  Supermassive black holes are commonly found in the center of galaxies and
evolve with their hosts. The supermassive binary black holes (SMBBH) are thus
expected to exist in close galaxy pairs, however, none has been unequivocally
detected. The square kilometre array (SKA) is a multi-purpose radio telescope
with a collecting area approaching 1 million square metres, with great
potential for detecting nanohertz gravitational waves (GWs). In this paper, we
quantify the GW detectability by SKA for a realistic SMBBH population using
pulsar timing array (PTA) technique and quantify its impact on revealing SMBBH
evolution with redshift for the first time. With only $\sim20$ pulsars, much
smaller a requirement than in previous work, the SKA PTA is expected to obtain
detection within about 5 years of operation and to achieve a detection rate of
more than 100 SMBBHs/yr after about 10 years. Although beyond the scope of this
paper, we must acknowledge that the presence of persistent red noise will
reduce the number of expected detections here. It is thus imperative to
understand and mitigate red noise in the PTA data. The GW signatures from a few
well-known SMBBH candidates, such as OJ 287, 3C 66B, NGC 5548 and Ark 120, will
be detected given the currently best-known parameters of each system. Within 30
years of operation, about 60 individual SMBBHs detection with $z<0.05$ and more
than $10^4$ with $z<1$ are expected. The detection rate drops precipitately
beyond $z=1$. The substantial number of expected detections and their
discernible evolution with redshift by SKA PTA will make SKA a significant tool
for studying SMBBHs.
