Hydrological Modeling of the Soils of the Chaparral and Coastal Sage Scrub Ecosystems of Southern California.

Nitrogen emissions from transportation and agriculture have greatly increased the amount of nitrogen deposition in many areas of southern California.  Elevated levels of nitrogen in ecosystems alter nitrogen cycling and can lead to changes in vegetation.  Evidence suggests that high anthropogenic nitrogen deposition often favors invasive plant species and makes the ecosystem more susceptible to fires by increasing the amount of biomass.  Data has been collected and modeling has been done to increase knowledge of the fate and transport of the nitrogen deposited in the soils of the chaparral, coastal sage scrub, and coniferous forest ecosystems in order to better predict how the species composition and fire cycle will change in the future due to the extra nitrogen.  Similar measurements are being taken in the desert creosote scrub ecosystem.  The model Hydrus-1D has been used to reproduce the observed hydrological data for the chaparral and coastal sage scrub ecosystems.  Once accurate models for the flow of nitrogen through soil are obtained using Hydrus-1D and HP1 models, data will be input into biological models to determine whether or not the plant biomass is increasing due to the nitrogen deposition.  The biomass estimates can then be used in a fuel load model to determine alterations to the fire cycle.  By modeling the flow of water and nitrogen through soil, a better understanding of the nitrogen cycle in arid and semi-arid regions can be obtained, providing insight to how these ecosystems will change in the future.  The modeling will be a useful contribution to testing the hypothesis that nitrogen deposition is aiding the spread of invasive species throughout several southern Californian ecosystems and facilitating the spread of fires by increasing fuel loads and changing species composition.