Canadian Prairie Hydrology and Runoff Generation John Pomeroy Centre for Hydrology, University of Saskatchewan, Saskatoon www.usask.ca/hydrology 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 Prairie Hydrological Cycle Prairie Runoff Generation Snow Redistribution to Channels Spring melt and runoff Dry non-contributing areas to runoff Water Storage in Wetlands Non-Contributing Areas to Streamflow a Prairie Characteristic Prairie Hydrology – don’t blink Smith Creek, Saskatchewan Drainage area ~ 450 km2 20 Average 1975-2006 15 Streamflow m 3 per second 25 1995 High Year 10 2000 Low Year 5 No baseflow from groundwater 0 27-Dec 27-Nov 28-Oct 28-Sep 29-Aug 30-Jul 30-Jun 31-May 01-May 01-Apr 02-Mar 31-Jan 01-Jan 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 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) Effect of Blowing Snow Sublimation on Prairie Snow Supply (losses, mm SWE) Location Calgary Peace River Swift Current Prince Albert Regina Yorkton Portage Winnipeg Stubble-field 19.7 6.6 28.2 24.9 39.4 18.6 23.5 27.4 Pomeroy and Gray, NHRI Science Report No. 7 (1995) Fallow-field 37.5 7.6 37.8 29.6 48.1 28.6 33.8 36.5 1970-1976 hourly simulations 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 Fallow Field Stubble Field Grassland Brush Trees Shelterbelts on Prairies Winkler, Manitoba Transport to shelterbelts depends on upwind fetch and vegetation roughness Conquest, Saskatchewan Spatially Distributed Snow Redistribution Snow mass balance equation St Denis, Saskatchewan Results – Spatially distributed SWE Fang and Pomeroy, Hydrol Proc, in preparation Spatially distributed SWE cont’ Spatially distributed SWE cont’ Spatially distributed SWE cont’ Spatially distributed SWE cont’ Spatially distributed SWE cont’ Spatially distributed SWE cont’ Spatially distributed SWE cont’ Spatially distributed SWE cont’ Spatial Pattern of Blowing Snow Sublimation Simulations vs. Snow Surveys 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 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) Diurnal Variation in Radiative Fluxes clear day near Saskatoon 700 600 500 400 Incoming SW Net SW Net Rad Net LW Radiation 300 (W/m²) 200 100 0 -100 -200 0:00 4:00 8:00 12:00 Time 16:00 20:00 24:00 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 Edmonton 1979-2000 Snowpack Albedo Decay CRHM Snowmelt Simulation 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 restricted limited Inf=SWE Inf=0 Inf = f(SWE, Saturation) Gray’s Model of Infiltration into Frozen Soils - Prairie Environment 120 Unlimited Restricted 100 1:1 80 Saturation 0.3 0.4 0.5 0.6 0.7 Infiltration 60 (mm) 40 20 0.9 0 0 30 60 90 120 150 Snow Water Equivalent (mm) 180 Effect of Thawed Soils on Prairie Spring Runoff Daily Flow, m 3/s 1.4 Observed Frozen Soil Unfrozen Soil 1.2 1 0.8 0.6 0.4 0.2 0 95 100 105 110 Julian Date 115 120 125 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 Cold Regions Hydrological Model Prairie hydrological modelling requires consideration of the following: 1. 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; 5. Episodic flow between soil moisture, groundwater, ponds and streams. 6. Variable storage, drainage and contributing area 7. Land use change CRHM Module Development DATA ASSIMILATION Data from multiple sites Interpolation to the HRUs PROCESSES SPATIAL PARAMETERS Basin and HRU parameters are set. (area, latitude, elevation, ground slope, aspect) 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 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 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 Bad Lake – Creighton Tributary Water Balance With 30% Summer Fallow Fallow Stubble Coulee Basin 500 mm water equivalent 400 300 200 Snowfall 100 Rainfall Runoff 0 Sublimation -100 Drifting Snow -200 Evaporation -300 -400 -500 Pomeroy, De Boer, Martz (2007) Stubble Coulee Basin 500 Changed to Continuous Grain Cropping mm water equivalent 400 300 Snowfall 200 Rainfall 100 Runoff Sublimation 0 Drifting Snow -100 Evaporation -200 -300 -400 runoff infiltration snowmelt sublimation drift evaporation snowfall rainfall -40 -20 0 20 % Change 40 60 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 1961-1990 average: 2050 2080 +2.6 ºC and +11% +4.7 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. 29/04/1975 15/04/1975 01/04/1975 18/03/1975 04/03/1975 18/02/1975 04/02/1975 21/01/1975 07/01/1975 24/12/1974 10/12/1974 26/11/1974 70 12/11/1974 60 29/10/1974 15/10/1974 01/10/1974 SWE (mm) Prairie Climate Change – Winter Snow Winter Snow Accumulation at Bad Lake, SK Normal SWE (Winter of 1974/75) 50 SWE (Winter of 2049/50) 40 SWE (Winter of 2079/80) 30 20 10 0 Prairie Climate Change – Spring Runoff 70 Spring Runoff from Creighton Tributary at Bad Lake, SK Normal Spring Runoff (Spring of 1975) 50 Spring Runoff (Spring of 2050) 40 Spring Runoff (Spring of 2080) 30 20 10 29/04/1975 15/04/1975 01/04/1975 18/03/1975 04/03/1975 18/02/1975 04/02/1975 21/01/1975 07/01/1975 24/12/1974 10/12/1974 26/11/1974 12/11/1974 29/10/1974 15/10/1974 0 01/10/1974 Runoff (mm) 60 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.