Quantifying Phosphorus Delivery Pathways in Forest Watersheds
Forest streams and lakes are renowned for their clarity, but concern that forest streams are carrying excessive amounts of phosphorus is increasing. Over the years, Forest Service scientists have helped the managers of forest watersheds better understand sources of phosphorus in Big Bear Lake in the San Bernardino National Forest, in Lake Tahoe on the California-Nevada border, and in the Great Lakes, where more than 60 percent of the basin is forested.
When phosphorus pollutes rivers and lakes, algal growth becomes excessive. When algae die and decompose, they can poison the water or make fish in the water poisonous for human consumption. The process also removes oxygen from the water, which sometimes leads to the death of vulnerable fish species. Current phosphorus models focus on agricultural systems where animal manure and chemical fertilizer tend to dominate phosphorus management and delivery issues. In these systems, phosphorus delivery is associated with surface runoff and erosion.
Agricultural models will not work in forest watersheds where minimal concern about livestock manure exists, where chemical fertilizers are seldom used, and where surface runoff and erosion rates are minimal compared with agricultural settings. Forest Service scientists developed the Water Erosion Prediction Project (WEPP) model for forest conditions, which has been in use for more than 20 years. One of the more recent improvements within WEPP technology is the addition of shallow lateral flow as one of the primary sources of runoff from steep forested watersheds.
Concurrent with this development, scientists observed that phosphorus concentration, although relatively low in forest soils, was relatively high in forest soil water. The scientists merged these two disparate pieces of information to develop a phosphorus delivery model that includes surface runoff and sediment delivery, plus delivery of phosphorus with lateral flow. The resulting predictive model clearly shows the importance of lateral flow in delivering phosphorus from steep forested hillslopes to forest streams.
One of the interesting findings is that the soil water phosphorus concentration is frequently lower after thinning or prescribed fire, suggesting that fuel management activities aimed at reducing the risk of wildfire may also reduce the delivery of phosphorus in lateral flow. To better understand these interactive processes that are associated with phosphorus delivery from forested watersheds, research continues. A user-friendly version of the model will be released soon for application in the Lake Tahoe Basin.