Figure 3. Nitrate (NO3-N) concentrations: (a)
Statistics of stream (Cstream, left box), shallow water
(Csw, middle box), and deep water (often groundwater,
Cdw, right box) under different land use conditions. The
Cstream was the direct field measurement from 228 sites
while Csw and Cdw were estimated and/or
measured end-member concentrations from 81 sites. The central line in
each box indicates the median, and the bottom and top edges are the
25th and 75th percentiles,
respectively. The whiskers extend to the most extreme data points not
considered outliers (not shown).
Outliers are defined as data
points more than 1.5 interquartile ranges (IQR, Q3-Q1) below the
first quartile (Q1) or above the third quartile (Q3), i.e., <
Q1-1.5×IQR or > Q3 + 1.5×IQR. (b) Cstreamas a function of agricultural land cover. The solid gray line is the fit
equation Cstream = 0.28e0.031 × %Ag(R2 = 0.54), and the dashed gray lines are confidence
interval at 99.9%, with upper and lower bound equations of
Cstream = 0.39e0.038 × %Ag and
Cstream = 0.20e0.025 × %Ag,
respectively.