One of the biggest problems encountered in water use planning is to obtain accurate information on the agricultural demand for water. The process, however, is time consuming and costly, and new, computer-based modeling is needed to streamline the process. Also, it is difficult to manage and apply the soil, crop, and climate information to estimate the evapotranspiration of applied water, which is needed to determine how much irrigation water, is needed to produce a crop.
The water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable depending on a wide range of climatic variables.
The water cycle is also essential for the maintenance of most life and ecosystems on the planet.
The sun, which drives the water cycle, heats water in oceans and seas. Water evaporates as water vapor into the air. Ice and snow can sublimate directly into water vapor. Evapotranspiration is water transpired from plants and evaporated from the soil. The water vapor molecule H2O has less density compared to the major components of the atmosphere, nitrogen and oxygen, N2 and O2. Due to the significant difference in molecular mass, water vapor in gas form gains height in open air as a result of buoyancy.
Evapotranspiration (ET) is the loss of water to the atmosphere by the combined processes of evaporation (from soil and plant surfaces) and transpiration (from plant tissues). It is an indicator of how much water your crops, lawn, garden, and trees need for healthy growth and productivity. Accurate estimates of ET are needed in many circumstances. In agricultural irrigation, for example, estimates of ET are necessary for system design, irrigation scheduling, water transfers, planning, and other water issues. For ET to take place, the following conditions have to be met. First, water has to be present at the surface. Second, there must be some form of energy to convert the liquid water into a water vapor. Third, there must be a mechanism to transport the water vapor away from the evaporating surface. Precipitation and irrigation are the two primary sources of water that plants use. Plant leaves and soil surfaces temporarily retain some part of the water applied to the field. This part is readily available for evaporation. The remaining part infiltrates into the soil. Plants extract the infiltrated water through their roots and transport it up to their leaves for photosynthesis, a process by which plants produce glucose (sugar). In addition to water, plants also need carbon dioxide (CO2) and light for photosynthesis. The light comes from the sun and CO2 comes from the atmosphere. In order to take in CO2 from the atmosphere, plants open their stomata’s, the microscopic pores on plant leaf surfaces. It is during this process that they lose their water to the atmosphere.
As mentioned earlier, the conversion of liquid water into water vapor requires large amounts of energy (about 540 Calories per gram of water at a temperature of 100 oC). This energy is provided by the sun in the form of solar energy. The solar energy is absorbed by water molecules and converted to latent heat energy, the energy that is tied up in vapor molecules. The water vapor thus produced escapes to the atmosphere because of a vapor pressure gradient between the surface and atmosphere. Once in the atmosphere, it is taken further away from the surface by wind (or other mechanisms), creating more gradient between the evaporating surface and the air above it. This process continues as long as the three conditions mentioned above are present. Estimating ET Many factors affect ET including: weather parameters such as solar radiation, air temperature, relative humidity, and wind speed; soil factors such as soil texture, structure, density, and chemistry; and plant factors such as plant type, root depth and foliar density, height, and stage of growth. Although ET can be measured using such devices as lysimeters, estimating ET using analytical and empirical equations is a common practice because measurement methods are expensive and time consuming. Most ET equations were developed by correlating measured ET to measured weather parameters that directly or indirectly affect ET. Since there are so many factors affecting ET, it is extremely difficult to formulate an equation that can produce estimates of ET under different sets of conditions. Therefore, the idea of reference crop evapotranspiration was developed by researchers. Reference ET is the ET rate of a reference crop expressed in inches or millimeters. Reference crops are either grass or alfalfa surfaces whose biophysical characteristics have been studied extensively. ET from a standardized grass surface is commonly denoted as ETo whereas ET from a standardized alfalfa surface is denoted as ETr. The American Society of Civil Engineers (ASCE) recommends the use of ETos and ETrs, respectively, where "s" stands for standardized surface conditions. The logic behind the reference evapotranspiration idea is to set up weather stations on standardized reference surfaces for which most of the biophysical properties used in ET equations are known. Using these known parameters and measured weather parameters, ET from such surfaces is estimated. Then, a crop factor, commonly known as crop coefficient (Kc), is used to calculate the actual evapotranspiration (ETc) for a specific crop in the same microclimate as the weather station site.
California Irrigation Management Information System CIMIS is using a well-watered actively growing closely clipped grass that is completely shading the soil as a reference crop at most of its over 120 weather stations. Therefore, reference evapotranspiration is mostly referred to as ETo on the CIMIS web site, although there are a few notable exceptions with ETr. The Equations There are many theoretical and empirical equations around the world to estimate ETo. The choice of any one method depends on the accuracy of the equation under a given condition and the availability of the required data. For reference surfaces with known biophysical properties, the main factors affecting ETo include solar radiation, relative humidity/vapor pressure, air temperature, and wind speed.
The Egyptian agriculture is facing water shortage problem. Which will need to improve irrigation techniques and to find out the possible ways for rationalize irrigation water. Water scarcity has become an increasing constraint to the economic development, particularly of agriculture which is the biggest water consumer. Disputes over water lead to tension within communities, and unreliable water services are prompting people to migrate in search of better opportunities. Water investments absorb large amounts of public funds, which could often be used more efficiently elsewhere. And the challenge appears likely to escalate.
Evapotranspiration numbers can be valuable tools in planning irrigations.