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Sub-damped Lyman alpha systems (subDLAs; HI column densities of19.0<=logN(HI)<20.3) are rarely included in the cosmic HI census performed atredshifts z>=1.5, yet are expected to contribute significantly to the overallHI mass budget of the Universe. In this paper, we present a blindly selectedsample of 155 subDLAs found along 100 quasar sightlines (with a redshift pathlength X=475) in the XQ-100 survey to investigate the contribution of subDLAsto the HI mass density of the Universe. The impact of X-Shooter's spectralresolution on sub-DLA identification is evaluated, and found to be sufficientfor reliably finding absorbers with logN(HI)>=18.9. We compared theimplications of searching for subDLAs solely using HI absorption versus the useof additional metal lines to confirm the identification, and found thatmetal-selection techniques would have missed 75 subDLAs. Using abootstrap-Monte Carlo simulation, we computed the column density distributionfunction (f(N,X)) and the cosmological HI mass density of subDLAs and comparedwith our previous work based on the XQ-100 damped Lyman alpha systems. We donot find any significant redshift evolution in f(N,X) or cosmological HI massdensity for subDLAs. However, subDLAs contribute 10-20 per cent of the totalcosmological HI mass density measured at redshifts 2<z<5 (agreeing withprevious measurements), and thus have a small but significant contribution tothe HI budget of the Universe.

Sub-damped Lyman alpha systems in the XQ-100 survey I -- Identification and contribution to the cosmological HI budget