## 2 Edge-of-Field Technologies
### Constructed Wetlands
Similarly, [PERSON] et al. [76] demonstrated through modeling that variations in the HRT presented clear effects on the percentage P retention. According to the above, however, the effect of hydraulic load on P retention may differ depending on the dominant P form in the drainage discharge, where higher loads would mostly benefit the retention of PP through sedimentation. The hydraulic efficiency (Equation (7)) quantifies the water flow distribution along the volume of the CW and is affected by the HRT distribution [96]. Thus, increasing the hydraulic efficiency improves the retention of P by enhancing the level of P mixing across the system and the active hydrological area of the SFCW. This consequently extends the contact time of P to its retention mechanisms in different parts of the system. Low hydraulic efficiency, on the other hand, promotes low P retention and is commonly characterized by the presence of preferential flow and zones with slow water flow or stagnant water with limited access to the incoming P loads. [PERSON] et al. [97], for example, reported low hydraulic efficiency in a SFCW due to preferential flow caused by varying density of submerged plants across the system, where areas with higher density slowed down the water flow and increased the retention of P. Sedimentation and plant uptake of P were consequently reduced in the areas with preferential flow, which markedly decreased the overall P retention. The study demonstrated that the varying retention of P across the SFCW occurred in response to the large distribution of HRTs. [PERSON] et al. [98] also reported low hydraulic efficiency in a SFCW receiving agricultural runoff due to preferential flow caused by the proximity of the inlet to the outlet. This resulted in large spatial variability in sedimentation rate and P sorption capacity of the soil, all affecting the retention of P.  
\[\text{hydraulic efficiency}=\frac{\text{time of peak outflow concentration of a tracer (yr)}}{\text{nominal hydraulic residence time (yr)}} \tag{7}\]  
The hydraulic efficiency is not only dependent on the hydraulic load, but also highly affected by the design of the CW [96]. [PERSON] et al. [99] found that design parameters such as the aspect ratio of length to width, configuration of the inlet and outlet and obstruction designation (i.e., the aspect ratio, number and location of designed obstructions) had an impact on the hydraulic efficiency. Thus, the design of a SFCW influences the retention of P. [PERSON] et al. (2007), for example, demonstrated that differences in water depth, plant spacing and configuration of the inlet and outlet in a SFCW influenced outflow total P concentrations. [PERSON] (2008) suggested that shallow SFCWs receiving agricultural runoff can optimize the sedimentation of particles and associated P due to short settling distance, which can be beneficial in SFCWs receiving mostly PP. Moreover, the study recommended the use of vegetation cover to mitigate resuspension--a secondary effect in shallow systems--which is supported in [PERSON] (2008). This latter study reported that vegetation cover reduced resuspension to approximately 40% five years after the construction of the wetlands, besides increasing the hydraulic efficiency. As discussed above, the hydrochemical and hydrological factors play a large role on the retention of P in SFCWs receiving agricultural drainage discharge. The release of previously retained P, however, is more likely affected by biogeochemical factors, which regulate transformation reactions and thus the stability of P in the different P pools (Figure 3). The export rate of P from a SFCW reflects both the amount of P not retained during its passage through the system and the amount of P released to the water column due to transformation reactions. As pointed out above, variations in the export rate of P may have higher consequences to the percentage P retention. In line with this, [PERSON] et al.