and water: Projections of global change effects on municipal water supplies,
irrigation water availability, and crop growth
The document here
takes a view complementary to the document on water wedges, found on another
page of this Website. This document does
not current incorporate the scholarly references; it does have hyperlinks to
some of our own relevant work.
components of global change:
in precipitation and temperature, from all the ultimate drivers
(greenhouse gases, aerosols, land use patterns)
in global means, latitudinal variations, and regional patterns (such as
increased moisture convergence - wet areas get wetter, dry areas get
drier, even with overall modest rise in global-total precipitation
closely following the Clausius-Clapeyron relation for mean water vapor
content in the air). Add these
changes on top of megadroughts currently in place in the SW US, Mexico, and Australia.
in the nature and degree of extreme events (floods, droughts, and
biological extremes in the physiology, development, and reproduction of
plants and animals. The biological
extremes are not simply related to point-in-time extremes of
meteorological variables such as temperature, but to sequences, and of
multiple variables; there are also extremes of biotic rather than
directly meteorological origin, such as emergent infectious diseases and
range expansions of formerly exotic competitors). We have published
and presented at national meetings on
these issues and continue to do so.
effects of CO2 on plants - not accounted in far too many
of stomatal conductance (total water use) and in water use per unit of
growth (water-use efficiency)
in nitrogen-use efficiency, reduction in plant N content (including
edible parts), and highly variable, species-specific changes in plant N
uptake from soil and symbiotic N fixation in plants
effects on how the geographic
distributions of plants will change
in land use patterns: conversion to or from crops, to or from wild land,
with consequent changes in direct water use; changes in regional climates
from atmospheric water recycling and temperature gradients (induced micro-
and mesoscale climates, such as are already seen in irrigated or
The issues can be developed along two lines:
a proper comprehensive framework - basically, laying long-term plans for
research and action, for entities with long planning horizons
"bite-sized" projects with short timelines, particularly to
address overlooked issues
Here are some specific comments,
plus some component issues - perhaps the issues are already bite-sized, or
useful to highlight in developing a comprehensive framework:
Predicting future regional climates, water supplies, crop yields: there
are many groups working on this, and many are quite innovative. There is relatively little leverage for
a new, small group such as we might form, a group lacking long-term
intellectual investment and contacts
Predicting changes in climatic extremes and in moisture convergence:
ditto, except that agencies, NGOs, and communities in selected areas need
to be made more aware of the issues and to plan for adaptation. For example, the state government in New Mexico has done
wishful thinking that precipitation will increase here, whereas greater
overall dryness and greater excursions to wet periods and drought are much
more likely. At least they already
are concerned about lower snowpack and earlier snowmelt in the Colorado
Rockies, source of about ¾ of our managed water supplies.
Changes in the nature of regional climates. John Williams and colleagues have
developed the concept, and the evidence for, the change to non-analog
climates, with combinations of seasonal temperatures and precipitation
that differ from all current climates.
This has potential consequences for wild plants, crops, heating and
cooling demand in homes, etc. Both
monitoring methods and adaptation strategies need developing.
Changes in biological extreme events: This is a rich area for research,
planning, and action. We have a
major scientific journal article on this,
plus one in review and another in preparation, as well as PowerPoint presentations. Biological
extreme events affect many functions in individual plants, communities,
and ecosystems, on the levels of physiology, ecology, and evolution. Here we focus on some effects relevant
to water availability for water supplies and for crop growth.
extremes, but not all, have negative consequences for plant
function. Extremes driven purely
by meteorology (e.g., drought, or sudden high temperature) or those of
biotic origin (pests, disease) commonly reduce plant stomatal
conductance, photosynthesis, leaf expansion, and other growth. Crop yield of edible or otherwise
usable parts is clearly reduced in the immediate term. This enhances partitioning of water
inputs (precipitation) or extant soil water to runoff and deep drainage
to aquifers, thus increasing surface water supplies relative to those
with fully functioning vegetation.
extremes of meteorological origin are positive for plant performance,
such as wet periods. An example is
wet years after forest fires that enable higher than normal establishment
rates of seedlings, leading to high stand densities. The dense stands evapotranspire more
water than normal stands. Although
strong quantitation is lacking, the high stand density has been
implicated in reduced yields of surface water, as well as higher
probabilities of later high-intensity forest fires. Land clearance by such fires leads to
increases in surface water yields, sealing a pattern of alternating
yields, but it also leads to increased soil erosion and attendant
sediment load in rivers.
- Issue Changes in plant water use (crops,
native plants) in non-extreme conditions: yes, higher temperatures at
approximately constant relative humidity increase the driving force for
transpiration by plants; the vapor-pressure deficit rises about 6-7% per
ºC. However, there are several
conductance per unit leaf area decreases, variously by species of plant
and conditions. There is a
reduction in direct response to CO2 (closely as 1/[CO2]) and
an increase in response to the rise in photosynthetic rate (approximately
as , from a combination of greater availability of CO2
as a substrate but a decrease in photosynthetic enzyme content as we and
our colleagues have shown mechanistic
reasons for). The net
reduction is approximately as the product of the two scaling factors,
that is, as 1/ - say, a 20%
drop in going from 370 to 550 ppm CO2.
This would counteract a 3.5 ºC rise in mean leaf temperature.
estimated change in photosynthetic enzyme content, which conditions
stomatal conductance and water use, is an average over plant
species. Different plant species
exhibit wide variations in how their uptake and
use of nitrogen for making photosynthetic systems responds to increased CO2. As a whole, trees follow the
prediction, grasses and herbs fall short (they have less growth
stimulation, and a decrease in water use). Among species in a group, variations are
temperature rises a bit more than air temperature, from the decrease in
transpirational cooling as stomatal conductance decreases. This boosts water use, and typically
more than it boosts photosynthesis, so that water-use efficiency
area per ground area may increase from the stimulation of photosynthesis
by elevated CO2 and the consequent gains in growth. These gains in photosynthesis and in
water use are tempered by:
inability to add non-shaded leaf area in regions where plant growth is
already dense. Sunlit area can be added in regions with sparser growth
in nutrient cycling, plus or minus, from a diversity of physiological
and ecological causes
duration (length of the growing season) changes. It commonly increases in the temperate
zones, leading to greater annual water use (but also greater water
recycling on large geographic scales).
In all zones, premature depletion of soil water can occur, and has
been observed in several wild landscapes.
Remember, plants use many environmental signals to set times of
leafout, reproduction, and senescence.
These signals, such as photoperiod, don't correspond (1, 2) to the
"real" environmental variables (T, water, …) the way they did
at lower CO2.
plants can adapt, meaning, evolve as populations to new patterns of
signalling, resource use, and growth, but many individuals within a
species die on the way. To
replace a gene in a population over
time requires the death of about as many individuals as in the current
population -not a pretty picture for long-lived species such as trees.
do plants need to change genetically, rather drastically, to alter their
physiological and developmental responses?
range of conditions to which plants (and animals, and fungi, and ….)
can adapt by changing the expression of extant genes is limited. Alternative forms of extant genes, or
new genes period, are necessary to meet the new conditions.
alternative forms (alleles) of extant genes abound in most species,
they don't cover the full range of new conditions at high CO2,
nor do alleles of multiple genes occur in the optimal combinations in
individuals. Recombination to
achieve the latter takes time (generations). If no alleles of some genes suffice,
natural mutation to generate useful alleles will take time to form and,
more so, to spread in the population.
might alleles for adaptive responses to high-CO2
environments not be present in current populations of plants? After all, CO2 was high in
the past. However, CO2
has been relatively low for over 20 million years, up till the last 100
years or so. Alleles adaptive
for performance at high CO2, those not needed at low CO2,
disappeared by a sort of random walk, or
genetic drift. Watch for some
Overall, the changes are specific
to regions, plant species, etc. - predictions are possible, but with
significant work. Overall crop water
use and crop water availability will be formed more strongly by human water
management, including land use.
in crop growth… and nutrient content: Can we grow as much later as we do
now, given the changes in temperature, precipitation, and CO2
content of the air? Should we
change crop types? Can we alter the
water-use efficiency of crops to grow more food, pasturage, fuel, and
timber with a given amount of water?
Many people have had a lot to say on
this. There are some novel ideas to
follow up, some important changes for which we need adaptive strategies in
water and crop management:
water use: how will rainfed crops fare, by region? What is the future availability of
irrigation water, which is applied to 8% of land area but supports 33% of
global food production? Again,
many people are working on this, but they each neglect certain phenomena
- most often:
in water-use efficiency at high CO2, as discussed above.
We can tie into work by crop/climate modellers (e.g., Jon Foley
at U. Wisc.)
events, specifically construed as biological extreme events. I have full discussions in several
papers and presentations, but a few examples are relevant:
(and water availability) at flowering and at seed set can have strong
effects on plant reproduction, and most of what we eat is reproductive
parts (seeds, associated fruits)
transpiration (cloudy or cool weather) at seedfill of rice in SE Asia can lead to great crop loss, because the plants
can't take up enough boron from the meager supplies in soil by
passively moving water (at now rates) to their roots.
in the seasonality of weather can offset flowering of plants from
activity of their pollinators
dramatically, the incidences of crop pests and diseases can change
markedly. As a rule, higher T
means more problems - greater activity of pests and diseases, wider
geographic ranges. The greatest
loss of timber in history (400,000 ha) occurred in British Columbia in
2000-2003 from the outbreak of bark beetles on 14 tree species, enabled
by higher T and drought occurring together (surprisingly, these two
phenomena don't often go together).
(It is also notable that the decay of the affected trees is
expected to pump 0.3 Pg of carbon back
into the atmosphere, or 4% of the global annual total attributable to
crop water use be ameliorated by breeding higher water-use efficiency
(WUE)? The short answer is, yes,
modestly. We can offer several of
our own and others' publications (not currently available as PDFs) on
the topic, and this summary:
single greatest physiological control of WUE is the ratio of CO2
partial pressure inside the leaf (Ci) to that outside the
leaf, in ambient air (Ca).
This is under stomatal control, and it is highly heritable. It has also been subject to very
strong natural selection, such that the ratio shows relatively little
variation between or within species.
The reason is that high values of Ci/Ca
increase rates of photosynthesis and growth, all else equal, but they
strongly degrade water-use efficiency.
The ratio varies with plant water stress, in some intriguing
patterns. In any event, the
genetic variability has been exploited to breed crops of higher
degree of improvement in WUE is modest, because lower Ci/Ca
arises from lower stomatal conductance (usually), which confers less
transpirational cooling and higher leaf temperatures that partially
deflate the rise in WUE. Also,
slower growth delays the cover of leaf area. There is more time that bare soil is
exposed, enabling more of the unproductive loss of water by evaporation
from soil. Finally, lower stomatal conductance and lower transpiration
rates mean that the plants do less humidification of their own
environment; reduced humidity also cuts into WUE.
WUE is under greater control by the environment - relative humidity and
temperature - than by physiology.
- The cost in yield for improving WUE is
variable: of itself, low Ci/Ca reduces
photosynthetic rate per unit mass of leaf, thus also the relative
growth rate under stable water availability. If yield is limited by total water
availability, the gain in yield may be significant
net result is that a few crops bred for lower Ci/Ca
have shown modest improvements, on the order of 5-10%, with increases,
it appears, up to about 15% in some pasture grasses grown at low
greatest change in WUE arose naturally, with the evolution of new plant
species (or, more rarely, ecotypes within a species) having a different
photosynthetic pathway, the so-called C4 pathway (most often
found in grasses, vs. forbs or woody plants) that contrasts with the C3
pathway of most species. The C4
plants have very low Ci/Ca and low stomatal
conductance. They maintain high
rates of photosynthesis and growth by having a biochemical
"pump" that boosts CO2 to very high levels inside
a restricted set of leaf cells (the bundle sheath) where photosynthetic
carbon fixation occurs. Only a
few major crop species are C4 plants, notably maize and
sorghum. A general limitation to
their nutritive value is that they have low contents of protein; they
need less than do C3's to do a given rate of photosynthesis,
because their photosynthetic enzymes operate at high efficiency (high
CO2 levels). Attempts
to breed the C4 trait into other crops have not succeeded;
the trait involves many genes.
hope for large increases in WUE is illusory. There is also widespread
misunderstanding of WUE, with many people confusing it with water use
per se. Many crops that use a
great deal of water also have very high yields. Water-use efficiency construed as
yield per mass of water used is often comparable in crops of high and
low rates of water use.
most promising way to increase WUE calculated on the basis of final
usable yield is to increase the partitioning of plant growth to the
usable parts, the so-called harvest index or HI. This has been very successful in
Green Revolution crops, where it has been achieved with the help of
dwarfing (reduced investment in support tissues). In those crops, HI is
near its biomechanical limit.
Promise remains in some other crops, provided that breeders can
be engaged to do such physiological breeding, while still spending the
usual - and absolutely necessary - 90+% of their time breeding pest and
in nutrient content. Nitrogen
content in plants virtually invariably declines significantly as CO2
rises. In our own research, we
came to a simple mechanistic
reason in formulating a model of functional balance between N uptake
from soil and N use in the photosynthetic apparatus. The model has been tested and
extended. The phenomenon is of
more than academic interest.
wheat, for example, has to have high N content. Breeders have to work to keep ahead
of the CO2-induced decline.
broadly, the content of most nutrients might be expected to decline, for
several reasons. Director
Gutschick was on an NSF panel in which a proposal came up to examine
this in wheat and rice grown at high CO2 in the so-called
FACE experiments (Free-Air CO2 Enrichment). The proposal was not funded, for
reasons that he did not much agree with.
The need is still there.
It is exacerbated by the global use of glyphosate herbicide
(Round-Up®). A side effect of
glyphosate is that its metabolite, aminomethylphosphonic acid, is both
stable in plant refuse (and in soil).
It is a potent chelator, binding to and tying up transition-metal
micronutrients, especially nickel and iron. Plant growth is affected, and so is
human nutrition. Notably, the UN
puts iron, zinc,
and manganese deficiencies as the number-two human health problem
globally, just behind HIV/AIDS and ahead of globalization (disease
spread, etc.) and malaria.
recycling in the atmosphere on the mesoscale: a change in land cover to
vegetation that uses more water (has a higher ET rate) will increase the
water content of the air mass over and downwind of that vegetation. This can lead to increased
precipitation downwind, that is, an increase in the amount of water
recycled through the atmosphere.
Increased convection in the air, with attendant convective storms,
can also be induced by the contrast between areas of high ET (where water
is put into the air) and areas of low ET (where air is heated more,
thereby rising to levels where water condenses). The phenomena are complex but the
documentation is considerable.
documentation is considerable.
effects are often modest but are exceptionally strong over some extensive areas
of the wet tropics. Recycling of water
there sustains the high rainfall levels.
Land clearance, such as by deforestation, can break the recycling and cause
major changes in climate. Climate models
indicate that clearance of Amazonia
will lead to very much higher temperatures (by 8-10ºC) and much reduced
precipitation. Furthermore, the
prospects for reversing the clearance are virtually nil, given the feedbacks
that cannot be reestablished. In
contrast, deforestation of SE Asia
might be reversible. The high rainfall
is sustained largely by water injected into the air from the adjacent oceans.
effects occur with land use change that affects water availability. The Aswan High Dam offers lessons. Impoundment of water has led to greatly
increased incidences of water-borne diseases, particularly
This whole writeup on water and crops is so long that we forgo
identifying people and groups to approach.
We can work on this later.