Coverage of the luminosity–redshift plane by upcoming absorption surveys. The parameter space is divided into cells of dz = 0.01 and dL = 0.1, and the colour coding indicates 0.1 (light), 1 (medium), and 10 (dark shading) objects per cell. As expected, the majority of objects lie close to the flux limits of the surveys. At z ≤ 0.26, SHARP in the northern hemisphere and WALLABY in the south cover very similar parameter space. The full MALS L-band survey covers 0 < z < 0.58, but is the only survey probing intermediate redshifts, 0.26 < z < 0.42. MALS UHF targets intermediate redshifts, 0.42 < z < 1. A similar redshift range is covered by FLASH, but with brighter flux limits. For clarity, we show the coverage of the plane out to z = 1, even though both surveys extend to z = 1.44 with the same behaviour. The black lines denote the minimum optical depth visible for sources of a given flux with the channel rms values given in Table 3.