HYDROLOGY

Climate

The Stephenfield Lake Watershed has a continental semi-humid climate characterized by significant variations in seasonal and annual temperatures, and precipitation.  Temperature extremes have ranged from -430^o C to 400^o C.

Precipitation is higher on the top of the Manitoba Escarpment than on the lowlands.  The mean annual precipitation for Somerset which is representative of areas on the top of the Manitoba Escarpment is about 555 mm (21.9 in.), as shown in Table 1.  Approximately 125 mm (4.9 in.) or 23% falls as a snow.  The mean annual precipitation for the lowlands varies between 515 mm (20.3 in.) in the Rathwell area (Table 2) to 490 mm (19.3 in.) in the Holland area (Table 3) with just under half occurring as rain between May through August. The wettest months are June and July.  At Rathwell during the 32-year period 1968 to1988, the minimum recorded annual precipitation was 376 mm (14.8 in.) in 1988 and the maximum was 681 mm (26.8 in.) in 1991. A graph of total annual precipitation for Rathwell and Somerset showing the effect of the difference in elevation is shown on Figure 1.

The average annual gross evaporation is in the order of 800 mm (31.5 in.). The fact that the evapotranspirative losses are greater than the annual precipitation in the region indicates that the study area is generally water deficient. The average annual gross evaporation estimated for Portage la Prairie is shown on Figure 2.

Surface Water Resources

General:
The Stephenfield Lake Watershed is characterized by a southeasterly curving waterway, known as the Boyne River, along the base of the Manitoba Escarpment which delineates the Pembina Hills on the south from the Lake Agassiz flood plain on the north and east.  The Boyne River Watershed is shown on Figure 3.  The Boyne has its headwaters originating south of Pellys Lake and in the Pinkerton Lakes area southwest of Treherne.  A smaller area of headwater originates four miles northwest of Treherne.  The Boyne River is fed by several tributaries off the escarpment before draining into Stephenfield Lake.  The tributaries mainly originate on the Manitoba Escarpment and traverse down and through the escarpment.  Two named tributary creeks, the Roseisle and Lyles, drain directly into Stephenfield Lake.  Lyles Creek is a north trending waterway while Roseisle Creek flows northeast along the southeast watershed boundary.  All tributaries are deeply incised where they pass through the escarpment.  In some areas the shale bedrock is exposed.
The Boyne River drops in the range of 500 feet from its headwaters in the Pembina Hills until it empties into the Stephenfield Lake some 70 miles downstream.

Hydrometric Data:
Streamflow and lake level data has been collected at five hydrometric gauging stations within the Stephenfield Lake Watershed. The locations of the five stations along with their gross drainage area boundaries are shown on Figure 3.                                               Table 4 indicates, for each of the stations, the type of data collected, the period of record, the effective and gross drainage areas, and the ratio of the effective to gross drainage area. The gross drainage area boundary is defined as the area at a specific location, enclosed by its drainage divide, which might be expected to entirely contribute runoff to that specific location under extremely wet conditions.  The effective drainage area is that portion of a drainage area which might be expected to entirely contribute runoff to the main stream during a median (1:2 year event) runoff year.  This area excludes marsh and slough area and other natural storage areas, which would prevent runoff from reaching the main stream in a year of average runoff.  The effective to gross drainage area ratio is an indication of how well an area is drained. A perfectly drained area has a ratio of one. The ratio within the Stephenfield Lake Watershed implies that the area is generally well drained.
Some basic streamflow statistics for the Boyne River and Roseisle Creek gauging stations are shown in Table 5. The mean monthly flows for the Boyne River near Treherne (05OF010), Boyne River near Roseisle (05OF011) and Roseisle Creek near Roseisle (05OF009) are shown in Table 6, Table 7 and Table 8 respectively. 

Table 4: Stephenfield Lake Watershed Hydrometric Gauging Stations

Table 5: Boyne River and Roseisle Streamflow Statistics

Annual Runoff:
The longer-term mean annual unit runoff from the watershed is 2 inches in the south half of the watershed and 1.6 inches in the north half which means that of the total precipitation which falls on the watershed, about 10 percent shows up as streamflow.  Based on the historic streamflow data, the mean annual runoff (MAF) volume for the Boyne River near Roseisle is 19,000 acre-feet (Table 7) or an unit runoff of 1.6 inches.  On the other hand, Roseisle Creek near Roseisle has a MAF of 11,300 acre-feet (Table 8) or a unit runoff of 2.5 inches.  Roseisle Creek is more productive as it is entirely an escarpmental stream.  

Flow Variability:
The Boyne River and Roseisle Creek are typical of small prairie streams. They experience a wide variability in flow regime. Streamflow variability is well illustrated by comparing extremes of daily discharge. The Boyne near Roseisle historic daily low flow is 0 cfs, recorded in numerous years, while the historic daily high is 7,300 cfs recorded in 1979. The Boyne River is considered to be an intermittent prairie stream as it experiences periods of no flow in the summer and/or fall of drier years.
The majority of slope off the Escarpment is north facing, which helps to further temper the climate (i.e. Slower, longer spring melt period). The data show on average about 85 per cent of the annual runoff occurs in the months of March to May, as illustrated on Figure 4.

Stephenfield Lake:
Stephenfield Lake is located centrally in and between NE 33 and NE 36 in Township 6, Range 7W and the west end is situated 2 miles northeast of the base of the Manitoba Escarpment.  The Boyne River empties in at the west end of the Lake (NE 33) while Lyles Creek and Roseisle Creek spill in at SE 34 on the south side.  A network of manmade drains take runoff from most of Township 7, Range 7W and empty mainly into the Boyne River and to a lesser degree directly into the Lake.
Stephenfield Dam was constructed in 1963 by P.F.R.A at a full supply level (fsl) of 970 feet. The Stephenfield Lake helps maintain an adequate water supply for Carman, provides water for stock and irrigation usage and other needs of licensed users. The Town of Carman is situated 12 miles east from the dam outlet on the Boyne River. The Lake also sustains recreational enjoyment by supporting public day-use and camping facilities.
The present spillway at the Stephenfield Dam conveys excess water above elevation 972 feet.  The spillway crest or fsl was raised to elevation 972 feet (a 2 foot increase) in the fall of 1990 to provide additional water for increasing water demands and to offset volume lost to sedimentation since the reservoir was originally constructed in 1963. A 1990 study had concluded that the Lake’s volume had been reduced by about 20% over the period 1963 to 1988.  Historic recorded water levels for Stephenfield Lake are shown in Table 9 and plotted on Figure 5.  The water level plot shows the effect of raising the Lake’s fsl in 1990. A re-survey of the Lake in 1992 indicates that at fsl, the Lake has a flooded area of 460 ac. and a storage volume of 3690 ac.-ft.  Figure 6 is a plot of the Stephenfield Lake Reservoir maximum, mean and minimum monthly historical water levels. The graph shows that the reservoir level is replenished in April and May of each year and steadily declines to a yearly low point in February to March. A plot of any year of reservoir levels will fall between the upper and lower curves.  For example, the 1983 year for Lake Stephenfield has been plotted on Figure 6 with the graphed line for 1983 situated between the median and minimum levels.

Operating rules for the Dam were established on July 25,1991 and are outlined in the following.

This is the normal release pattern. Greater discharges may be made in special situations if the reservoir level is above the rule curve elevation.  On the other hand, releases may not be any greater than this if the reservoir is below the levels indicated on the rule curve.  Furthermore, in severe drought situations, if the reservoir falls below established cutoff elevations, summer releases will be reduced from the established release pattern to forego irrigation and to protect municipal supplies.
The Boyne River near Roseisle and Roseisle Creek near Roseisle streamflow gauging stations have at least 30 years of data each. This data has been used to produce the graph on Figure 8, which shows the sum of the open water inflows to Lake Stephenfield.  The graph shows that annual volumes for the years 1967 to 1996 were more than sufficient to fill the Lake every year. However, the critical period for water supply on Lake Stephenfield, which was used to determine the Rule Curve, is the drought of the 1930’s. 

Municipal Water Use:
The three communities of Notre Dame Des Lourdes, St. Claude, and Treherne have historic water use data. The recorded monthly water use for Notre Dame Des Lourdes, St. Claude, and Treherne are shown on Table 10, Table 11, and Table 12 respectively. The annual water use in the three communities has been increasing is illustrated on Figure 9 to 11. 

Groundwater Discharge:
The Odanah shale of the Riding Mountain formation and granular deposits on and through the escarpment provide a nominal amount of groundwater to the surface water system in the spring and fall seasons.  Most of the groundwater contribution is introduced to the Boyne River via the tributary creeks upstream of the gauging stations. Approximately, five square miles of the watershed on the south side of the Boyne River and Lake Stephenfield contributes ungauged groundwater discharge from the escarpment complex into Stephenfield Lake.