Water requirement of a crop is dependent on evapotranspiration (ET), soil textures and chemistry and also crop’s maximum allowable depletion. Therefore, I do think that lysimeter simply measures evapotranspiration (Et) from vegetation or crop, if it is well spread and dense. However, evapotranspiration is not enough other variables as depth of irrigation, return-flow to aquifer, and those mentioned above are to be taken into consideration, therefore. There are certain other ways in which crop-water requirement may be determined. See:

http://www.fao.org/docrep/t7202e/t7202e06.htm

Since, you have placed emphasis on tool (to me device/instrument) with the example of lysimeter, there are developed for the sake of precision farming and to avoid wastage of irrigation water, certain computer programmes and devices are available which not only predict time of irrigation, but also depth of water to be applied. For example Hawaiian Irrigation Water Requirement Estimation Decision Support System (IWREDSS), an ArcGIS based numerical simulation model and also Osama Osman Ali (2013): A Computer program for Calculating Crop Water Requirements; Greener Journal of Agricultural Sciences, 3 (2):150-163. Besides TDR, FDR and C-Probes and soil moisture profile station which includes four Hydra Probe Soil Sensors, Stevens DL3000 data logger, radio, antenna and accessories are available which are found quite effective in predicting water requirement of a crop.

Hi Nehrooz,

Why not measure crop transpiration?

The less transpiration, the higher the water requirement of the crop.

The higher transpiration, the less the requirement for water of the crop.

By the way transpiration is evapotranspiration minus evaporation.

So measure evapotranspiration and evaporation and you got a grip on crop water requirement.

You can also measure leaf water content in function of time by measuring middle IR reflectance, and invert a leaf RTF with these reflectance measurements. Gives a very useful result as well.

Cheers,

Frank

Water requirement of a crop is dependent on evapotranspiration (ET), soil textures and chemistry and also crop’s maximum allowable depletion. Therefore, I do think that lysimeter simply measures evapotranspiration (Et) from vegetation or crop, if it is well spread and dense. However, evapotranspiration is not enough other variables as depth of irrigation, return-flow to aquifer, and those mentioned above are to be taken into consideration, therefore. There are certain other ways in which crop-water requirement may be determined. See:

http://www.fao.org/docrep/t7202e/t7202e06.htm

Since, you have placed emphasis on tool (to me device/instrument) with the example of lysimeter, there are developed for the sake of precision farming and to avoid wastage of irrigation water, certain computer programmes and devices are available which not only predict time of irrigation, but also depth of water to be applied. For example Hawaiian Irrigation Water Requirement Estimation Decision Support System (IWREDSS), an ArcGIS based numerical simulation model and also Osama Osman Ali (2013): A Computer program for Calculating Crop Water Requirements; Greener Journal of Agricultural Sciences, 3 (2):150-163. Besides TDR, FDR and C-Probes and soil moisture profile station which includes four Hydra Probe Soil Sensors, Stevens DL3000 data logger, radio, antenna and accessories are available which are found quite effective in predicting water requirement of a crop.

Yes Dr. Mohammed is right the basis of the CWR is the penman-monteith equation, you will find it in the FAO document. However as frank suggested TCWR = Irrigation requirement + ET +E. You may find few papers on my RG dealing with the aspect. It is very critical for arid countries. we did experimental studies and by using CWR and changing the practice from gravity irrigation to center pivot and sprinkler there was over 80% reduction in irrigation.

Dear all

thank you so much for your applied opinion

Dear colleagues

What your idea about using lysimeter for measure water requirement in natural vegetation (for example rangeland and forest covers)

thank you all of you for a nice discussion