Measuring crop evapotranspiration

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One of the project development objective indicator is "Irrigation efficiency (Water withdrawn used for crop production) - This indicator measures crop evapotranspiration as percentage of the total amount of water supplied" (I copied it as it was in the TOR).


Question: how can it be measured in practice (the project area is about 30,000 ha)? If someone did similar job, please share your experience.

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8 Answers

  1. Estimation of evapotranspiration can be made from evaporation data, because close relationship exists between the rate of consumptive use by crops and the rate of evaporation from a properly located evaporation pan.

    Open "Pan-Evaporimeter" can be used for the same. You will have to put several pans at representative places having uniform characteristics; as the area is large. 

    If you want further information, please write me on my mail east5star@gmail.com

    1 Comment

  2. Estimation of evapotranspiration can be made from evaporation data, because close relationship exists between the rate of consumptive use by crops and the rate of evaporation from a properly located evaporation pan.

    Open "Pan-Evaporimeter" can be used for the same. You will have to put several pans at representative places having uniform characteristics; as the area is large. 

    If you want further information, please write me on my mail east5star@gmail.com

    1 Comment

    1. Dr. Morey suggestion good. but one needs to know the Pan coefficients depending on the Agroclimatic regions . Also Fetch is another important factor. Apart from that Advection plays a crucial role in ET So the sensible Heat flux need to be monitored . In fact more Microclimate studies needed and will be interesting for students and scientists. Hope Morey  will agree.

      K.K.Nathan

      IARI, New Delhi, India

  3. I am Almost sure that you know about direct evaporation measurement. A meteorologist could help you with that. The Other variable is the "consumptive use" which is a crop-dependant factor (k). To get this value seems not to be highly complicated (We have values for almost any crop); read something about "consumptive use estimation" or something like that.

    2 Comments

    1. Thank you for advising. I'll take into account your suggestion.

    2. One of the Direct ET evaluation is by LYSIMETERS which I hope the scientist is aware off. There are many type of LYSIMETERS  are available, particularly Electronic type with high precision of ET to 0.1mm may be monitored.This will also help in Climate Crop Water Balance studies.Many good references are available which may be tried.

      K.K.Nathan

      Principal Scientist, IARI, New Delhi

      INDIA

  4. What you are trying to do is to quantify the water balance in your field area. The parameter of interest is the percentage of applied irrigation water consumed by the crop. Assuming no rainfall, you have 3 elements to the equation: Applied water = Evapotranspiration + Runoff + Infiltration. Your available monitoring approaches are to quantify at least 2 of the unknowns.  As Feki Mouna has commented, evapotranspiration can be measured directly using Eddy Covariance (basically sophisticated weather stations) or can be estimated from remote sensing data. Field runoff to drains can be measured directly with weirs. Infiltration can be measured by examining groundwater levels or careful vertical characterisation of soil moisture profiles. In practice projects will often use field measures for runoff/infiltration data, as these are fairly easy and cheap to measure, and calculate evapotranspiration, or use remote sensing products for evapotranspiration and estimate infiltration.  The direct measurement of evapotranspiration is complex and expensive and usually only done by academic projects to validate the remote sensing products.

    An alternative approach is to model based on meteorological measurements, crop types and irrigation technology - but of course how well models match the real world depends on both the model and the quality of its inputs.

    I think its valuable to consider issues of irrigation efficiency holistically. For an individual field high irrigation efficiency may mean more 'crop per drop' but water that runs off one field or infiltrates may support ecosystems or be the water resource for other farmers., so at a command area scale pursuing efficiency can be subject to a law of diminishing returns.

    1 Comment

    1. Thank you a lot for explanation.

  5. Good attempt. But Crop ET varies from crop to crop and different Phenological stages of the crop .Abiotic factors like temperature, wind and radiation plays a crucial role in Crop ET. My suggestion may be to work out the crop coefficients at Phenological stages of the main crops in your region. it will be useful research crop ET contribution.

    Thanks 

    K.K.Nathan, Principal Scientist, IARI, New Delhi India

    1 Comment

  6. I think the development objective should be 'improved irrigation efficiency."  Consumptive use can be determined from a 10-day calculation of ET from the local climate data and crop types.  Note I say types because the large area has a low probability of being monocropped.  Irrigation efficiency is the ratio of net water applied.  The net water applied is the total water input minus seepage from canals and percolation losses into the soil from the canal/pipe to the plant.  In most irrigation systems main canals are generally lined and their unit area seepage loss can be estimated from the type of lining.  If the canal is instead a piped system, then there should be no losses. Similarly if the irrgation application system is piped (e.g. trickle) the area of soil losses will be from percolation losses around the plant only.

    1 Comment

    1. Thank you. I agree that the development objective should be 'improved irrigation efficiency".

  7. It's possible to measure with scada but some fundamental data without irrigation will first has to be established. Secondly, the parameters which defines effective irrigation will have to be supplied. 

    1 Comment

  8. Using eddy covariance or remote sensing data

    2 Comments

    1. My name is Galina Stulina. I have been calculate  in water consumption, requirement  of crops in Uzbekistan for many years. I advise you to read FAO issues 24 and 56.

      Estimated evapotranspiration of crops with CROPWAT program ( FAO) _ is the most common method that is used is determined by the potential evapotranspiration ETo depending on climate and estimated ETc based crop coefficients determining the evapotranspiration for each crop. The disadvantage of this program is that it does not take into account recharge from groundwater in the balance.

      This way you will get the amount of water consumption required for each plant and will be able to calculate the irrigation efficiency knowing the actual water supply. That is, you can say that the plant needed so much water , but was served on the field of water more or less. This will be the efficiency of irrigation management.

      Another question is to evaluate the actual evapotranspiration of the ETa . This can be done in two ways - by installing lysimeters with plants on the field and periodically weighing them to find out the actual evapotranspiration The second method is to determine the ETa from satellite images using the leaf surface index NDVI.

      1 Comment reply

      1. Very informative. Extrapolated data can also be  used for selective irrigation distribution based on the acreage.

    2. Thank you.

      1 Comment reply

      1. I am Dr.K K Nathan . I have already mentioned regarding the direct method of estimating or monitoring crop Et by Lysimeters. There are Gravimetric Lysismeters available. Also Electronic Balance Lysimeters are also  available which can measure accurarately + or - , 0.5% mm.  

        There are climatological methods available and can work out the climate Water Balance of any crop.