SQUARE GRID TRANSPORT MODEL RELATING LAND-USE EXPORTS TO NUTRIENT LOADS IN RIVERS

The present transport model relates a nutrient‐oriented land use data bank to mass discharges of total nitrogen and total phosphorus on an annual or seasonal basis. This bank is based upon already available Canadian statistics distributed upon drainage units of the Universal Transverse Mercator square grid system; it contains for both types of sources (direct and diffuse) information on the intensities and spatial distributions of the produced loads. Using two constants only, the contribution of a drainage unit to a downstream site can be written as L = CTk [P + (CI × NP)], where P and NP represent the point and nonpoint production of nutrients; CI is an internal transfer coefficient of nonpoint sources to the river, CT a transport coefficient from one unit to the following unit, and k a distance expressed in number of units covered. Stable sets of coefficients were obtained for each season, reflecting the hydrological regime and differences in mobility of phosphorus and nitrogen. Once it is calibrated, the model can be used to assess the relative contributions of each type of land use and predict the effect of impoundments or changes in land use. Copyright 1979 by the American Geophysical Union.