estimation of moisture index and aridity index

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PresentationOnEstimation 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.

Moisture indexA term based on the computation of an annual moisture budget by C. W. Thornthwaite (1955), and calculated from thearidityandhumidityindices, asIm= 100 (SD)/PE, whereImis the moisture index,Sis the water surplus in months when precipitation exceedsevapotranspirationDis the water deficit in months when evapotranspiration exceeds precipitation, andPEis the potential evaporation.Moisture index

Moisture Index Formula

Moisture regions and their limits in Thornthwaite Classification-(1955)Climatic Type SymbolMoisture Index RangePerhumidA100 and aboveHumidB480 to 100HumidB360 to 80HumidB240 to 60HumidB120 to 40Moist Sub-humidC20 to 20Dry Sub-humidC1-33.3 to 0Semi-aridD-66.7 to -33.3AridE-100 to -66.7

Agro climatic Classification of India based on moisture index

Matric potentialWhen water is in contact with solid particles (e.g.,clay orsandparticles withinsoil),adhesiveintermolecular forcesbetween the water and thesolidcan be large and important. The forces between the water molecules and the solid particles in combination with attraction among water molecules promotesurface tensionand the formation ofmenisciwithin 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 (alsocapillary actionand differing Pa at ends of capillary) and the chemical composition of the solid matrix (meniscus, macroscopic motion due to ionic attraction).

Pressure potentialPressure potential is based on mechanical pressure, and is an important component of the total water potential within plantcells. Pressure potential increases as water enters a cell. As water passes through thecell wallandcell 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 maintainturgor, which allows the plant to keep its rigidity. Without turgor, plants will lose structure andwilt.

Seepage: Horizontal flow of water channel is called seepage.

Infiltration is the movement of water into the soil surface

PercolationDown ward movement of water through saturated or nearly saturated soil in response to the gravity. Leaching It refers to the loss of water-soluble plantnutrientsfrom thesoil, due torainandirrigation.Soil structure, crop planting, type and application rates offertilizers, 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 excessirrigationwhere the water has a high salt content to avoidsalts from building up in the soil(salinity control). Where this is practiced,drainagemust also usually be employed, to carry away the excess water.

Field CapacityAmount of water in soil after free drainage has removed gravitational water (2 3 days)

Soil is holding maximum amount of water available to plantsOptimal aeration (micropores filled with water; macropores with air)

Wilting PointAmount of water in soil when plants begin to wilt.

Plant available water is between field capacity and wilting point.

Hygroscopic coefficient

Amount of moisture in air dry soil

Difference between air dry and oven dry amounts Ultimate wilting pointThe 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.

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

Permanent wilting pointPermanent wilting point(WP) is defined as the minimal point ofsoil moisturethe plant requires not towilt. If moisture decreases to this or any lower point a plant wilts and can no longer recover itsturgiditywhen placed in a saturated atmosphere for 12 hours. The physical definition of the wilting point (symbolically expressed as pwpor wp) is defined as thewater contentat 1500 J/kg (or -15 bar) of suction pressure, or negativehydraulic head.

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 betweenwater contentatfield capacity(fc) andpermanent wilting point(pwp):

a fc pwp

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. Richardsremarked 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.

PHPH: Given by SPL SorensonThesoil pHis a measure of theacidity or alkalinityinsoils.pHis defined as the negativelogarithm(base10) of theactivityofhydroniumions (H+or, more precisely,H3O+aq) in asolution. In water, it normally ranges from -1 to 14, with 7 being neutral. A pH below 7 isacidicand above 7 is alkaline. Soil pH is considered a master variable in soils as it controls many chemical processes that take place. It specifically affectsplant nutrientavailability by controlling the chemical forms of the nutrient. The optimum pH range for mostplantsis between 5.5 and 7.0,however many plants have adapted to thrive at pHvalues outside this range.

Methods of Soil Moisture Estimation Laboratory & Field MethodsBy 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.

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 contentWet weight Dry weightOn weight basis= ----------------------------- X 100 Dry weight

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

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.

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) Tensiometer2) Gypsum block3) Neutron probe4) Pressure plate and pressure membrane apparatusIn 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.

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.

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.

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.

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.

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.

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.

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.

Aridity IndexAnaridity index(AI) is a numerical indicator of the degree of dryness of the climate at a given location. A number ofaridityindices 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.

Historical background and indices

At the turn of the 20th century,Vladimir KppenandRudolf Geigerdeveloped the concept of aclimate classificationwhere 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.

In 1948,C. W. Thornthwaiteproposed 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.

ClassificationAridity IndexGlobal land areaHyperaridAI < 0.057.5%Arid0.05 < AI < 0.2012.1%Semi-arid0.20 < AI < 0.5017.7%Dry subhumid0.50 < AI < 0.659.9%

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.

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.

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.

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 25C. Nights are distinctly chilly with the average minima in the neighborhood of 10C.

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 Penmans (1948) equation. The percentage value of aridity index was computed as the ratio of water deficiency to potential evapotranspiration.

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

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