Dear Md. Nurul Kadir,

You are right. Part of the task is to calculate ET. The other to determine the effective rainfall (part of the rainfall that infiltrates into the root zone, and does not penetrate further down into groundwater), which will satisfy a portion of the demand. See the article: Chapter 4: Irrigation water needs:

http://www.fao.org/docrep/s2022e/s2022e08.htm.

You can also look at the previous chapter, Crop Water Needs:

http://www.fao.org/docrep/s2022e/s2022e07.htm#TopOfPage

However, there is an exception. While modelling Lucerne in the Peel System (NSW, Australia), I noticed that the irrigators were ordering only half of the calculated net crop demand. Lucerne, with its deep roots was able to reach to the capillary fringe above the aquifer and tap in effect into groundwater.

Regards

Aleksandra Rancic

Dear Md. Nurul Kadir,

Thank you for your interesting question, I hope I am able to give you some help with my response below...

As you (& Aleksandra) suggest, determining ET is part of the solution however, this will tell you only what the (approximate) total amount of water the crop is using, which is not necessarily (nor usually) the amount of this water that is supplied by irrigation alone.  The amount of water used by an irrigated crop can be made up from a number of sources, the major ones are likely to be irrigation water, effective rainfall (as per Aleksandra answer above) +/- a contribution from groundwater (I'll call this "effective groundwater" from now on although this is not a accepted term).  Therefore, in the simplest terms, working out how much of the crop water use is supplied from irrigation (your unknown) can approximately be determined from the following equation:

Crop Irrigation Use = ET - (Eff. Rainfall + Eff. Groundwater)

While determining crop ET and effective rainfall is (relatively) simple, determining the 'effective groundwater' can be more difficult, especially if you have a shallow watertable.

There are plenty of good references about determining 'effective rainfall' and the one quoted by Aleksandra above is a good place to start.

When determining 'effective groundwater', the first thing to do is determine what the average depth to the watertable is below your crop.  If you have a deep watertable (more than 2 to 3 meters below the rootzone of your crop), then it is likely that the contribution from groundwater will be very low, and it can reasonably be considered to be zero in the above equation (send me a note if you need a reference for this).

If however, you have a shallow watertable (less than 2 to 3 meters below the rootzone of your crop) below your crop then determining the 'effective groundwater' can be much more difficult and will likely depend on a large number of factors (the relative contribution of each will also vary widely from place to place).  These factors may include the actual watertable depth, its variation through the growing season, the soil type (composition, grain size, texture etc) the groundwater chemistry (especially salinity and sodicity), the type of crop grown and a number of others.

If you do have a shallow watertable under the crop you are working with, and would like some more information/references about the factors I have described in the above paragraph, please let me know (or you may be interested in reading some of my listed publications, which (in part) deal with this topic).

I hope my response has been of some help to you (although I realise it only expands a little on the excellent answer already given by Aleksandra).

Best wishes,

Alister

I think Alister's answer your question fully. However, you may  need to account for water transfer and irrigation system efficiency since there are some losses during the process.

I think you can do rough calculations about inflow to the field. Estimate the volume of water entering into a specific field. This can be done by estimating discharge of irrigation water (in lps) divided by the total irrigation time (in seconds). Discharge can be calculated by velocity area method. In addition, soil samples can be taken to the lab to determine the field capacity and permanent wilting point of a certain soil. Also, the rainfall data can be obtained from nearby met-station or from internet.

Zafar

Dear Md. Nurul Kadir,

I can provide you 2 tools:

1) Use small buckets as lysimeters, plant and irrigate as is carried in the field, and conduct a simple water balance: ET~Ir-Dr. The Irr is the applied irrigation amount applied to the bucket, Dr is the drained water collected from the bottom of the bucket. Using this simple water balance, and averaging over a period of about 7-10 days will give you the ET. There are some limitations to this method (similarities to real field conditions and boundaries), however, if the plants are similar to those in the adjacent field, you will have a good estimation.

2) To Use Penman-Montieth method to estimate potential ET, and then multiply the calculated value by  common Kc, specific to the grown crops.

Effi

Thanks to all for your cordial answers...

Dear Alister Terry,

Thank you for your interesting response and explanation. I am very interested by the how to evaluate the contribution of the shallow groundwater table to the crop ET and how to compute it in the field water balance. In my case, I have a very shallow salty groundwater table (less than 2 m), a sandy gypsyferous soil, monthly irrigation events, a perennial crop with deep root system. Can you please recommand some properly documentations and advices for my study case. Thanks in advance,

Imed

Dear Alister Terry,

Thank you for your interesting response and explanation. I am very interested by the how to evaluate the contribution of the shallow groundwater table to the crop ET and how to compute it in the field water balance. In my case, I have a very shallow salty groundwater table (less than 2 m), a sandy gypsyferous soil, monthly irrigation events, a perennial crop with deep root system. Can you please recommand some properly documentations and advices for my study case. Thanks in advance,

Imed

How do I determine the total amount of irrigation water from real crop field? - ResearchGate. Available from: https://www.researchgate.net/post/How_do_I_determine_the_total_amount_of_irrigation_water_from_real_crop_field [accessed Oct 28, 2015].

Your question has been very well answered in detail by Mr Alister and Alexandra and others and I only want to add that while calculating irrigation water slightly modify the formula given above. The irrigation water required by a crop should be  = ET- ER-EGW-SM + losses in the field.  Here, ER is effective rainfall, EGW is effective ground water, and SM is the soil moisture contribution(amount of available moisture present in the soil prior to applying irrigation). So while calculating irrigation water you need to add the losses like conveyance loss, application losses, distribution losses etc, as the total irrigation water requirement is more than the water actually consumed by the crop. In most of the Asian countries where water is applied through field channels there is huge amount of water losses from irrigation source to the field therefore you need to add all such losses while calculating irrigation water.

Also you can calculate the kc from remote sensing techniques and refine the actual ET.

wonderful question really I have the same problem, thanks all.

Please check the research paper - Wireless sensor network application development: an architecture cenric MDE approach. They discuss MITRA a precision agriculture system developed in SPAIN

Discharge multiplied by time will give volume.

Dear Md. Nurul Kadir .

I strongly recommend to you take a look at the FAO documents (FAO document 66) and software (Aquacrop). The water balance in the "soil reservoir" is complex and need careful to estimate the ET (will depend a lot of parameters from soil, plants, infra structure, etc). Also you need consider how big is your field (due to the rainfall distribution and total amount). Follow the links to the material from FAO.

http://www.fao.org/3/i2800e/i2800e00.htm

http://www.fao.org/aquacrop/en/

https://www.youtube.com/watch?v=nWEzr2O8Dm0

Cheers.