Question

according to which concept depreciation is to be charged as per one particular method of the Year

Answer 1

as per consistency concept , The idea in accounting that once an accounting method is adopted, it should be followed consistently from one accounting period to the next.

so if one method of depreciation is selected it should be followed to rest of years . 

exception to the concept of consistency 
method can be changed following reasons :
1)better presentation of financial statements 
2)statutory compliance 

Answer 2

there is a method called diminishing method