PUB in practice: case studies - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 286

Figure 11.16. Prairie hydrological
cycle: (left) winter processes, (right)
summer processes.

Prairie winter hydrological processes Prairie summer hydrological processes

Snowfall Snowfall

Rainfall

Evaporation

Blowing snow sublimation

Evaporation

Transpiration

Blowing snow
Transport

Snow
Runoff

Accumulation

Runoff

Melt
Frozen soil infiltration

Soil water movement Infiltration

Groundwater flow Groundwater flow Percolation/discharge

characterised by good water-holding capacity, low to mod-
erate levels of soil moisture and high unfrozen infiltration
rates. Figure 11.16 shows the major hydrological processes
in winter and summer in typical Canadian Prairie land-
scapes that contribute to hydrological cycling on small
stream drainages.
Surface runoff generation is strongly affected by the
climate variation over the Prairies. A synthetic drought
analysis at a typical semi-arid prairie site suggested that
spring stream discharge drops substantially under warmer
and drier conditions and ceases completely when winter
precipitation decreases by 50% or winter/spring air tem-
peratures rise by 5 C, as is common during drought. Water
supply to wetlands is insufficient during drought due to
lower discharge of surface runoff from local catchments.
The hydrography of the Canadian Prairies is very repeti-
tive, with many drainage basins consisting of assemblages
of small ephemeral streams that feed into depressions, and
their associated uplands. The vast majority of these small
drainage basins are ungauged and the majority of larger
basins in the region do not meet WMO standards for
gauging density. Runoff in these small basins cannot be
modelled using traditional techniques due to poorly
developed drainage systems and mild topography.

by Pomeroy et al.( 2007b ). CRHM permits the assembly
of a purpose-built model from a library of processes, and
interfaces the model to the basin based on a user-selected
spatial resolution. The hydrological processes are simu-
lated on landscape units called hydrological response
units (HRU, see Sections 10.2.2 and 10.2.3). HRUs are
defined as spatial units of mass and energy balance calcu-
lation corresponding to hydrobiophysical landscape units,
within which processes and states are represented by single
sets of parameters, state variables and fluxes. HRUs can be
finely scaled (hillslope segment), or coarsely scaled (sub-
basin). HRUs in the prairies typically correspond to agri-
cultural fields (stubble or fallow fields), natural cover
(grassland or forest woodland) and bodies of water (lake
or pond) (Fang and Pomeroy, 2008 ). CRHM has shown
good simulations in a semi-arid, well-drained prairie basin
(Fang and Pomeroy, 2007 ) and in a sub-humid, poorly and
internally drained prairie basin (Fang and Pomeroy, 2008 ).

Virtual basin
Because the hydrography of the Canadian Prairies is repeti-
tive, and because of the interaction amongst HRUs via
blowing snow transport and runoff, a virtual basin can be
defined based on a basin with well-known characteristics
and can be considered a fundamental first-order runoff
generation unit. The concept is very useful as an index
for vast regions where, due to lack of gauging and subtle
topography, not only is runoff unknown but drainage area
and drainage network characteristics are poorly defined.
A virtual basin was defined based on Creighton Tributary
of the Bad Lake International Hydrological Decade (IHD)
research basin in the south-western portion of the Canadian
Prairies. Bad Lake is an internally drained wetland basin
and Creighton is a small catchment (11.4 km 2 ) flowing into
Bad Lake. Approximately 85% of the basin area was
cultivated land (grain stubble and summer-fallow fields),
and the rest of basin consisted of grassland for the periods

Method
Cold Regions Hydrological Model platform
The Cold Regions Hydrological Model (CRHM) platform
is a physically based hydrological model based on a modu-
lar, object-oriented structure in which component modules
represent basin descriptions, observations or physically
based algorithms for calculating hydrological processes.
The component modules have been developed based on
the results of over 45 years of research by the University of
Saskatchewan and Environment Canada in cold regions
environments. A full description of CRHM is provided