Souris River Basin Spring Runoff Outlook As of March 15, 2018 Prepared by: Flow Forecasting & Operations Planning Water Security Agency
Basin Conditions Summer rainfall in 2017 in the Saskatchewan portion of the Souris River Basin was extremely low (Figure 1). As shown in Figure 2, precipitation in the 60 days prior to freeze-up was approximately 50% of average. Prior to freeze-up, cropland topsoil moisture was generally considered to be low in the southeastern portion of the basin, dropping to very low in the central and northwestern portion of the basin. As shown in Figures 3 and 4, point snowfall data collected within the basin indicates cumulative winter precipitation to date is near to slightly below average (0 to 10mm below or 60 to 100% of average) across the Saskatchewan portion of the Souris River Basin. A precipitation event in early March (Figure 8) brought an estimated 15 to 35 mm of moisture to the basin. Snow Surveys Airborne gamma snow surveys were completed by the US National Weather Service (NWS) on March 7, 2018. This survey indicated snow water equivalent (SWE) values vary from approximately 50 to 65 mm (Figure 5). This product may be overestimating the SWE of the snowpack due to moisture from mid-winter melt events that would have went into the soil. With that said, this product does correlate reasonable well with the SWE values estimated via passive-microwave satellite (Figure 6). Water Security Agency (WSA) conducted 2 manual snow surveys near Weyburn on March 12, 2018. These samples indicated only 20mm of SWE which correlates better with the NWS SNODAS image (Figure 7). Additional snow surveys are planned for March 22-23. Generally speaking, the snowpack is thought to be below normal on the west side of the basin and closer to normal on the east side. 2018 Spring Runoff Forecast As per the terms of the 1989 Canada-United States Agreement on Water Supply and Flood Control in the Souris River Basin, on February 1 and thereafter on the 15 th and last day of the month until runoff occurs, the WSA, in coordination with the NWS, will prepare forecasts of the 30-day and 90-day runoff volumes for the basin. The full agreement is available here: 1989 Agreement Runoff volume estimates based on conditions as of March 15, 2018, are outlined in Table 1. These volumes are the average of WSA and NWS forecasts. As shown in the table, runoff yields are expected to be below median values at all forecast points. The forecasted 50 th percentile 30-day local runoff volume below the Canadian Reservoirs is well below the 37,500 dam 3 threshold for the initiation of Flood Operations under the terms of the 1989 Agreement. The forecasted unregulated 90 day volume at Sherwood is well below 216,000 dam 3, the other criteria which would initiate flood operations under the 1989 Agreement. Operations will proceed based on non-flood operating rules and no further reservoir drawdowns will be initiated at this time. Also included in Table 1, in order to illustrate the uncertainty in these forecasted volumes, are estimates of the volumes associated with a 10% and 90% confidence of exceedance for the 30-day volumes. 1
Table 1: March 15, 2018, Runoff Forecast for the Souris River Basin Forecast Location Long Creek Near Noonan Inflow into Rafferty Reservoir Diversion to Rafferty Reservoir Inflow into Grant Devine Lake 90% Confidence 30-Day Runoff Volume (dam 3 x 10 3 ) Best Estimate 10% Confidence 90-Day Runoff Volume (dam 3 x 10 3 ) 90% Best Confidence Estimate Peak Flow (m 3 /s) Event Return Period (years) 2 5 20 2 5 5 <1:2 2 4 10 2 4 5 <1:2 0 0 10 0 0 5 <1:2 2 4 15 2 4 5 <1:2 Local Runoff 4 6 12 4 6 5 <1:2 Sherwood Crossing Projected Sherwood Crossing Natural Sherwood Crossing Unregulated 6 8 20 6 8 5 <1:2 12 22 50 12 15 5 <1:2 10 20 60 10 15 5 <1:2 Notes for the Forecast The 90-Percent 90-Day inflow forecast to each of the reservoirs is used in Plates A-1, A-2, A-3 of the 1989 Agreement to determine target drawdown levels at Boundary, Rafferty, and Grant Devine Lakes respectively (Long Creek near Noonan, Inflow into Rafferty Reservoir, and Inflow into Grant Devine Lake in the above table). Inflows into Rafferty Reservoir do not include diversion from Boundary Reservoir. Local Runoff is the volume of runoff that is expected at the Sherwood Crossing from the basin below the Canadian reservoirs (Boundary, Rafferty, and Grant Devine). If this 30-day Local Runoff volume exceeds 37,000 dam 3, Flood Operations, as per page A-26 of the 1989 Agreement, are initiated. Sherwood Crossing Projected is equal to the expected runoff less planned reservoir storage at the Canadian Reservoirs to the end of the forecast period. This is the volume (and associated peak flow) that is expected to occur at the Sherwood Crossing based on local runoff and planned releases from the Canadian Reservoirs. Sherwood Crossing Natural is equal to the expected runoff less the volumes from the Yellow Grass and Tatagwa Marshes and minor project diversions. This is an estimate of the natural flow volume at the Sherwood Crossing. Sherwood Crossing Unregulated is equal to the expected runoff (including runoff from the Yellow Grass and Tatagwa Marshes). If this best estimate 30-day Sherwood Crossing Unregulated volume exceeds 216,110 dam 3, the Flood Operations, as per page A-26 of the 1989 Agreement, are initiated. The best estimate 30-day volume is used in Plates A-5 and A-6 to determine target flows at Sherwood and Minot. 2
Proposed Reservoir Operations for the Spring of 2018 Please note that these are preliminary operating plans based on forecasted inflows. Actually operating decisions will be made based on information available at the time. Boundary Reservoir On March 15, 2018, Boundary Reservoir was at an elevation of 559.15 m, approximately 9800 dam 3 below full supply. Due to very low projected runoff, flood operations are not in effect and no pre-runoff drawdown is required. With average conditions going forward to spring melt, Boundary Reservoir is not projected to reach its full supply level (FSL) in 2018. The preliminary operating plan for Boundary Reservoir is to store all inflows. Should the reservoir reach FSL, any additional runoff will be diverted into Rafferty Reservoir. In the event that Rafferty Reservoir reaches its full supply level, releases will be made from Boundary Reservoir into Long Creek rather than diverting over to Rafferty Reservoir for release directly into the Souris River. Rafferty Reservoir On March 15, 2018, Rafferty Reservoir was at an elevation of 549.5 m. This is 47,000 dam 3 or 1 m below its full supply level. Due to very low projected runoff, flood operations are not in effect and no pre-runoff drawdown is required. With average conditions going forward to spring melt, Rafferty Reservoir is not projected to reach its full supply level in 2018. The present operating plan for Rafferty Reservoir is to store all inflow in an attempt to raise the reservoir to its full supply level. In the event that Rafferty Reservoir fills, any diversions from Boundary Reservoir will cease and releases will be made at a controlled rate, respecting downstream constraints including the target flow at the Sherwood Crossing. If the reservoir surcharges above full supply, it will be returned to full supply (550.5 m) no later than June 1. Grant Devine Lake On March 15, 2018, Grant Devine Lake was at an elevation of 561.0 m. This is 11,100 dam 3 below its full supply level (562.0 m). Due to very low projected runoff, flood operations are not in effect and no pre-runoff drawdown is required. With average conditions going forward to spring melt, Grant Devine Lake is not projected to reach full supply in 2018. The preliminary operating plan for Grant Devine Lake is to store all inflows in an attempt to reach full supply. If the reservoir happens to reaches its full supply level, release will be made at a reasonable rate, respecting downstream impacts and the target flow at Sherwood, with the objective of being at full supply on or before June 1. Grant Devine Lake will also be used to meet any apportionment requirements in 2018 not satisfied by prior releases and local flows below the reservoirs. 3
Figure 1: 2017 Growing Season Rainfall Percentile 1
Figure 2: Percent Normal Precipitation 60-days Prior to Freeze-up (September 3 to November 1, 2017) 2
Figure 3: Percent Average Winter Precipitation (Nov. 1, 2017 Mar. 11, 2018) 3
Figure 4: Departure from Average Winter Precipitation (Nov. 1, 2017 Mar 11, 2018) 4
Figure 5: Snow Water Equivalent from Airborne Gamma Surveys (March 7, 2018) 5
Figure 6: Snow Water Equivalent as Estimated via Satellite (March 8, 2018) 6
Figure 7: SNODAS Modelled Snow Water Equivalent (March 15, 2018) 7
Figure 8: March 3-6 Snowfall Accumulations 8