We present a channel-resolved study of strong-field ionization and Coulomb explosion dynamics in ${\mathrm{H}}_2\mathrm{O}$ using real-time time-dependent density functional theory (RT-TDDFT) on a real-space grid with explicit inclusion of the driving laser field. By analyzing an ensemble of trajectories with randomized laser polarizations and thermally sampled nuclear velocities, we identify the microscopic mechanisms governing competing fragmentation pathways. Three-body fragmentation is found to dominate over undissociated and two-body channels. Fragmentation branching is determined during the early stage of the laser pulse and is strongly correlated with orbital-specific ionization. Enhanced ionization of the HOMO-2 $\left(1b_2\right)$ orbital promotes higher-ionization three-body breakup, whereas ionization dominated by the highest occupied molecular orbital (HOMO) $\left(1b_1\right)$ and HOMO-1 $\left(3a_1\right)$ favors two-body dissociation. Laser-molecule orientation plays a decisive role by selectively enhancing ionization from different molecular orbitals. The calculated kinetic-energy-release and angular distributions reproduce the principal experimental observations, while revealing that the three-body kinetic energy release is strongly correlated with the total ionization degree, confirming Coulomb repulsion as the primary determinant of the fragment energetics. These results establish a microscopic connection between orbital-specific ionization, molecular orientation, charge-state formation, and fragmentation branching, demonstrating the capability of RT-TDDFT for interpreting channel-resolved Coulomb explosion dynamics in polyatomic molecules.

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• Light-matter interaction
• Photodissociation
• Ultrafast phenomena
• Density functional theory