  We experimentally study the effect of different scattering potentials on the
flicker noise observed in graphene devices on silica substrates. The noise in
nominally identical devices is seen to behave in two distinct ways as a
function of carrier concentration, changing either monotonically or
nonmonotonically. We attribute this to the interplay between long- and
short-range scattering mechanisms. Water is found to significantly enhance the
noise magnitude and change the type of the noise behaviour. By using a simple
model, we show that water is a source of long-range scattering.
