Wetland Flowpaths Mediate Nitrogen and Phosphorus Concentrations across the Upper Mississippi River Basin.

Eutrophication, harmful algal blooms, and human health impacts are critical environmental challenges resulting from excess nitrogen and phosphorus in surface waters. Yet we have limited information regarding how wetland characteristics mediate water quality across watershed scales. We developed a large, novel set of spatial variables characterizing hydrological flowpaths from wetlands to streams, that is, "wetland hydrological transport variables," to explore how wetlands statistically explain the variability in total nitrogen (TN) and total phosphorus (TP) concentrations across the Upper Mississippi River Basin (UMRB) in the United States. We found that wetland flowpath variables improved landscape-to-aquatic nutrient multilinear regression models (from R2 = 0.89 to 0.91 for TN; R2 = 0.53 to 0.84 for TP) and provided insights into potential processes governing how wetlands influence watershed-scale TN and TP concentrations. Specifically, flowpath variables describing flow-attenuating environments, for example, subsurface transport compared to overland flowpaths, were related to lower TN and TP concentrations. Frequent hydrological connections from wetlands to streams were also linked to low TP concentrations, which likely suggests a nutrient source limitation in some areas of the UMRB. Consideration of wetland flowpaths could inform management and conservation activities designed to reduce nutrient export to downstream waters.

Agriculture, diversions, and drought shrinking Galilee Sea.

In water-limited regions worldwide, climate change and population growth threaten to desiccate lakes. As these lakes disappear, water managers have often implicated climate change-induced decreases in precipitation and higher temperature-driven evaporative demand-factors out of their control, while simultaneously constructing dams and drilling new wells into aquifers to permit agricultural expansion. One such shrinking lake is the Sea of Galilee (Lake Kinneret), whose decadal mean level has reached a record low, which has sparked heated debate regarding the causes of this shrinkage. However, the relative importance of climatic change, agricultural consumption, and increases in Lebanese water consumption, remain unknown. Here we show that the level of the Sea of Galilee would be stable, even in the face of decreasing precipitation in the Golan Heights. Climatic factors alone are inadequate to explain the record shrinkage of the Sea of Galilee. We found no decreasing trends in inflow from the headwaters of the Upper Jordan River located primarily in Lebanon. Rather, the decrease in discharge of the Upper Jordan River corresponded to a period of expanding irrigated agriculture, doubling of groundwater pumping rates within the basin, and increasing of the area of standing and impounded waters. While rising temperatures in the basin are statistically significant and may increase evapotranspiration, these temperature changes are too small to explain the magnitude of observed streamflow decreases. The results demonstrate that restoring the level of the Sea of Galilee will require reductions in groundwater pumping, surface water diversions, and water consumption by irrigated agriculture.