eChapters

Need of an average

Through frequency distribution are the condensed form of raw data, but these are not sufficient for making comparison between two or more data. Therefore further condensation of frequency distribution into a single figure is made. This single f igure is always representative of the frequency distribution and such a figure is known as average.

Measure of Central Tendency

According to Croxten and Cowden “An average is a single value within the range of the data that is used to represent all the values in the series”. A statistical measure used for representing the centre of central value of a set of observations is known as “Measure of Central Tendency”. It is also called as “Measure of f irst order”.

Characteristics of an ideal Measure of Central Tendency

An ideal measure of central tendency should possess the following properties:

1) It should be rigidly defined i.e. it has a fixed and finite value

2) It should be easy to understand 3) It should be easy to calculate

4) Its calculation should be based on all the observations

5) It should be capable for further algebraic treatment

6) It should not be affected by extreme values of the observations

7) It should be least affected by fluctuations of sampling

Dispersion

Measure of central tendency tells about the central position of the series. They do not throw like on the formation of series because in some cases, two series may have a same measure of central tendency but differ in their composition. In order to study the formation, composition, structure and scatteredness of the items of the series, measure of second order are necessary. The measure of second order will give idea about the formation of the series. As the representation of a series on the basis of its measure of central tendency is not sufficient but when a series is represented on the basis of its measure of central tendency as well as its measure of dispersion, then it will give the complete adequate representation of the series. Therefore, it is defined as the extent or degree of which data tend to spread around an average is called the dispersion. It is also known as variability or measure of second order.

Characteristics of an ideal Measure of Dispersion

An ideal measure of dispersion should possess the following properties:

1) It should be rigidly defined i.e. it has a fixed and finite value

2) It should be easy to understand

3) It should be easy to calculate

4) Its calculation should be based on all the observations

5) It should be capable for further algebraic treatment

6) It should not be affected by extreme values of the observations

7) It should be least affected by fluctuations of sampling

Object: Measurement of absorption spectrum of chloroplastic pigments.

Material required: Spectrophotometer, plant leaves, ethanol, electric balance, beaker, test tubes and water.

Temperature influences most plant processes, including photosynthesis, transpiration, respiration, germination, flowering, adaptation and distribution of crops. As temperature increases (up to a point), photosynthesis, transpiration, and respiration increase. When combined with day-length, temperature also affects the change from vegetative (leafy) to reproductive (flowering) growth. Depending on the situation and the specific plant, the effect of temperature can either speed up or slow down this transition.

Temperature is the measure of mean kinetic energy per molecule of the molecules in an object, while the heat is the measure of total kinetic energy of all the molecules of that object. A large object may  have a much lower temperature than a small object and may still have greater heat content by virtue of larger number of molecules in it. Thus heat and temperature are not the same.

Cloud is a visible aggregate of minute water droplets and/or ice particles, or both in the air usually above the general ground level. Clouds are the most important form of suspended water droplets caused by condensation. There is no precipitation without clouds.

11.1. Measuring Instrument

The cloudy hours are measured with the help of eye or sensory parts of the body. No particular instrument is required for cloud amount or cloud cover measurement in the sky.

The most established method for detecting crop water stress remotely is through the measurement of a crop’s surface temperature. The correlation between surface temperature and water stress is based on the assumption that as a crop transpires, the evaporated water cools the leaves below that of air temperature. As the crop becomes water stressed, transpiration will decrease, and thus the leaf temperature will increase. Other factors need to be accounted for in order to get a good measure of actual stress levels, but leaf temperature is one of the most important.

Introduction

All biological reactions in soil are catalyzed by enzymes. Soil enzyme activities are believed to indicate the extent of specific processes in soil and in some cases, can act as indicators of soil fertility. Biological oxidation of organic compounds is generally a dehydrogenation process and there are many dehydrogenases that carry out the following reaction:

XH₂ is an organic compound and is a hydrogen donor and A is a hydrogen acceptor. These systems are integral part of the microorganisms, so dehydrogenase activity can be correlated with active population.

Principle

Measurement of dehydrogenase activity involves determination of Triphenyl formazan (TPF) produced by the reduction of Triphenyl tetrazolium chloride (TTC). TTC is a redox indicator, a yellow coloured water soluble salt which possesses the property of being easily transformed into, methanol soluble formazan (intensely coloured, water insoluble) by reduction. 

Dew acts as a source of moisture for the crop plants. It causes reduction in transpiration rate and saving of soil moisture reserves. Dew fall helps in insect-pest and diseases spread in crop plants.

14.1 Dew and Conditions for Dewfall

The radiational cooling during night causes the moisture to saturate and further cooling results into condensation and depositing this moisture in the form of tiny droplets on the grasses, vegetative and other non-conducting surfaces. This deposition form is known as dew. Clear nights, low wind velocity in the order of 1.0 to 3.0 m/s, presence of moderate to high moisture (RH = 75%) and sufficient radiational cooling are the four necessary and conductive conditions for the occurrence of dew.

Sunshine means incoming solar radiation, which consists of a bundle of rays of different wavelengths. At the top of the atmosphere, the amount of radiant energy received is found to be constant (1.94 cal/cm2/min). The receipt of the radiant energy, which is received on the earth and on crop surface, is influence by the cloudy and sunny or clear sky conditions. The brightness hours of the sunshine are an important parameter, which not only influence microclimate of a place but also the photosynthesis of the crop. The radiant energy of intensity of 0.17 cal/cm²/min or 120 W/m² and above is known to specify the bright sunshine.

The change of state of water form solid or liquid to gaseous form is known as evaporation. It may be defined as the water loss in vapour form from a soil/water surface. The energy used in this process is called as latent heat. The energy of 540 cal/g of water is used in this process. This process plays a major role in the redistribution of thermal energy between the earth and the atmosphere. Water vapour diffuses into the atmosphere from the different natural surfaces like ocean, river, lake, pond, soil and plant etc. It is difficult to measure the evaporation from the natural surfaces. To measure the evaporation from open water surface is easy and is normally in practice. The rate of evaporation is dependent on the size of the measuring surface (Pan) at constant wind speed below saturation point. Therefore a measured surface (Pan) is designed to measure the evaporation.

Evapotranspiration (ET) is the combined loss of water from vegetation and soil. When water is not a limiting factor at a site, the rate of ET is primarily controlled by meteorological factors like solar radiation, temperature, wind and relative humidity. The measurement of ET under natural conditions is of great importance in the water resource management and irrigation planning. Potential evapo-transpiration (PET) is the evaporation from a surface if water was never a limiting factor. PE-can also be calculated using empirical formulae.

It is the actual minimum temperature experienced by the plants near ground surface. The grass minimum temperature is lower than the air temperature at 4 ft level. It is the temperature recorded in open air ground on short turf, with the bulb of the thermometer just in contact with the tips of the blades of grass. It is also described as the temperature at 5.0 cm (2 inches) above ground. It is used during October to March period only when frost occurrence is expected. The readings of the grass minimum thermometer indicate the possibility of ground frosts, which is important information for farmers, gardeners and growers.

Aim

To measure horizontal distance on the level ground using chain

Materials required

Chain

Ranging rods

Chain pins

Field note book

The presence of moisture in atmosphere is termed as humidity and plays an important role in formation of weather system and plant as well as insect-pest growth and development. An abundance of moisture results in a rich natural flora and makes possible a wide choice of crops. Deficiency of moisture, on the other hand, permits only narrow range of crops. Humidity also affects the water requirement and the rate of transpiration. Reduction in transpiration reduces the translocation of food material and uptake of nutrients. High humidity with high temperature favours the outbreak of pest and disease. Moisture stress condition reduces the cell size and shape and ultimately plant growth and yield is reduced. High soil moisture and humidity condition cause failure in establishing stand of legumes and pulses. Excess of soil moisture in earlier growing season may result in more vegetative growth and delayed flowering.

Aim

To measure irrigation water by area-velocity method by using (i) float and (ii) current meter

Apparatus required

Float

Current meter

Theory

The rate of flow of water in open channel can be determined by multiplying the cross sectional area of the flow section at right angles to the direction of flow by the average velocity of water. In area-velocity method, the water can be measured at motion. The water is measured in motion by means of rate of flow or discharge and is expressed in litre per second (lps), litre per minute (lpm), cubic metre per second (m³/s), etc. Litre per second: A continuous flow amounting to 1 litre passing through a point each second. Cubic metre per second: A flow of water equivalent to a stream of 1 m long, 1 m wide and 1 m deep flowing at a velocity of 1 metre per second. In area-velocity method, the velocity of water is measured either by float or current-meter

Aim

To measure the irrigation water by using parshall flume

Construction of Parshall flume

Parshall flume is a water measuring device that can be used to measure water flowing in an open channel and intended primarily for use in irrigation. The flume consists of a converging inlet section, a throat section with straight parallel sides, and a diverging outlet section (Fig.61.1). The floor of the converging section is level with walls which converge in the direction of flow, towards the throat. The throat section constitutes the middle portion of the Parshall flume. The floor of the throat section is dips downward in the direction of flow, but the walls are parallel. The throat width specifies the dimensions of the parshall flume. The diverging outlet section of the flume has a floor which rises in the direction of flow. The walls become further separated in the direction of flow (diverge).The parshall flume can be constructed by G.I. sheet, aluminum sheet, M.S. sheet, brick masonry, reinforced concrete or timber. Flumes made of G.I. or aluminum sheets are easily portable. Brick masonry and concrete flumes are permanently constructed in the channels. Stilling wells should be constructed at the side to take measurements.

Aim

To measure the irrigation water by using orifice

Orifices

An orifice is an opening in a bulk head with closed perimeter usually circular or rectangular cross-section through which water flows. For measurement of water, orifices are fabricated by making an accurate cut of proper size and shape in a mild steel sheet, aluminum plate etc. Proper machining is done to have a sharp edge through which water flows (Fig.60.1).

Free discharge

If the downstream water level is lower than the bottom of the orifice opening, the water jet discharges freely into the air and then it is said to be free discharge. The depth of water on the upstream side of the orifice that causes the flow is called the head. The orifice flow formulae can be applied only when the orifice flows fully.

Aim

To measure the irrigation water by using weirs

a) Weirs

A weir is a notch or opening of some definite shape installed on a channel or a stream through which waterfalls (Fig.59.1). Materials like wood, concrete, mild steel, rigid PVC etc. can be used for construction of a weir. Mainly a cut on a sheet metal is used for fabrication of weirs, to be installed at various locations in a channel. Advantages of weirs are (i) capable of accurately measuring a wide range of flows (ii) easy to construct, portable and adjustable (iii) tends to provide more accurate discharge rating than flumes and orifices.

Aim

To measure irrigation water by volumetric method

Apparatus required

Siphon tube

Stop watch

Measuring bucket

Aim

To measure the irrigation water flowing through a pipe by using Venturimeter

Theory

A venturimeter is a device used to measure the rate of flow of a fluid through a pipe and is often fixed permanently at different sections of the pipeline to know the discharge there. Venturimeter consists of three parts: (i) a short converging part (ii) a throat (iii) a diverging part (Fig.62.1). In the venturimeter, the fluid is accelerated through a converging cone of angle 15-20° and the pressure difference between the upstream side of the cone and the throat is measured, and provides the signal for the rate of flow. Due to the constriction there is an increase in the flow velocity and hence an increase in the kinetic energy. The fluid slows down in a cone with smaller angle (5-7°) where most of the kinetic energy is converted back to pressure energy. Because of the cone and the gradual reduction in the area there is no “vena contracta”. The flow area is at minimum at the throat.