4.1. The Variability of Fertilizer Concentration in the Surface Water Flow
The supply of water and fertilizer was combined in irrigation, i.e., the fertilizer was broadcasted onto the basin surface in advance, the water flowed onto the field, and fertilizer was dissolved and infiltrated into the soil as the flow advances. Under the broadcast fertilization method, the surface water flow carried fertilizer and closely related to the distribution of fertilizer in the soil. The temporal and spatial distribution of fertilizer in the surface water flow would affect the distribution of fertilizer in the root zone of the soil. Therefore, distribution migration characteristics of ions in the surface water flow could better reflect the influence of the diffusion of water under basin irrigation on the distribution of fertilizer.
Figure 5 shows the changes in average concentration of
in the surface water flow at different observation points along the basin length under different treatments over time. Different experimental treatments had a certain effect on the change process of the average concentration of
in the surface water flow along the basin length.
Figure 5a,d show that Under the condition of uniform broadcast fertilization, the
concentration in the surface water flow increased gradually along the basin length at the first time (the water flow reached the observation point), because the ammonium sulfate was an easily soluble fertilizer, and when the flow advanced to the observation point the water depth was shallow; with the increase in irrigation time, the water level in the basin rose gradually and was accompanied by the forward advance of the flow front, the flow advance could drove the solute and increased
concentration at the flow front.
Under the non-uniform broadcast fertilization (
Figure 5b,c,e,f), the
concentration decreased gradually along the basin length at the first sampling time. As the result of interaction between the non-uniform broadcast fertilization and the increased solute concentration at the flow front, the effect of the non-uniform broadcast fertilization was the opposite of the increased solute concentration at the flow front. Additionally, the effect of the non-uniform application coefficient on
concentration was higher than that of the flow advance under the experimental condition, which resulted in that the
concentration decreased along the basin length when the flow reached the observation point under the condition of the non-uniform broadcast fertilization, indicated that the non-uniform application coefficient had a significant effect on
concentration at the first sampling time for each observation point.
For all experimental treatments, the general trend of change in
concentration in the surface water flow over time was that the value decreased sharply and then stabilized after the water advanced to the observation points.
concentration at 10 m, 30 m, 50 m, and 70 m were closed to 30 mg/L at the fourth sampling time (the water flow reached the basin tail after 15 min). At the last observation point (90 m)
concentration was related to the non-uniform application coefficient, as the non-uniform application coefficient increased,
concentration decreased gradually, and under the condition of uniform broadcast fertilization (
Figure 5a,d),
concentration was the highest. When the non-uniform application coefficient was 1.5 and the inflow rate was 6 L·s/m,
concentration of the five observation points were similar at the fourth sampling time. The results showed that the non-uniform application coefficient had a significant effect on
concentration at different moments for the observation points, and with the value increased,
concentration tended to be the same at the fourth time point, i.e.,
concentration was more uniform.
The corresponding statistical characteristics of the spatial distribution of
concentration along the basin length are given in
Table 4. At the first sampling time, the difference in
concentration in the surface water between observation points was not obvious under the different experimental treatments; the coefficient of variation
Cv was between 0.12 and 0.42. At the second, third and fourth sampling time, when the inflow rate was the same, the
Cv decreased with the increases in the non-uniform application coefficient, indicated that the
distribution in the surface water flow was non-uniform under the condition of uniform broadcast fertilization; moreover, the uniformity of the
distribution in the surface water flow increased with the increase of the non-uniform application coefficient.
4.2. The Spatial Distribution of Fertilizer Concentration in the Soil
Figure 6 shows the spatial distribution of the average increment of
concentration in the 0~80 cm soil layers under different fertilization treatments 2 days after irrigation, and
concentration was the average value from the three repetitions. Under different fertilization treatments, the increment of
concentration in the soil decreased with the increase of soil depth, and this process was affected by the soil moisture in the effective root layer of the crop. The increment of
concentration in the soil was larger in the 0~40 cm soil layer, and the increments were both smaller and had little differences in the 40~60 cm and 60~80 cm soil layers. At treatment I and treatment IV, the increment of
concentration in the soil at the basin tail (80~100 m) was higher than that at the beginning of the basin. At treatment III and treatment VI, the increment of
concentration in the soil from 0~60 m of the basin beginning was higher than that at the basin tail. At treatment II and treatment V, the difference in the increment of
concentration in the effective root layer (0~40 cm) of crops was small along the basin length for each observation point and reached a relatively uniform state, which was related to the
distribution in the surface water flow. The uniform distribution of the
concentration in the surface water flow was good along the basin length, and the uniformity of the average increment of
concentration in the soil was high.
The results in
Table 5 also show that in the same soil layer, especially in the effective root depth, the variation coefficient
Cv values of treatment II and V are smaller than those of other treatments. The results showed that non-uniform broadcast fertilization could effectively improve the non-uniform situation of fertilizer distribution caused by “backward warping” in relation to solutes in the soil at the basin tail.
4.3. The Evaluation of the Fertilization Performance
Table 6 shows the fertilization performance under different fertilization treatments for winter wheat in the returning green stage.
In conclusion, non-uniform application coefficient and inflow rate had effects on fertilization uniformity and fertilization storage efficiency, especially significant for fertilization uniformity. Under the condition of uniform broadcast fertilization (under treatment I and treatment IV), the smaller inflow rate was higher than the larger inflow rate in the uniformity of distribution and its fertilization storage efficiency, and its values were increased by 5.1% and 5.2%, respectively. The results showed that under uniform application coefficient, the smaller inflow rate had higher fertilization performance.
The results of the field evaluation of the fertilization performance showed that non-uniform broadcast fertilization could improve effectively the fertilization uniformity and fertilization storage efficiency. Under the non-uniform broadcast fertilization method, with non-uniform application coefficient increased, the fertilization uniformity and fertilization storage efficiency first increased and then declined. When the inflow rate was 2 L/(m·s), fertilization uniformity increased from 69.8% to 76%, and fertilization storage efficiency increased from 55.5% to 62.5%, increased by 6.2% and 7%, respectively. The results showed that non-uniform broadcast fertilization could improve effectively the fertilization uniformity and fertilization storage efficiency under smaller inflow rate. When the inflow rate was 6 L/(m·s), the fertilization uniformity increased from 64.7% to 85.3%, and fertilization storage efficiency increased from 50.3% to 71.4%, increased by 20.6% and 21.1%, respectively. The results showed that under larger inflow rate non-uniform broadcast fertilization could improve effectively the fertilization performance. When the non-uniform application coefficient is the same, fertilization performance of the larger flow rate is better.
From six treatments of experimental designed, the best combination of fertilization performance was q6-s1. Its fertilization uniformity and fertilization storage efficiency were 85.3% and 79.4%, respectively. The above results showed that the reasonable combination of non-uniform application coefficient and inflow rate was beneficial to improve the fertilization performance. In practical applications, it was possible to obtain a better fertilization effect by selecting an appropriate non-uniform application coefficient in combination with the actual inflow rate.
4.4. Simulation Analysis of Influence of Non-uniform Application Coefficient on Fertilization Performance
4.4.1. Model Validation
Before model validation, the model parameters are determined. Firstly, the field double-loop experiment was carried out to obtain the infiltration parameters. Secondly, the model parameters were calibrated according to the experimental results of treatment I, II, IV and V. Finally, the model was validated according to the experimental results of treatment III and VI.
Figure 7 shows the dynamic time change process of simulated and measured surface
concentration at different observation points, and
Table 7 shows the corresponding ARE values. It can be seen that the initial value of
concentration in surface water is relatively high due to the thin surface water layer at the beginning of sampling. As the irrigation water flows downstream, the surface water depth increases, and the
concentration in both simulated and measured surface water showed a trend of gradual decline. The ARE values under different inflow rates are between 6.94%~13.91% and 6.60%~9.74%, respectively. This showed that the model can better simulate the change process of
concentration.
4.4.2. The Variation of Fertilization Performance with Non-uniform Application Coefficient
Figure 8 shows the variation trend of fertilization uniformity and fertilization storage efficiency under different basin lengths, inflow rate and non-uniform application coefficient. In general, fertilization uniformity and fertilization storage efficiency have similar laws, that is, the combination with higher fertilization uniformity has higher fertilization storage efficiency, and vice versa. Therefore, the non-uniform broadcast fertilization method has the dual effect of improving fertilization uniformity and fertilization storage efficiency. With the increase of non-uniform application coefficient, the fertilization uniformity and storage efficiency increased first and then decreased. The simulation results were consistent with the experimental results. This phenomenon can be explained as follows: when the non-uniform application coefficient is 0, the larger fertilizer concentration at the basin tail will reduce the uniformity and storage efficiency of fertilization; when the non-uniform application coefficient is 1.5, the amount of fertilizer applied at the basin head will increase, and the driving effect of irrigation water is limited; when the irrigation is over, the fertilizer concentration at the basin head will reduce the uniformity and storage efficiency of fertilization. Therefore, choosing appropriate non-uniform application coefficient can effectively improve the performance of fertilization.
For the same basin length, when the non-uniform application coefficient is 0, the fertilization performance decreases with the increase of the inflow rate. The simulation results were consistent with the experimental result. With the increase of non-uniform application coefficient, the highest fertilization performance can be obtained, and the corresponding optimal non-uniform application coefficient increases with the increase of inflow rate. Under the optimal non-uniform application coefficient, the highest fertilization performance increases with the increase of inflow rate, which indicates that for non-uniform broadcast fertilization method the higher inflow rate had a greater potential to improve fertilization performance.
For different basin length, when the non-uniform application coefficient is 0, the fertilization performance tends to decrease with the increase of the basin length, which indicates that with the increase of the basin length, the effect of water flow on the accumulation of fertilizer in the basin tail is more significant. With the increase of the non-uniform application coefficient, the highest fertilization performance can be obtained, and the corresponding optimal non-uniform application coefficient increases with the increase of basin length, which indicates that a longer basin length requires a larger non-uniform application coefficient to achieve better fertilization performance.
According to the highest fertilization performance in
Figure 8,
Table 8 gives the non-uniform application coefficient under different conditions, which are convenient for practical production. When the theoretical maximum fertilization performance is reached, the fertilization uniformity of different schemes is above 82.5% and the fertilization storage efficiency is above 70%. Therefore, the non-uniform broadcast fertilization method has great application value.