4 Methodology

Abstract

In this section a detailed description of the methodology developed for modeling agrichemical transport is presented. Two agrichemicals are selected: a nutrient, nitrate plus nitrite as nitrogen, and a herbicide, atrazine. These chemicals are introduced in Section 4.1. Section 4.2 describes two geographic analysis regions: one region used for developing a statistical model, and another smaller region used for GIS model development and its verification. The regression equations that relate chemical concentrations to the chemical application on the field, selected watershed descriptors, climatic variables, and the month of a year, are developed utilizing the USGS data collected in more than 150 watersheds scattered over the Mississippi-Missouri Basin above Thebes, Illinois, and the Ohio Basin above Grand Chain, Illinois. A GIS application of these equations is developed and verified for the Iowa-Cedar River watershed located in Iowa and Minnesota. A detailed mathematical description of the statistical model is presented in Section 4.3. It provides an overview of the transport equations, introduces the explanatory variables which explain the spatial and seasonal (monthly) variations of the nitrate and atrazine concentrations in surface waters, and explains the concept of spatio-temporal " cascade " modeling within GIS. The watershed parameters of the statistical model are estimated from the 500 m DEM. The GIS model of agrichemical transport in the Iowa-Cedar River basin is presented in Sections 4.4-4.6. Section 4.4 describes the methodology of discretizing the watershed, i.e., subdividing the basin into small (about 30 km 2) hydrologic units and determining the flow connectivity between these units utilizing the 3 arc-sec DEM.