Hi Anil,

I am not sure I understand your question. However, I can think of these ideas which might help you:

- PAR is equal to the total incident radiation multiplied by 0.48. You can get monthly PAR values from NASA at https://eosweb.larc.nasa.gov/sse/RETScreen/. Ideally, you should get those values from a meteorological station near your experimental site.

- To get the intercepted PAR of your crop you need to measure light interception. There are several devices you could use (Sunscan, http://www.delta-t.co.uk/product/sunscan/; LI-3000, https://www.licor.com/env/products/leaf_area/LI-3000C/). You will generally measure on a weekly or 2-week basis, depending on your crop and the goal of your research. You then generally assume that between two measurement dates there was a linear accumulation of radiation.

- An equation that is generally accepted to define yield is the following:

Yield = [1-Exp (-k x LAI)] x PAR0 x RUE

"k" is the extinction coefficient, which depends on plant architecture.

"LAI" is leaf area index

PAR0 is the total amount of PAR that was accumulated within the period of measurement

RUE corresponds to radiation use efficiency

I hope this helps you.

It is related to LAI and stage of growth.

See:

Remote Sensing of Environment

Volume 14, Issues 1–3, January 1984, Pages 65–75

Intercepted photosynthetically active radiation estimated by spectral reflectance

https://doi.org/10.1016/0034-4257(84)90008-7