.: ✿ Transpiration

Figure 1.1 : Transpiration Process

What is mean by Transpiration

There are involves three type of processes which is Evaporation, Condensation and Precipitation.

Evaporation is when the heat from the sun warms up water and turns it into water vapour. In more scientific terms, evaporation is the change of liquid water into gaseous water. Any water in ponds, lakes, streams, or oceans evaporates. Even water in your glass will evaporate eventually.

Water even evaporates from plants. This is called transpiration. Plants, such as trees, lose water out of their leaves after they have absorbed water from the ground. Transpiration helps keep plants cool in the same way that perspiration keeps animals, such as humans, cool. The rate at which water will evaporate or transpire from plants depends on the temperature, wind, and humidity.

Condensation

You have already learned that condensation turns water vapour back to a liquid that forms clouds. This process is part of the water cycle. Water vapour condenses into the atmosphere to form clouds. Condensation occurs when the temperature of the air declines causing cloud formation. As clouds form, they move from the winds. This causes water vapour to spread out. When the clouds can’t hold all the moisture that has made them, they release the water in the form of precipitation.

Precipitation

Precipitation is also part of the hydrologic cycle. You’ve already learned that the main forms of precipitation are rain, hail, snow, sleet, and freezing rain. Precipitation brings the water back to the ground where it originated.

Accumulation

Accumulation of the precipitation is the final stage of the water cycle before it starts all over again. There is subsurface and regular surface runoff from hills and mountains, and the water then accumulates back into lakes, rivers, and oceans. If it ends up on land with no close body of water it will soak into the earth and become ground water that plants and animals use.

Equation

ET_o = p · (0.46·T_mean + 8)

Where:

ET_o is the reference evapotranspiration [mm day⁻¹] (monthly)

T_mean is the mean daily temperature [°C] given as T_mean = (T_max + T_min )/ 2

p is the mean daily percentage of annual daytime hours.

ESTIMATING TRANPIRATION BY USING BLANEY CRIDDLE

Here only the Blaney-Criddle method is given. The Blaney-Criddle method is simple, using measured data on temperature only (see also Fig. 11). It should be noted, however, that this method is not very accurate; it provides a rough estimate or "order of magnitude" only. Especially under "extreme" climatic conditions the Blaney-Criddle method is inaccurate: in windy, dry, sunny areas, the ETo is underestimated (up to some 60 percent), while in calm, humid, clouded areas, the ETo is overestimated (up to some 40 percent).

Figure 1.2 : The Blaney-Criddle method

The Blaney-Criddle formula: ETo = p (0.46 T mean +8)

ETo = Reference crop evapotranspiration (mm/day) as an average for a period of 1 month.
T mean = mean daily temperature (°C).

p = mean daily percentage of annual daytime hours.

The use of the Blaney-Criddle formula

Step 1: Determination of the mean daily temperature: T mean

The Blaney-Criddle method always refers to mean monthly values, both for the temperature and the ETo. If, for example, it is found that T mean in March is 28°C, it means that during the whole month of March the mean daily temperature is 28°C.

If in a local meteorological station the daily minimum and maximum temperatures are measured, the mean daily temperature is calculated as follows: