# Controlled Drainage Effectiveness in Reducing Nutrient Outflow in Light of Climate Changes
## 4 Discussion
These highest values refer to the initial water depth of 40 cm b.s.l. According to the UKESM model results for the CD practice started on 1 and 15 March, no significant increases or decreases of \(\pm\)2% were observed. Thus, the number of days with GWT above the drainage level (>90 cm) increases. In wet years, the number of days for the initial depth of 40 cm b.s.l. was the highest, with 67, 64, and 60 days for the GFDL, MPI, and UKESM models, respectively. For a depth of 60 cm b.s.l., it was 63, 60, and 53 days, while, for 80 cm b.s.l., it was less than 54 days for the GFDL, MPI, and UKESM models, respectively. [PERSON] et al. [30], who simulated the impact of CD from March to September with RCP 4.5, found that blocking outflow at the 1 March date in the near-future GWT will maintain an average of 85 days regarding the drain level. The average sum of the drainage outflows for CD started on 1 March, allowing them to be reduced, compared to FD, by 52 mm (58, 54, and 45 mm for wet years), by 44 mm (34, 47, and 52 mm for normal years), and by 35 mm (32, 41, and 34 mm for dry years) based on the results of the three models. The lowest number of days with outflow is 17-19, 18-21 and 15 days in wet years for the GFDL, MPI, and UKESM models, respectively. In normal years, it is 12-14, 15-17, and 16-18 days, and, in dry years, it is 12-14, 13-15, and 11-13 days for the GFDL, MPI, and UKESM models, respectively. Relative to the actual climate conditions given by [PERSON] et al. [34], the highest average outflow increase of 33-80% was modeled for the near future according to the GFDL model using CD on 1 March, with GWT initial depths of 60 and 80 cm b.s.l. The MPI and UKESM models showed increases of 30-40% and 7-18%, respectively, for wet years and 17-23% and 13-25%, respectively, for normal years, both with initial depths of 60 and 80 cm b.s.l. On the other hand, when the outflow was stopped on 15 March for the MPI model in all three wet, normal, and dry years, reductions of 69-79%, 85-91%, and 88-90% were obtained. Regardless of the initial GWT in wet, normal, and dry years, the resulting reductions were 100%. Moving the date to two weeks later (15 March) yields a significant reduction for the MPI model above 85%. At this date, the other two models indicate much lower reductions, at 10-55%. Stopping the outflows from 1 April to 15 April indicates the lowest reductions and/or no reductions at all. Using CD on 1 March allows for lower amounts of NO\({}_{3}\)-N in wet years (on average, 22 kg per ha), as well as 19 kg per ha in normal years and 15 kg per ha in dry years. For PO\({}_{4}\), it is 0.9, 0.7, and 0.6 kg per ha for wet, normal, and dry years. [PERSON] et al. [30] indicated an average reduction in outflow at the start of CD on the 1 March at an average level of 82% and 84% in the near and far future for the drainage networks with a spacing of 7 m. However, that research has shown that there will be a decrease in outflows from the drainage network in the analyzed time period, along with an increase in precipitation in that period. [PERSON] et al. [33] have indicated that CD efficiency was projected to increase during summer by a 42.7% and 81.8% reduction in subsurface drainage outflow for RCP 4.5 and RCP 8.5 under late-century climate conditions. According to [PERSON] et al. [47], the role of CD in paddy field simulations showed a significant decrease of 61% in the drainage discharge in the near future for RCP 8.5. These results further showed that there would be a 99% decrease in runoff rate in scenarios under RCP 8.5 in the near future. Other results have shown that, under future climate change conditions, the subsurface outflow will increase, along with an increasing mean yearly precipitation [48-50]. Selected climate change design models for precipitation showed an increase in the average value of surface runoff for the GFDL model for wet years on the 1 March CD start date (above 30 mm).