On a sunny day our body temperature tends to rise, the sensors on our body tells our brain to induce sweat but we cool off easily by losing 1-2% body fluid. Plants on the other hand lose gallons of water everyday, upto 99% water absorbed from roots is lost through evapotranspiration. You may now wonder why plants have not evolved to prevent such loss of water. Actually, some already have such as cacti! Other plants, however, continue to transpire because of the benefits. Evapotranspiration cools the plant and as the roots replace transpired water, they also draw in more nutrients from the soil. The presence of this water in the soil also provides support for the plant so that it does not fall over. It is also an important phase of the water cycle, responsible for water and energy distributions on land surfaces.
Evapotranspiration(ET) is the sum of evaporation and transpiration and is strongly influenced by the spatial environment. The heterogeneity of environmental factors makes it cumbersome to use traditional methods to simulate or predict regional ET. Thus regular attempts are made to accurately estimate the rate of ET from different land surface covers in agriculture, meteorology, hydrology, soil science, and other related disciplines.
ET is a vital rate for plants that impacts a wide variety of performance indicators.
ET in agriculture can be understood as the amount of water needed to replace in the ground through irrigation. The inability to satisfy ET demand often leads to reduced photosynthesis in row crops, wilting in leafy greens, slow rates of fruit growth, indicates potential root pathogens in many crops and leads to increased incidence of insect damage owing to deteriorated health. But fulfilling reduced ET through deficit irrigation is very helpful to accelerate fruit development and improve quality. Factors affecting ET are presented below.
The primary factors are radiation, air temperature, humidity and wind speed. The combination of these factors are represented in Reference Evapotranspiration(ETo). It is a conceptual value for what would be evaporating off of a surface, “assuming well-watered conditions and a canopy of uniform height and coverage.”
Crops grown in well managed fields influence the rate of evapotranspiration through crop type, variety and development stage. Crop Evapotranspiration(ETc) is derived by multiplying ETo with a crop coefficient (Kc) ranging from 0–1. For bare fields Kc is zero and in complete canopy it is 1. Kc is simple in concept but complicated in practice: planting density, bed width, crop stress, growth rate all have some impact, and it has been characterised rigorously in a few experiments. But there are always huge data gaps which are being fulfilled to date. In practice, most people use the “eyeball method” to estimate Kc.
Management and environmental conditions
Adding to the complication ET assessment requires ground cover, plant density and the soil water content which vary hugely even within the same field. Water deficiency and type of soil affects ET directly and on the other hand, too much water results in water logging which damages the root and limits root water uptake. Also factors such as soil salinity, poor land fertility, limited application of fertilisers, the presence of hard or impenetrable soil horizons, the absence of control of diseases and pests and poor soil management may limit the crop development and reduce the evapotranspiration.
In 1875 Edward Lewis Sturtevant, a botanist from Massachusetts, built the first lysimeter in the United States to measure actual evapotranspiration. It was a bulky setup that used a tank to grow crops and estimate the difference in weight between the rain input and water lost. But not until 1948 much was known to estimate ET at large scale, Thornthwaite had just published a simple equation that used only temperature data from weather stations across the globe to estimate ET at a monthly resolution. The wide acceptance came when Howard Penman and John Monteith in 1948 laid the foundation for the evapotranspiration model. They combined all parameters that govern energy exchange and corresponding latent heat flux. Most of the parameters are readily calculated from weather data. It was initially meant for grass surfaces but with further development researchers extended it to cropped surfaces.
Understanding ET may help growers
Recent rise in temperatures have led to unexpected and unusual weather patterns. Plants are sessile which means they stay in one place and can’t move around like we can. They can’t pull up their roots and relocate to a shady or damp spot. Therefore, plants somehow need to deal with these ever-increasing unusual conditions, or they will simply die. Plants definitely have a number of genes for defence strategies encoded in their DNA but they do need our help in managing their needs on a day to day basis. It's their only vocabulary through which they can speak and we as humans are responsible to understand their vocabulary and take action. Solutions as simple as placing a weather station or taking inputs from satellite data to estimate ET can help manage crops very efficiently.