Without models, Running's team could miss important details. This is a simple diagram of the model used by Steve Running to predict the rate at which large douglas fir trees exchange water with the air. The model incorporated exchange of water from the soil to the roots, storage of water in the root system and tree trunk, and release of water from the needles to the atmosphere. Additionally, Running points out that models allow his team to compute variables that cannot be directly measured. But models allow us to compute that variable on a global scale.
Models provide the only means for comprehensively examining the terrestrial biosphere.
Models allow scientists to integrate multiple measurements across varying scales of time and space into a single tool for visualizing the system and predicting future changes. There is no way you can look at data and extrapolate into the future. The only way we can imagine what the terrestrial biosphere may be doing 10 or 100 years from now is through modeling. Because plants stop photosynthesizing when the ground is frozen, soil temperature is an important factor in determining net primary productivity.