Fig. 2: Global patterns of the time needed to achieve the water quality goal (≤1 mg N L^–1) under different watershed N input reduction scenarios.

Three scenarios are (a) 2% (low-level reduction, SC1), (b) 5% (intermediate-level reduction, SC2), and (c) 10% (high-level reduction, SC3) reduction in annual watershed N input compared to those in each preceding year. The sub-graphs in the lower left corner show the relationship between the years needed and the exceedance of targeted N concentration in lake waters (the difference between the current concentration and the water quality goal of ≤1 mg N L^–1, ETNC). The pie charts represent the proportion of lakes that require 0 − 10, 10 − 20, 20 − 30, 30 − 40, and >40 years to achieve the water quality goal. Data are shown at the HydroSHEDS level-5 watershed scale. Blue polygons indicate that the lake water quality is ≤1 mg N L^–1, and there is no need to reduce watershed N input. Gray areas indicate areas with no data available.