TRANSPIRATION

TRANSPIRATION

The process of evaporation from stomata and the ascent of sap is called transpiration.

Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves. Transpiration also includes a process called guttation, which is the loss of water in liquid form from the uninjured leaf or stem of the plant, principally through water stomata.

ESTIMATING TRANSPIRATION

Blanney - Criddle Method

Blanney and Criddle (1945 , 1950 , 1962 ) developed an empirical relation between ET,

mean air temperature, and mean percentage of daytime hours: which are taken as measure of

solar radiation at an area.

ET varies directly with the sum of the products of mean monthly air temperature and

monthly percentage of daytime hours with an actively growing crop with sufficient soil

moisture.

EVAPOTRANSPIRATION

Evapotranspiration (ET) is a term used to quantify all evaporative losses in a watershed.

1) Evaporation of water intercepted and stored by vegetation

2) Water transpired by vegetation

3) Water evaporated from soil

ET accounts for about 60 to 70% of the water loss from a slope.

 Potential evapotranspiration(PET)

Potential evapotranspiration(PET) is a maximum ET assuming a sufficient supply of soil water to meet the highest demand by plants.

 Actual evapotranspiration(AET)

Actual evapotranspiration(AET) is the actual ET that occurs under natual field conditions (i.e., variable soil water contents).

Models are used to estimate PET and AET

1)      Thornthwaite Model is a simple, temperature index empirical model for estimating PET

2)      Penman-Monteith is a physically based model that incorporates many meteorological, vegetation, and soil water characteristics.

Estimating Evapotranspiration

a)      Penman's Method

Theory is based on two requirements which must be met if continuous evaporation is to occur :

·         There must be a supply of energy to provide latent heat of vaporization.

·         There must be some mechanism for removing the vapor, once produced.

Penman's equation

Usual range of values of r are given below

Surface	Range of r value
Closed ground crops	0.15 - 0.25
Bare lands	0.05 - 0.45
Water surface	0.05
Snow	0.45 - 0.95

Example

Calculate the PET from a certain area in the month of November by Penman's formula.

The following data are available.

Latitude                                  = 28° 4' N

Elevation                                 = 230 m above mean sea level.

Mean monthly temperature     = 19oC

Mean relative humidity           = 75%

Mean observed sunshine hour = 9 hr

Wind velocity at 2 m height   = 85 km / day

Nature of surface area             = close ground green crop

Table 3.1

Table 3.3 Mean Monthly Values Of Possible Sunshine Hours, N .