Evapotranspiration and measures to reduce
evapotranspiration
11.1 Definition
Under dry land conditions soil moisture is the most limiting factor for crop
production. It is lost as evaporation from soil surface and as transpiration from the
plant surfaces. The combined loss of moisture through these two processes is known
as evapotranspiration.
11.2 Methods to reduce evaporation
There are three principles of evaporation control under field conditions.
a. Decreasing the turbulent transfer of water vapour to the atmosphere by
growing plants, raising wind breaks, straw mulches etc.,
b. Decreasing capillary conductivity by rapid drying of the surface soil layers.
c. Decreasing the capillary flow and moisture holding capacity of the surface
soil layers.
For evaporation control, mostly mulches are used.
11.2.1 Mulches
Mulch is any covering material applied on the soil surface to reduce
evaporation losses. This material may be grown and maintained in place, or any
material grown and modified before placement or any material processed or
manufactured and transported before placement.
Types of mulches
a. Soil mulch or dust mulch: soil mulch is a thin layer of loose soil surface that can
be created by frequently stirring the soil with surface tillage implements like danthis,
guntakas (blade harrows) etc., Soil mulch of surface 5-8 cm dry soil effectively
reduces the evaporation losses by obstructing the raise of soil moisture through
capillary action. The soil mulch also prevents deep cracks in soils (especially black
soils) by reducing the direct action of atmosphere and hence evaporation is also
reduced. The repeated intercultivations done in rabi crops even in the absence of
weeds help in reducing evaporation losses. Among the different mulches soil mulch is
the cheapest.
b. Straw and stubble mulch: Straw and other crop residues like stubbles, groundnut
shells, cotton stalks etc; can be used as mulches on soil surface for moisture
conservation. Straw mulches reduce both the amount of energy absorbed by the soil
and its movement above the soil and hence reduce evaporation (Fig. 11.1) However,
the availability of adequate crop residues is a problem for use as mulches.
Fig .11.1 Straw mulch in mango
c. Plastic mulches: Plastic mulches are very effective as mulches for evaporation
control provided cost is not a limiting factor. The plastic mulches may be either white
or black. Black plastic mulches will absorb the solar radiation and enhance the soil
temperature for hastening the germination of winter crops like wheat; barley etc.,
White plastic mulches will reflect the incident radiation and reduce evaporation of
soil moisture.
d. Chemical mulches: Chemicals like hexadeconol, a long chain alcohol when mixed
with surface 5 mm of soil can reduce evaporation by about 40%. The surface layer of
a treated soil dries out more rapidly than that of untreated soil, creating a diffusional
layer to evaporation.
e. Vertical mulching: It is a technique wherein trenches of 40 cm wide, 15 cm deep
are dug at 2 to 4 m interval across slope and filled with stubbles or organic wastes to
a height of 10 cm above soil surface. Runoff is checked, collected in the shallow
trenches and redistributed to adjoining soil layers and infiltration is increased in
black soils.
f. Live mulching: Is the term used to describe the covering of soil surface through
the plant canopy in intercropping system.
Eg. Sorghum + forage cowpea, sorghum + sword bean
g. Pebble mulch: Where small pebbles like stone are placed on the soil surface
(Fig. 11.2). This mulching will be successful in dryland fruit tree culture. The pebbles
placed on the basins of trees not only reduce evaporation but also facilitate
infiltration of rain water into the basin.
Fig. 11.2 Pebble mulch in Mango
Mulching is more advantageous during rabi/summer months than in kharif
season. Organic mulches particularly under receding soil moisture conditions
increase crop growth by conserving soil moisture.
11.2.2 Effect of mulches on soil properties
1. Soil structure: Surface mulches reduce the impact of falling raindrops, thus
reducing dispersion and sealing of soil pores leading to crust formation. Hence the
soil structure is protected. The mulches also improve soil structure due to
decomposition of mulch.
2. Soil salinity: Under dry land conditions due to limited precipitation, soluble salts
move only to a limited depth and readily return to the surface as the soil water
evaporates. Due to salt accumulation in surface layers the germination and seedling
establishment may be adversely affected. Hence, mulches will reduce soil salinity
problem by increasing infiltration and reducing evaporation.
3. Soil water: The soil moisture content is improved by induced infiltration, reduced
evaporation and reduced transpiration by weeds. Surface mulches also obstruct the
free exchange of water vapour from soil surface into the atmosphere and hence
increase soil water content.
4. Soil temperature: The effects of mulches on soil temperature are highly variable
and depend up on the type of mulch material. White or reflective type of plastic
mulches generally decrease soil temperature, while black plastic mulches may
increase soil temperature. Crop residues moderate temperature by decreasing it in
summer and by increasing in winter season. This is due to combined effect of
radiation interception and evaporative cooling. The sugarcane trash mulch will
enhance the germination of sugarcane setts during summer by temperature
reduction.
5. Soil erosion: The ease by which soil particles are moved by wind and water is
related to size of soil particles and wind and water velocity. The particles of size
greater than 0.84 mm in diameter are generally not eroded by wind but they are
easily eroded by water. The mulches reduce the direct impact of falling raindrops on
soil, there by preventing soil dispersion and consequent sealing of soil pores leading
to reduced soil erosion.
11.3 Reducing losses due to transpiration
Nearly 99% of water absorbed by the plant is lost in transpiration. Hence
transpiration reduction is needed for maintaining favorable water balance in the
plants. Transpiration has become unavoidable evil as the stomata, which allow CO2
exchange also allows water vapour transfer into the atmosphere.
There are four principles of transpiration control
a. By increasing leaf resistance to water vapour transfer by application of
materials, which tend to close or cover stomata (ex: both stomatal closing
and film forming type of antitranspirants).
b. By reducing amount of energy absorbed by leaf surface (Eg: leaf
reflectants)
c. By reducing top growth of plants ( Eg: Growth retardants )
d. By increasing air resistance to water vapour transfer by shelter belts/
wind breaks
The transpiration losses can be controlled by use of antitranspirants, use of
wind breaks/shelter belts and efficient weed control
11.3.1. Anti-transpirants: Any material that is applied on transpiring plant surface
for reducing water loss is called antitranspirants. The antitranspirants are also
known as transpiration suppressants. The best anti transpirants reduce transpiration
losses up to 30-40%. There are four types of anti transpirants.
a. Stomatal closing type: Transpiration mostly occurs through stomata on the leaf
surface. Some fungicides like PMA (phenyl mercuric acetate) and herbicides like
atrazine in low concentrations serve as anti transpirants by closing of stomata. PMA
is known to inhibit mesophyll photosynthesis. Though the success was reported from
glasshouse studies, their effectiveness under field conditions is limited.
b. Film forming type: The plastic and waxy materials, which form a thin film on the
leaf surface, retard the escape of water due to formation of physical barrier. The
success of these chemicals is limited since they also reduce photosynthesis. The
desirable characteristics of film forming type of antitranspirants are: they should
form a thin layer, they should be more resistant to the passage of water vapour than
carbon dioxide and the film should maintain continuity and should not break. These
film forming anti transpirants may be of either thin film or thick film.
Thin film forming type: Hexadeconol
Thick film forming type: Mobileaf, Polythene S-60
c. Leaf reflectant type: These are the white materials, which form a coating on the
leaves and increase leaf reflectance (albedo). By reflecting the radiation they reduce
leaf temperatures and vapour pressure gradient from leaf to atmosphere and hence
reduces transpiration. About 5% of kaolin spray reduces the leaf temperature by 3-
4°C and decrease in transpiration by 22 to 28 per cent. Celite and hydrated lime are
also used as reflectant type of anti transpirants.
d. Growth retardant type: These chemicals reduce shoot growth and increase root
growth and thus enable the plants to reduce transpiring surface and resist drought
conditions. They increase root/shoot ratio.
Eg : Cycocel – (2-chloroethyl) Trimethyl ammonium chloride (CCC), Phosphon–D,
Maleic Hydrazide (MH)
Antitranspirants generally reduce photosynthesis. Therefore, their use is
limited to save the crop from death under severe moisture stress. If crop survives, it
can utilise the rainfall that is received subsequently. Antitranspirants are also useful
for reducing the transplantation shock of nursery plants. They have some practical
use in nurseries and horticultural crops. Waxy materials are used for reducing post
harvest shrinkage of fruits.
11.3.2. Use of wind breaks and shelterbelts: Wind breaks are any structures that
obstruct wind flow and reduce wind speed while shelterbelts are rows of trees
planted for protection of crops against wind. The direction from which wind is
blowing is called windward side and direction to which wind is blowing is called
leeward side. Shelterbelts are planted across the direction of wind. They do not
obstruct the wind flow completely. Depending upon their porosity, certain amount of
wind passes through the shelterbelts while the rest deflects and crosses over the
shelterbelts (Fig. 11.4). It thus reduces wind speed without causing turbulence. The
protection offered by the shelterbelts is dependent on the height of central tree row
in the shelterbelts. Generally, shelterbelts give protection from desiccating winds to
the extent of 5 to 10 times their height on windward side and up to 30 times on
leeward side. Due to reduction in wind speed, evaporation losses are reduced and
more water is available for plants. The beneficial effect of shelterbelts are seen more
clearly in drought years. In addition, shelterbelts reduce wind erosion.
Fig 11.4 Wind Deflection due to shelterbelts
11.3.4 Effective weed control: Weeds transpire frequently greater amount of water
per unit of dry matter production than the crop plants. Therefore controlling weeds
especially at early stages of crop growth will be most effective means of increasing
the amount of water available for crops. This is the most useful method to reduce
transpiration losses.