## 2 Edge-of-Field Technologies
### Constructed Wetlands
Furthermore, attempts to estimate the percentage P retention (Equation (5)) with hydrochemical factors have not attained similar explanatory power as for mass P retention [73; 89], which makes the percentage retention more difficult to predict. Therefore, other factors shall also be taken into account to understand the variability in P retention. \[\text{percentage P retention}=\frac{\text{P load (g m}^{-2}\text{ yr}^{-1})-\text{P load at the outlet (g m}^{-2}\text{ yr}^{-1})}{\text{P load (g m}^{-2}\text{ yr}^{-1})}\times 100 \tag{5}\]  
The hydraulic load (Equation (6)) normally correlates weakly to the mass and percentage P retention, as it does not account the P concentration [68; 73; 89]. However, it regulates the nominal HRT as well as the HRT distribution (i.e., the time for all parcels of water in the system) throughout the SFCW. This is because the hydraulic load has an inverse relationship to the nominal HRT (see Equations (4) and (6)). Moreover, the hydraulic load affects flow dynamics in SFCWs, and therefore the hydraulic efficiency (i.e., the proximity of the water flow in a CW to plug-flow conditions). During peak-flow events, for example, the hydraulic load increases and favors preferential flow and lower level of P mixing in the water. This consequently reduces the active hydrological area of the SFCW and HRT for proper P retention by the retention mechanisms. Base to moderate flow, on the other hand, attenuates the hydraulic load which tends to improve the distribution of water and P throughout the system. This condition increases the active hydrological area of the SFCW and HRT for P treatment and thus optimizes the retention. It is also important to note that variations in the hydraulic load commonly influence the concentration and dominant form of P in the drainage discharge and therefore in the inlet water of a SFCW, where higher hydraulic loads tend to increase the concentration and amount of PP [93]. Thus, SFCWs are subject to seasonal variations in P loads and forms due to common oscillations in hydraulic load between warm and cold periods. Therefore, hydrological factors also play a marked role on the retention of P.  
\[\text{hydraulic load}=\frac{\text{water flow (m${}^{3}$ yr${}^{-1}$)}}{\text{ wetland area (m${}^{2}$)}} \tag{6}\]  
In a systematic review accounting mostly SFCWs receiving agricultural runoff, [PERSON] et al. [94] demonstrated through a spline regression model that percentage P retention clearly increased under higher P concentrations when the hydraulic load was within a low to moderate range, not exceeding a critical threshold. Under this condition, the HRT is likely sufficient to allow proper retention of P by the retention mechanisms. However, hydraulic loads exceeding this threshold, as commonly occurring during peak-flow events, reduce the HRT and favor preferential flow, which can compromise the retention. This condition not only mitigates the P retention mechanisms, but also favors the resuspension of deposited sediments and subsequent export. This was observed, for example, in [PERSON] et al. [95], where sedimentation and accumulation of P in three SFCWs increased under higher hydraulic loads--suggestively up to a critical threshold. A fourth SFCW, however, showed minor sedimentation of P owing to a markedly high hydraulic load (400 m yr\({}^{-1}\)). In line with this, [PERSON] et al. [94] found that wetlands receiving event-driven hydraulic load were less effective (%) at retaining P than wetlands presenting lower variations in the hydrological regime. The model for mass P retention presented in this study, on the other hand, demonstrated that increases in both P concentration and hydraulic load resulted in higher retention. Therefore, changes in the hydrological regime may have higher consequences to the percentage P retention due to variations in the export rate of P. This study suggests that an optimum hydraulic load shall be high enough to allow considerable mass P retention, although not exceeding the threshold which would decrease the percentage P retention.