Knowledge on relationships between diet composition and water kinetics of dairy cows will assist in understanding water utilization and determining water footprint. The objective of the study was to investigate the effects of dietary forage and CP contents on water partitioning in lactating dairy cows. Twelve Holstein cows were randomly assign to a 2 × 2 factorial arrangement of 2 forage levels [38 (LF) vs. 53% (HF)] and 2 CP levels [15.2 (LP) vs. 18.5% (HP)] in a 4 × 4 Latin square design with 4 periods. Cows were kept in metabolic cages indoors, where drinking water intake (dWI), DMI, urine weight, feces weight, and milk yield were measured over 3 d. The dietary treatment effects were analyzed in mixed-effect models including random cow effect. Average DMI, dWI, water via feed (fWI), milk yield, and ambient temperature (T) were 20.0 ± 2.2, 100.6 ± 14.3, 2.5 ± 0.2 and 31.4 ± 5.8 kg/cow/d, and 25.9 ± 1.4°C, respectively. Drinking water intake was positively related to DMI (P < 0.01). Independent of DMI, dWI increased respectively by 5.0 ± 2.1 and 5.9 ± 2.2 kg/cow/d (P < 0.05) for the increases in dietary fiber and CP contents. Irrespective to DMI and the diet composition, dWI increased by 2.2 ± 0.8 kg/cow/d (P = 0.02) for unit increase in T. Fecal water output (fWO) increased as DMI increased (P < 0.01). Regardless of DMI, fWO increased (FL = 27.2 vs. FH = 30.4 kg/cow/d) as dietary fiber content increased (P < 0.01). Urinary water output (uWO) was positively related to dWI (P < 0.01). When adjusted for dWI, HP and HF diets were associated with greater uWO (by 3.4 ± 1.1 and 2.0 ± 1.1 kg/cow/d, respectively) than LP and LF diets (P < 0.08). Each dietary nutrient modification independently and equally increased total manure water output (fWO+uWO) by 4.7 ± 1.2 kg/cow/d (P < 0.01). Water in milk (mWO) and BW were not affected by the dietary treatments. Overall, 26.8 ± 5.4, 28.1 ± 5.7, and 23.5 ± 6.8% of total water intake (fWI+dWI) were partitioned to mWO, fWO, and uWO, respectively. The majority of the rest (21.6 ± 10.7%) may be respiratory-cutaneous water losses. Forage content, dietary composition and temperature should be included for a better estimate of water utilization and partitioning in dairy cows.