eq = Sum[k1 (f[i + 1, j] - f[i, j]) ** (f[i + 1,
         j] - f[i, j]), {i, 0, nx - 2}, {j, 0, ny - 1}] +
   Sum[k2 (f[i, j + 1] -f[i, j]) ** (f[i,
         j + 1] - f[i, j]), {i, 0, nx - 1}, {j, 0, ny - 2}];