Published by the American Geophysical Union as part of the Water Science and Application Series, Volume 3. Land surface hydrology integrates various physical, chemical and biological processes that occur above, on, and below the surface of the Earth. As a result, it is critical to accurately account for land surface processes within predictive models of hydrology, meteorology, and climate. One of our main difficulties, however, concerns the broad range of spatial and temporal scales that characterize land surface hydrological processes. For example, we determine infiltration by pore scale physics, while soil hydraulic conductivity remains a field scale property. Photosynthesis, respiration, and transpiration occur at the leaf scale. Runoff is a catchment scale process, and the variability of groundwater storage is a regional scale issue. Turbulence in land–atmosphere exchanges of heat, moisture, and momentum occur on the order of seconds to minutes, while variations in land surface and air temperatures occur much more gradually: on the order of hours. The persistence of floods and droughts is seasonal to annual, and so is the effect of El Nino on regional hydrology. Long–term climate effects occur much more slowly, on the order of years to decades.