Heat pulse velocity techniques were developed for effectively monitoring water movement in aspen (Populus ttremuloides), subalpine fir (Abies lasiocarpa), and Englemann spruce (Picea engelmannii). Once the techniques were perfected, transpiration was monitored in replicated trees of each species for one year. This data was used to modify the plant activity index and the crop coefficient for each species within the model ASPCON, a deterministic, lumped-parameter model describing the hydrology of aspen to conifer succession. Results of the modeling indicate 18.5 cm(7.3 in) net loss of moisture available for stream flow when spruce replace aspen, and a loss of 7.1 cm (2.8 in) when fir forests cover the watershed. The aspen to conifer successional trend is therefore significantly reducing water yields within the Colorado River Basin, water that could be used to dilute salt downstream from the high water-yielding watersheds.
Gifford, Gerald F.; Humphries, William; and Jaynes, Richard A., "A Preliminary Quantification of the Impacts of Aspen to Conifer Succession on Water Yield Within the Colorado River Basin (A Process Aggravating the Salt Pollution Problem)" (1983). Reports. Paper 464.