Temporal Variation in Nitrate Leaching Across Watersheds of the Mississippi River Basin

Abstract:

Large-scale numerical studies have suggested different temporal and spatial trends in total nitrogen fluxes across the continental United States. For example, total nitrogen fluxes have increased over the middle Mississippi and lower Ohio regions, whereas the Arkansas-White-Red region has experienced small decreases and increases over the last few decades. This contrasting variability is largely attributed to historical changes in agricultural practices, anthropogenic nitrogen, and climate. Large-scale trends are important to understand regional nitrogen budgets; however, these trends may average out nitrogen variability at a smaller scale. We developed a regional agro-ecosystem model (Agro-IBIS) to simulate annual runoff and nitrate leaching across the Mississippi River Basin at a spatial resolution of ~10 km, for the 1948-2007 period. We aggregated these results at a HUC8 (Hydrologic Unit Code) scale (853 watersheds) and divided the original time window into two periods of 30 years: a first period between 1948 and 1977, and second period between 1978 and 2007. For each watershed, we computed the median and standard deviation for each period and evaluated the relative change of the second period with respect to the first one. Our numerical results indicate that the median and standard deviation of nitrate leaching nearly tripled in some watersheds of the upper and middle Mississippi and decreased by half in some watersheds of the upper Missouri and Arkansas regions. Increases in total annual and springtime precipitation may have contributed to the nitrate leaching dynamics. Temporal changes in nitrate leaching may affect long-term agroecosystem services and the performance of conservation practices.