Using APEX to model nutrient retention in riparian buffers in an agricultural watershed in western Washington: Issues in model parameterization and calibration
Providing financial incentives for projects that enhance ecosystem services is a promising avenue for improving agricultural watershed management. Establishment of riparian buffers is a common management practice for attenuating nutrients and trapping sediments in runoff. Such benefits can be monetized in the form of credits to be traded in ecosystem service markets. However, efficiency of nutrient uptake for riparian buffers varies greatly depending on a number of environmental variables including buffer width, soil hydraulic conductivity, and agricultural nutrient loading. Conducting N and P measurements in all projects in an ecosystem service trading market is generally not feasible, so developing well-validated models is essential. This project uses the Agricultural Policy/ Environmental Extender (APEX) model to estimate potential benefits of buffer establishment on nutrient retention in a small watershed in Whatcom County, Washington. Here we discuss key steps in data collation and manipulation for accurate calibration and validation of APEX in a new location.
The APEX model requires detailed inputs of area soils, climate, cropping system, and agricultural management data. We modeled the Upper Kamm Creek watershed, a 228 hectare watershed that contains a diverse representation of Whatcom County cropping systems. Developing data layers to run the model in a new location necessitated several key approximations. First, we required parameterization for new crop (blueberry and raspberry) and buffer (shrubs and trees) species, which we gained by measuring light interception by leaves and nutrient partitioning of biomass. Second, we converted SSURGO soils data to APEX-specific soil layer data by using the Nutrient Tracking Tool. Third, site-specific data on crop management and yield were unavailable due to confidentiality, so we consulted with the local Conservation District to come up with county-wide averages. Finally, no detailed measurements of nutrient and sediment flux were available for calibration, so we relied on modeled hydrologic fluxes. In conclusion, a simulated watershed is presented using the APEX model for the first time in western Washington. However, the issues above may compromise model accuracy during validation and in simulations examining potential benefits of buffer establishment on nutrient retention.