useful generally for agricultural student for gaining knowledge about dry land agriculture

2. Presentation
On
Estimation of Moisture Index and Aridity Index
Course no: Agron-512
Course Title: DRYLAND FARMING
Submitted To:
Dr. K. M. GADIYA
Associate Research Scientist,
B. A. College of Agriculture,
Anand Agricultural University,
Aanad-388 110
Submitted By:
Roll no.05 = Chaudhary Pravin J.
Roll no.06 = Dharva Mitesh D.
Roll no.07 = Gamit Niraj H.
Roll no.08 = Joshi Sanjay.

3. Moisture index A term based on the
computation of an annual moisture budget by C.
W. Thornthwaite (1955), and calculated from
the aridity and humidity indices,
as Im = 100 × (S − D)/PE,
where Im is the moisture index, S is the water
surplus in months when precipitation
exceeds evapotranspiration D is the water
deficit in months when evapotranspiration
exceeds precipitation, and PE is the potential
evaporation.
Moisture index

4. Moisture Index Formula

5. Moisture regions and
their limits in
Thornthwaite
Classification-(1955)
Climatic Type
Symbol
Moisture Index Range
Perhumid A 100 and above
Humid B4 80 to 100
Humid B3 60 to 80
Humid B2 40 to 60
Humid B1 20 to 40
Moist Sub-humid C2 0 to 20
Dry Sub-humid C1 -33.3 to 0
Semi-arid D -66.7 to -33.3
Arid E -100 to -66.7
Agro climatic Classification of India
based on moisture index

8. Matric potential
When water is in contact with solid particles (e.g.,clay
or sand particles within soil), adhesive intermolecular
forces between the water and the solid can be large and
important. The forces between the water molecules and the
solid particles in combination with attraction among water
molecules promote surface tension and the formation
of menisci within the solid matrix. Force is then required to
break these menisci. The magnitude of matrix potential
depends on the distances between solid particles the width
of the menisci (also capillary action and differing Pa at
ends of capillary) and the chemical composition of the
solid matrix (meniscus, macroscopic motion due to ionic
attraction).

9. Pressure potential
Pressure potential is based on mechanical pressure, and is an
important component of the total water potential within plant cells.
Pressure potential increases as water enters a cell. As water passes
through the cell wall and cell membrane, it increases the total amount
of water present inside the cell, which exerts an outward pressure that
is opposed by the structural rigidity of the cell wall. By creating this
pressure, the plant can maintain turgor, which allows the plant to
keep its rigidity. Without turgor, plants will lose structure and wilt.

10. Seepage: Horizontal flow of water channel is called seepage.

11. Infiltration is the movement of water into the soil surface

12. • Percolation
Down ward movement of water through saturated or
nearly saturated soil in response to the gravity.
• Leaching
It refers to the loss of water-soluble plant nutrients from the soil,
due to rain and irrigation. Soil structure, crop planting, type and
application rates of fertilizers, and other factors are taken into
account to avoid excessive nutrient loss. Leaching may also refer
to the practice of applying a small amount of
excess irrigation where the water has a high salt content to
avoid salts from building up in the soil (salinity control). Where
this is practiced, drainage must also usually be employed, to carry
away the excess water.

13. Field Capacity
• Amount of water in soil after free drainage has removed
gravitational water (2 – 3 days)
• Soil is holding maximum amount of water available to plants
• Optimal aeration (micropores filled with water; macropores
with air)

14. Wilting Point
• Amount of water in soil
when plants begin to
wilt.
• Plant available water is
between field capacity
and wilting point.

15. •Hygroscopic coefficient
• Amount of moisture in air dry soil
• Difference between air dry and oven dry amounts
• Ultimate wilting point
The moisture content at which wilting is complete and the
plant die is called UWP.
At UWP the soil moisture tension is as high as -60 bars.

16. Not all capillary water is equally available to
plants
• Plants can extract water easily from soils that
are near field capacity
• Wilting point is not the same for all plants
Sunflowers can extract more water from soil than
corn

17. •Permanent wilting point
Permanent wilting point (WP) is defined as the minimal
point of soil moisture the plant requires not to wilt. If
moisture decreases to this or any lower point a plant
wilts and can no longer recover its turgidity when
placed in a saturated atmosphere for 12 hours. The
physical definition of the wilting point (symbolically
expressed as θpwp or θwp) is defined as the water
content at −1500 J/kg (or -15 bar) of suction pressure,
or negative hydraulic head.

18. •Available water capacity
available water content (AWC) is the range of available water that can be stored in
soil and be available for growing crops.
The concept, put forward by Frank Veihmeyer and Arthur Hendrickson,[ assumed
that the water readily available to plants is the difference between water
content at field capacity (θfc) and permanent wilting point (θpwp):
θa ≡ θfc − θpwp
Daniel Hillel criticized that the terms FC and PWP were never clearly defined, and
lack physical basis, and that soil water is never equally available within this range.
He further suggested that a useful concept should concurrently consider the
properties of plant, soil and meteorological conditions.
Lorenzo A. Richards remarked that the concept of availability is oversimplified. He
viewed that: the term availability involves two notions: (a) the ability of plant root to
absorb and use the water with which it is in contact and (b) the readiness or velocity
with which the soil water moves in to replace that which has been used by the plant.

19. PH
PH: Given by SPL Sorenson
The soil pH is a measure of the acidity or alkalinity in soils. pH is
defined as the negative logarithm (base 10) of
the activity of hydronium ions (H+or, more precisely, H3O+aq) in
a solution.
In water, it normally ranges from -1 to 14, with 7 being neutral. A pH
below 7 is acidic and above 7 is alkaline. Soil pH is considered a
master variable in soils as it controls many chemical processes that
take place. It specifically affects plant nutrient availability by
controlling the chemical forms of the nutrient. The optimum pH range
for most plants is between 5.5 and 7.0, however many plants have
adapted to thrive at pH values outside this range.

20. Methods of Soil Moisture Estimation Laboratory & Field
Methods
By measuring soil moisture at regular interval and at several
depths within the root zones, information can be obtained as
to the rate at which moisture is being used by the crops at
different depths. This provides the base for determining when
to irrigate and how much water to be applied.
For practical purpose, irrigation should be given when about
50 percent of available moisture in the root zone is
depleted. The amount of water to be applied is directly
related to the water already present in the soil. The methods
of measuring soil moisture are divided in to:
A) Direct method: Measurement of moisture content in the
soil (wetness)
B) Indirect methods: Measurement of water potential or
stress or tension under which water is held by the soil.

21. A) Direct methods:
I) Gravimetric methods: In the gravimetric method, basic measurement
of soil moisture is made on soil samples of known weight or volume. Soil
sample from the desired depths are collected with a soil auger. Soil
sample are taken from desired depth at several locations of each soil type.
They are collected in air tight aluminum containers. The soil samples are
weighed and they are dried in an oven at 105 oC for about 24 hours until
all the moisture is driven off. After removing from oven, they are cooled
slowly to room temperature and weighed again. the difference in weight
is amount of moisture in the soil. The moisture content in the soil is
calculated by the following formula:-
Moisture content Wet weight –Dry weight
On weight basis = ----------------------------- X 100
Dry weight

22. PROBLEM: Undisturbed soil sample was collected from a field, two days after irrigation when the soil moisture
was near field capacity. The inside dimension of core sampler was 7.5 cm diameter and 15 cm deep. Weight of core
sampling cylinder weight of the core-sampling cylinder was 1.56 kg. Determine the available moisture holding
capacity of soil and the water depth in centimeter per meter depth of soil.
Solution:
Weight of moist soil = 2.76-1.56 = 1.20kg
Weight of oven dry soil = 2.61-2.56 = 1.05 kg
1.20-1.05
Moisture content = ------------- X 100
1.05
= 14.28%
Volume of core sampler = ----------------------------X d2 x h
= ------------X7.5X7.5X15
4
= 662 cu. Cm
Wt. of dry soil in grams
Apparent specific gravity = --------------------------------
Volume of soil in cu. Cm
1.05
= ------ = 1.58
662
Available moisture = Ap. Sp. Gr. X moisture content
= 1.58 X 14.28
= 22.56 cm / m depth of soil

23. The method is though accurate and simple it is used mainly for
experimental purpose. Sampling, transporting & repeated weighing
give errors. It is also laborious and time consuming. The errors of
the gravimetric method can be reduced by increasing the size and
number of samples. however the sampling disturbs the experimental
plots and hence many workers prefer indirect methods.
III) Using Methyl Alcohol: Soil sample is mixed with a known
volume of methyl alcohol and then measure the change in specific
gravity of school with a hydrometer. This is a shot cut procedure but
it is no in common use.
IV) Using calcium chloride: Soil sample is mixed with a known
amount of calcium chloride. calcium chloride reacts with water and
removes it in the form of acetylene gas. The moisture is determined
has come in common use.

24. B) Indirect methods:
In those methods, no water content in the soil is directly measured
but the water potential or stress or tension under which the water is
held by the soil is measured. The most common instrument used for
estimating soil moisture by indirect method is:
1) Tensiometer
2) Gypsum block
3) Neutron probe
4) Pressure plate and pressure membrane apparatus
In all these methods, the reading from above instruments and
corresponding soil moisture content is determined by oven drying
method are plotted on a graph. Subsequently, these calibration
curves are used to know soil moisture content from the reading of
these instruments.

25. 1) Tensiometer: Tensiometer is also called irrometers since they are used in
irrigation scheduling. Tensionmeters provide a direct measure of tenacity (tension)
with which water is held by soil. It consist of 7.5 cm porous ceramic or clay cup, a
protective metallic tube, a vacuum gauge and a hollow metallic tube holding all
parts together. At the time of installation, the system is filled with water from the
opening at the top and rubber corked when set up in the soil. moisture from cup
moves out with drying of soil, creating a vacuum in the tube which is measured with
the gauge. Care should be taken to install tensiometer in the active root zone of the
crop. When desired tension is reached, the soil is irrigated. The vacuum gauge is
graduated to indicate tension values up to one atmosphere and is divided in to fifty
divisions each of 0.2 atmosphere value. The tensiometer works satisfactory up to
0.85 bars of atmosphere.
Merits of tensiometer:
1. It is very simple and easy to read soil moisture in situ.
2. It is very useful instrument for scheduling irrigation to crops which require
frequent irrigations at low tension.
Limitations:
Sensitivity of a tensiometer is only up to 0.85 atmospheres while available soil
moisture range is up to atmosphere and hence is useful more on sandy soils wherein
about 80% of available water is held within 0.85 ranges.

26. 2) Gypsum Blocks: Gypsum blocks or plaster of Paris
resistance units are used for measurement of soil moisture is
situ. These were first invented by Bouycos and Mick in 940.
the blocks are made of various materials like gypsum, nylon
fiber, glass, plaster of Paris or combination of these materials.
The blocks are generally rectangular shaped. A pair or
electronics is usually made of 20 mesh stainless steel wire
screen soldered to copper lead wire. The common dimensions
of screen electrodes are 33.75 cm long and 0.25 cm wide. The
usual spacing between the electrodes is 2 cm. A similar block
is 5.5 cm long, 3.75 cm wide and 2 cm thick.

27. Principal of working: It works on principal of conductance of
electricity. When two electrodes A and B are placed parallel to
each other in a medium and then electric current is passed, the
resistance to the flow of electricity is proportional to the moisture
content in the medium. Thus, when the block is wet, conductivity
is high and resistance is low. Generally these read about 400 to
600 ohms resistance at field capacity and 50,000 at wilting point.
the readings are taken with portable Wheatstone Bridge Bouycos
water Bridge operated by dry cells.
While placing the gypsum block in soil, care should be taken that
the blocks must have close contact with undisturbed soil. After
placing, the blocks get wetted with soil moisture due to capillary
movement. Pure gypsum block sets in about 30 minutes. The
gypsum block is sensitive to soil to moisture from 1.0 atm tension
to 20.0 atm. How ever, the gypsum blocks are not reliable in wet
soils.

28. 3)Pressure membrane and pressure plate
apparatus:
Pressure membrane and pressure plate apparatus is
generally used to estimate field capacity, permanent
wilting point and moisture content at different
pressures. The apparatus consists of air tight metallic
chamber in which porous ceramic pressure plate is
placed. The pressure plate and soil samples are
saturated and are placed in the metallic chamber. The
required pressure, say 0.33 bar or 15 bars is applied
through a compressor. The water from the soil sample
which is held at less than the pressure, Applied trickles
out of the outlet till equilibrium against applied
pressure is achieved after that, the soil samples are
taken out and oven dried for determining the moisture
content.

29. 4) Neutron meter (neutron scattering method):
Soil moisture can be estimated quickly and continuously
another with neutron moisture meter without disturbing
the soil. Another advantage is that soil moisture can be
estimated from large volume of soil. This meter scans the
soil to about 15 cm. diameter around the neutron probe in
wet soil and 50 cm in dry soil. it consists of a probe and a
scalar or rate meter. The probe contains fast neutron
source, which may be a mixture of radium and beryllium
or Americium and beryllium. Access tubes are aluminum
tubes of 50 to 100 cm length and are placed in the field
where moisture to be estimated.

30. Limitations: The two drawbacks of the instruments are that it is
expensive and moisture content from shallow top layers cannot be
estimated. The fast neutrons are also slowed down by other source of
hydrogen (present in the organic matter). Other atoms such as
chlorine, boron and iron also slow down the fast neutrons, thus
overestimating the soil moisture content.
5) Gama Ray absorption method:
it is the technique of measurement of changes in soil water content
by change in amount of gamma radiation absorbed. The amount of
radiation passing through soil depends on soil destiny which varies
chiefly with change in water content. This is suitable where change in
bulk destiny is very small.

31. 7) Soil moisture characteristic curve:
The energy status of water and amount of water in the
soil are related with the soil moisture characteristic curve.
As the energy status of water decreases (moisture towards
more negative values) soil water content also decreases. In
other words, as soil moisture content deceases, more
energy has to be applied to extract moisture from the soil.
the relation between suction (externally applied force) and
water content of the soil are represented graphically by a
curve which is known as a soil are moisture characteristic
curve.

32. Aridity Index
An aridity index (AI) is a numerical indicator of the degree of
dryness of the climate at a given location. A number of aridity indices
have been proposed (see below); these indicators serve to identify,
locate or delimit regions that suffer from a deficit of available water,
a condition that can severely affect the effective use of the land for
such activities as agriculture or stock-farming.

33. Historical background and indices
At the turn of the 20th century, Vladimir
Köppen and Rudolf Geiger developed the concept of
a climate classification where arid regions were defined as
those places where the annual rainfall accumulation (in
centimeters) is less than R/2 where:
• R = 2 x T if rainfall occurs mainly in the cold season,
• R = 2 x T+14 if rainfall is evenly distributed throughout
the year, and
• R = 2 x T+ 28 if rainfall occurs mainly in the hot season.
where T is the mean annual temperature in Celsius.

34. In 1948, C. W. Thornthwaite proposed an AI defined as:
where the water deficiency is calculated as the sum of
the monthly differences between precipitation and
potential evapotranspiration for those months when the
normal precipitation is less than the normal
evapotranspiration; and where stands for the sum of
monthly values of potential evapotranspiration for the
deficient months (after Huschke, 1959). This AI was later
used by Meigs (1961) to delineate the arid zones of the
world in the context of the UNESCO Arid Zone Research
programme.

35. Classification Aridity Index Global land area
Hyperarid AI < 0.05 7.5%
Arid 0.05 < AI < 0.20 12.1%
Semi-arid 0.20 < AI < 0.50 17.7%
Dry subhumid 0.50 < AI < 0.65 9.9%

37. The causes of aridity are following:
1. Distance:
One of these causes is the separation of the region
from oceanic moisture sources by topography or by
distance. Part of the desert area of the United States and
the Monte-Patagonian Desert to the leeward of the Andes
in South America is a result of the acidifying effect, Major
Mountain barriers have on air masses which move over
them. One of the causes of the Takla-Makan, Turkestan,
and Gobi deserts of Central Asia is the great distance from
major moisture sources.

38. 2. Wind System:
A second general cause of aridity is the formation of dry,
stable air masses that resist convective currents. The Somali-
Chalbi desert probably owes its existence to a stable
environment produced by large-scale atmospheric motions.
Deserts dominated by the eastern portions of subtropical
high-pressure cells originate in part from the stability
produced by these pressure and wind systems.
The deserts of the subtropical latitudes are particularly
sensitive to the climatology of cyclones. The Arabian and
Australian deserts and the Sahara are examples of regions
positioned between major wind belts with their associated
storm systems.

39. 3. Rain:
Widespread rains almost unknown over large parts of the
hot deserts, most of the precipitation coming in violent
convectional showers that do not cover extensive areas.
The wadis, entirely without water during most of the year,
may become torrents of muddy water filled with much
debris after one of these flooding rains.
Because of the violence of tropical desert rains and the
sparseness of the vegetation cover, temporary local runoff
is excessive, and consequently less of the total fall
becomes effective for vegetation or for the crops of the
oasis farmer. Much of the precipitation that reaches the
earth is quickly evaporated by the hot, dry desert air.
Rainfall is always meager.

40. 4. Temperature:
Skies are normally clear in the low latitude deserts so that
sunshine is abundant. Annual ranges of temperature in the
low latitude deserts are larger than in any other type of
climate within the tropics. It is the excessive summer heat,
rather than the winter cold, that leads to the marked
differences between the seasons.
During the high-sun period, scorching, desiccating heat
prevails. Midday readings of 40 to 45° C are common at
this season. During the period of low sun the days still are
warm, with the daily maxima usually averaging 15 to 20°
C and occasionally reaching 25°C. Nights are distinctly
chilly with the average minima in the neighborhood of
10°C.

41. Methodology of Computing Aridity:
Aridity index is a useful parameter to study stress on
growing plants quantitatively (Carter & Mather,1966). The
various components of the water balance required in the
analysis of aridity were computed using procedure of
Thornthwaite and Mather (1955). It can be done
climatologically on book-keeping procedure either week by
week or month by month or year by year.
The potential evapotranspiration (PE) required for aridity
index computation was estimated using Penman’s (1948)
equation. The percentage value of aridity index was
computed as the ratio of water deficiency to potential
evapotranspiration.

42. The aridity index (la) is given as:
Ia- Water Deficiency (WD) X 100
Water Need (WN)
Aridity index is a ratio between water deficiency and water
need (potential evapotranspiration = PE). An aridity index
can be calculated on annual or monthly or weekly basis by
using annual, or monthly or weekly values of water
deficiency and water need
e.g. annual aridity index (la) is
Ia = Annual WD X 100
Annual PE