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FEATURES HOW DO PLANTS USE WATER? by Grant Cardon and Dan Smith Department of Soil and Crop Sciences Colorado State University Estimates of crop consumptive water use are useful for a number of purposes, ranging from site - specific applications, such as irrigation scheduling, to broader uses related to stream or basin depletions. The most accepted method of obtaining these estimates is to use locally derived reference evapotranspiration (ET) values and adjust these values to specific cropping conditions. The water requirement of irrigated crops varies widely depending on a number of factors. Crude studies conducted in the early 1900s using a diverse array of crops revealed that the amount of water used to produce a pound of dry matter varied from 300 to 1000 pounds. The plant tissue associated with each pound of plant dry matter contains only a little over four pounds of water. This amounts to less than one -tenth of one percent of the total water requirement, assuming the best -case scenario of 300 pounds of water use per pound of dry matter. So where does the rest of the water go? The best answer is, "...into thin air." More than 99.9 percent of the total water requirement of an irrigated crop is consumed by evaporation from water occurring on either soil or crop surfaces, and transpiration, the evaporation that occurs from water on internal plant surfaces. The combined water loss from the processes of evaporation and transpiration is called evapotranspiration, or ET. The cumula- tive amount of ET for a crop over an entire growing season is roughly equivalent to that crop's seasonal water requirement. Reference ET -- For irrigated crops that reach complete ground cover for most of the growing season, most of the seasonal ET is from transpiration. Transpiration water losses from a crop that completely covers the ground are similar in magnitude to what would be observed from the surface of an open water body such as a pond or lake of comparable area. Although transpiration losses are high, they are directly linked to The Effect of Soil Salinity on Crop ET Coefficients Soil salinity is widely recognized for its potential to affect crop water use. Mechanistic relationships between salinity and ET are known to exist, and some attempts have been made to formalize the use of these concepts in practical water management situations. However, little effort has been devoted to developing practical crop coefficients for use under variable saline soil conditions. Soil salinity can affect plant water use through both direct and indirect effects. Direct effects are those related to physiological responses of plants to salinity. Water moves along a path in the direction of free energy gradient, from a higher energy state to a lower energy state. Crop water uptake is an active, energetic pro- cess and is influenced by the energy state of water in the soil. Soil salinity lowers the free energy of water in the soil, thereby reducing the potential uptake of water from the system. Plant responses to soil salinity also have been observed at the leaf- atmosphere inter- face in the form of reduced stomatal conductance. Because photo- synthesis relies directly on leaf gas exchange, salinity can exert a direct effect on dry matter accumulation. Based on the direct physiological responses noted above, the over- all expression of salinity on the plant, except in the less common cases of specific elemental toxicities, is an induced drought, even if soil water content is high enough to otherwise allow for normal water uptake under non - saline conditions. Thus, the indirect effect of soil salinity is to produce stunting of plant growth, which re- duces crop water use for either a part or the entire growing season, depending on the sensitivity of the plant to saline soil conditions and other factors related to canopy cover and distribution. In some instances, the stunting can be permanent, even if the saline condi- tion is relieved, if the crop cannot immediately recover to produce compensatory growth or restore transpiration to its potential level. Soil salinity effects, therefore, are reflected in crop ET through both direct and indirect effects. Conceivably, salinity- induced reductions in crop ET could be accounted for by use of specially calibrated crop coefficients. These coefficients would be especially useful in the Arkansas River valley, where salinity problems are widespread, and water accounting procedures that account for sa- line soil conditions are needed. The lysimeter studies proposed for this region could provide the data needed to formulate these special crop coefficients. �� 7