Ecohydrologic Nutrient Dynamics Associated with Biomass Changes Due to Wildfires

Abstract:

Watershed nutrient dynamics are difficult to accurately measure, especially when ecosystems experience disturbance events. Changes in climate and land use interactions with complex hydrodynamic-biogeochemical processes are essential in understanding the influence of riparian biomass on watershed dynamics. This is vital to protection of aquatic ecosystem health and drinking water quality. Furthermore, “the percentage growth in wildfire activity in Pacific northwestern and southwestern US forests has rapidly increased over the last two decades (Westerling, 2016).” Ecohydrologic models, such as VELMA (Visualizing Ecosystems for Land Management Assessments), simulate the effects of disturbances including burned watersheds. Thermal remote sensing instrumentation, synthetic aperture radar (SAR), and lidar imagery can quantify ecohydrologic watershed characteristics at higher spatial and temporal resolution, which can increase model accuracy.

Using satellite imaging sensor products and SARs, including Landsat, PALSAR/ALOS, MODIS/TERRA, and Sentinel-1, backscatter retrievals, biomass height and type, land cover type, soil moisture, and wildfire disturbance are used as initialization parameters in VELMA. VELMA includes a utility to flat process digital elevation data, and spatial and temporal meteorological data are used as hydrologic drivers. Improved model performance is indicated by an increase the Nash- Sutcliffe Efficiency (NSE) coefficient for each simulation, where comparisons are made between modelled and observed discharge. This case study seeks to optimize NSE and model improvement through isolation of each backscatter retrieval product obtained between shortwave and longwave polarized transmissions for the same initialized state variables between VELMA model runs. To demonstrate VELMA model performance against observed data, two historic wildfires, the Rim fire in California (2013) and the Hayman fire outside of Denver, CO (2002) were used as case studies. Since studies of physical wildfire interactions within watersheds can’t be performed, VELMA provides a vehicle to examine various scenarios with nutrient dynamic modeling and evaluation of best management practices for maintaining watershed health and water quality pre-and post-wildfire.