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Canadian Prairie Hydrology and Runoff Generation

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Presentation on theme: "Canadian Prairie Hydrology and Runoff Generation"— Presentation transcript:

1 Canadian Prairie Hydrology and Runoff Generation
John Pomeroy Centre for Hydrology, University of Saskatchewan, Saskatoon

2 *evaporation used here as transpiration + evaporation + sublimation
Prairie Hydrology Major river flow is primarily from mountain runoff, but prairie runoff supplies smaller rivers, streams, wetlands, and lakes Prairie Runoff forms in internally drained (closed) basins that are locally important but non-contributing to river systems that drain the prairies, OR drains directly to small prairie rivers (Battle, Souris, Assiniboine) >80% of runoff during snowmelt period Redistribution of snow to wetlands and stream channels in winter is critical to formation of runoff contributing area Drainage of small streams and wetlands ceases completely in summer when actual evaporation* consumes most available water. Baseflow from groundwater often nonexistent. Prairie streams are almost completely ungauged and often altered by dams, drainage, water transfers, etc *evaporation used here as transpiration + evaporation + sublimation

3 Prairie Hydrological Cycle

4 Prairie Runoff Generation
Snow Redistribution to Channels Spring melt and runoff Dry non-contributing areas to runoff Water Storage in Wetlands

5 Non-Contributing Areas to Streamflow a Prairie Characteristic

6 Prairie Hydrology – don’t blink
Smith Creek, Saskatchewan Drainage area ~ 450 km2 No baseflow from groundwater

7 Information Needed to Estimate Runoff
Snow accumulation and redistribution Melt rate Infiltration to frozen soils Infiltration excess forms runoff >80% of all runoff is snowmelt runoff

8 Blowing Snow: Transport, Sublimation and Redistribution of Snow
Pomeroy and Gray, Wat Resour. Res. (1990) Pomeroy and Male, J Hydrol. (1992) Pomeroy, Gray and Male, J Hydrol. (1993) Pomeroy and Gray, NHRI Science Report No. 7 (1995)

9 Effect of Blowing Snow Sublimation on Prairie Snow Supply (losses, mm SWE)
Location Stubble-field Fallow-field Calgary 19.7 37.5 Peace River 6.6 7.6 Swift Current 28.2 37.8 Prince Albert 24.9 29.6 Regina 39.4 48.1 Yorkton 18.6 28.6 Portage 23.5 33.8 Winnipeg 27.4 36.5 Pomeroy and Gray, NHRI Science Report No. 7 (1995) hourly simulations

10 Distribution of Blowing Snow over Landscapes
Blowing snow transport, and sublimation relocate snow across the landscape from sources to sinks depending on fetch, orientation and area. Source Sink Stubble Field Grassland Brush Trees Fallow Field

11 Shelterbelts on Prairies
Winkler, Manitoba Transport to shelterbelts depends on upwind fetch and vegetation roughness Conquest, Saskatchewan

12 Spatially Distributed Snow Redistribution
Snow mass balance equation St Denis, Saskatchewan

13 Results – Spatially distributed SWE
Fang and Pomeroy, Hydrol Proc, in preparation

14 Spatially distributed SWE cont’

15 Spatially distributed SWE cont’

16 Spatially distributed SWE cont’

17 Spatially distributed SWE cont’

18 Spatially distributed SWE cont’

19 Spatially distributed SWE cont’

20 Spatially distributed SWE cont’

21 Spatially distributed SWE cont’

22 Spatial Pattern of Blowing Snow Sublimation

23 Simulations vs. Snow Surveys

24 Snowmelt Degree Day Method has problems in open environments with late melt, & in forests. Energy Balance snow CAN be estimated using reliable and readily applicable methods

25 Coupled Mass and Energy Equations for Snowmelt
MELT of SWE = QM/(w Lf Bi) Melt Energy QM = Q* - QE – QH – QG – dU/dt Q* Net radiation (+ to snow surface) QE Evaporative energy (+ away from snow surface) QH Sensible energy (+ away from snow surface) QG Ground heat flux (+ downward from snow) dU/dt Internal energy change (+ loss from melt)

26 Diurnal Variation in Radiative Fluxes - clear day near Saskatoon
700 600 Incoming SW Net SW 500 Net Rad 400 Net LW Radiation (W/m²) 300 200 100 -100 -200 0:00 4:00 8:00 12:00 16:00 20:00 24:00 Time

27 Empirical atmospheric transmittance equations
Qsi can be calculated directly if the atmospheric transmittence is known Many similar relationships, all give similar results: Bristow and Campbell and Walter et al. Annandale All use a simple relationship between daily atmospheric transmittance and the range of daily air temperatures

28 Edmonton

29 Snowpack Albedo Decay

30 CRHM Snowmelt Simulation

31 Infiltration to Frozen Soils
Frozen soils can be permeable, but show reduced infiltration compared to unfrozen conditions ‘Frozen’ means a frost depth of at least 0.5 m Simple grouping of soil types Three classes of infiltrability: unlimited Inf=SWE restricted Inf=0 limited Inf = f(SWE, Saturation)

32 Gray’s Model of Infiltration into Frozen Soils - Prairie Environment
120 Unlimited Restricted 100 Saturation 1:1 80 0.3 Infiltration (mm) 0.4 60 0.5 0.6 40 0.7 20 0.9 30 60 90 120 150 180 Snow Water Equivalent (mm)

33 Effect of Thawed Soils on Prairie Spring Runoff

34 Local Scale Prairie Runoff
Because of frozen soils and rapidly melting snowcovers in the spring, 80% - 90% of prairie runoff is produced from snowmelt Snowmelt runoff is strongly controlled by snow drift location and size, soil moisture and mid winter thaws. In wet years, there is often excess water to dryland cereal grain growing needs. Hydrological computer simulations may tell us something about the reliability and behaviour of local prairie water supplies

35 Cold Regions Hydrological Model
Prairie hydrological modelling requires consideration of the following: Transport of water in liquid, vapour and frozen states (runoff, percolation, evaporation, sublimation, blowing snow); 2. Coupled mass and energy balances; 3. Phase change in snow & soils (snowmelt, infiltration in frozen soils, soil freezing and thawing); 4. Snow and rain interception in forest canopies; Episodic flow between soil moisture, groundwater, ponds and streams. Variable storage, drainage and contributing area Land use change

36 CRHM Module Development
DATA ASSIMILATION PROCESSES Data from multiple sites Interpolation to the HRUs Infiltration into soils (frozen and unfrozen) Snowmelt (prairie & forest) Radiation – level, slopes Evapotranspiration Snow transport Interception (snow & rain) Sublimation (dynamic & static) Soil moisture balance Sub-surface runoff Routing (hillslope & channel) Advection SPATIAL PARAMETERS Basin and HRU parameters are set. (area, latitude, elevation, ground slope, aspect)

37 Creighton Tributary, Bad Lake as a typical Prairie Basin
Moderately well drained plateau of grains and fallow drains into a coulee Semi-arid to sub-humid climate Typical drainage and landcover for much of southern prairies

38 Snowmelt Runoff over Frozen Soils
Bad Lake: Semi-arid SW Saskatchewan Soil moisture is FALL soil moisture Snowmelt runoff is Spring Physically based Infiltration equations (Zhao & Gray, 1999) Cold Regions Hydrological Model

39 Bad Lake – Creighton Tributary Water Balance
With 30% Summer Fallow Pomeroy, De Boer, Martz (2007)

40 Changed to Continuous Grain Cropping

41 Prairie Streamflow & Climate Change “first more, then less”
Three most “reliable” climate change scenarios for hydrology suggest increases in annual prairie winter temperature and precipitation from the average: ºC and +11% ºC and +15.5% Using these scenarios in the virtual upland basin results in a 24% rise in 2050 spring runoff, but a 37% drop by 2080, compared to conditions in the mid 1970s.

42 Prairie Climate Change – Winter Snow

43 Prairie Climate Change – Spring Runoff

44 Conclusions Prairie hydrological processes that control water balance and runoff generation have been largely quantified and described and model requirements are known, but have not been widely implemented in models. Major unknowns are the changing contributing area and its interaction with surface storage terms in poorly defined drainages.

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