Prediction of annual runoff in ungauged basins - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 74

(and runoff enhanced) in catchments where precipitation
and E p are out of phase.
Of course, within-year climatic variability on all time
scales can impact annual runoff variability. For
example, the statistics of rainfall inter-arrival, modified
by runoff generation processes and vegetation uptake,
have been shown to predict the mean and variance of
annual runoff (Porporato et al., 2004 ;Zanardoet al.,
2012 ). A detailed example of the effects of precipitation
timing was presented by Montanari et al.( 2006 ), who
showed that annual runoff in the monsoonal area of
Northern Australia could vary by a factor of 100%
between two years with equivalent annual precipitation,
solely due to precipitation in the wet year arriving
slightly later in the wet season when evaporation poten-
tial was smaller.
Analyses of the effects of within-year climate variability
on annual runoff have to be put in the context of co-
evolution of climate, soils and vegetation, because over
time the landscape and vegetation adapt to the climate
forcing and develop functional features unique to a par-
ticular region. This was illustrated by a comparative study
by Jothityangkoon and Sivapalan ( 2009 ) in Australia,
which showed that the dominant climate regimes (e.g.,
seasonality dominated in Western Australia, storminess
dominated in Queensland) governed the inter-annual vari-
ability of annual runoff.

1.0

0.5

0.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0

E P / P

Figure 5.4. Budyko curve and points representing 331 catchments in
Australia. Large, hollow circles denote the 30 moderate-sized
catchments ( ≥ 1000 km 2 ) and small circles denote the remaining 301
smaller catchments ( < 1000 km 2 ). From Donohue et al.( 2007 ). Data
are from Peel et al.( 2000 ) and Raupach et al.( 2001 ).

and annual potential evaporation (used as a surrogate for
energy available, see Milly et al., 1994a , b and Potter et al.,
2005 ). These may be either in phase

where maxima in
potential evaporation (E p ) coincide with annual maxima in
precipitation (P)

-

where annual maxima
in E p coincide with annual minima in P ( Figure 5.5a ).
Many regions of the world exhibit strong seasonality in
climate forcing, ranging from completely in phase to com-
pletely out of phase. The relative seasonality of precipita-
tion and potential evaporation has significant impacts on
mean annual runoff and inter-annual variability. In catch-
ments where rainfall and potential evaporation are out of
phase, runoff production is enhanced, and evaporation
reduced, and vice versa. If P and E p are out of phase (solid
lines in Figure 5.5a ), there is an excess of water compared
to energy during the wet season. When this water accumu-
lates beyond the ability of the catchment to store it, runoff
is generated. In contrast, when P and E p are in phase
(dashed lines in Figure 5.5a ) or there is no seasonality at
all, evaporation reduces the accumulation of water, and
thus reduces runoff generation. This phenomenon explains
why runoff is observed in otherwise arid regions: although
annually the Mediterranean climates of the south-west of
Western Australia and Southern California have a deficit
of precipitation compared to energy, during cool wet
winters there is a localised water excess that generates
runoff. (Note that in other arid places seasonal phasing is
less important, because infiltration excess is the dominant
runoff mechanism and storage of water in the catchment
matters less). The effects of in-phase and out-of-
phase seasonality are highlighted in Figure 5.5b , which
presents annual water balance data from the USA within
a Budyko style framework, with the catchments stratified
by whether precipitation and E p are in phase or out of
phase. The observations show that evaporation is reduced

- or out of phase -

Catchment (physical) processes
If the Budyko curve is taken as representing the first-order
effects of water and energy availability on annual runoff
variability, then the scatter around the curves shown in
Figures 5.4 and 5.5 is evidence of the second-order effects
of catchment storage on annual runoff. Based on detailed
analysis of hundreds of catchments across the continental
USA, Wolock and McCabe ( 1999 ) concluded that, to
improve predictions of mean annual runoff beyond the
Budyko relationship, soil moisture storage capacity, sea-
sonality in water supply and seasonality in water demand
had to be accounted for.
Catchment storage includes temporary storage in the
snowpack and/or soil moisture and longer-term storage in
lakes, glaciers and groundwater. Climate fluctuations that
lead to an excess of water, relative to the capability of the
catchment to infiltrate and store water, will favour the
generation of runoff at the expense of evaporation. On
the other hand, climate fluctuations that promote the infil-
tration and storage of water for extended periods favour
evaporation, since they provide the opportunity for water
to be evaporated. The storage effect can be pronounced on
seasonal time scales where soil water storage can provide
water for evaporation during extended precipitation-free
periods, sustaining vegetation that otherwise would not