Irrigation management

   The role of irrigation in the global food supply is indisputable; 8% of arable land that is irrigated supplies 33% of our food.  Yet, irrigation is challenging to maintain against two major problems:

• Progressive salinization of land.  Soils contain soluble salts, especially salts of sodium, that are toxic at some level to all plants and impose a significant metabolic cost in those plants that have physiological routes to deal with salt.  Salt loads in soil are notably high in drier areas, precisely where irrigation is most needed. 

            Consider surface waters first as sources of irrigation water.  The rise in river salinity with irrigation is at heart a simple matter of mass balance.  In the absence of irrigation, rivers carry salt loads to the sea, where it is ultimately disposed to sediments and recycled by tectonic action.  In carrying the load, rivers attain mean salt concentrations that are set as input rates (largely from weathering) divided by mean water flow.  Agriculture does not add sodium salts in any quantity, so the effect of diverting river flows to crops, thus increasing the loss of water to the atmosphere, is to decrease river flow and thus decrease the dilution of salts.      

            Increased river salinity can be modest.  It is, however, not the only determinant of soil salinity.  If surface waters are applied in merely sufficient quantities for crop growth, all the water is lost and all the salt remains, thereby building up progressively.  Flushing the salts, at least periodically by overwatering, returns the salts to the river source.  On a long-term average, salt loads in the river increase as 1/(fraction not lost to ET).  This may be tolerable on average, but on the lowest reaches the salinity is highest and can be lethal to crops.  The river may even run dry.  A number of rivers in the US West and China match this description.    

                        Mitigation methods include:

• Removing salt at supportable expense, by leaching salt to evaporation ponds on farms, where it is either stored or disposed of remotely.  This requires diversion of part of the land area, but the ability to sustain crop production on the rest of the area is an overwhelming benefit.   Note that decreasing the salt load by desalination of input water is far too expensive.  The best technologies at present cost about US $2 per 1000 gal (US $0.50 per 1000 L).  Water-use efficiency for crop growth has an effective upper limit of about 2 kg biomass per 1000 L water.   Edible or marketable yield from the 2 kg of total biomass may be 0.8 kg for field crops or 0.2 kg for higher-value horticultural crops.  The economic value of either is below the cost of desalination, and the other costs to the grower (fertilizer, machinery, etc.) must still be met.
• Using or developing crops that tolerate salinity.  A moderate amount of research around the world is devoted to this.  Basic physiological research has revealed a number of mechanisms of tolerance, such as exclusion at roots, sequestration in tissues or in organelles of tissues, and enzyme insensitivity to salt.  To achieve good tolerance, defined as maintenance of high fraction of unstressed yield, requires operation of several mechanisms in concert.  Therefore, introgressing salt tolerance genes from wild salt-tolerant relatives into crop varieties is a difficult task that has not yet yielded major gains.  An alternative is switching to naturally salt-tolerant species.  Only some of these are palatable, even to livestock.
• Richard Richards of CSIRO in Australia analyzed the research as far back as  1991 and concluded that it is more economical, and achievable, to raise yields on unsalinized lands than to develop salt-tolerant crops.
• As a niche for our consulting work, we must at least bring these facts to the fore.  Beyond this, we could develop decision-support tools with details of water supplies and quality (salinity), evapotranspiration, crop yields, climatic variability, drainage practices, and more.  There are competing firms or statal extension services, but few have the full context.  The latter can, however, be very valuable collaborators, for their long-term contacts with growers and public bodies.
• Recurring water deficits that reduce yield periodically or, worse, increase as a secular trend and threaten to kill perennial tree crops that represent major investments.  Severe reductions in water availability have obtained in Australia for a decade and have developed in California, along the Colorado River, and the Yellow River (Huang He) in China.   Climate and humans are both causal:
• Droughts, including megadroughts (those droughts with high impacts, long duration, and long return times).  Climatic cycles have been documented over centuries with methods such as tree ring widths.  Humans may have altered the climatic cycles recently, with the primary route likely being addition of greenhouse gases to the atmosphere; El Niño-Southern Oscillations have accelerated in recent decades.
• Expropriation on irrigation water by municipal and industrial use (M&I).  Commonly, there is no alternative irrigation source or site in the affected region, so that there is a net loss of irrigated agriculture in the region.  Crop production is displaced to other regions, as is the case with California urbanization pushing crop production to Mexico, Chile, and diverse other areas, depending upon the crop species displaced.  Extra water is used in the new regions; the market served by the original region is then importing the products of that water use, or "virtual water."  This is often problematic.  The new producer region becomes ever shorter of water, and food security is reduced in the market served by the original farming region.
• Water conflicts between statal entities.  The Middle East is rife with examples.   Turkey, Syria, and Iraq are at odds over the Tigris and Euphrates Rivers; Israel, Jordan, and the Occupied Territories are at odds over three regional rivers. States of the western US and Mexico are in nearly constant litigation.  Wins and losses occur to such statal entities and to individual irrigators.

   In light of growing problems with sufficiency of irrigation water, a number of research groups have attempted to develop management strategies for growers, including:

o       More efficient irrigation methods.  Subsurface / drip irrigation reduces the exposure of applied water to surface evaporation.  The reduction in total water use is apparent mainly for crops that have lengthy stages of low leaf cover, such as row crops; for orchards, the savings are considerably smaller.  The need remains to irrigate to excess periodically to flush inevitable salt accumulations to drainage areas. 

                 The complete irrigation system is also scrutinized.   Water delivery to the point of use is improved by lining irrigation canals.  There are some downsides, including reduction of canal-side wild vegetation that may support insects and birds that offer pest control.

o       Better decisions on which crops to grow, partly on the basis of economic yield per unit water use.  Decisions can be implemented by individual growers.  On larger scales, the need for irrigation water can be reduced by changes in the agricultural system.   Large decreases in crop production required to sustain a population and thus in water use attend the switch from using grain to feed livestock to using grain and other crops directly in the human diet.  Among livestock, ruminants such as cattle are far less efficient in converting crop biomass to edible animal products.  Regrettably, current global trends are in the opposite direction, particularly in the large and growing economics of China and India.

o       Identifying stages of crop development at which reductions in water supplies have minimal or no effect on final yield and on the potential for future yield.  The full, quantitative optimization of such deficit irrigation, as it is termed, is a project in which we are currently involved.  The research is being carried out by a consortium of 13 investigators at three universities in CA, NM, and TX.  We offer a few details here.

·        One of the investigators, Ted Sammis at New Mexico State Univ., has engaged our consulting firm to develop:

o       A detailed model of pecan photosynthesis and transpiration, under varying degrees of water stress, using all that we know about the physiology and development of the plants, the soil water balance and water transport, soil and plant N dynamics, and the weather drivers

o       Simpler, more empirical models that growers could ultimately use.  The holy grail is a model that predicts performance (photosynthesis, growth, nutfill, water use) as a function of the degree of water stress imposed.  In turn, the simplest measure of imposed stress is the fraction of unstressed ET allowed by managing irrigation inputs.

o       Consequently,  prediction of the optimal stress to impose at various growth stages.

o       Ideally, the responses of ET, photosynthesis, growth, and yield to low-nitrogen stresses.  We could then develop joint water and N management strategies.  We won't present these ideas here.

·        The detailed model predicts many plant responses that can be tested, to see if the model is accurate enough:

o       ET, of course, at all times of day and of the season

o       Photosynthetic rate, ditto - Measure both this and ET with eddy covariance, on selected sites (here and in California; the funding comes from a grant to a consortium in CA, NM, and TX)

o       Additional physiological responses:

§         Stomatal conductance at all times and in all local environments on the tree (measured by gas exchange)

§         Photosynthetic capacity, which is the potential peak rate of photosynthesis at high light and high CO₂ level.  It is modulated by actual light and CO₂ levels, with the latter controlled by stomtal conductance and the CO₂ level in ambient air (this capacity is measured by gas exchange)

o       Leaf water stress at all times, which can be measured by pressure bomb and also by remote sensing of leaf reflectance

o       Leaf N content over the season, which can be sampled

o       Soil water balance, ditto

              We already have a detailed model that predicts:

·        Light interception on all leaves (statistically), using canopy architecture (including neighboring trees) and solar angles at all times (in time steps of one's choice - e.g., hourly)

·        The rest of the leaf environment - canopy air temperature and humidity (including changes induced by canopy  ET and PS), windspeed, CO₂ level

·        Resulting transpiration and photosynthesis of all leaves (again statistically), including:

o       The stomatal response to the immediate (aerial) environment of the leaf and to achieved leaf PS rate, and to leaf (or root) water potential, using an extended version of the well-verified Ball-Berry model

o       The photosynthetic rate, from the robust Farquhar-von Caemmerer-Berry enzyme-kinetic formulation

o       Leaf temperature from the energy balance on each leaf (inputs of solar radiation in both the PAR and NIR; thermal IR from the sky and from surrounding vegetation; thermal IR output radiated by the leaf; transpirational cooling; and convective heat transfer between leaf and air)

We also have generated a simpler, more empirical model of whole-tree ET, PS, and nut growth, using:

·        The predicted changes in stomatal conductance (g_s) from water stress

o       The calculation is iterative.  Water stress depends on soil water withdrawal, hence, on ET and g_s themselves.  At each time step, g_s is adjusted to maintain the desired fraction of unstressed water use.

·        The consequent changes in PS rate, with the assumption that PS capacity is not changed by water stress but only by leaf N content

·        The predicted seasonal trend in leaf N content, assuming that leaf N is progressively withdrawn to support nut growth.  (This need work.)

·        An estimate of how much the final leaf area is reduced by water stress during leafout. (Also needs work)

This model makes some interesting predictions:

·        In short, if one has only 50% of normal irrigation water available, one should apply irrigation to support 36% of normal unstressed ET during leafout and 60% of normal unstressed ET during nutfill.  One expects 67% of normal nut yield. 

·        The early-season stress reduces leaf area development to 71% of normal.

o       Combined with reduced stomatal conductance, the water use becomes 36% of what a full-area, unstressed canopy would have.  The reductions in area and in ET rate per unit leaf area are both important.

o       During nutfill, the canopy uses less water, computed as leaf area multiplied by transpiration rate per unit leaf area.  Leaf area is reduced to 71% of normal, and the leaves are stressed to use less water per area.   As a saving grace, the leaves have far smaller proportional reduction in photosynthesis per area

            Other schedules of stress are worse for yield.  This is a preliminary model; it needs improvement to resolve more growth stages.  It also needs better data on leaf area development.

            The results are expected to be applied in agricultural extension in the three states.  There is a niche for using these developments more widely.  Provided that the application is effective in the original crops (pecans, almonds, pistachios) and states, the method will have credibility in application to other states and in being developed for other crops.  Decision support can also be offered to irrigation districts and state water boards.