Role of Temperature, PET and ET in determination of Irrigation Requirement

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What is Irrigation Requirement?

Irrigation Requirement is the amount of water the crop requires in order to stay alive. It is given on the basis of the moisture content of the soil. The irrigation interval is known as the difference between two successive irrigations in days. It depends on ET (Evapotranspiration) which is the combined effect of evaporation and transpiration. In bare lands or soils with seeds just planted or for small seedlings, transpiration contributes very little % in ET while Evaporation takes the main credit. While in the case of grown crops transpiration plays a major role in ET. More is the canopy cover more is be the effect of transpiration in ET. In fully grown crops, transpiration has the 90% share in ET while 10% is occupied by evaporation.

Why calculate Irrigation Requirement?   

Irrigation Requirement tells us the amount of depth of water needed to be given in the field depending on the area. This data will prevent the case of over and under-irrigation.

How to calculate Irrigation Requirement?

Various methods have been given in the studies by various researchers. Penman, Papadakis, Penman-Montieth, Blaney-Criddle each take different input variables such as temperature, pressure, solar radiation, wind speed, crop type, soil type, crop density.....etc. Kc(Crop Coefficient) is the value depending upon the type of crop and stage of the crop. These values can be found on the FAO website.


The easiest of these are Panman and Papadakis.
1. Penman method consists of calculation of PET under realtime data by putting a pan in an open area and then measuring the water level every day at 8:00 AM (India) and then multiplying the amount of water depth decreased by Pan coefficient.     
2. Papadakis method consists of an equation that takes maximum and minimum daily temperature data as input and in result gives PET (Potential Evapotranspiration).

 PET=[0.5625(emax-emin-2)*10]/number of days in month

emax =Saturation Vapour Pressure (mb or millibar) corresponding to daily maximum temperature.

emin-2=Saturation Vapour Pressure (mb or millibar) corresponding to daily dew point temperature.

 Irrigation requirement is the product of PET and crop coefficient.

ET=PET*Kc
IR=ET/Ea
Ea=Water Application Efficiency
Ea=50% (Flood Irrigation)
Ea=95% (Drip Irrigation)
Ea=60% (Furrow Irrigation)

For example as on 1 Jan 2007, Tmax =14°C &Tmin=1°C.

Hence applying the PAPADAKIS method, we get

emax=(33.8639*(0.00738*Tmax+0.8072)^8)-(0.000019*(1.8*Tmax+48)+0.001316)=15.99
emin-2=(33.8639*(0.00738*Tmin-2+0.8072)^8)-(0.000019*(1.8*Tmin-2+48)+0.001316)=5.66
PET=[0.5625*(15.99-5.66)*10]/31
            PET=1.87 mm
WHEAT
Kc=1.15 (For Crop Development stage in January) 
ET=1.87*1.15=2.15mm
ET=2.15mm
For flooding, Ea=50%
Hence IR=2.15/0.5
IR=4.301 mm
        Now farmer needs to have around 4.301 mm of depth over his entire field for 1st January for wheat crop in Punjab. As temperature data taken is of Punjab. 

    








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