Global change 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.

 

   Three components of global change:

• Changes in precipitation and temperature, from all the ultimate drivers (greenhouse gases, aerosols, land use patterns)
• Changes 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.
• Changes 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.
• Direct effects of CO₂ on plants - not accounted in far too many studies!:
• Reduction of stomatal conductance (total water use) and in water use per unit of growth (water-use efficiency)
• Increases 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
• Major effects on how the geographic distributions of plants will change
• Changes 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 desertified areas)

   The issues can be developed along two lines:

• Creating a proper comprehensive framework - basically, laying long-term plans for research and action, for entities with long planning horizons
• Developing "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:

• Comment: 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
• Comment: 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.
• Issue: 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.
• Issue: 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.
• Many 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.
• Some 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 other changes:
• Stomatal 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.
• The 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 very wide.
• Leaf 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 decreases
• Leaf 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:
• The 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
• Changes in nutrient cycling, plus or minus, from a diversity of physiological and ecological causes
• Leaf 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. 
• Sure, 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.
• Why do plants need to change genetically, rather drastically, to alter their physiological and developmental responses?
• The 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.
• While 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. 
• Why 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 big surprises.

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.

• Changes 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:

• Crop 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:
• Increases 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.)
• Extreme events, specifically construed as biological extreme events.  I have full discussions in several papers and presentations, but a few examples are relevant:
• Temperatures (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)
• Low 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.
• Changes in the seasonality of weather can offset flowering of plants from activity of their pollinators
• Most 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 human activities.)
• Can 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:
• The single greatest physiological control of WUE is the ratio of CO₂ partial pressure inside the leaf (C_i) to that outside the leaf, in ambient air (C_a).  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 C_i/C_a 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 WUE. 
• The degree of improvement in WUE is modest, because lower C_i/C_a 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.
• Overall, 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 C_i/C_a 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
• The net result is that a few crops bred for lower C_i/C_a 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 density.
• The 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 C_i/C_a 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.
• The 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.
• The 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 disease resistance.
• Changes 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. 
• Bread wheat, for example, has to have high N content.   Breeders have to work to keep ahead of the CO₂-induced decline. 
• More 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.
• Land use change:
• Water 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.                                                                          

            The 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.

• Corollary  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 schistosomiasis.

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.