Our default report includes the day's max/min air temps, total precip,
average RH and wind speed, as well as solar radiation and grass reference crop ET.
The term evapotranspiration combines evaporation from soil and plant surfaces with transpiration from plants to describe the total water loss from a crop to the air.
Water evaporates from any moist surface to the air as long as the air is not saturated. This process is called evaporation. Water surfaces in contact with the air—like lakes, moist soils, and even plant leaves—all evaporate water.
Evaporation from plants is called transpiration. Plant leaves evaporate water through tiny, adjustable openings called stomates that are scattered across leaf surfaces. Water moves from moist soil into plant roots, through the plant, and finally out through leaf stomates. The evaporation and transpiration processes are illustrated in Figure 1.
Evapotranspiration is often abbreviated ET. It is often called crop water use. Water is lost to the air both from moist soils and from plants as the final step of the transpiration process. This combination of evaporation and transpiration common to crops is often shortened to one term, evapotranspiration. ET is an energy driven process. Major energy processes are illustrated in Figure 2. There would be more ET occurring on a hot, sunny, windy day then on a cool, cloudy, calm day. The amount of ET that occurs, can be measured but also can be accurately calculated.
Early in the season, when plants are small, most water loss is from soil evaporation. When plants are large, most is from plant transpiration.
Crop water use determines how much water is needed by rain or irrigation. Too little water can reduce crop yield. Too much irrigation can:
Smart irrigation management begins with knowing crop water use. The goal is to give the plants exactly what they need when they need it. If a crop does not get enough water to meet its maximum demand, yield will decline. In fact, crop yield increases as water availability increases to the level of peak crop water use. More irrigation beyond this level does the crop no good, and may actually cause harm. Increased pumping and water costs erode profits. Excess irrigation water may run off or percolate below the root zone, removing nutrients and chemicals from where they are needed. Irrigating at a level less than maximum crop water demand lowers operating costs, but it also can decrease profitability since yield is proportional to crop water use. To attain the highest profits for most crop and situations, irrigation must be managed to prevent both crop water stress and excess irrigation.
Water has three functions in plants:
A second function of water is to transport nutrients throughout the plant. As water moves from the soil into the roots and to the leaves, nutrients are carried along as well. Though nutrients are very important for crop growth, nutrient transport is considered a secondary function of water movement, since much more water is needed to meet the cooling needs of a plant than to meet the nutrient transport needs.
Finally, a very small portion of water, less than 1 percent for most crops, remains in the plant tissue. This may be surprising since water makes up more than 90 percent of the weight of most crops. This shows how much water plants use for other purposes.
Plants and soil usually maintain surface temperatures very near to air temperature. But how can they do this when they receive so much heat from the sun? Why don’t they overheat? The answer is ET. Evaporation of water uses energy, whether the evaporation is from a pan of water, a moist soil surface, or a leaf. Energy from the sun or a hot, dry breeze is used to evaporate water instead of increase the leaf or soil temperature. Brighter sunlight means the plant needs to evaporate more water through ET to keep leaf temperatures near normal. Drier air, hotter air, and stronger winds also pull water away from crops at a faster rate.
Many formulas have been developed to calculate ET. In Kansas, the formulas used to estimate ET are called combination equations because the energy available to drive the ET process needs to account for all the energy sources that the plant is exposed to. This includes energy from solar radiation and advective energy. Advective energy is heat energy transported to a crop by wind. Since Kansas Climatic conditions are very advective, ET methods must account for this. The amount of energy available to drive the ET process in the equations used, are routinely measured at a number of weather stations across the state. These factors are temperature, relative humidity, solar radiation, and wind. Reference ET values that are available today were developed by researchers who would carefully measure weather conditions, while at the same time observe and measure the water use of a reference crop.
Two commonly used reference crops are alfalfa and grass. These crops were used as references since they can be maintained at a certain height and stage of growth for long periods of time during a growing season. A common method of developing reference ET formulas involved the use of weighing lysimeters. Weighing lysimeters are essentially containers of soil that are mounted on a very accurate scale system. Whenever, the reference crop being grown in the container was at the reference growth stages, weather observations and water use data was collected. The productive equations were developed and calibrated using many sites and seasons to achieve the necessary accuracy. On the reference crop ET value can be accurately and reliably predicted the reference ET can be translated into actual crop ET. The same type of research process is needed to determine what portion of the reference ET value that the current crop of interest will use for the given weather condition. Figures 4a-c illustrates the research process required to develop the crop coefficients (Kco's) needed to translate the reference ET into crop ET. In this research, the crop water use for the crop of interest is observed for all stages of it's life cycle. The Kco would be the ratio of the observed crop water use to the reference ET.
Crop ET can be thought of as a percentage of a reference ET, as illustrated in Figure 5, however, Kco values can be greater than 1, if the actual crop use of the crop of interest is greater then the reference crop. This is possible, especially for grass reference ET values, since the reference grass is short as compared to many tall field crops. Obtaining reference ET values is becoming easier in the major irrigated areas of the state. Radio, newspapers, and computer network services often report reference ET. Reference ET values are used in many irrigation scheduling programs, which help irrigators minimize irrigation pumping costs without causing yield loss. Reference ET values are also calculated using information from the K-State weather station network. These values are available via the web at www.oznet.ksu.edu/wdl, which is the address for the Kansas State University weather data library.
The crop coefficient reflects these differences and is used to estimate ET by comparison with ET from a specific reference crop.
Different types and varieties of plants are capable of moving water at different rates. They also have different maximum rates of ET demand. Even the same crop may have very different water-use rates depending on the stage of growth or the relative amount of canopy cover.
Many of these differences are included in a term called the crop coefficient. The crop coefficient reflects how the ET of one crop compares with the ET of a reference crop. For example, alfalfa is often used as a reference crop; and ET from a full canopy of alfalfa is given the term reference ET. Compared to the reference crop under a specific set of conditions, a corn crop typically evaporates some percentage less water. The exact percentage changes for different crops and canopy characteristics. Thus, if reference ET is known, we can use crop coefficients for corn or other crops to quickly estimate crop ET.
If water is not available in the root zone, no ET can occur.
Soil, water, and plant relations are very important to crop ET, and they are critical to effective irrigation management. Soil characteristics determine how much and how tightly water can be held in the soil and how quickly water can move to plant roots to replace absorbed water. Rooting depth determines the volume of soil water the plant can access. Together, these factors control the amount of water available to the crop for ET. The goal of irrigation is to keep soil water availability from limiting crop ET.
These relationships are discussed in detail in K-State Research and Extension Bulletin L-904, Soil, Water and Plant Relationships. These soil, water , plant relationships provide the basis of developing a crop water budget or an irrigation schedule. Irrigation scheduling refers to the use of procedure to determine when and how much irrigation water should be applied to meet a specific management goal. A common irrigation goal is to prevent yield loss due to deficient levels of available soil water. An additional goal may also to prevent excess irrigation application. An ET-based irrigation schedule uses ET information to determine the crop water withdrawals from the crop root zone, which can retain a fixed amount of water based on root depth and soil type. The withdrawals by the crop are off set by additions of water by rain or irrigation. This water balance is illustrated in Figure 6. KanSched is an ET-based irrigation scheduling software program that has been developed by K-State Research and Extension irrigation specialists to assist irrigators wishing to use ET based scheduling. KanSched is avaiable via the web at www.oznet.ksu.edu/mil.
Soil characteristics determine how much and how tightly water can be held in the soil and how quickly water can move to plant roots to replace absorbed water. Rooting depth determines the volume of soil water the plant can access. Together, these factors control the amount of water available to the crop for ET.
The goal of irrigation is to keep soil water availability from limiting crop ET.