and 3,000,000 kg (435 kg/ha/yr) of sediment in 1993 based on flow, water chemistry, and suspended sediment measurements carried out near Miami.  In contrast to the Twin Watersheds, summer runoff contributed less than 20% of the total load at the South Tobacco Creek outlet.  Each of the summer rain events delivered loads exceeding those obtained from spring runoff.  

Figures 8 show how these values compare for various other non-point sources cited in the literature.  These preliminary observations indicate that nitrogen concentrations in South Tobacco Creek are within the ranges found within other rural sources.  Conversely, total phosphorus output from the South Tobacco Creek watershed in 1993 was low in comparison to most other known non-point sources.  
 

Figure 8.  Various non-point sources cited in literature.

Conclusions  
The data presented in this report, combined with the results of soil investigations at the Twin Watersheds (Michalyna, 1994 cited within the Zakrevsky paper) are leading to a better understanding of the hydraulic and chemical processes which govern the release and transport of nutrients and sediments.  This information is still relatively sparse and preliminary.  

A valid estimate of the relationships between certain South Tobacco Creek agricultural practices and their nutrient impacts on Lake Winnipeg is required.  Otherwise, it may be difficult to provide farmers with reliable advice regarding best agricultural management practices and their possible positive impacts.  

Verifying the possible link between agricultural practices and enhanced eutrophication will likely have to await completion of the Twin Watershed study.  A solid understanding of nutrient cycling and the extent to which the underlying physical, chemical, and biological processes are impacted by farming practices will also have to form the basis of a rational solution to Lake Winnipeg's excessive eutrophication problem.  Several major gaps in our knowledge of these processes are identified below.  

Knowledge Gaps and Future Work  - Nutrient Sources and Fate  
Ongoing work should help identify the major sources of nutrients in the South Tobacco Creek watershed within the next few years.  Several years of intensive spring runoff and rainfall data collection will be required to formulate accurate cycling models for nitrogen, phosphorus, and carbon.  

The Role of Sediments  
Preliminary results indicate that sediments may act as a conveyor of nutrients through the South Tobacco Creek watershed and may therefore have a significant impact on the fate of nutrients in the Red-Assiniboine basin.  There is a need to determine the major sources of sediments (stream bank vs. farm soil) in the South Tobacco Creek, and to understand the processes involved in their release and movement.  

Atmospheric Input  
Based on very limited snow data, it appears that atmospheric inputs of nutrients to South Tobacco Creek are insignificant.  However, this hypothesis should be further verified as well.  

The Role of Climate  
Climate is believed to be a key factor controlling the release of nutrients and sediments in the South Tobacco Creek.  There is a need to collect detailed data on the frequency, duration, and intensity of rain events, and to link this information to the results of nutrient and sediment investigations.  

A Final Word  
"I think all farmers are naturally curious about their land and how they farm it.  We are constantly juggling weather, soil, crop and livestock variables in trying to make the right management choices .  The practical research relating environmental conditions in the South Tobacco Creek watershed to our farming practices as well as quantifying the natural processes in the watershed will give us some of those answers"  -   Gordon Orchard, Deerwood Secretary  

Agriculture: Making a Living.

Future generations explore their South Tobacco Creek watershed.

---

Previous  Page  HOME