PUB in practice: case studies - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 288

Figure 11.18. Stations used as
model inputs in western Canada. The
prairie ecozone is shaded. Projection
is UTM 13.

N

Alberta Manitoba

Saskatchewan

ColdLake

Edmonton

PrinceAlbert

NorthBattleford

RedDeer

Saskatoon

Calgary

Yorkton

Regina

SwiftCurrent

Winnipeg

MedicineHat

Brandon

Lethbridge

Estevan

0 200 400 Kilometres

North Dakota

Montana

Because the distribution is computed for each location
modelled, the frequencies are comparable from location
to location. The exceedance probabilities were interpolated
between stations using a continuous curvature spline
( Smith and Wessel, 1990 ), gridded and plotted using the
Generic Mapping Tools, which is a collection of
command-line GIS tools for Linux and Unix available at
gmt.soest.hawaii.edu.

2001

2005

Results
The gridded exceedance probabilities of the annual dis-
charge from the virtual basin model are plotted in Figure
11.19 for a typical drought year (2001) and the post
drought year (2005). Whilst 1999 was a moderate to wet
year, in 2001 the hydrological drought was at its max-
imum and extended over large areas of the western and
eastern Prairies; in 2002, it retreated from most of the
region. In 2005 the annual discharge map shows a large
volume of runoff, which was due to spring rainfall that
was widely distributed across the region. This corres-
ponded to record flooding in Alberta. The results show
that hydrological drought included extremely wet regions,
record dry regions and large spatial variability in surface
water availability.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Normal annual outflow exceedance fraction

Figure 11.19. Annual runoff exceedance probability of the virtual
basin over the 1999 - 2005 drought compared to the 30-year normal
period (1961 - 90) with the maximum drought year (2001) and
post-drought year (2005) selected. The Mercator projection was used
for all maps.

annual discharges modelled for each of the years between
1999 and 2005, inclusive. All calculations were performed
using the open source statistical language R ( Ihaka and
Gentleman, 1996 ).
The use of the normal ECDF permitted regional com-
parison of the virtual basin behaviour and provides an
index to hydrological drought

Discussion
Calculation of exceedance probabilities using physically
based hydrological modelling for virtual basins is a new
methodology for describing hydrological droughts in

for ungauged basins.