The treatment of processes in numerical models of the land-surface is largely determined by the spatial and temporal resolution of the model employed in the “host” environment. Atmospheric models have short time steps (less than 0.5 hours), but large spatial resolution of tens to hundreds of kilometers due to numerical and dynamical constraints. Hydrological and ecosystem models operate under other constraints and have time steps on the order of days to months. Because their numerics are relatively simple compared to those of atmospheric models, there is more flexibility in specifying their spatial resolution, which is often constrained by the availability of spatially explicit data sets. Ecological models focus on still longer time scales but treat complex processes such as photosynthesis.