Check the following paper:

Li, Jing, Yu Sheng, Jichun Wu, Ji Chen, and Xiumin Zhang. "Probability distribution of permafrost along a transportation corridor in the northeastern Qinghai province of China." Cold Regions Science and Technology 59, no. 1 (2009): 12-18.

Also, you can contact the authors for more information related to Potential Solar Radiation (PSR) & Topographic Wetness Index (TWI).

Hi Kazem, thank you vẻy much for your reference, i will look through the paper, kind regards

What is your definition of "Potential solar Radiation (PSR)"? There are so many ways of evaluating solar radiation...