about energy spectrum

[Shengfeng233]

still have another question about the reference energy spectrum line(k^-4)
I looked up in some books but did not find any 2D HIT that presents a reference energy spectrum line of k^-4, but I did find out in a published paper that also uses the reference line of k^-4(but did not leave any reference).
Could you please tell me where to find about why 2D decaying turbulence has the cascade of k^-4

Thank you in advance!