We found that the window phase for the total N loss mainly appeared in the rice-growing season. However, the window phase for the total P loss was more dispersive, attributed to the different behavior of the total N and P in the runoff and the fertilizer management during the planting of rice. The demand for N in rice is much higher than that of P. In this experiment, N fertilizer (i.e., 186 kg N ha−1) was applied to the soil on three occasions, mainly in July-August. However, P fertilizer (i.e., 120 kg P2O5 ha−1) was applied as a base fertilizer before rice planting.
Additionally, studies show that soil P has a much lower migration rate than soil N . Some studies [39–41] confirm that N losses with both forms, i.e., organic nitrogen (Org-N) and nitrate (NO3-N), are predominantly conditioned by the soil fertilization type, while P losses are mainly associated with soil particles, and the agricultural land uses at any intensity. Therefore, the window phase of total N is mainly concentrated in the rice-planting season following fertilization. Contrarily, the window phase of total P is relatively dispersed throughout the year, attributed to the surface runoff. The study showed that the peak flows in the window phase usually contained the majority of N and P loss [42, 43].
Meanwhile, the loss window phases of total N and P are consistent with the runoff pattern. When runoff occurs, higher total P loss loading also occurs (Fig. 4). However, the concentration does not show the same pattern. The concentration of total P in the runoff during heavy rainfall periods (especially in the rice-planting season from June to July) is rather low. The low total P concentration may be due to the strong effect of P immobilization and the adsorption onto the soil . Some studies  show that the total P in the paddy soil is mainly organic P and inorganic Al oxide-P and Fe oxide-P, which is wrapped in aggregates, and it is not easily released into the runoff. Some studies [46, 47] showed that P migrates mainly through small-sized organic-inorganic composite colloids. Therefore, its mobility is relatively less affected by runoff, and its concentration increases when soluble P and colloidal P enter the water body with an increasing runoff amount. During heavy rainfall, all the soluble P and colloidal P in the soils carried by the runoff, and the concentration is diluted, resulting in a lower concentration . However, owing to the fertilization that occurs during the rice-planting period, the N concentration in the soil tends to be remarkably high, with a peak value consistent with the peak value of the heavy rainfall.
Moreover, the results of their study revealed that the total N and P concentrations in irrigation water have an impact on the drainage total N and P loads. However, the total N and P transport to the tile-drains was primarily regulated by the macropore flow and was not significantly affected by storms . Furthermore, a study by van der Salm et al.  on the nutrient losses from a heavy clay soil in a fluvial plain in the Netherlands found that the contribution of pipe drains to the total discharge of N and P was strongly related to the length of the dry period in the preceding summer, attributed to the very low conductivity of the soil matrix and the formation of shrinkage cracks. Another study  reports that the total P concentration in the drainage flow is independent of the Olsen P, while the antecedent soil moisture had a strong influence on the total P loss in the drainage flow. Furthermore, higher concentrations were recorded above a certain soil moisture deficit threshold.
The Δtotal N, in this study, showed negative values, while the Δtotal P showed positive values. This finding indicates the role of the stream on total N and P at different periods. The field runoff or channel drainage often influences the stream water quality. A study by Tomer et al.  on runoff in the Tipton Creek watershed (Iowa) shows that the channel drainage dominated, and was an essential source of the total N and P discharge of the watershed. In the present study, total P concentrations were high in the water samples, especially in the water sample from the retention ditch. More than 75% of the drainage water samples had higher P concentrations than that stated in the United States Environmental Protection Agency (US EPA) guidelines (i.e., 0.10 mg total P L−1 for streams) , and more than 90% of the samples exceeded 0.02 mg total P L−1. This value is a critical total P concentration value, above which the eutrophication in the surface water would accelerate . These data indicate that the water monitoring and forecasting of the retention ditch are essential to protect the water quality of the adjacent streams.
The annual average N runoff from the paddy field was 11.6 kg ha−1, which is consistent with the average loss loading of 12.7 kg ha−1 confirmed by field-scale experiments [8, 53, 54]. The annual average P runoff from the paddy field was 1.5 kg ha−1, which was higher than the 0.75 kg ha−1 of total P by field-scale experiments in TaiHu region of China [17, 55]. Other field-scale studies [12, 18] found a higher P runoff, which ranged from 5.49 to 17.68 kg ha−1. We also found that the total N and P loss loadings, rather than the concentrations of total N and P in the runoff, were linearly related to the surface runoff amount, which confirmed our hypothesis. This result shows that we can predict the loss loadings of total N and P using the cumulative runoff amount. As the total N and P concentrations fluctuate across the rice seasons, the total N and P loss loadings cannot be directly calculated as the product of the concentration and runoff at each time point. This relationship provides a simple tool for predicting the nutrient loss loading in the retention ditches of the paddy field. Consistent with our results, a study by Xue et al.  in the Baltic Region also shows no correlations between the nutrient concentrations and precipitation in the Mrzezino canal. Meanwhile, other studies  indicate the similarities in total P loss in the tile drainage and surface runoff, as well as the strong correlations between total P loss and storm hydrographs.
Using high-resolution monitoring at 6-h intervals, we found that the rainfall amount was an essential factor that affects the total N and P concentrations in the retention ditch. These findings are supported by the results of other studies that were conducted in eastern China and other Asian monsoon areas [57, 58]. However, the SOM showed weak relationships between the rainfall at 6, 12, 18, 24, 36, and 48 h intervals with the total N and P concentrations. Some studies using high temporal resolution monitoring (i.e., at 0.5 h intervals) show that using a one-hour sampling frequency may lead to a higher error in assessing the total pollution load than when using 0.5 h frequency . However, determining whether this error is an over- or under-estimation depends on which one-hour sample set was used. This is also consistent with our result, which revealed that using more than 24 h monitoring data did not improve the relationship between the total N and P in the retention ditch and the accumulated rainfall amount than when using shorter monitoring periods, i.e., < 24 h.
In addition to the rainfall, there are many other factors, such as soil conditions (i.e., soil moisture, soil depth, soil macropore, available N and Olsen P concentrations) and management practices (i.e., irrigation N and P concentrations, dry period, land cover, and fertilization), that influence the drainage water N and P concentrations or loadings [4, 47, 60]. Therefore, future studies should consider such additional environmental and management factors in the SOM approach to improve prediction accuracy. The SOM approach, developed in this study, helps to predict the total N and P in the drainage based on high-resolution monitoring data, i.e., at 6-h intervals. This approach was easy to implement by river managers in designing a long-term protection plan to maintain the water quality; however, it may be unable to predict the daily total N and P concentrations based on the daily rainfall forecast. In the present study, we only investigated the total P and N in the drainage water. Other P and N forms, especially particulate, colloid, soluble, reactive, and unreactive P, NH3+-N, and NO3--N should be tested further because several studies have shown that their response to rainfall in storm events was much closer than that of the total P .