Evapotranspiration: Understanding and Predicting Plant Water Requirement
Irrigation scheduling and planning is essential for crop growth. While plants need water to grow properly, water is often a scarce resource in many agricultural areas. This scarcity leads to many problems for farmers. By understanding and predicting the evapotranspiration needs of the crop, the farmers can better utilize the water from rainfall and other natural sources and effectively schedule and manage irrigation.
Evapotranspiration (ET) is the combined processes of evaporation of water from the soil surface and plant surfaces and transpiration of water through the plant tissues.
The overall amount of ET for a crop or landscape over an entire growing season is about the same as the seasonal water requirement. So it is a direct measure of, how much irrigation farmers need to do to maintain required water stress level for the plant.
But in some crops like grapevine, maintaining irrigation below ET demand - is used to accelerate fruit development and improve quality by controlling sugar (brix), berry size and color.
How does ET occur?
The majority of the seasonal ET happens from transpiration. Transpiration losses are usually high and are directly linked to plant growth and productivity. This is because the pathway for transpiration in plants is the same one that allows for plant intake of carbon dioxide. Both exchange processes occur through pores called stomates on the leaf surface. Stomates are fully open when plants receive enough water through the soil and when both transpiration and photosynthesis are occurring at maximum rates. The inability to satisfy the need to ET can leads to reduced photosynthesis in crops, wilting, slow rates of fruit growth, indicates potential root pathogens in many crops, and also leads to increased incidence of insect damage owing to reduced health.
How is ET Estimated?
Most of the present day work on Evapotranspiration is based on Howard Penman’s paper titled “Natural Evaporation from Open Water, Bare Soil and Grass” published in 1948 by Royal Society.
Penman derived the fundamental physics of evaporation, which is driven by energy arriving by the sun and sky, along with the temperature and humidity of the air. Penman’s complicated formula just says what is intuitively true: if it’s sunny and warm, there is more evaporation. If it’s humid out, it’s hard to evaporate to cool off.
The most important factors needed to estimate ET are: 1) the local weather conditions and 2) the cropping system for which estimates are needed (type of crop, planting date, crop development).
Local weather conditions are important because ET is driven by weather factors that determine the drying power of the air. We can accurately predict ET losses in a given area from the measurements of four local weather variables:
- solar radiation
- temperature
- humidity
- wind
The data from these measurements are then plugged into equations that accurately predict the daily rate of ET for those conditions. These values are called reference ET and refer to the ETo of the reference crop.
An important point to note here is ETo is a “reference ET”, to estimate the water use of a well-watered grass field under a set of local weather conditions. ETo expresses the evaporating power of the atmosphere at a specific location and time of the year and does not consider the crop characteristics and soil factors.
On the other hand crop evapotranspiration(ETc) is the evapotranspiration from disease-free, well-fertilized crops, grown in large fields, under optimum soil water conditions, and achieving full production under the given climatic conditions.
Crop evapotranspiration can be calculated from climatic data and by integrating directly the crop resistance, albedo and air resistance factors in the Penman approach. As there is still a considerable lack of information for different crops, the Penman method is used for the estimation of the standard reference crop to determine its evapotranspiration rate, i.e., ETo. Experimentally determined ratios of ETc/ETo, called crop coefficients (Kc), are used to relate ETc to ETo or ETc = Kc x ETo.
Differences in leaf anatomy, stomatal characteristics, aerodynamic properties and even albedo cause the crop evapotranspiration to differ from the reference crop evapotranspiration under the same climatic conditions. Due to variations in the crop characteristics throughout its growing season, Kc for a given crop changes from sowing till harvest.
Fasal ET
Fasal ET is ETo and derived from Penman’s equation which was originally introduced by the pioneers Howard Penman and John Monteith and adapted by the Food and Agriculture Organization (FAO) and American Society of Civil Engineers.
We not only calculate ETo based on real-time farm data but also with the power of Fasal μClimate, we make ETo predictions which helps the farmer plan their irrigations schedule in much advance and much accurate to meet optimal plant growth.
Crop is growing, we are coding. Stay tuned for our engineering tale :)
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