Irrigation Chapter 6 - Crop Water Use

Good water management requires that the irrigator apply only enough water to meet the needs of the crop plus some additional amount to compensate for the inefficiencies of the irrigation system. Estimate of both crop water use and irrigation efficiency are necessary to calculate total irrigation amount. This chapter will deal only with crop water use.

Authors: Darrell Watts, University of Nebraska-Lincoln Biological Systems Emeritus Extension Engineer Norm Klocke, Former University of Nebraska Irrigation Engineer.

Crop water use is made up of two parts: evaporation from the soil surface and transpiration from the leaves of the crop. The sum of these is called evapotranspiration (ET) (Figure 6.1). The terms evapotranspiration and crop water use will be used interchangeably.

Over a growing season, 70 to 80 percent of all evapotranspiration is made up of transpiration.  Water­ is drawn from the soil through the roots and up through the plant to be evaporated from the surface of the leaves. This is highly useful water consumption. It cools the leaves and helps move nutrients from the soil into the plant.

When the plant is small, the rate of transpiration is controlled by two factors, if soil water is not limiting: 1) the evaporative demand of the atmosphere and 2) the amount of leaf area available for evaporating the water. (The primary source of evaporative energy­ is solar radiation; however, atmospheric demand­ is also affected by temperature, wind speed and relative humidity of the air.) As the plant grows and leaf area expands, the atmospheric demand becomes­ the major determining factor. When the soil becomes dry, the movement of water from the soil to the root system cannot keep up with evaporative demand­. Evapotranspiration is reduced in comparison to that of a well-watered crop. Normally, the ET rate will decline some time before wilting is visible.

Evaporation from the soil makes up 20-30 percent of the total evapotranspiration. It represents a direct­ flow of water vapor from the soil to the air above. To a considerable extent, evaporation is a waste. It can’t be avoided; however, it can be controlled to some degree by residue cover and by when and how much tillage is done.

The evaporation rate from the soil depends mainly on two factors: 1) the evaporative demand at the soil surface and 2) how moist the soil is in the top few inches. On a bare soil exposed to direct sunlight, the evaporative demand at the soil surface will be essentially­ the same as the atmospheric evaporative demand. If the surface is wet, the evaporation rate will be essentially equal to the atmospheric demand. After three or four days, however, the evaporation rate will decrease to only 5-10 percent of demand.

The situation is different under a growing crop. As leaf area expands and shades the soil from solar radiation, the evaporative demand at the soil surface decreases in comparison to a soil exposed to direct sunlight. Under a well developed crop canopy, the initial evaporation rate for a wet soil surface will be lower and the surface will dry much more slowly. These points are illustrated in Figure 6.2, which shows how the evaporation rate changes over several days.

The amount of soil surface cover influences soil evaporation. Crop residues can reduce soil evaporation by one to three inches during the irrigation season. The amount of water savings depends on the time interval between irrigations or rainfall events. When the soil surface is wet, the residue insulates the soil from solar radiation, reducing the evaporation rate. Water that would quickly evaporate from a bare soil may be retained under residue long enough for the next rain or irrigation to push it deeper into the soil profile, where it will be available for plant use. If there is no rain or irrigation for a long period, the surface moisture under the residue will slowly continue to evaporate and eventually will be lost. When soil is wetted more often, as in the case of sprinkler irrigation, there is greater potential for evaporation loss. In this situation, crop residue can contribute significantly to evaporation reduction.

When a crop is small, actual evapotranspiration will be low compared to atmospheric demand. As the growing season progresses, leaf area expands to give more “evaporating surface”, so that the crop’s evapotranspiration­ rate comes closer and closer to the atmospheri­c demand. At full cover, when the crop fully shades the ground, the ET rate will be approximately­ equal to atmospheric demand. Around beginning dent in corn or pod fill in soybean, the plant begins to lose its capacity to transpire at high rates. The actual ET rate begins to fall off in comparison to atmospheric demand, even though the crop is still at full cover. These points are illustrated in Figure 6.3, which shows average ET rates over the growing season for corn and soybean in east-central Nebraska. Average values will be slightly greater in western Nebraska because of lower humidity.

Actual crop water use can be very different from the average because of variability in the weather. This is shown in Figure 6.4, where actual daily ET measurements across a growing season are compared with the average for corn that was shown in Figure 6.3. In any one year, average values can give only a rough guideline to water use. That’s why irrigation scheduling­ is much more accurate when it’s based on evapotranspiration estimated from current daily weather data rather than average values.

Leaf Area Index (LAI) defines the amount of leaf surface available to evaporate water. The leaf area index­ is the ratio of leaf surface area (one side) to land surface area. For example, if there is an average of 15 square feet of leaf surface for each 5 square feet of land surface, then the LAI would be 3.0. When this value is greater than 2.7, the crop’s evapotranspiration is determined only by atmospheric demand and is not limited by leaf area, provided that the plant is not under water stress. For irrigated populations of corn, an LAI value of 2.7 is reached when the crop is five to six feet tall and the ground is almost completely shaded.

When the leaf area index is less than 2.7, evapotranspiration is determined by atmospheric demand, LAI, and soil surface wetness. If the soil surface is dry, when the leaf area index is below 2.7 the actual evapotranspiration will be less than atmospheric demand­; however, when leaf area index is below 2.7 and the soil surface is wet, evapotranspiration approaches­ the full cover value for a short time because­ of additional evaporation from the wet soil surface.

Figure 6.5 shows a typical leaf area index curve for irrigated corn, as it changes throughout the growing season. Before the crop reaches the peak LAI value in Figure 6.5, it crosses the full ET threshold of 2.7. From that point on, leaf area becomes­ a non-limiting­ factor for water use. A corn crop planted to a high irrigated population might have a peak leaf area index of 5. The same variety planted at the same time to a lower population might have a peak leaf area index of 3. When both crops have passed the full cover threshold leaf area index value of 2.7, they will have essentially the same evapotranspiration rate, if soil water is not limiting. Total­ seasonal ET may be nearly the same; however, under typical conditions­ the yield potential­ of the higher population is greater.

The effect of plant population on leaf area index over the growing season is illustrated in Figure 6.6 for a typical growing season. The curves represent four populations of corn planted on the same date and with the same variety. Once the “high” and “medium­” populations cross the full ET threshold (LAI = 2.7), the rate of water use is essentially­ the same for both populations. Leaf area index is not the limiting factor in water use in either case; however, the high population crosses the threshold level sooner than the medium population. After the high population has passed a leaf area index of 2.7 and before the medium population reaches it, transpiration by the high population would be greater for a few days.

The “low population” just barely reaches a peak leaf area index of 2.7 and only for a short time. When the index is below 2.7, the transpiration rate in the low population should be less than that of the medium and high populations.

The leaf area index in the very low population never gets to 2.7. It is likely that total evapotranspiration in this population would be less than that of the others when soil water is not limiting. When the soil surface is dry, the further the leaf area index is below 2.7, the more transpiration is reduced; however, if it rains frequently, additional evaporation from the wet soil will offset some transpiration reduction. The sum of the evaporation plus transpiration for the very low population may be closer to that for the other three plant populations than would be projected based only on leaf area index. This will be very year specific, depending on the rainfall frequency. Infrequent rainfall will let the soil surface stay dry under the very low population and less total evapotranspiration will result.

To get any significant savings in water use under irrigation, populations of modern, upright leaf corn varieties (115-120 day maturity range) would have to drop below 13,000-14,000 plants per acre. Substantial savings would come only when populations are in the range of 8,000-10,000. For shorter season varieties (with fewer leaves), populations can be 10-20 percent greater and attain water savings, but the principle is the same. There may be good reasons to reduce plant populations on some soils or in certain areas of an irrigated field; however, water savings is probably not one of them.

Hail can cause a near instantaneous reduction of effective leaf area in any crop. When this happens transpiration will be reduced if the leaf area index is below the 2.7 threshold. Whether total evapotranspiration is reduced­ depends on how much soil evaporation increases­ as the canopy is opened up. The effect of green snap on corn is similar. Plant population is lost. If enough plants are killed, transpiration will be reduced­; however, this may be temporary. If weed growth develops as the result of the opened canopy, the additional leaf surface of the weeds may bring total­ transpiration back to full crop canopy levels.­

Under irrigation, the relative maturity range of a particular corn variety has an important effect on seasonal crop evapotranspiration. At the same location, a variety with maturity of 120 days will use more water­ than a 100-day variety. If both varieties are able to mature fully, the grain produced for each inch of evapotranspiration is approximately equal.

Longer season corn varieties use more water, but they also produce more grain if the heat units and water supply are available. The difference in water use is due to total days of water use, not a difference in daily water use. If a long season variety and a short season variety are both at full cover (LAI above 2.7), the daily ET rate will be the same for both until the short season variety begins to mature.

The ability of soil to transmit water to plant roots and the evaporative demand from the environment together determine actual crop evapotranspiration. This transmitting ability is different for every soil and depends on the water content. Generally, when 50-70 percent of  available water is depleted from a soil, crops start to experience stress. Crop evapotranspiration falls below evaporative demand. The drier soil is unable to transmit water to the roots fast enough to meet the demand at the leaf surface. Stress may not be visible, but plant processes begin to slow down and plant temperature increases.

In the field the actual amount of water depletion at which stress begins depends on the evaporative demand. Under very low demand (a few hundredths of an inch per day), 75-80 percent of the available soil water may be depleted and no stress will occur. As soon as the evaporative demand goes up, however, stress reappears. In general, irrigating when no more than 50 percent of available water has been depleted is a conservative approach to avoiding plant stress. This approach leaves some room for error without risk of serious crop stress. This is discussed in more detail in a later chapter.

Table 6.1 shows the effect of reduced soil water availability on growing season evapotranspiration for corn in an experiment at North Platte, Nebr. The dryland treatment received rainfall only, while the limited irrigation treatment received an additional six inches of water. The fully irrigated treatment was managed to replace all the water used for evapotranspiration. Crop evapotranspiration and grain yields were reduced when soil water became limiting. The limited irrigation was applied to give the most benefit to grain yield.

Under limited irrigation little or no water was applied during vegetative growth. Water was applied during pollination and grain fill to maximize yield with limited water. Limited water applications targeted­ to critical growth stages can be very effective for grain production if there is not enough water for a full crop.

Under water limiting conditions, grain yield increases­ in a straight line manner as more water is added to increase evapotranspiration. This is true up to the point ET max, where water availability is no longer limiting and crop evapotranspiration depends only on atmospheric demand. Figure 6.7 is a graph of the yield and ET data from Table 6.1. It shows that from the dryland yield up to full irrigation, each additional­ inch of evapotranspiration gave about 14.9 bu/ac of grain. This emphasizes the value of con­serving water and reducing evaporation with residue covers when water­ is limiting. A backward projection of the line suggests that about 10 inches of evapotranspiration would have been needed to produce the first increment of grain yield. Note: These results represent­ an average­ over several years at a particular location­ for one corn variety. The yield per inch of additional­ ET will vary with variety and soil and climatic conditions. Timing of irrigations is important­ to receive the greatest yield increase per inch, when water is limiting.

During crop development there are critical periods­ when water stress should be avoided. A small reduction of evapotranspiration below the atmo­spheric demand (i.e., an evapotranspiration deficit) during the critical period will reduce yield much more than would the same reduction during another growth period. In general, the most critical period for grain and seed crops is during the reproductive stage (flowering). Crops such as corn, winter wheat, determinate soybean, and dry edible beans are all highly sensitive to stress during this period. They also are more sensitive to stress during grain filling than they are in the vegetative period before flowering.

Indeterminate soybean will flower over an extended time period. They can tolerate water stress well through vege­tative growth and into reproduction, but are more sensitive to stress during pod fill. Excessive growth may result from early irrigation, which may lead to lodging later in the season. Late irrigation can delay maturity­ and drying of the plants in advance of harvest.

Sugarbeet is particularly sensitive to water deficits­ at the time of crop emergence and for about a month after emergence. In general, except for this period­ the crop is not very sensitive to moderate water­ deficits.

For some crops moderate  stress early may not be harm­ful, and in a few cases may be beneficial. For exam­ple, limited stress in corn during vegetative growth can be tolerated well with little or no impact on yield. For sugarbeet, a moderate evapotranspiration deficit during the later stages of growth may reduce beet tonnage, but will increase sugar content with little or no effect on final sugar yield.

In crops grown for vegetative yield such as alfalfa or grass pasture, any water shortage results in a reduction in dry matter yield. In general, the percent reduction in yield is proportional to the percent reduction­ in evapotranspiration, although for alfalfa the reduction is not on a one-to-one basis unless stress is severe. For example, an evapotranspiration deficit of 25 percent would result in a yield reduction of 18-20 percent.

Reference Crop Evapotranspiration

Techniques have been developed to use weather data to estimate the atmosphere’s evaporative potential and translate it into evapotranspiration (ET) estimates for different crops. In practice, daily data on solar radiation­, air temperature, wind speed and relative humidity are used to calculate evapotranspiration for a “reference crop.” This is designated as ET_R. In the central Great Plains alfalfa is used as the reference. ET_R is calculated for alfalfa, assuming full cover and adequate soil water. Long-term averages for ET_R across Nebraska are shown in Figure 6.8.  ET_R peaks in the first half of July. ET_R on a given day can be above or below the average value because of variable weather conditions. To be closer to actual values, ET_R must be calculated from daily weather data. The University­ of Nebraska’s High Plains Climate Center calculates the ET_R, and makes it available to the public­, along with estimates of actual crop water use, ET_crop.

The ET_R does not always give a good estimate of actual evapotranspiration in an alfalfa field. When the crop is cut, the only water loss for a few days is the evaporation from the soil. As the crop regrows and leaf area expands, the actual crop evapotrans­piration increases to the ET_R value, which is always calculated for full cover conditions.

Another method to calculate ET_R is the use of an atmometer or ETgage®.  An atmometer uses a ceramic plate covered with canvas to simulate the surface of a leaf.  The amount of water that is evaporated from the simulated leaf surface is an estimated measurement of ET_R.   For additional information on the atmometer, see NebGuide G1579, Using Modified Atmometers (ETgage®) for Irrigation Management. The Nebraska Ag Water Demonstration Network is source of ET_R for most of Nebraska:  http://water.unl.edu/cropswater/nawmdn

Estimating ET Using ET_R and a Crop Coefficient

To estimate actual evapotranspiration for different crops, ET_R is multiplied by a “crop coefficient” (K_c), according to the crop’s growth stage. For exam­ple, ET_crop = ET_R x K_c.

Figure 6.9 shows a crop coefficient curve for corn. You can see how the coefficient varies according to the growth stage. Instead of a time scale across the bottom, the most common scale is accumulated growing degree days from planting to maturity. In absence of growing degree information, the best way to estimate the K_c value is to observe the growth stage and obtain a value from Table 6.2.  This table presents crop coefficients for several important Nebraska­ crops, according to growth stage.

In applying the crop coefficient curve (or table) it is important to understand a few assumptions and limitations.

1. When a crop has just emerged, the transpiration will be essentially zero, but there will be evaporation from the soil. Because of evaporation the crop coefficient will be more than zero, probably about 0.1-0.2. Its exact value for bare or near-bare soil depends on specific soil characteristics.
2. When the crop is at full cover, the crop coefficient will be very close to 1.0. The exact value depends on local and regional conditions that are difficult to estimate. That’s why soil water content has to be checked in the field periodically, even when the best possible estimates of evapotranspiration are put into irrigation scheduling routines.
3. If the soil is wet and K_c is less than 1.0, an additional calculation is required to arrive at the correct­ value of daily evapotranspiration. After full cover, no adjustment is needed. This omission is normally not too important to Nebraska irrigators, since irrigation usually does not begin until the crop is near full cover. Values of ET_crop that are published in local newspapers or broadcast on radio or TV do not include any estimate of additional evaporation due to wet soil conditions.

The University of Nebraska’s Great Plains Climate­ Center has developed a network of automated weather stations throughout the state. These stations measure and record solar radiation, air temperature, relative humidity, and wind speed. Weather information from each station is collected daily by a central computer in Lincoln. These data are used to calculate ET_R and to estimate ET_crop, by assuming planting date and one or more values of growing degree­ days required from emergence to maturity. The evapotranspiration is calculated using the pro­cedures previously outlined. Complete information is available on-line at http://hprcc.unl.edu/.  The information is sometimes available­ from news media sources.

Regional crop evapotranspiration estimates are an excellent starting point for tabulating water use from a particular irrigated field.Periodic checks of soil water in each irrigated field are necessary to confirm the water use from that field.

Sometimes those using ET estimates­ from the weather station network have difficulty­ matching information­ from the station to actual­ field conditions. The problem may be with the station, but more often it is due to the user’s failure to make proper adjustments­ before using the data for irrigation­ scheduling on a particular field. Listed below­ are a few problems that may be encountered.

1.     Weather station site with improper exposure (can result in greater or lesser ET values than actual). Watch ET from other surrounding weather stations.

2.     Crop growth stage assumed in the ET estimates does not match the actual growth stage for the field being scheduled. Check growth stage in the field. If the growth stage in the field does not match that assumed in the published evapotranspiration data, a better answer can usually be obtained by selecting a K_c value from Table 6.2 and multiplying it by the ET_R (the value for alfalfa)­.

3.     Wet soil surface in the field from rain or irrigation before full canopy cover. Actual crop evapotranspiration rates may be slightly greater than the estimates­.

4.     Dry root zone in the field before irrigation begins. Actual crop evapotranspiration rates may be lower than the estimates.

5.     Differences in planting dates. Planting dates can be customized if the Internet bulletin board is used directly.

Remember: crop evapotranspiration measurements are estimates from weather stations that are reasonably close in most situations, but certainly are not perfect. They are useful for estimating water availability in soil during the growing season, however, periodic field checks of soil water are very important to keeping any irrigation scheduling program on track.

The most important use for evapotranspiration information is in irrigation scheduling. A detailed guide for the procedure is available in Extension Circular EC709, Irrigation Scheduling: Checkbook Method. ET estimates are a key component for tracking how much water the crops are using­, when to irrigate and how much to apply. This is discussed in more detail in Chapter 9.