On the form of local conservation laws for some relativistic field   theories in 1+1 dimensions
  We investigate the possible form of local translation invariant conservation laws associated with the relativistic field equations $\partial\bar\partial\phi_i=-v_i(\bphi)$ for a multicomponent field $\bphi$. Under the assumptions that (i)~the $v_i$'s can be expressed as linear combinations of partial derivatives $\partial w_j/\partial\phi_k$ of a set of functions $w_j(\bphi)$, (ii)~the space of functions spanned by the $w_j$'s is closed under partial derivations, and (iii)~the fields $\bphi$ take values in a simply connected space, the local conservation laws can either be transformed to the form $\partial{\bar{\cal P}}=\bar\partial\sum_j w_j {\cal Q}_j$ (where $\bar{\cal P}$ and ${\cal Q}_j$ are homogeneous polynomials in the variables $\bar\partial\phi_i$, $\bar\partial^2\phi_i$,\ldots), or to the parity transformed version of this expression $\partial\equiv(\partial_t+\partial_x)/ \sqrt{2}\rightleftharpoons\bar\partial \equiv (\partial_t-\partial_x)/\sqrt{2}$. 
E.G.B.Hohler and K.Olaussen