4.1 Impacts of Climate change and Water

INTRODUCTION
In assessing the anticipated impacts of climate change on agriculture and agricultural
water management, it is clear that water availability (from rainfall, watercourses
and aquifers) will be a critical factor. Substantial adaptation will be needed to
ensure adequate supply and efficient utilization of what will, in many instances,
be a declining resource. However, the long-term climatic risk to agricultural
assets and agricultural production that can be linked to water cannot be known
with any certainty. While temperature and pressure variables can be projected by
global circulation models with a high degree of ‘convergence’, the same cannot be
said of water vapour in the atmosphere. The levels of risk associated with rainfall
and runoff events can only be determined with provisional levels of precision.
These may not be sufficient to define specific approaches or levels of investment
(e.g. the costs of raising the free-board on an hydraulic structure) in many locations.

The evidence for climate change is now considered to be unequivocal, and trends in
atmospheric carbon dioxide (CO2), temperature and sea-level rise are tracking the
upper limit of model scenarios elaborated in the Fourth Assessment (AR4) undertaken
by the International Panel on Climate Change (IPCC). There remain many scientific
questions related to cause and effect that are not yet fully explained, but the probable
future costs of climate change are so significant that action now is considered to
be a prudent insurance. Current negotiations focus on stabilizing end-of-century
temperatures at no more than 2 ˚C to minimize negative impacts. The criticism that
climate science has recently taken does not detract from the reality nor the gravity of
the clear trends in global climate.
The prediction of impacts relies heavily on simulation modelling with global climate
models (GCMs) that have been calibrated as closely as possible to historical climate
data. Modelling scenarios have been standardized from a set defined by the IPCC
Special Report on Emissions Scenarios (SRES) to allow more consistent comparison of
predicted impacts. The predictive ability of climate models is currently much better for
temperature than for rainfall. Indeed, models tend to solve primarily on temperature and
pressure. The spatial and temporal patterns of rainfall are affected by land-atmosphere
interactions that cannot be accomodated in the existing algorithms, and the models’
spatial resolution is anyway too coarse to capture many topographic effects on climate
patterns. The predictions for one scenario of economic development vary considerably
from model to model, and contradictory predictions, such as increased or decreased
precipitation, can result for specific parts of the world. Ensemble modelling has become
increasingly useful in identifying both the range, and the most likely future conditions
for a given scenario, and rapid progress is being made with the development of finer
resolution Regional Climate Models (RCM) to downscale predictions to national and
river basin scales.
The impact of climate change on water and agriculture requires the use of simulation
models to predict the distribution and extent of change in key variables that
govern crop growth (temperature and evaporative demand) and water availability
(rainfall, evaporation, stream flow and groundwater recharge). Water management

for agriculture encompasses all technologies and practices that sustain optimum soil
moisture conditions for plant growth; these range from enhancing the capture and
retention of rainfall to full-scale irrigation of crops where there may be no rainfall at all.
It also includes the provision of drainage, and the avoidance and mitigation of flooding.
Irrigated agriculture is the largest user of raw water and therefore the main concern of
this publication.
The anticipated impacts of climate change pose an additional stress on food
production systems under pressure to satisfy the food needs of a rapidly growing
and progressively wealthier world. As agriculture develops and becomes more
intensive in its use of land and water resources, its impact on natural eco-systems
becomes more and more apparent. Damaging the integrity of these ecosystems
undermines the food-producing systems that they support. The assessment of
viable and effective adaptations to the impacts of climate change on water and
agriculture will require a sound understanding and integration of agronomic science
with water management and hydrology. Due regard for the resulting environmental
interactions and trade-offs will be essential.

The long downward trend in commodity prices made an abrupt turnaround in 2007–2008
when a combination of run-down strategic reserves, poor harvests, droughts and a sudden
rush to plant biofuels in the United States and Europe reduced trade volumes. Prices for
rice doubled and although commodity prices have fallen back since, the fundamentals
(oil price, biofuel development and continued rising food demand) are now expected to
drive a period of high volatility in food prices.
In the wake of this market turmoil, food security and agricultural livelihoods have
regained importance in development planning, although some countries such as China
seem ever more likely to balance further agricultural development and investment with
imports.
The world has a large stock of under-performing canal irrigation infrastructure, and
a vibrant groundwater sector that is competitively depleting its own lifeblood. Both
create significant environmental externalities, which need to be managed. Not only
that, there are calls for water to be reserved to maintain environmental flows in rapidly
developing river basins and restored to ecosystems in over-allocated ones.
SUMMARY OF IMPACTS OF CLIMATE CHANGE
ON WATER MANAGEMENT IN AGRICULTURE
Climate change will significantly impact agriculture by increasing water demand,
limiting crop productivity and by reducing water availability in areas where irrigation
is most needed or has comparative advantage.
Global atmospheric temperature is predicted to rise by approximately 4 ˚C by 2080,
consistent with a doubling of atmospheric CO2 concentration. Mean temperatures are
expected to rise at a faster rate in the upper latitudes, with slower rates in equatorial
regions. Mean temperature rise at altitude is expected to be higher than at sea level,
resulting in intensification of convective precipitation and acceleration of snowmelt
and glacier retreat.

In response to global warming, the hydrological cycle is expected to accelerate as
rising temperatures increase the rate of evaporation from land and sea. Thus rainfall
is predicted to rise in the tropics and higher latitudes, but decrease in the already dry
semi-arid to arid mid-latitudes and in the interior of large continents. Water-scarce areas
of the world will generally become drier and hotter. Both rainfall and temperatures are
predicted to become more variable, with a consequent higher incidence of droughts
and floods, sometimes in the same place. Runoff patterns are harder to predict as they
are governed by land use as well as uncertain changes in rainfall amounts and patterns.
Substantial reductions (up to -40 percent) in regional runoff have been modelled in
southeastern Australia and in other areas where annual potential evapotranspiration
exceeds rainfall. Relatively small reductions in rainfall will translate into much larger
reductions in runoff, for example, a 5 percent fall precipitation in Morocco will result
in a 25 percent reduction in runoff. In glacier-fed river systems, the timing of flows will
change, although mean annual runoff may be less affected.
As temperature rises, the efficiency of photosynthesis increases to a maximum and
then falls, while the rate of respiration continues to increase more or less up to the
point that a plant dies. All other things being equal, the productivity of vegetation
thus declines once temperature exceeds an optimum. In general, plants are more
sensitive to heat stress at specific (early) stages of growth, (sometimes over relatively
short periods) than to seasonal average temperatures. Increased atmospheric
temperature will extend the length of the growing season in the northern temperate
zones, but will reduce it almost everywhere else. Coupled with increased rates of
evapotranspiration, the potential yield and water productivity of crops will fall.
However, because yields and water productivity are now low in many parts of
the developing world, this does not necessarily mean that they will decline in the
long term. Rather, farmers will have to make agronomic improvements to increase
productivity from current levels.

Increased atmospheric concentrations of CO2 enhance photosynthetic efficiency
and reduce rates of respiration, offsetting the loss of production potential due to
temperature rise. However, early evidence was obtained from plant level and growth
chamber experiments and has not been corroborated by field-scale experiments; it has
become clear that all factors of production need to be optimal to realize the benefits of
CO2 fertilisation. Early hopes for substantial CO2 mitigation of production losses due
to global warming have been restrained. A second line of reasoning is that by the time
CO2 levels have doubled, temperatures will also have risen by 4 ˚C, negating any benefit.
Agriculture will also be impacted by more active storm systems, especially in the
tropics, where cyclone activity is likely to intensify in line with increasing ocean
temperatures. Evidence for this intuitive conclusion is starting to emerge. Sea-level rise
will affect drainage and water levels in coastal areas, particularly in low-lying deltas,
and may result in saline intrusion into coastal aquifers and river estuaries.
Estimates of incremental water requirement to meet future demand for agricultural
production under climate change vary from 40–100 percent of the extra water needed
without global warming. The amount required as irrigation from ground or surface water
depends on the modelling assumptions on the expansion of irrigated area – between
45 and 125 million ha. One consequence of greater future water demand and likely
reductions in supply is that the emerging competition between the environment and
agriculture for raw water will be much greater, and the matching of supply and demand
consequently harder to reconcile.

The future availability of water to match crop water requirements is confounded in
areas with lower rainfall – those that are presently arid or semi-arid, in addition to
the southern, drier parts of Europe and North America. Runoff and groundwater
recharge are both likely to decline dramatically in these areas. Where rainfall volume
increases and becomes more intense (Indian monsoon, humid tropics), a greater
proportion of runoff will occur as flood flow that should be captured in dams or
groundwater to be useable.
About 40 percent of the world’s irrigation is supported by flows originating in the
Himalaya and other large mountain systems (e.g. Rocky Mountains in the western
United States and Tien Shan in Central Asia). The loss of glaciers worldwide has been
one of the strongest indicators of global warming. At present, the estimates of the rates
of glacier mass loss are being reviewed by the IPCC. Notwithstanding the long-term
evolution of glacier mass balance, the contribution of snowmelt to runoff is important
in terms of base flows and timing of peak flows, but is more variable in its proportion
of total runoff. The impacts on some river systems (such as the Indus) are likely to be
significant and will change the availability of surface water for storage and diversion as
well as the amount of groundwater recharge. In general, the probable impacts of climate
change on groundwater recharge have not been sufficiently explored, but aquifers in
arid and semi-arid areas, where runoff will decline, can expect severe reductions in
replenishment.

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