Due to the depth you specified, many of the normal devices will not fit.  I think it is possible to install a TDR probe at each depth.

Additionally, I highly recommend that you read the attached reference to compare all the soil-moisture measuring methods, technologies, and devices. Through It, you will be an expert for selecting the appropriate method/device for any case.

Dear Mohammad　＆ Raheem,

Thank you very much for adding very for adding valuable references and answers to my questions.　Your help is highly appreciated. 

Only TDR can do that.

In our lab, we have TDR probe but I am wondering whether I can take periodic measurements across a big plot with that unit. Thanks all for your useful answers and sharing the expertise. 

Nalaka, interesting question. You can take periodic measurements, but any TDR measurement only reflects the conditions of the area immediately surrounding the probe. You would need several sampling points across a large area if the goal is to estimate soil water across a big plot (of course it depends of what you mean by big plot). Best wishes to you!

Agree that TDR may do what you want. There are other soil water measurement devices. You may take a look to the web utilizing the appropiate key words.

TDR would be preferable, but as others noted multiple randomized sampling points would be the best way to gather quality data. What is the total area of each plot?

Thanks everyone again for adding useful answers. @Jenkins the plot size is 100X100m

A Theta probe could help. You can take as many measurements as required, and in some of the devices, the information is stored for downloading into a computer package. Additionally, some of the devices can be calibrated for the type of soil/media you want to test.

The most effective method would be using a probe to take soil measurements. Also take subsamples using traditional method to verify and calibrate electronic methods

Thank you for the answervFranklin Mairura