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As water supplies diminish, land will necessarily be removed from irrigation or water will be applied more sparingly. Further, much of the agriculture of the semiarid to subhumid regions will continue to be practiced on "dryland". Science in support of agricultural production must concentrate on increasing water supplies and diminishing water consumption. Increasing the water use efficiency in crop production by increasing photosynthesis, by decreasing transpiration, or both, is another way to make limited water supplies go further.
An analysis of technological options to further the objectives described above was made for the Great Plains region in a workshop sponsored by the National Science Foundation. Ways to increase supply include water harvesting, minimum tillage, snow management, improved cultural practices, soil evaporation reduction, phreatophyte control, and use of small water impoundments. Findings with respect to these methods are briefly summarized in this chapter.
Ways to decrease demand include the use of alternative crops, microclimate modification, selection for efficient water use, and irrigation scheduling. Microclimate modification and selection for efficient water use are illustrated through description of an integrative scientific approach involving manipulation of plant reflectance. Mathematical models, chamber studies, and field studies with artificial reflectants and with specially bred pubescent isolines of soybeans, have led to the development of plants better adapted to limited water supply.
Much of the land in semiarid and subhumid areas is already being used for food production. If we can save and use more of the uncertain and sometimes inadequate water supply, if we can beneficially alter the water supply and the microclimatic conditions in which these plants grow, and if we can develop plants
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that use water more efficiently, we can produce more and better crops. In my view, this is one of the major challenges facing agronomic scientists, agronomists, engineers and meteorologists in the years to come.
Others in this volume emphasize ways of getting more water to sustain agriculture in the semiarid West. My emphasis will be on ways to decrease demand by crops grown on dryland.
Of course, many of the same principles I review can apply to irrigation agriculture, and reductions in water demand mean an increase in water supply. Sustained agricultural production on the lands that have remained dry in the semiarid West will be given primary emphasis. However, realism tells us that some lands that are now in irrigation will, when their water supplies are depleted or cannot be economically brought to the surface, revert to dryland agriculture. Perhaps some intermediate type of cultivation such as strategic or critical-stage irrigation will be used in such regions to extend the useful life of the aquifer.
We may also foresee that some lands that have not yet been introduced to irrigation may be irrigated for only a short time. Evidence of satellite-provided imagery shows that in Nebraska in 1980, 18,785 center pivot systems were operational and 1,348 were inactive or had been abandoned. Economic factors such as commodity prices and input costs (e.g., fertility, erodibility) may necessitate the abandonment of lands now under irrigation or their reconversion to dryland culture. We should be prepared for that development.
The problem is to increase water supply and to demand less. To accomplish the former in dryland agriculture is not so easy as it is (conceptually, at least) in irrigation. You do not open the tap more widely. The task is to capture and hold in soil storage as much of the precipitation as possible. There are ways in which plants can be made to demand less water: the microclimate in which they grow can be modified, the plant can be treated, or the plant can be altered in certain ways. In the discussion which follows, these concepts will be further explained and examples will be given.
The issues of water supply and demand, especially as it applies to Great Plains agriculture, was reviewed in a workshop that addressed the task of developing drought management strategies for the Great Plains. In March 1979 a group of knowledgeable individuals-climatologists, agronomic scientists, farmers, and persons in other sectors of the economy and government-were invited to assemble a tabulation of strategies, technological,
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political, economic and social, in anticipation of future droughts in the region. The report of the Panel on Technology, chaired by Dr. Harold E. Dregne of Texas Tech University, provides a comprehensive list of emergency, short-term, and long-term tactics worthy of further study and development as a means of minimizing the impacts of drought and in improving agricultural stability.
Eleven technologies were identified. These can be ordered according to their primary purpose-increasing water supply or decreasing water demand.
1) Water harvesting
2) Minimum tillage
3) Snow management
4) Improved cultural practices
5) Soil evaporation reduction
6) Phreatophyte control
7) Use of small impoundments
8) Use of alternate crops
9) Microclimate modifications
10) Selection for efficient water use
11) Irrigation scheduling
Of these, items 6, 7 and 11 do not fall within the scope of this paper. The brief review of items 1-5 and 8 draws heavily on the report of the Panel on Technology.
"Water harvesting" is the capture of runoff water which spreads over depressional areas in fields or the floodplains of streams. The practice has ancient roots. Evenari et al. have described reconstruction of ancient Nabateaen settlements in Israel's Negev desert where water harvesting provided the basis for all food production. On this continent, water spreading has been limited to mostly a few high-value crops grown in the Southern Plains region, but the potential exists for greater use of microcatchments in low rainfall areas. Microcatchments (microwatersheds) within fields can cause severe flooding of planted areas when precipitation is heavy, so that, except in the very arid regions, this technique may be of limited use.
Methods have been developed to reduce the number of tillage operations needed in crop production. Certain plowing, harrowing, and cultivation operations can be eliminated for many crops, particularly where chemical herbicides are effective in weed control. The benefits of minimum tillage in reducing soil erosion by wind and water have been demonstrated, as has the increased soil moisture availability in times of drought. About 20 percent of U.S. crop production was on minimum or no-tilled land in 1979. Much greater adoption is predicted in the future. Some unsolved problems of minimum tillage include uneven seed germination, low soil temperature in spring, possible disease and insect outbreaks, and possible undesirable environmental effects due to reliance on chemical herbicides. Nonetheless, the potential of minimum-tillage methods for improving soil moisture conditions, minimizing losses of topsoil, and reducing the energy and labor costs in crop production indicates that adoption should increase in coming years.
In the Canadian Prairie provinces, the northern Great Plains, and into Nebraska and Kansas, a significant portion of the annual precipitation occurs in the form of snow. Unless controlled, snow either blows off or runs off over frozen ground as it thaws. Thus, snow often contributes little to the reservoir of soil moisture available to the crop in the spring. Snow is best controlled by reducing windspeed near the surface. This can be accomplished with constructed barriers, tree windbreaks, sown windbreaks, or annual or perennial crops, or by leaving stubble of the previous crop standing in the field.
Barriers cause some problems. Efficiency of tillage operations is decreased to a degree where barriers such as trees create traffic obstacles. Windbreaks may harbor insects that attack the sheltered crop, but they may also, of course, harbor beneficial creatures. The incidence of fungal diseases is thought to increase where the windbreak creates a more humid environment that can favor such disease. Nonetheless, the benefits of wind barriers, particularly in snow management, are important. In the semiarid regions where grains are grown, even small increments of water lead to very significant yield increases. Windbreaks are discussed in greater detail below.
Cultural practices of benefit in moisture conservation and yield improvement are many and varied. They include tillage, fertilization, strip cropping, crop (plant) selection, skip-row planting, and crop rotation. The objective is to maximize crop production while achieving water and soil conservation. Many highly efficient management practices have been developed in the past 30 years. The development of minimum tillage and no-tillage practices are recent examples.
One of the major needs in dryland agriculture is to find better ways to supply nitrogen to crops without running the risk of injuring the plant or of prolonging the vegetative period. Plant and row spacing to meet varied climatic conditions is also a tactic in need of further research.
Systems of reduced tillage which maintain crop residues, promote aggregation, increase infiltration, reduce evaporation and erosion, and control weeds are key factors in increasing stored soil moisture. Crop residues and other mulches on the soil surface reduce evaporation for one or more of the following reasons: a vapor barrier is created; soil temperature is lowered; wind speed at the soil surface is diminished. Weed control by mechanical means depletes soil moisture by exposing moist soil to the elements. Rational use of chemical weed control for moisture conservation is indicated. Winter moisture is stored more efficiently than is summer rainfall, so that in the northern plains windbreaks of perennial grasses can be used to increase snow catch and soil moisture storage.
Fallowing systems are widely used over the Great Plains, and further research to integrate chemical and mechanical weed control will be needed to adapt such methods to areas of varying rainfall and soil texture, organic matter content and pH, so that soil water storage can be maximized and evaporation minimized without injury to the crop.
Where limited water is available for supplemental irrigation, evaporation can be reduced and water use efficiency increased by applying the water at critical stages of crop growth like tasseling and silking, head emergence, or pod and bean development. Tests of strategic (limited) irrigation of sugarbeets in the High Plains of Texas have shown a considerable overall saving of water with little loss of yield. Irrigation water was used most efficiently when application was adequate to maintain a nearly full canopy,
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with no periods of major water stress or excessive water. One irrigation prior to the onset of major water stress in July promoted high water use efficiency in sugarbeets. A considerable body of work has also been done on limited irrigation and conjunctive use of rainfall for sorghum and wheat in the High Plains of Texas. Similar strategies of limited irrigation and conjunctive water use need to be worked out under a variety of cropping, environmental, and stress conditions.
One way to face a problem may be to avoid it. Thought is being given to introduction to the semiarid and subhumid regions of new crops which are less sensitive to moisture stress than those currently grown. Possible new food crops for use in dryland farming systems or with limited irrigation include pearl millet, amaranth, and guar. Specialty crops considered for introduction include guayule as a source of latex and forage sorghum for biomass conversion. Kochia and fourwing saltbrush (Atriplex canascens ) are potentially useful new plants for Great Plains rangeland.
Breeding, cropping systems development, and marketing research will be needed before these crops can be introduced.
A number of microclimate modification methods have proven effective in reducing demand for water and/or increasing water use efficiency in crop production. A few of these are described in detail here:
Strong and damaging winds often reduce agricultural productivity. Cold winds in spring and fall may cause mechanical damage to the whole plant, as well as freezing damage to certain tissues. Winds blowing from arid into semiarid and subhumid areas can also cause mechanical damage. But these winds, because of their high temperature and low humidity, also impose severe moisture stress on the growing crops and cause wilting, desiccation, and loss of potential productivity. In regions where the land is not well protected by vegetation, wind erosion may occur and initiate a decline in productivity. Young, tender vegetation may be damaged or destroyed by "sand blasting" when soil is
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eroded by wind. For a detailed review of the direct effects of wind on plant growth, see Sturrock, and Grace.
Properly designed windbreaks can aid greatly in stabilizing agriculture in regions where strong winds are common. The windbreak aids in uniformly distributing snow over the fields, thereby increasing the supply of soil moisture in spring.,  Windbreaks have a considerable impact on the crops they shelter during the growing season as well.
Considerable experimentation with tree windbreaks and with windbreaks constructed of such materials as snow fencing, plastic screens, the land to the point where farmers begrudge its occupation by tree windbreaks. Windbreaks may interfere with the mechanical operation of the large center-pivot sprinkling systems that are revolutionizing irrigation in the Great Plains region. There is urgent need for windbreak designs compatible with current and foreseeable agricultural systems in windswept regions.
What do we know of the actual mechanis
and reed mats has shown that the climate that prevails in the sheltered area is more moderate than that in adjacent unsheltered fields.  , The air is slightly warmer by day and slightly cooler by night, but absolute humidity is greater by day and by night. The overall effect on the microclimate is to moderate evaporative demand and moisture stress on the sheltered plants. Since moisture stress leads to wilting, closure of the plant stomates, and cessation of photosynthetic activity, the windbreak should permit the achievement of improved crop yields. Evidence from around the world showing this to be true is given in reviews of "shelter-effect" by Van Eimern et al., Marshall, Rosenberg, and Grace.
Despite the proven beneficial effects of windbreaks planted in the Great Plains during the drought years of the 1930s, many of them are now being removed. Changes in agricultural land use that involve larger fields and expensive irrigation systems have increased the value of
ms of windbreak effects on the sheltered plants? From the reviews cited above and particularly from our work in Nebraska with annual windbreaks, constructed windbreaks, and tree windbreaks, the following synthesis is made:
· Shelter is normally beneficial to plant growth. Moisture conservation for later use by the plant is probably the major direct benefit of shelter in dryland agriculture. Even under liberal irrigation, however,
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shelter effect is beneficial to plant growth and yield; turbulent exchange is reduced in shelter; the amplitude of the temperature wave is increased; the air becomes more humid. CO2 concentration is affected very little.
· If the shelter-affected microclimate did not induce physiological differences in plant stomatal resistance to diffusion, evapotranspiration would be consistently reduced in shelter. Sometimes, however, unsheltered plants exposed to strong evaporative demand exhibit incipient wilting with large increases in the stomatal resistance. Then it is possible for the sheltered plants to transpire more water. At such times, of course, the opportunity for photosynthesis remains greater for the plants in shelter.
In an attempt to better understand the problem of shelter effect on water use, Brown and Rosenberg developed a resistance model to predict actual evapotranspiration. The model is derived from the Daltonian concept of evaporation from a free water surface, and requires knowledge of the following meteorological inputs: net radiation and soil heat flux (Rn + S), air temperature (Ta ), vapor pressure of the air (ea ), and aerial diffusion resistance (ra ). The stomatal diffusion resistance to vapor, rs , must also be known. To apply the model to a crop canopy rather than to a single leaf, rc , a measure of the canopy resistance may be substituted for rs .
Using data developed in an experiment conducted in western Nebraska where double rows of corn sheltered irrigated sugarbeets, the model closely predicted measured windbreak influence on evapotranspiration. The model predicted major water savings, particularly under conditions of strong sensible heat advection (strong winds, high temperature, low humidity). Miller et al. demonstrated, with precision weighing lysimeters, that soybeans in shelter also transpire significantly less water when such conditions prevail.
We see no convincing evidence to suggest that photosynthesis rate is decreased in shelter, either because of reduced turbulence or lowered CO2 concentration. In fact, the physiological responses of the sheltered crop under the conditions of reduced evaporative demand in shelter are all conducive to greater photosynthetic activity. The sensitivity of stomatal resistance to the interacting influences of soil water potential and shelter climate
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strongly suggests that windbreaks, valuable as they are in most climates, can be particularly important in the development and stability of agriculture in semiarid lands.
Shelter effect and its influence on crop growth now seems fairly well understood. There is great need for engineers, foresters, and agronomists to develop and/or adapt windbreak designs which will be most effective in each region. Tall annual crops such as corn, sorghum, or ryegrass can be planted in fields of shorter crops in order to provide shelter or to augment shelter provided by widely-spaced tree windbreaks. This idea is not new, but considerable adaptive research will be needed to develop appropriate management systems.
Speculations by Seginer and Aboukhaled et al. suggested that, by increasing the albedo of plants, the net energy load upon them could be reduced, and this should result in diminished evaporation and transpiration. Rosenberg and Brown modeled the effects of reflectants on evapotranspiration. For their calculations, they assumed a 20 percent reduction in net radiation. No more than 15 percent has actually been achieved in our subsequent field studies, however. Whatever the weather condition, lower Rn (higher reflectance) leads to reduced evapotranspiration. The influence of reflectance is of least consequence under high temperature and low humidity since, then, advection of sensible heat is a major source of energy. Savings of water are significant at high temperature and high humidity, and even greater if the reflectant increases canopy resistance to a degree by plugging the stomates.
That reflectants actually do reduce evapotranspiration has been demonstrated with artificial coatings applied to rubber plants by Aboukhaled et al. in growth chambers, and by Doraiswamy and Rosenberg, Lemeur and Rosenberg, and Baradas et al.,  for soybeans grown in the field.
We do not yet fully comprehend the reasons for the reflectant effect on water use, although Baradas et al. consider that increased stomatal resistance, reduced net radiation, reduced longwave emissivity, and other factors are involved.
Reflectants may indeed reduce evapotranspiration, but water use efficiency can increase only if photosynthesis is not reduced concomitantly. With soybeans we anticipated no major decrease in photosynthesis since, under field conditions, that crop is light-saturated by a global radiation flux density of about 700 W m-2 .
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In experiments conducted in the early 1970s at Mead, Nebraska (cited above), we found that photosynthesis and yield were not reduced at all, apparently because the materials with which the plants were coated increased multiple reflection of light deep into the canopy where the plant is usually light-un saturated.
One would expect that reflectorizing a C4 plant such as sorghum which is light-saturated might diminish photosynthesis in that crop. Moreshet et al. found, indeed, that net photosynthesis in sorghum was reduced by 23 percent (solar radiation by 26 percent) immediately after application of a kaolinite coating. But grain yield was consistently increased by the treatment. They attribute this result to specific beneficial physiological effects at the time of panicle initiation and to early senescence in the treated plants that hastened translocation to the developing grain.
The application of reflectant materials may be impractical on a large scale. The practice may prove useful as an emergency technique in times of drought or severe water shortage, especially where labor to apply the material is available. However, from the studies described above, another different approach to increasing water use efficiency has developed. This is described in the following section.
As shown above, there is theoretical support for the ideal that increased reflectance should reduce evapotranspiration. There is experimental evidence that, by artificially increasing reflectance, evapotranspiration may be reduced. There are ways by which reflectance can be naturally modified.
Albedo varies from species to species and within species according to age of the leaf, turgidity, presence of waxes or other materials on the surface, and concentration of chlorophyll. For example, Fergus et al. have reported on barley plants bred isogenically for greater albedo through a reduction in chlorophyll concentration. Wooley, Ghorashy et al., and Gausman and Cardenas have found in the case of soybeans that leaf pubescence increases reflectivity slightly in the visible waveband, more significantly in the near infra-red. Wooley, and Ghorashy et al. also found, for single leaves, that pubescence decreased transpiration, both because of reduced radiation absorption and because of an increased boundary layer resistance. Ehleringer and Bjorkman and Ehleringer and Mooney have observed a
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greater visible reflectance, a reduced transpiration, and a slightly reduced photosynthetic rate in Encelia farinosa due to increased pubescence. However, Ghorashy et al. found no reduction in photosynthetic rate in soybeans, and Hartung et al. found increased yield rate associated with leaf pubescence in that crop.
Drs. James Specht and James Williams of the University of Nebraska Agronomy Department have developed a range of paired soybeans bred isogenically to differ in a single gene only. In their work pairs differ only in the gene that controls a certain expression of the plant's architecture. For example, they have provided us with seed of isogenically paired Harosoy cv. soybeans which differs only in the degree of pubescence on the leaves and stems.
These isolines were grown in plots of about 1.5 hectares in experiments conducted during 1980 at our Agricultural Meteorology Station near Mead, Nebraska. Detailed measurements of radiation balance, energy balance, photosynthesis, and evapotranspiration were made in the field during the course of the summer. Results of the study are being reported in a number of papers now in press (Baldocchi et al., ,  ).
For our purpose, however, it is sufficient to cite the following findings: ET was reduced overall by approximately 7 percent in the densely pubescent isoline of Harosoy cv. Photosynthesis and yield were unaffected by pubescence over the season. But on single days, especially days of strong regional sensible heat advection, the CO2 /H2 O flux ratios were increased by 30 percent. The pubescence greatly altered the partioning of Rn with deeper penetration into the canopy of the pubescent isoline.
In practical terms, the research on pubescence described above provides an important opportunity for strengthening and stabilizing agriculture at the edge of the semiarid and subhumid zones. Soybeans are currently grown about as far west as Lincoln, Nebraska. To the west, corn, especially irrigated corn, predominates. Corn is strongly sensitive to dry and hot weather, especially as it enters the reproductive stage. Soybeans are less so, since indeterminate cultivars predominate. The agronomic trials of Hartung et al. and the microclimate-physiology studies of Baldocchi et al., ,  support the plan to introduce pubescent soybean lines into the drier areas to the west. Pubescent isolines are now being increased so that by the end of this decade they may be introduced to commercial use.
In order to sustain dryland agriculture in the semiarid and subhumid West, proven techniques for increasing the supply of available water and decreasing demand must be applied. Snow management, limited irrigation, minimum tillage, evaporation suppression, and other such techniques are mentioned in the foregoing pages. These and other techniques need improvement, but a considerable body of research work indicates that there is significant potential for their adaptation.
Microclimates can be beneficially modified with well designed windbreak systems. The energy balance of the crop may also be artificially altered by the application of special materials such as reflectants. Perhaps most logical and economical, plants may be bred for improved adaptation to the moisture stresses that typify the arid, semiarid, and subhumid environments. Increased reflectance and/or increased pubescence has been shown to improve water use efficiency in the soybean crop.