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 (NO-3---N) and orthophosphate (PO3-4) 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 NO-3---N and PO3-4 loads without reforestation and the other was selected to estimate the impacts of reforestation upon NO-3---N and PO3-4 and 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 NO-3---N and PO3-4 loads and occurred because forest soils enriched in organic matter absorb water and nutrients and NO-3---N and PO3-4 reduce the surface water runoff. Overall, a two-fold increase in forest land would result in approximately two-fold decrease in annual average NO-3---N and PO3-4 loads. On average, over a 10-year simulation, the specific NO-3---N and PO3-4 load reductions were, respectively, 0.06 and 0.004 ton/ha/y. Although the annual average NO-3---N and PO3-4 loads always decreased with increasing forest land conversion, the optimal specific NO-3---N and PO3-4 load reductions were found at a 75% reduction of agricultural land for the simulation conditions used in this study. Additionally, the annual average NO-3---N load was about 16 times higher than that of PO3-4 in the LYRW. This study suggests that reforestation in or around the batture of streams is a beneficial practice for NO-3---N and PO3-4 load reductions.