Here is an essay on ‘Scheduling Irrigation’ for class 8, 9, 10, 11 and 12. Find paragraph, long and short essays on ‘Scheduling Irrigation’ especially written for school and college students.
Essay on Scheduling Irrigation
Essay Contents:
- Essay on the Meaning of Irrigation Scheduling
- Essay on the Factors Influencing Irrigation Scheduling
- Essay on the Approaches for Irrigation Scheduling
- Essay on the Advantages of Irrigation Scheduling
Essay # 1. Meaning of Irrigation Scheduling:
Once we know the crop water requirements, the most important step is to supply the right quantity of water at right time through an appropriate application method to satisfy the crop water requirements. This is called irrigation scheduling and serves the objectives of high yield of good quality, attaining high water use efficiency without any damage to soil productivity and applying water at a reasonable cost.
The importance of irrigation scheduling is that it enables the irrigator to apply the exact amount of water to achieve the goal. This increases irrigation efficiency. A critical element is accurate measurement of the volume of water applied or the depth of application. A farmer cannot manage water to maximum efficiency without knowing how much was applied.
Essay # 2. Factors Influencing Irrigation Scheduling:
Soil characteristics, climatic factors, crop characters and management factors are to be considered in determining irrigation schedules.
However, salient points are listed below:
When to Irrigate:
1. Maintenance of soil moisture around field capacity (-0.33 bar) is ideal for many crops
2. As the soil moisture content drops from FC due to evapotranspiration and other losses, soil moisture tension increases and crops can’t extract needed moisture from soil for optimum growth
3. Crops start wilting leading to retarded growth and permanent wilting
4. Crops should not experience moisture stress between two irrigations, as in the case of light textured (sandy and sandy loam) soils
5. By knowing the ASM in the crop root zone and the evapotranspiration demands of the crop and atmosphere, it is easy to determine when irrigation is needed
6. There are several approaches to decide ‘when to irrigate’ based on soil, plant and atmospheric parameters and critical crop stage approaches.
Essay # 3. Approaches for Irrigation Scheduling:
Crops differ in their tolerance to depletion of soil moisture. Crops such as rice respond to continuous land submergence or very high frequency irrigation. Some crops such as potato and most winter vegetables, require moist conditions (< 40 to 50 DASM).
Other crops such as small millets and fruit trees and several others with deep root system may show little reduction in yield until nearly all the available water has been depleted in the soil depth from which extraction has been most rapid. It must be emphasised, however, that irrigation programmes for specific crops should vary according to prevailing situation.
Irrigation scheduling decisions are to be made for mainly two situations:
1. Where water is expensive, irrigation should be scheduled to maximise crop production per unit of applied water.
2. Where good land is scarcer than water, irrigation should be scheduled to maximise crop production per unit of planted area.
The following approaches are, generally, considered for scheduling crop irrigations:
1. Soil moisture monitoring approach:
In these methods, soil moisture content is estimated to know the deficit available soil moisture at which it is proposed to irrigate based on predetermined soil moisture content to bring the soil to field capacity.
2. Soil-water content approach:
Irrigation scheduling on the basis of gravimetric soil- water content is based on two assumptions: assuming threshold moisture content below which the plant growth is adversely affected or determination of optimum fixed interval between two successive irrigations (allowable soil moisture depletion). Both these assumptions (threshold moisture content and fixed time interval) vary with soil texture. As such, the soil-water content approach may not be an ideal approach for irrigation scheduling.
3. Depletion of available soil moisture approach (DASM):
This approach essentially consists of determining the lower limit of available water in an assumed shallow root zone of the crops at which irrigation must be applied to avoid yield reduction. Soil moisture content is estimated by gravimetric sampling. Though, this approach has been widely used and the DASM has been-fixed for many crops (Table 6.10), it suffers from serious drawbacks.
TABLE 6.10: Irrigation guide of important field crops
Assumption of a fixed root zone depth runs counter to dynamic and ever-growing character of plant roots. Even a small fraction of root system located in a deeper wet soil may extract enough moisture to meet the ET requirement of the crop, although the assumed root zone depth may be dry enough to warrant irrigation. Periodic determination of root growth and soil moisture is beyond the capability of farmers.
4. Soil-water potential or soil moisture tension (suction) approach:
Available water in soils at specific soil depths can be measured in terms of corresponding range of soil water potential or soil water tension (suction) by tensiometers or electrical resistance blocks for scheduling irrigation. Soil moisture tension approach is more useful for orchards and vegetable crop, particularly on coarse textured soils, where most of the ASM is held at low tensions. In soils with higher moisture holding capacity, properly calibrated gypsum blocks can be used for irrigation scheduling.
Use of tensiometers and gypsum blocks could not become popular due to many shortcomings. Further, these approaches do not indicate the amount of irrigation to be applied directly but require soil moisture characteristic curve to interpret the soil moisture content. Table 6.11 compares different methods of irrigation scheduling by monitoring soil moisture content or tension.
Atmospheric Measurements and Water Balance Techniques:
An integrated approach to SPAC is essential for an ideal irrigation schedule. Climatological approach for scheduling irrigation is also based on two assumptions: ETcrop is, generally, a function or at the most equal to evaporation from free water surface and PET is considered desirable for optimum crop yield.
Measurement of crop evapotranspiration (ETc):
Daily crop-water use (crop evapotranspiration: ETc) is calculated using a crop coefficient (Kc) and a reference evapotranspiration (ETo) obtained from atmospheric measurements as:
ETc = Kc × ETo
For further information 5.7.4 Empirical methods may be referred. Full details on this aspect can be found in FAO 56: Crop evapotranspiration – Guidelines for computing crop water requirements by Allen et al 1998.
The volume of soil-water readily used by the crop (ASM) may be estimated from texture-based soil-water characteristic curve data and maximum acceptable crop stress.
The volume of the effective crop root zone depth may be used to convert the ASM in to volume appropriate to the crop area.
Irrigation interval may then be calculated from the ETc and ASM values:
PET measurement:
Irrigation can be scheduled if the level of DASM in the root zone and evaporation during the crop period is known.
IW/CPE approach:
Prihar et al (1974) suggested relatively simple IW/CPE approach for scheduling crop irrigation. Evapotranspiration mainly depends up on climate. The amount of water lost by evapotranspiration is estimated from climatological data and when ET reaches a particular level, irrigation is scheduled. The amount of irrigation given is either equal to ET or fraction of ET.
In IW/CPE approach, a known amount of irrigation water (IW) is applied when cumulative pan evaporation (CPE) reaches a predetermined level. The amount of water given at each irrigation ranges from 4 to 6 cm, most common being 5 cm. Generally, irrigation is given at 0.75 to 0.8 ratios with 5 cm of irrigation water.
Example:
Calculate cumulative evaporation required for scheduling irrigation at 0.5, 0.6, 0.75 and 0.8 with 5 cm of irrigation water?
Solution:
Cumulative pan evaporation at IW/CPE ratio of 0.5 – IW/CPE = 0.5
Irrigation of 5 cm is given when CPE is 10 cm
CPE at 0.6 ratio = 5/0.6 = 8.33 cm
CPE at 0.75 ratio = 5/0.75 = 6.66 cm
CPE at 0.8 ratio = 5/0.8 = 6.25 cm
In IW/CPE ratio approach, irrigation can also be scheduled at fixed level of CPE by varying amount of irrigation water.
Example:
Calculate the amount of water for each irrigation for scheduling irrigation at 0.5 and 0.8 IW/CPE with 10cm of CPE?
Solution:
Amount of water to be given at IW/CPE ratio of 0.5 = IW/10=0.5
IW = 0.5 × 10= 5 cm.
Amount of water to be given at IW/CPE ratio of 0.8 = IW/10 = 0.8,
IW = 10 × 0.8 = 8 cm.
Irrigation schedules developed using atmospheric measurements are widely used. However, there are a number of limitations, particularly as the spatial scale of discriminations required for management decreases. For example, while the crop coefficient can be adjusted for a range of factors (crop age, canopy cover) appropriate values for different crop cultivars and management conditions (deficit irrigation, partial root zone drying) are not normally available.
Similarly, the soil moisture characteristic data commonly used for estimating the soil-water volume is often based only on the soil textural properties. However, ASM content of a particular soil will also depend on soil structure and organic matter content. Estimates of effective root zone depths are also commonly based on small sample of point measures which may not adequately reflect the variations observed in the field.
The technique is also not suitable for differential (variable rate) irrigation at small spatial scales as the atmospheric measurements are normally obtained from a single local weather station and assumed constant over the surrounding area.
Plant Based Monitoring:
Any physical measurement related directly or indirectly to plant water deficits and which responds readily to integrated influence of soil-water-plant factors and evaporative demand of the atmosphere may serve as a criterion for timing of irrigation to crops.
The sensitivity of plant factor to monitor water deficiency and the deleterious effects of the latter on plant growth depends upon plant species, its development stage and the prevailing environment. It seems difficult to evolve any universal plant-based index for scheduling irrigation to various crops under a variety of field conditions.
Symptoms of plant wilting (visual plant symptoms) such as drooping, curling and rolling of leaves are visual indicators of plant needs for water. Although, this approach is simple and rapid, it suffers from many deficiencies like personal error in judgment, poor wilting of certain crops under moderate to severe stress and growth reduction in some crops even before the plant is visibly wilted.
Distinct change in foliage colour has been used to time irrigation to beans, but colour change may be misleading because several other factors like nutritional disorders, insect and disease incidence, varietal characters and growth stage can also cause such change in colour.
In view of the deficiencies in scheduling irrigation based on visual plant symptoms, measurement of plant-water stress has been the topic for a number of years and there are a number of plant sensing tools for both research and commercial crop irrigation scheduling. Summary of review on plant based sensors for scheduling irrigation by White and Raine (2008) is presented.
Plant Based Sensing:
Plant based sensors can be broadly classified into contact and non-contact sensors. Contact sensors are those that are physically mounted so that they are in direct contact with the plant.
Contact sensors normally provide single point or point source data. Non-contact sensors are further divided into either proximal (near to the plant) are remote (aerial and satellite based data acquisition) sensors depending on how close to the crop the sensor is located. Proximal sensors can be either hand-held, fixed or vehicle mounted. Advantages and limitations of plant based sensing have been summarised.
Depending on the crop, plants can exert significant degree of control over their water status by manipulating their response to soil moisture availability and imposed evaporative demand.
Hence, plant based sensors which either directly measure plant-water status or the response of plant to imposed conditions have an advantage over other methods of scheduling as they provide an integrated measure of the plant’s response to both the soil moisture availability and evaporative conditions.
During the periods of moisture stress or high evaporative demand, a reduction in stomatal aperture results in a reduction in transpiration and hence, an increase in canopy temperature. This change may be measured using a canopy temperature sensor and presented as a crop water stress index value.
However, in some plants, measurements of tissue water content may not be a reliable indicator of irrigation stress because autonomous stomatal control may be used to maintain the same tissue water content under a wide range of environmental conditions.
A significant current limitation in the application of plant based sensors for commercial irrigation scheduling is that these techniques do not provide a direct measurement of the irrigation volume required to be applied. Hence, plant based sensing is commonly used in conjunction with soil moisture measurement equipment and/or water balance approach.
Contact Methods of Plant Based Sensing:
Response of a plant to combined effect of soil moisture availability, evaporative demand, internal hydraulic resistance and resistance/uptake capacity of plant/root interface is principally measured in terms of plant-water status. Plant-water status can be determined by measuring either the tissue water status (potential or content) or the plant’s response to a change in tissue water status.
Wilting is an extreme example of plant response to decrease in plant/cell water content and plant-water potential. Plant-water potential is a direct measure of the energy status of the water within the plant and reflects both the soil-water availability and atmospheric conditions.
Pressure chamber:
Pressure chamber measures the tension on the water that is within the water conducting tissue of the plant (xylem).
Psychrometer, dew point hygrometer and osmometer:
These instruments are used to measure RH (water vapour potential) of the atmosphere equilibrated with plant tissue in a closed chamber. The main difference between the instruments is the method of measuring water vapour potential in the air chamber. Plant sample is required to equilibrate with air volume within a sample chamber.
Hence, measurement of equilibrated water potential in the sample chamber (Fig 6.8) provides a measure of plant-water potential. Tissue sample (leaf disc) is place in a small sealed container insulated to ensure temperature stability. Tissue sample and the surrounding air are allowed to equilibrate and the water potential of the air is then measured.
A large number of sensor techniques are used to measure water potential of the air in the chamber. In wet bulb psychrometric method, temperature of a wet thermocouple varies according to evaporative cooling effect and RH of air in the chamber. Since the thermocouple is wet and the tissue sample has a water potential below zero, a water potential gradient exists between the thermocouple, surrounding air and plant tissue sample, causing a movement of water away from the surface of thermocouple in the form of evaporation.
The resultant change in temperature due to evaporative cooling is assumed to be linearly related to the water potential of the sample for a constant chamber temperature. A correction factor is applied to the measured output to convert the reading at a given temperature back to a standard measure at 25°C.
An alternate method of measuring water potential is the chilled mirror dew point technique (Fig 6.8). This method relies on measurement of RH in the closed sample chamber. A mirror within the chamber is chilled. As the mirror cools, a dew point sensor detects when moisture condenses on the surface and an infrared temperature sensor measures the temperature at which the dew point is reached.
Osmometers use a reference droplet of standard solution with known concentration. Depending on the water potential gradient between the reference droplet and sample tissue, there is a measured change in temperature of the reference droplet. Hygrometers (Fig 6.8) can be used to measure either leaf or stem water status. Stem hygrometers involve clamping the instrument to the stem of the sampled plant.
A small cut is made to expose the sapwood and thermocouple in the sample chamber is placed in contact with the exposed sapwood. A second thermocouple in the sample chamber is used to measure the chamber air temperature. A third thermocouple is imbedded in the sample chamber body to measure the instrument temperature and provide temperature compensation correction. Stem water potential measurement is achieved using either a psychrometer (wet bulb) or hygrometric (dew point) measurement.
These instruments are considered valuable, thermodynamically based measure of water status which with some particular devises (stem hygrometer) is able to automated for continual logging on a single plant. However, devises which require a leaf disc and leaf hygrometers are able to take multiple samples but are labour intensive and time consuming.
Measurement of tissue water content:
There are a range of sensors which function to indirectly measure the water content of leaf, fruit or stem plant tissues. Sensors include dendrometers, stem micro-variation (diameter) sensors, linear variable differential transformer gauges, beta gauges, leaf thickness sensors and direct measurement of relative water content. A key advantage for many of these measurements is that they are nondestructive and can be conducted in situ on growing plants.
Dendrometers are primarily used for growth measurement of trees. The measurement and continuous recording of the dia of stem and fruits is also referred to as micro-morphic method.
Stem diameter (micro-variation) sensors measure the daily diurnal change in stem diameter (Fig 6.9). This sensor measure the change in tissue water status within the plant as transpiration increases after sunrise and subsequently decreases at the end of the day. Root originated sap (water) is transport within the plant through xylem, associated with living cells (cambium and phloem). Stem shrinkage is the result of water loss and turgor in these living cells as a result of redistribution of water in response to the imposed water potential gradient.
Water potential in the xylem decreases due to an increase in transpiration after sunrise. This results in a radial flux between xylem and surrounding living cells. A water flux as a result of imposed potential developing causes tissue water loss and a reduction in cell volume and hence, a trunk diameter constriction.
As translocation decreases in the late afternoon, there is a re-hydration of the living cell and swelling of the trunk occurs into the night. There is a close association between the stem water potential and that of the amplitude of daily change in stem dia contraction. Under water limiting conditions, there is an increase in water potential gradient and a resulting increase in water movement out of the living cells and an increase in stem dia contraction.
Daily contractual amplitude (DCA), maximum daily shrinkage (MDS) or trunk daily fluctuations (TDF) is the diurnal change in stem dia measured as the difference between night time maximum to the following day minimum. Under constant weather conditions and limiting water situation both an increase in DCA and decrease in growth rate will occur.
As plant-water status is controlled to some degree by stomatal aperture and other mechanisms, sensitivity in using plant-water status indicator may be limited in application amongst isohydric plants which maintain their plant-water status as evaporative demand and/or soil moisture availability becomes limited. Logging dendrometer devises are advantageous for irrigation scheduling due to non-destructive measurement method and continual measurement capacity.
Relative water content (relative turgidity) is calculated as the ratio of leaf water content when sampled compared to the water content of the same leaf sample when re-hydrated. Leaf discs from predetermined position on the leaf and plant are sampled in the field and placed in closed weighing bottles. After weighing, discs are re-hydrated for a predetermined period of time in distilled water before being blotted dry of excess moisture and re-weighed (turgid weight).
The discs are then oven dried and relative water content is calculated:
Although, this is a low cost option for direct leaf water content, there is large sample variation, necessitating use of a large number of samples. As this is a measure of plant-water content rather than plant-water potential, there is need for consideration of plant’s ability to maintain water content under limiting soil moisture availability and/or increased evaporative demand. This method is not considered practical for commercial irrigation scheduling.
β gauges (leaf thickness sensors) are used to measure thickness of leaves based on attenuation of beta particles. Non-destructive measure of leaf water content can be made by using a calibration equation relating attenuation with a range of known leaf water contents.
Beta gauge consists of two main components: a radiation isotope which provides the source of beta particles and a radiation detector to measure the level of beta particles being received. As the density of leaf (presence of moisture) changes, there is a corresponding change in the quantity of radiation allowed to pass through the leaf and hence a change in the level of radiation measured on the opposite side of the leaf with the detector.
Calibration of a beta gauge is achieved by measuring similar leaves with a range of known relative water content which were manually measured. Re-sampling in the same canopy location on leaves of the same age and regular re-calibration will reduce problems associated with growth and variability in leaf material.
Beta gauge gives reliable relative water content with non-destructive sampling. However, a large number of samples are required to overcome leaf sampling variation.
Measures of crop response:
Crop response measures include sap-flow sensors, stomatal conductance and plant growth rate.
Sap-flow sensors:
Sap-flow sensors measure the velocity of sap flow by monitoring changes in sap temperature when heat is applied to the stem. These measures can be used to calculate plant transpiration rate. Three main types of sap flow sensor (heat balance, thermal dissipation, heat pulse) use heat as a tracer.
Energy or stem heat balance (SHB) sensors:
Energy or stem heat balance (SHB) sensors work by measuring heat transfer due to movement of the sap when the stem is continuously heated (Fig 6.10). As heat is applied to the stem, the flow of the sap is resolved by balance of the fluxes of heat from vertical heat loss by conduction in the stem, radial heat loss by conduction and by heat loss from sap flow.
Dynagage sensor consists of a flexible circumferential heater, a thermocouple to measure radial heat loss and differential thermocouples pair to measure the temperature difference along the stem. Heat balance sensors can either be stem type-defined where the heat is applied racially around the circumference of the stem or by trunk type-where by heat is applied to only a segment of the trunk.
Thermal dissipation (TD):
Thermal dissipation (TD) probe (Fig 6.10) uses an empirical determination (Granier’s) from a measure of sapwood temperature upstream of a heater probe inserted into the sapwood and continuously heating. The smaller the temperature difference between the two probes the more rapidly the heat is being dissipated and the higher the sap flow velocity.
Heat pulse sensors:
Heat pulse sensors apply a short pulse of heat and measures the corresponding velocity of heat pulse in the stem sap. A single heating probe (heat pulse probe) is installed into the sapwood and temperature probe is placed upstream and another downstream of the sap flow from the heater probe at predetermined distance.
Once a short pulse of heating is applied via the heat-pulse probe, there is dissipation of heat through conduction away from the heat-pulse probe. However, due to upward movement of sap there is a shift in the maximum heat pulse (signature) from the heat-pulse probe towards the downstream thermistor probe. Time taken to move the peak heat pulse half way between the two thermistors (equal temperature readings) is recorded.
Use of sap flow sensors has enabled automation and real time control of irrigation scheduling based on an estimate whole plant transpiration rates. However, sensors only provide an estimate of transpiration and hence water use is limited by point based measures being made on a per plant basis with a sap flow sensor. Therefore, an appropriate sample size (number of plants) must be measured to ensure a representative estimate is achieved.
Porometers (stomatal conductance):
Porometers (stomatal conductance) are used to measure the ability of a leaf to lose gases (CO2 and water vapour) by diffusion in particular through leaf stomata (Fig 6.10).
There are four types of porometers:
1. Mass flow porometers:
Air is forced through a leaf and the flow rate of air through the leaf is measured. This method appears to be inaccurate due to the physiological disturbance it causes to the leaf and stomatal aperture.
2. Null balance porometer:
A constant humidity in the chamber containing the leaf is achieved by changing rate of dry air flow through the chamber. A stirring fan is used to overcome boundary layer resistance. The rate of applied air is measured and along with the known leaf area within the chamber is used to calculate a resistance value which can then be converted into a measure of stomatal conductance (reciprocal of resistance).
3. Dynamic diffusion porometer:
Dry air is pumped over a leaf sample which is sealed within a small cuvette until a preset humidity level (close to ambient temperature) is reached. The time is then measured for the humidity within the cuvette to change to a second preset value of humidity. This value is then compared to readings obtained with a calibration plate of known conductance to account for ambient temperature and pressure and to calculate conductance.
4. Steady state porometer:
It measures the vapour pressure and vapour flux of the leaf surface. A fixed diffusion path with the chamber is used to measure the vapour pressure. Flux and gradient can be calculated from the vapour pressure measurements and the known diffusion path. Steady state porometer which monitor the time required to reach equilibrium by changing the rate of air flow through the chamber are similar to the null balance system although they measure the time taken to reach a steady state.
Commonly used porometers are either dynamic or static systems. Porometer only measure conductance on one side of the leaf. However, plants have stomata on both the adaxial (top) and abaxial (bottom) side of the leaves. A single measurement may take 20 to 50 seconds depending on the conductivity of the leaf.
Stomatal conductance of the leaf measured with porometers can be used to estimate whole plant-water use if the whole leaf area is measured. As porometers measure stomatal conductance and not plant-water status, these instruments are not suitable for anisohydric plants like cotton that show limited stomatal response even under conditions of limiting soil moisture and high evaporative demand.
Main limitations with porometers for commercial irrigation scheduling are high labour requirement, time required for measurement and requirement for specific measurement timing due to diurnal fluctuations and environmental constraints (cloudy days). Need for calibration and large number of leaf samples required to adequately represent the field reduce the potential to use these instruments for commercial irrigation.
Plant growth rate:
Plant growth rate sensors which are suitable for irrigation scheduling include auxanometers and stem or fruit diameter sensors (dendrometers).
Auxanometers are devises used to measure plant height and the extinction in plant height over time. It uses a clamp or similar device clipped into the growing point of the plant and is attached to a logger by a thin line which is under a small degree of tension. As the plant grows in height, the line is retracted or metered by the logger and the height and/or change in height is recorded. Dendrometers can also be used to measure plant growth rate.
Logging auxanometer and dendrometer systems provide a non-destructive continual measurement capability which is useful for irrigation scheduling. Major limitation in the use of such devises is their point based measurement and the need for adequate representation of the field.
Non-Contact Methods of Plant Based Sensing:
Site specific crop management and irrigation:
Non-contact plant sensors typically measure the reflectance of electromagnetic radiation from crop surface. These platforms have been used for various applications including identification of nutrient deficiencies and weed or disease infestations in crops. These sensors can also be used to detect crop water stress and irrigation scheduling.
Non-contact plant sensors may be mounted on a platform which may be proximal (hand held or ground rig devise) or remote (aircraft or satellite based) from the crop. It enables the identification of variations in crop water stress and crop water requirements within fields.
This combined with advent of enhanced computer processing capacity and the ability for real-time irrigation control allows for irrigation application to be varied both across the field and at different times to maximise economic potential of each management unit within the field. This can yield significant benefits in terms of improved water use efficiency, agronomic crop management, and critical irrigation timing/management and reduce off-field environmental impacts.
Plant spectral responses:
Electromagnetic radiation transmitted by the sun on to the surface of the earth is absorbed, transmitted or reflected off all surfaces, including crop canopy and soil. However, electromagnetic waves which are reflected from various surfaces differ in length and frequency due to characteristics of the surface material.
It is this difference in reflectance (spectral response) that is measured and can be used to infer crop stress and water requirement. Platforms which have been used to capture spectral reflectance data include hand held devises, tractor or machine mounted devises, balloons, unmanned aerial vehicles, planes and satellites.
Radiometric sensors:
These sensors include multispectral sensors, hyperspectral sensors and thermal sensors.
Multispectral sensors:
Multispectral sensors measure the electromagnetic reflectance from a surface across a number of band widths. Proximal multispectral units are typically handheld or vehicle mounted. The sensor is connected to data logger (computer) and pointed at the plant canopy. These devises come with logging and GPS capability to enable maps of the field measurements produced. There are wide ranges of satellite based multispectral sensors from which data can be obtained for agricultural use. As of now, these devises are used in research applications.
Hyperspectral sensors:
Hyperspectral sensors are used to measure the spectral reflectance of an object at a high spectral resolution. These may be hand held units or satellite based systems. Their use is, at present, limited to research applications.
Thermal sensing:
Thermal sensing provides crop canopy temperature, a relative measure of transpiration and an indication of crop water stress. Canopy temperatures of well-watered plants are about 12°C cooler than the temperatures of surrounding air under high evaporative demand and about 6°C less than the air temperature under low evaporative demand. Water stress, however, causes canopy temperature to increase.
Thus, irrigation can be scheduled for some crops by measuring the temperature between canopy and surrounding air and comparing that difference to that of a well-watered plant. A number of methods are available, ranging from hand-held infrared thermometers/guns (Fig 6.11), which can be pointed at an individual tree or field canopy, to satellite-based remote sensing systems, which can measure the average canopy temperature over a much larger area.
Uses of infrared thermometer for canopy temperature measurements require calculating a crop water stress index (CWSI). This in turn, requires developing two base line curves (Fig 6.12).
The lower baseline curve is the difference between canopy temperature and air temperature for a well-watered crop (non-water stressed baseline).The second baseline curve is the temperature difference of a severely water-stressed crop where little or no transpiration occurs (severe water stressed baseline). Both sets of temperature differences are plotted against vapour pressure deficit (VPD). This deficit is the difference between saturation vapour pressure at the air temperature at the time of measurement and actual vapour pressure of the air at the time of measurement.
The CWSI is calculated as follows:
1. Measure canopy temperature of the crop
2. Calculate the difference between measured canopy temperature and air temperature
3. Determine the vapour pressure deficit using an aspirated psychrometer.
4. Plot the value calculated in step 2 against the VPD on the baseline curves, which is point B on Fig 6.12
5. Determine the difference between canopy and air temperature at the measured VPD (step 3) of a well-watered crop using non-water stressed baseline, which is in point C in Fig 6.12
6. Determine the difference between canopy and air temperature at the measured VPD (step 3) for a severely stressed crop using the severe water stressed baseline, which is point A in Fig 6.12
7. Determine the CWSI by calculating the ratio of the difference between the values of step 2 and step 5 to the difference between the values of step 5.
Baseline for well-watered crop can be developed by measuring temperature differences and VPDs over a range of climatic conditions for a crop adequately irrigated. One approach for developing the baseline for a severely stressed crop is to extend the curve for the well- watered crop back to a VPD of zero. The temperature difference at that point is assumed to be the baseline of a severely stressed crop.
Baselines must be developed for each crop where the infrared thermometer is to be used to schedule irrigations. For some crops such as barley and wheat, different baseline curves occur for pre-heading and post-heading growth stages. Table 6.12 lists coefficients for equations describing non-water stressed baseline curves for various crops.
The equation used with these coefficients is as follows:
Tc – Ta = A1 – A2 × VPD
where, Tc = Canopy temperature (°C)
Ta = Air temperature
VPD = Vapour pressure deficit (kPa)
A1, A2 = Coefficients in Table 6.12.
What is the CWSI at which irrigations should be occur? Interestingly, in spite of the large amount of information on non-water stressed baselines; little information exists on the CWSI at which irrigations should occur. Thus, irrigators will need to determine appropriate CWSIs for their crops and site-specific conditions.
Crops tolerating moderate water stress are best suited for this method. Canopy temperature of these crops under moderate water stress can increase sufficiently to measure differences in canopy temperature after irrigation. Crops that are water-stressed sensitive are not suitable for this method. Canopy temperature of these crops will not increase sufficiently to determine temperature differences after irrigation.
Unfortunately, there are many drawbacks with these methods:
1. Leaf canopy temperature will only increase after the stomata reduce water loss by closing and by that time some negative effects of the water stress may have already occurred, hence the information may not be timely
2. Remote sensors give an average temperature value over a large area, hence any inclusion of sky (if pointing from the ground up into the tree canopy) or of soil (pointing down from above) can cause substantial measurement errors
3. In order to interpret the measured value of canopy temperatures, a number of additional environmental variables such as air temperature, relative humidity, wind speed and sunlight intensity must be obtained and combined together to form an index of stress. This index must then be compared to a previously determined reference value. These reference values have not been determined for most tree and vine crops but they are available for some field and row crops.
Critical growth stages of crops:
As a result of extensive experimentation, critical growth stages (moisture sensitive stages) of various crops for water demand. If irrigation water is scarce, irrigations are to be scheduled at least at these critical growth stages for maximum water use efficiency.
Comparison of the above irrigation approaches leads to the conclusion that as estimation of permissible soil moisture depletion is essential before the crop yield starts declining. Among the approaches, the IW/CPE ratio method appears to be ideal because of its simplicity when the water supply is adequate. When irrigation water is inadequate, irrigation may be scheduled at critical growth stages or at variable IW/CPE ratios. However, emphasis has to be shifted from the potential yield to optimum yield with high water use efficiency.
Feel and appearance of the soil (soil samples made into balls and tossed into the air and caught in hand for rough estimation of soil moisture), transpiration ratio approach (ratio of quantity of water required to produce a unit quantity of dry matter) and soil cum sand mini plot technique (artificial reduction in available soil moisture by mixing sand with the soil) were the other methods for fixing irrigation schedules in the early 1900s.
Essay # 4. Advantages of Irrigation Scheduling:
Irrigation scheduling offers several advantages:
1. It enables the farmer to schedule water rotation among the various fields to minimise crop water stress and maximise yields
2. It reduces the farmer’s cost of water and labor through fewer irrigations, thereby making maximum use of soil moisture storage
3. It lowers fertiliser costs by holding surface runoff and deep percolation (leaching) to a minimum
4. It increases net returns by increasing crop yields and crop quality
5. It minimises waterlogging problems by reducing the drainage requirements
6. It assists in controlling root zone salinity problems through controlled leaching
7. It results in additional returns by using the “saved” water to irrigate non cash crops that otherwise would not be irrigated due to water shortage.