Canadian Prairie Hydrology and Runoff Generation John Pomeroy Centre for Hydrology, University of Saskatchewan, Saskatoon www.usask.ca/hydrology
Water Use in Saskatchewan Precipitation on average 350 mm Grain Growing 125 mm soil water reserves needed 175 mm spring rainfall needed Roughly 300 kg/ha increased yield for each extra 25 mm of water added Livestock, industrial, ecosystem and human use surface and groundwater stores in excess to plant use South Saskatchewan River water originates in the mountains and loses discharge as it crosses the Prairies irrigation, domestic, livestock, industrial use
Prairie Hydrological Cycle Precipitation - snowfall, rainfall - highly variable. Interception & Storage snow cover, glaciers, lakes, wetlands, plant canopies. Infiltration - to soils for plant use limited by frozen status Evaporation/Transpiration - return to atmosphere, surface cooling, plant productivity. Runoff & Streamflow - peak events often associated with snowmelt or extreme rainfall - erosion, sedimentation, flooding, lake/wetland recharge. Groundwater - slow storage, long-term resource. Precipitation Storage Infiltration Evaporation Groundwater Runoff
Prairie Hydrological Cycle
Distinctiveness of Prairie Hydrology Dry relatively small precipitation, water deficit, low moisture reserves Cold long frozen season, snow cover, frozen soils Flat gentle topography, poorly defined drainage Variable Inter-annual sequences of drought and floods Intra-annual dry and wet years Episodic intense snowstorms, snowmelt and rainfall, intense heat, rapid growing season
Prairie Runoff Generation Snow Redistribution to Channels Spring melt and runoff Dry non-contributing areas to runoff Water Storage in Wetlands
Donald M. Gray, 1964
Prairie Hydrological Connectivity The fill and spill hypothesis Lack of groundwater connections in this landscape heavy tills
Non-Contributing Areas to Streamflow a Prairie Characteristic
Prairie Hydrology don t blink Smith Creek, Saskatchewan 25 Drainage area ~ 450 km 2 Streamflow m 3 per second 20 15 10 5 0 Average 1975-2006 1995 High Year 2000 Low Year No baseflow from groundwater 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 Spring 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)
Substantial losses of winter snowpack due to blowing snow transport and sublimation 300 250 200 Snowfall Snowfall - Blowing Snow Snowfall - Blowing Snow - Melt Measured Accumulation SWE (mm) 150 100 50 0 305 320 335 350 365 15 30 45 60 75 Julian days, 1973-1974, Regina
Prairie Blowing Snow Losses Blowing Snow Disposition on 1-kmFetches 100 Stubble Fallow Stubble Fallow % of Annual Snowfall 80 60 40 20 RESIDUAL SUBLIMATION SUSPENSION SALTATION 0 PRINCE ALBERT REGINA
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 = Q M /(ρ w L f B i ) Melt Energy Q M = Q* - Q E Q H Q G du/dt Q* Net radiation (+ to snow surface) Q E Evaporative energy (+ away from snow surface) Q H Sensible energy (+ away from snow surface) Q G Ground heat flux (+ downward from snow) du/dt Internal energy change (+ loss from melt)
Diurnal Variation in Radiative Fluxes - clear day near Saskatoon Radiation (W/m²) 700 600 500 400 300 200 100 0-100 Incoming SW Net SW Net Rad Net LW -200 0:00 4:00 8:00 12:00 16:00 20:00 24:00 Time
Empirical atmospheric transmittance equations Q si 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
Point Field Measurements of Frequency Distribution of Snow Water 100 80 Percent Greater Than 60 40 20 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 SWE (mm)
Depletion in Snow-covered Area on a Natural Watershed During Ablation 100 80 Percent Snowcover 60 40 20 0 7-Mar 12-Mar 17-Mar 22-Mar 27-Mar 1-Apr
Meltwater Movement - before and after reaching soil surface Heterogeneous domain Melt Homogeneous domain Ice lens Wetting front Soil Before Basal Ice layer Soil After
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 Infiltration (mm) 120 100 80 60 40 20 0 Unlimited Restricted 1:1 Saturation 0 30 60 90 120 150 180 Snow Water Equivalent (mm) 0.3 0.4 0.5 0.6 0.7 0.9
Effect of Thawed Soils on Prairie Spring Runoff Daily Flow, m 3 /s 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Observed Frozen Soil Unfrozen Soil 95 100 105 110 115 120 125 Julian Date
Modelling Prairie Hydrology Need a physical basis to calculate the effects of changing climate, land use, wetland drainage Need to incorporate key prairie hydrology processes: snow redistribution, frozen soils, spring runoff, wetland fill and spill, non-contributing areas Frustration that many hydrological models do not have these features and fail in this environment Streamflow calibration does not provide information on basin non-contributing areas and is not suitable for change analysis
Cold Regions Hydrological Model Platform: CRHM Modular purpose built from C++ modules Modules based upon +55 years of prairie hydrology research at U of S Parameters set by understanding, abduction, calibration Hydrological Response Unit (HRU) basis landscape unit with characteristic hydrological processes single parameter set horizontal interaction along flow cascade matrix Model tracks state variables and flows for HRU HRUs assumed to represent one response type, basis for coupled energy and mass balance HRUs connected aerodynamically for blowing snow, via dynamic drainage networks for streamflow, via subsurface gradients for groundwater Incorporate wetlands and variable contributing area directly using fill and spill algorithm
Physically Based Hydrological Modelling using the Cold Regions Hydrological Model
CRHM Modules DATA ASSIMILATION Data from multiple sites Interpolation to the HRUs SPATIAL PARAMETERS Basin and HRU parameters are set. (area, latitude, elevation, ground slope, aspect) Infiltration into soils (frozen and unfrozen) Snowmelt (open & forest) Radiation level, slopes Wind speed variation complex topo Evapotranspiration Blowing snow transport Interception (snow & rain) Sublimation (dynamic & static) Soil moisture balance Pond/depression storage Surface runoff PROCESSES Sub-surface runoff Routing (hillslope & channel)
Hydrological Response Units Sequential HRU landscape connectivity Grouped HRU or Tile must drain to stream HRU 1 HRU 2 HRU 3 outlfow
Prairie Simulations
Recent Prairie Wetland/Soil Module No If pond Yes Snowmelt Snowmelt Infiltration Rainfall Rainfall Infiltration Recharge Zone Soil Column Evapotranspiration Subsurface Discharge Saturated Overland Flow = 0 No If soil column is full Runoff No Yes Saturated Overland Flow If depression Yes Runoff to Depression Surface Runoff Evaporation Snowmelt Snowmelt Infiltration No fill-and-spill No If depression is full Yes Rainfall Rainfall Infiltration Wetland Pond Groundwater fill-and-spill Evaporation No fill-and-spill No If pond is full Yes fill-and-spill Subsurface Discharge Groundwater Discharge Depression Subsurface Discharge Groundwater Groundwater Discharge Groundwater Groundwater Discharge
CRHM Prairie Model Structure
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
Results Snow Regime Test at Bad Lake
Results Snowmelt Runoff Test at Bad Lake
Results Snowmelt Runoff Test at Wetland 109, St Denis
Bad Lake Creighton Tributary Water Balance With 30% Summer Fallow 500 Fallow Stubble Coulee Basin mm water equivalent 400 300 200 100 0-100 -200-300 -400 Snowfall Rainfall Runoff Sublimation Drifting Snow Evaporation -500 Pomeroy, De Boer, Martz (2007)
Vermilion HRU Delineation
HRU Hydraulic Routing within a Sub-basin Note: i) modified Hack s Law used to parameterise length scales for weighting routing. ii) blowing snow aerodynamic routing from smooth to rough land covers
Wetland Representation in CRHM from PCM
Hydrological Response Unit Routing and Wetland Network Representation
Sub-basin Routing RB 1 RB 2 RB 3 RB 4 RB 5 Smith Creek basin outlet (b)
CRHM Surface Water Drought Modelling CRHM was used to create virtual model of typical prairie upland basin Model was run over climate normal period (1961-1990) Output during drought period was compared to normal period and spatially interpolated
Simulating Water Supply from Virtual Prairie Drainage Basins over 46 years Upland Drainage Basin Wetland Drainage Basin 4 1 3 2
Drought Hydrology Simulations Station locations, Prairie ecozone and Palliser Triangle boundaries
Prairie Spring Discharge 2000 Wetlands Uplands
Prairie Spring Discharge 2001 Wetlands Uplands
Prairie Spring Discharge 2003 Wetlands Uplands
Prairie Spring Discharge 2005 Wetlands Uplands
Spatial Variation of Prairie Soil Moisture (Drought vs Wet) Mean for normal period 332 mm Drought period: distribution wide, variance large, median > mean Wetter period: distribution loses low soil moisture, variance smaller, median < mean Probability density of soil moisture Drought Wet
Spatial Variation of Evapotranspiration (Drought vs Wet) Mean for normal period 352 mm Drought period: distribution wide, variance large, median >> mean Wetter period: distribution symmetric, variance greatly reduced Drought Wet Probability density of evapotranspiration
Conclusions Process algorithms can form the basis for physicallybased hydrological model structure and parameter selection Flexible model structure and physically based components can lead to appropriate and robust hydrological simulation Errors in simulation identify gaps in understanding of processes, structure or parameters A process based modular model is able to simulate key components of the cold regions hydrological cycle from an understanding of principles, and without calibration of parameters except for routing Modular models are relatively simple to update as our science advances.