Crop Water Requirement For Competitive Exam

Crop Water Requirement

 

Crop Water Requirement

It is defined as the entire amount of water required by a crop in a certain period of time, regardless of its source, for normal growth and development under field circumstances at a given location. It is the total amount of water required to mature an adequately irrigated crop in order to cover losses due to evapotranspiration (ET), as well as losses during irrigation water application (unavoidable losses) and special operations such as land preparation, transplanting, salt leaching below the crop root zone, frost control, and so on. It is measured in terms of depth per unit of time.

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In other terms, crop water demand is the entire amount of water required to cultivate a crop. In terms of supply, crop water demand may be stated as:

WR = IRR + ER +ΔS +GWC                       

Where:

WR = Total depth of water required during the life of crop irrespective of source

CU = Consumptive use (total water required for all plant processes)

ER = Effective rainfall received during crop life

ΔS = Profile water use i.e., difference in soil moisture in the crop root zone at the beginning and end of the crop

GWC= Groundwater contribution, if any

IRR = Irrigation

Crop ET estimate based on reference crop ET and crop coefficient technique was discussed in the previous lecture. There are a variety of ways for estimating reference crop ET (ETo) using meteorological data. Crop ET can also be assessed with a lysimeter or a field water balance. Because these approaches are time-consuming and labor-intensive, indirect methods of crop ET estimate are widely utilised and will be discussed in this lecture.

To various people, the word “effective rainfall” means different things. For example, hydrologists define effective rainfall as runoff, but irrigation engineers and agriculturists define it as helpful or utilisable rainfall for crop development. Effective rainfall, according to Dastane (1974), is “that percentage of total annual or seasonal rainfall that is beneficial directly or indirectly for fulfilling agricultural water demands in crop production at the site where it falls but without pumping.” As a result, it is the percentage of rainfall that excludes losses from surface runoff, unnecessary deep percolation, and leftover moisture after harvest.

This idea of effective rainfall is offered for use in irrigation project design and operation. Rainfall features, land topography, soil and crop characteristics, management strategies, carryover moisture content, and groundwater contribution are all elements that influence effective rainfall. For calculating effective rainfall, a variety of approaches are used. Field water balance methodology, rice drum cultivation approach, and empirical connection are some of them (SCS method). Different crops’ water requirements are listed.

Net Irrigation Requirement

This idea of effective rainfall is offered for use in irrigation project design and operation. Rainfall features, land topography, soil and crop characteristics, management strategies, carryover moisture content, and groundwater contribution are all elements that influence effective rainfall. For calculating effective rainfall, a variety of approaches are used. Field water balance methodology, rice drum cultivation approach, and empirical connection are some of them (SCS method). Different crops’ water requirements are listed.

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Where,                 

NIR = net amount of water to be applied during an irrigation, cm

Mfci = gravimetric moisture content at field capacity in the ith layer of the soil, (%)

Mbi = gravimetric moisture content before irrigation in the ith layer of the soil, (%)

ρbi = bulk density of the soil in the ith layer, g/cm3

D= depth of the ith soil layer, cm, within the root zone, cm

N = number of soil layers in the root zone D.

Gross Water Requirement

The total amount of water applied by irrigation, including losses, is known as gross irrigation need, or net irrigation requirement plus application and other losses.

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Duty of Water (D)

Throughout the baseperiod, this is defined as the area that may be watered with a constant non-stop supply of irrigation water at the rate of one cumec or cusec. It’s measured in acres per cusec or hectares per cumec.

Base Period (B)

This is the time frame for supplying irrigation water for the cultivation of any crop. This is usually equivalent to the time between the first and last watering of a crop.

Delta (Δ)

This is the amount of water a crop needs to satisfy its requirements during the growing season. This has nothing to do with the size of the cropped field. It is measured in millimetres or centimetres.

Relationship between D, Δ and B

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Where, Δ in cm, B in days and D in ha/cumec.

Methods of Crop Water Requirement Determination

Direct Measurement of Evapotranspiration

Because plant water usage is a key management input, knowing ET is essential. Several techniques for measuring evapotranspiration have been developed, as previously described (see section 25.3), and a few are summarised below.

Aerodynamic Methods

At different elevations above a plant canopy, the air vapour pressure and air flow velocities may be monitored. The instantaneous evapotranspiration rate may be calculated by assessing these observations. The evapotranspiration for a day is calculated by adding these immediate observations. Because the air flows irregularly above the canopy, this approach necessitates extremely precise equipment.

Soil Water Balance Methods

Evaporation is caused by changes in soil water, and numerous approaches have been employed to link changes in soil water to plant water usage. Figure 26.1 depicts the key components of the soil water balance. The soil water balance may be calculated as follows:

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Where,

ET = amount of evapotranspiration during the period,

AWe = amount of soil water in the root zone at the end of a period,

AWb = amount of soil water in the root zone at the beginning of a period,

P = total precipitation during the period,

dg = gross irrigation during the period,

Uf = groundwater contribution to water use during the period,

Ri= surface water that runs onto the area during the period,

Ro = surface runoff that leaves the area during the period, and

dp = deep percolation from the root zone during the period.

 

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Sketch illustrating the components of the soil water balance.

Neutron scattering or other methods can be used to determine the amount of water in the soil. Deep percolation is difficult to quantify, and it’s frequently considered to be minimal unless there’s a lot of rain or a lot of irrigation. The need for repeated measurements throughout the season is a fundamental drawback of the soil water balance approach. The shortest interval for estimating ET using the soil water balance approach is generally one week.

Lysimetry

Lysimeters are devices that are used to calculate evapotranspiration. It comprises of subterranean open-top tanks filled with undisturbed soil and planted with the same crop as the surrounding region. The soil water within the tank must be utilised for ET by the plants growing in the lysimeter. ET may be assessed by monitoring soil water content and irrigation or rainwater applications. The soil tank separates soil water from the rest of the system, preventing runoff, upward groundwater movement, and drainage from entering.

Drainage is permitted in some applications, and the volume of deep percolation is monitored. Traditional technologies, such as neutron probes, can be used to measure the soil water within the tank. Weighing the tank, soil, plants, and soil water can also be used to calculate the amount of water in the tank. The change in weight matches the quantity of water needed for ET since soil water is the only thing that varies considerably during short time periods.

 

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Cutaway drawing of weighing type lysimeter.

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