Estimating effects of reforestation on nitrogen and phosphorus load reductions in the Lower Yazoo River Watershed, Mississippi

Surface water quality in the Lower Mississippi River Basin (LMRB) and the adjacent Gulf of Mexico has degraded over the past several decades primarily due to deforestation to agricultural lands and the loss of wetlands. This study investigated the benefits of reforestation upon nitrate-nitrogen (NO3−N) and orthophosphate (PO43−) load reductions in the Lower Yazoo River Watershed (LYRW) within the LMRB using the BASINS-HSPF model. The model was calibrated and validated with available experimental data prior to its applications. Two simulation scenarios were then performed: one was chosen to predict the NO3−N and PO43− loads without reforestation and the other was selected to estimate the impacts of reforestation upon NO3−N and PO43− load reductions following the conversion of 25, 50, 75, and 100% of the agricultural lands (with most lands near or in the batture of the streams) into forests. In general, an increase in forests reduced NO3−N and PO43− loads and occurred because forest soils enriched in organic matter absorb water and nutrients and reduce the surface water runoff. Overall, a two-fold increase in forest land would result in approximately two-fold decrease in annual average NO3−N and PO43− loads. On average, over a 10-year simulation, the specific NO3−N and PO43− load reductions were, respectively, 0.06 and 0.004 ton/ha/y. Although the annual average NO3−N and PO43− loads always decreased with increasing forest land conversion, the optimal specific NO3−N and PO43− load reductions were found at a 75% reduction of agricultural land for the simulation conditions used in this study. Additionally, the annual average NO3−N load was about 16 times higher than that of PO43− in the LYRW. This study suggests that reforestation in or around the batture of streams is a beneficial practice for NO3−N and PO43− load reductions.