PHOSPHORUS LOADS TO SURFACE WATERS - A SIMPLE-MODEL TO ACCOUNT FOR SPATIAL PATTERN OF LAND-USE

Modeling nonpoint-source phosphorus (P) loading from land to surface w aters can be both complex and data intensive. Our goal was to develop a simple model that would account for spatial pattern in topography an d land use using geographic information system (GIS) databases. We est imated areas of the watershed that strongly contributed to P loading b y approximating overland flow, and modeled annual P loading by fitting three parameters to data obtained by stream monitoring. We calibrated the model using P loading data from two years of contrasting annual p recipitation for Lake Mendota, a Wisconsin eutrophic lake in a watersh ed dominated by agriculture and urban lands. Land-use scenarios were d eveloped to estimate annual P loading from pre-settlement and future l and uses. As much as half of the Lake Mendota watershed did not contri bute significantly to annual P loading. The greatest contribution to l oading came from a heterogeneous riparian corridor that varied in widt h from 0.1 km to approximate to 6 km depending on topography and runof f conditions. We estimate that loading from pre-settlement land use wa s one-sixth of the loading from present land use. A future scenario, r epresenting an 80% increase in existing urban land (from 9 to 16% of t otal watershed area, which would be reached in 30 yr with current land -use trends), showed only modest increases in annual P loading but pos sible significant effects on water quality. If the watershed were to b ecome entirely urbanized, P loading to the lake would double and poten tial effects on water quality would be severe. Changes in P loading we re strongest with conversions of undisturbed vegetated lands, especial ly riparian areas, to either urban or agricultural uses. Variability i n total annual rainfall leads to variability in the riparian area that affects P loading, with implications for policies intended to control nonpoint nutrient inputs.