No! That is away outside my field, but an intriguing question. On a dry, sunny, day, there is probably a detectable zone of lowered water activity where the rootlets are. This might be detectable as soil conductivity. (Building surveyors in UK detect dampness in the wall with a probe: two steel (Pt?) spikes a fixed distance apart; set a voltage difference; measure current flow between the spikes.)

Dear Ian

Thank you so much for your answer.,

You are right. Let me explain my question. Please imagin there is a tree more than 50 years old and i want to irrigate this tree according to the crop water need and soil limitation. Is this method you explained( two steel spikes) help me to estimate the depth of the root?

If yes, could you please send me the reference or references for that?

Thank you so much.

Dear Bita, to apply my suggestion will require much invention on your part. I was talking about the instrument used to assess dampness in old or badly constructed buildings. (http://www.damp-meter-direct.co.uk/how-to-use-protimeter-surveymaster.htm#.VCfEqxznSjA) . I do not know the literature on root depth. You must search, then invent.  

Dear Ian.,

Thank you so much 

I think the best way is to dig a profile beside the tree but it is destructive.

Dear Mr Kiani

Thank u for your answer. So... Is there any validate reference to estimate that base on tree  type?

Many thanks.

Dear Bita

Roots depth depends  many things Includ type species, moisture content, and etc.

destructive method is best method with hight accurat .

ِDear Younes

Thank you so much. 

Could u please tell me Which  method is the best? 

Did you test that?

Many thanks.

Dear Bita,

have you seen the following references:

- Mean root depth estimation at landslide slopes

Original Research Article

Ecological Engineering, Volume 69, August 2014, Pages 118-125

S. Tron, A. Dani, F. Laio, F. Preti, L. Ridolfi

- The lateral spread of tree root systems in boreal forests: Estimates based on 15N uptake and distribution of sporocarps of ectomycorrhizal fungi

Original Research Article

Forest Ecology and Management, Volume 255, Issue 1, 20 February 2008, Pages 75-81

S.G. Göttlicher, A.F.S. Taylor, H. Grip, N.R. Betson, E. Valinger, M.N. Högberg, P. Högberg

- Measuring tilt of tree structural root zones under static and wind loading

Original Research Article

Agricultural and Forest Meteorology, Volume 168, 15 January 2013, Pages 160-167

Ken James, Craig Hallam, Chris Spencer

Dear Dr Shiri., 

Thank u so much for your kindly helps. 

I should study these references.

Many thanks.

A question:

Have u ever estimated tree root depth in the field?

Thanks

Best regards.

Bita

Many thanks, Bita. I have never estimated it in field, sorry!

Dear Dr shiri

Thanks for your help.