Figure 1 . A conceptual figure and hypothesis about nitrate export patterns, exemplified in Concentration-Discharge (C-Q) relationship under different land use conditions (i.e., Ag = Agriculture, Undev = Undeveloped, and Urban). Triangle wedges at the left of each hillslope describe the depth profile of N abundance with dark and light red represents high and low N abundance, respectively. Here we define two “loose” end-members: the “shallow” water (from, for example, surface runoff, shallow soils, or shallow pipes) and the “deep” water (e.g., from deeper zones such as groundwater). Stream water is often dominated by shallow water under high flow conditions and by deep water under low flow conditions. We propose the shallow versus deep hypothesis: nitrate concentration contrasts in shallow water (Csw) and deep water (Cdw) drive the export patterns. If this hypothesis is true, we expect higher concentrations in shallow water in agricultural lands, leaning toward a flushing pattern. In contrast, in urban watersheds, concentrated nutrients accumulated in leaky pipes in deeper subsurface are often higher than shallow, rapid runoff on impervious surfaces, possibly leaning toward a dilution pattern. Nitrogen in forests and pristine sites can come from the decomposition of organic matters in shallow soils and leaching from N-containing rocks in deeper zones. They are often tightly cycled with very low concentrations. These characteristics can lead to diverse patterns