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Information about soil bulk density (SBD) is important for soil-water conservation, carbon content, and nutrient reserves. The authors studied the spatial variability of SBD in gravel-mulched fields of different ages in an arid area of northwestern China. The SBDs of a new gravel-mulched field (NGM), a moderately aged gravel-mulched field (MGM), an old gravel-mulched field (OGM), and bare land (CK) ranged from 1.07 to 1.41, 1.15 to 1.41, 1.18 to 1.38, and 1.04 to 1.29 g/cm(3), respectively, with a general trend of OGM>MGM>NGM>CK. The coefficients of variation were less than 7.58%, indicating weak spatial variability. SBD was strongly correlated with soil-water content (SWC), indicating that the distribution of high and low SBD can be approximately estimated using SWC distribution. Kriging could effectively predict the spatial distribution of SBD in the study area with a high precision of interpolation, but estimating SBD using cokriging by measuring SWC was more accurate. (C) 2018 American Society of Civil Engineers.