MODELING OF PHOSPHORUS TRANSPORT IN SURFACE RUNOFF FROM AGRICULTURAL WATERSHEDS

SEYFOLLAH AMIN-SICHANI, Purdue University

Abstract

Eutrophication, the enrichment of waters by nutrients especially phosphorus, has become a serious problem in the United States. Models have great utility for planning the reduction of eutrophication and other water quality problems. The majority of models now used are of the lumped parameter type which predict values based on average watershed conditions. A distinct disadvantage to this approach is the lack of information about the causes of the pollution. In contrast to lumped parameter models, distributed parameter models can predict the spatial effect of land use, soil type, fertility, rainfall, and other spatially distributed inputs. In this study, a phosphorus component was added to ANSWERS, a distributed parameter model, which has already been demonstrated to handle runoff and sediment transport. Because of the complexity of the phosphorus cycle in nature, only sediment-bound and soluble phosphorus transport were considered. In predicting sediment-bound phosphorus, a negative logarithmic equation was derived for the phosphorus enrichment ratio using Black Creek Project data. For soluble phosphorus, the empirical relationship between Bray-P1 and desorbed soluble phosphorus presented by Romkens and Nelson (1974) was used. However, the equation was modified using different water-soil ratios. Sensitivity analyses were performed on the slopes of the enrichment ratio and the desorbed phosphorus isotherm. A series of simulations were then made on two small watersheds in the Black Creek Watershed area, northeast of Fort Wayne, Indiana and for the another watershed located southwest of Indianapolis. Simulated and recorded data, where available, compared favorably for each watershed. The phosphorus component has the capability to simulate phosphorus loadings for a wide variety of input conditions. The total and Bray-P1 phosphorus level over a watershed can be provided from soil samples, but, in many cases, values of phosphorus levels could be obtained from soil maps and fertility tests. With proper input values of Bray-P1, total phosphorus, and desorbed phosphorus isotherms for the soils of a particular watershed, the ANSWERS model with the new phosphorus component is now capable of predicting phosphorus yields from agricultural lands.

Degree

Ph.D.

Subject Area

Agricultural engineering

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