Soil water dynamics reflect the integrated effects of climate conditions, soil hydrological properties and vegetation at a site. Consequently, changes in tree density can have important ecohydrological implications. Notably, stand density in many semi-arid forests has increased greatly because of fire suppression, such as that in the extensive ponderosa pine (Pinus ponderosa Laws.) forests that span much of western USA. Few studies have quantified how soil water content varies in low- versus high-density stands both by depth and years, or the inter-relationships between water content, stand density, and ecohydrological processes. Over a 4-year period, we measured the soil water content throughout the soil profiles in both low-density (250 trees/ha) and high-density (2710 trees/ha) ponderosa pine stands. Our results document significantly greater soil water contents in the low-density stands over a wide range of conditions (wet, dry, winter, summer). We observed substantial differences in water contents at depths greater than are typically measured. Our results also show that differences in monthly average soil water contents between the low- and high-density stands fluctuated between 0·02 and 0·08 m3 m−3 depending on the time of year, and reflect a dynamic coupling between infiltration and stand evapotranspiration processes. The difference in soil water availability between low- and high-density stands is substantially amplified when expressed as plant-available water on a per tree, per biomass or per leaf area basis. Our findings highlight important ecohydrological couplings and suggest that restoration and monitoring plans for semi-arid forests could benefit from adopting a more ecohydrological focus that explicitly considers soil water content as a determinant of the ecosystem process. Copyright © 2008 John Wiley & Sons, Ltd.