Agriculture in an even more sunburnt country - Four Degrees of Global Warming - page 95

of mean rainfall decline, notwithstanding the positive impacts of elevated CO 2
on yield (Howden, 2002).
Increases in extreme daily rainfall seem likely (Braganza et al., 2013, this
volume). For example, the intensity of the 1-in-20-year daily rainfall event is
likely to increase by 5 to 70 per cent by the year 2050 in Victoria, up to 25 per
cent in northern Queensland by 2050 and up to 30 per cent by the year 2040
in south-east Queensland (Hennessy et al., 2007). Intense rainfall is particularly
relevant to soil erosional processes and climate change could increase already
problematic rates of soil erosion leading to longer-term yield decline (McKeon
et al . , 1988; 2004; Littleboy et al . , 1992).
Agriculture is also significantly affected by soil degradation processes such as
dryland salinization. This could potentially reduce under a scenario with less
rainfall due to a reduction in groundwater recharge (van Ittersum, et al ., 2003).
However, John et al . (2005) project that if summer rainfall increases to offset
less winter rainfall, salinity will remain the same, or might even increase under
climate change. Other soil degradation processes such as wind erosion are likely
to increase due particularly to expected reductions in ground cover arising from
longer dry spells (Crimp et al . , 2010).
Evaporation
Evaporation is a key factor for both dryland and irrigated agriculture as it
strongly influences yields in many situations and is the primary determinant of
water demand. Evaporation (or evaporative demand) is likely to increase with
climate change (CSIRO and BoM, 2007; Whetton et al., 2013, this volume)
and this, combined with anticipated reductions in rainfall, suggests a significant
increase in drought risk over most of Australia (Hennessy et al . , 2008). There
has been some debate over how evaporation may change in the future and the
methods appropriate to evaluating this (Gifford et al., 2005). To examine this
issue we briefly explore potential changes in evaporation, calculated using the
well-established Penman-Monteith equation (Allen et al., 1998), for three
sites across the cropping zones: Emerald (Queensland), Birchip (Victoria) and
Kellerberrin (WA). 1 The increase in potential evaporation was calculated to be
consistently higher (30 to 50 per cent) using the GCM output ( Table 6.1 ). The
increase in potential evaporation was caused by not just a change in the mean
temperature in the GCM output but a change in the temperature distribution
(increased variance) with the increasing frequency of very hot days dispropor-
tionately increasing potential evaporation. This effect has also been found in
assessments of urban water demand under climate change (Howden and Crimp,
2008). Consequently, global warming is likely to significantly increase potential
evaporation increasing water demands by agriculture but the estimated degree of
change is critically dependent on the method used: another source of uncertainty
in future projections.