Modeling phosphorus transport using the WEPP model

The Water Erosion Prediction Project (WEPP) model has been widely tested for its ability to predict soil erosion, runoff, and sediment delivery over a very wide range of conditions and scales for both hillslopes and watersheds. However, WEPP has not been used to estimate nutrient loss, in particular phosphorus losses. It is well known that most of the phosphorus transported from cropland is attached to sediment; consequently management practices that reduce erosion and sediment transport could reduce phosphorus losses. Management practices like vegetative filter strips, grass barriers, and multispecies riparian buffers have been proposed to reduce the transport of sediment and nutrients from agricultural lands to streams. However, research has not specifically addressed the question of how different amounts of perennial vegetative cover distributed in the landscape could affect soil loss and water quality. The first objective of this dissertation is to incorporate and test the ability of WEPP to estimate phosphorus loss with sediment at the watershed scale. The second objective is to study the effect of different landscape configurations on sediment yield and phosphorus loss using the WEPP model. The hypothesis related to the first objective is that WEPP can be coupled with a simple algorithm to simulate transport of phosphorus bound to sediment at the watershed outlet. Two hypotheses are related to the second objective. The first hypothesis is that increasing the amount of perennial cover located at the bottom of the hillslope will reduce sediment yield and phosphorus loss within a corn-soybean rotation at the watershed scale. The second hypothesis is that the strategic placement of perennial cover strips distributed in the hillslopes will reduce sediment yield and phosphorus export from the watershed compared to the same proportion of perennial cover located at the bottom of the hillslope. Two watersheds (side by side) in corn-soybean rotation were used to test the model. Watershed sizes were 5.05 and 6.37 ha. Total phosphorus (TP) loss at the watershed outlet were simulated as the product of TP in the soil (kg of TP kg-1 of soil), amount of sediment at the watershed outlet (kg of soil ha-1), and an enrichment ratio (ER) factor. One approach (P-empirical) estimated ER according to an empirical relationship, and the other approach used ER calculated by WEPP (P-WEPP). To address the first hypothesis of the second objective the scenarios were: 2.5% (2.5_B scenario), 5% (5_B scenario), 10% (10_B scenario), 15% (15_B scenario), and 20% (20_B scenario) of the area converted to perennial cover and placed at the bottom of the hillslope. To address the second hypothesis of the second objective the scenarios included 10% of the area in perennial cover placed at the bottom of the hillslope (10_B scenario), 10% in perennial cover with 50% at the bottom of the hillslope and the other 50% placed 40 m upslope (10_S scenario), 10% in perennial cover where WEPP simulated maximum detachment (10_WEPP scenario), and 20% in perennial cover with 50% at the bottom of the hillslope and the other 50% at 40 m upslope (20_S scenario). The same baseline scenario was used to address both hypotheses of the second objective. This scenario corresponded to a corn soybean rotation. The t-test failed to reject the null hypothesis that there was no statistical difference between the mean measured and simulated TP loss. This was the case for both methods (p=0.49 and p =0.40, P-empirical and P-WEPP, respectively). The Nash-Sutcliffe coefficient was 0.80 and 0.78 for the P-empirical and P-WEPP method, respectively. The inclusion of perennial cover at the bottom of the hillslope decreased the amount of sediment delivered to the channel; the reductions modeled for the various scenarios when compared to the baseline scenario were in the range of published field work. However, sediment yield and phosphorus losses at the watershed scale were affected by erosion in th...