**6. Cropland–Riparian–Stream Hydrologic Continuum**

Hydrologic processes are important for understanding the relative contribution of different P sources to watersheds and the relative effectiveness of P transport mitigation practices such as RBZs [15–22,38–49,69,75,86–91]. Site hydrology is also critical for P transport from cropland to RBZs and from RBZs to streamflow. We define the cropland–RBZ– stream continuum as the contiguous land area between active cropland (including pastures) and the nearest perennial or intermittent stream capable of transporting P to downstream systems. Runoff pathways along the cropland–RBZ–stream continuum contributing to streamflow (Qsf) include Hortonian and Dunne overland flow (Qof), groundwater flow (Qgw), and interflow (Qif) (Figure 2). Subsurface tile drainage is a fairly common practice for farms in CCs with poorly drained soils to improve agronomic performance. Tile drain flows are a mix of shallow groundwater and vadose zone water fluxes. Since tile drainage represents a form of subsurface lateral flow, it is included with Qif for simplicity. Stream flow (Qsf) at any given time is thus the sum of individual flow components:

$$\mathbf{Q\_{sf}} = \mathbf{Q\_{of}} + \mathbf{Q\_{if}} + \mathbf{Q\_{\S^W}} \tag{1}$$

**Figure 2.** Hydrologic pathways contributing to streamflow along the cropland–riparian–stream continuum.

Note that groundwater flow components (Qgw) are lumped for simplicity and likely include a mix of both shallow/younger and deeper/older flow paths. Stream baseflow is defined as those times when Qgw is the main flow source contributing to Qsf. Interflow (Qif) includes infiltrated water subject to gravitationally driven lateral movement in the unsaturated zone often induced by the presence of a flow boundary. To reiterate, the uncultivated area between cropland edge-of-field areas and stream bank edges is defined as the RBZ (Figure 2).

RBZs include both semi-natural and unmanaged systems in addition to designed and well managed RBZs. The critical assumption is that RBZs must have permanent vegetation

maintained with no agricultural operations (i.e., no tillage or agrichemical applications) occurring. Riparian areas are largely owned and managed by farms in the US and exist in a wide range of field conditions. The grass buffer in Figure 2 is approximately 2 to 3 m wide, which is narrow relative to NRCS riparian forest buffer specifications (minimum width = 10.7 m). Maintaining minimum width RBZs is mandatory in some US states. For example, in Vermont, State Required Agricultural Practices mandatea3m permanent RBZ along drainage ditches and 7.6 m wide RBZ along perennial streams and lakes.

#### **7. Riparian Buffer Zone Impacts on Phosphorus Transport**
