The first scenario consisted of applying the same amount of fertilizer spread across all irrigation pulses , except for the last irrigation pulse to enable flushing. The second scenario consisted of continuous irrigation of the same duration and irrigation amount as under pulsed treatments, with fertigation at all times , except for the same period of flushing at the end of irrigation. The fertigation scheme in PF1, PF2, PF3 and continuous scenarios was assumed to start from 17 August 2010. All fertigation simulations were run as for the irrigation experiment, that is for 29 days .The water content distribution in the soil reflects water availability to plants, and plays a crucial role in water movement through and out of the root zone. Volumetric water contents simulated by HYDRUS 2D/3D are compared in Fig. 5 with the measured values obtained using EnviroSCAN probes 15 cm away from the dripper. Simulated values matched measured values well, both spatially and temporally. However, deviations between simulated and measured values were observed at day 19 of simulation, particularly in the upper 50 cm of the soil profile; at later times this difference was not observed. Simulated and observed daily and cumulative drainage are compared in Figs. 6 and 7, respectively. It can be seen that simulated daily drainage remained slightly below observed values , except for the initial higher leaching on day 1. However,vertical hydroponic nft system the total drainage observed in the lysimeter was matched closely by the model.
The high peak on day represents the effect of high rainfall on that day, which also was very well predicted by the model. However, the cumulative drainage remained slightly over predicted during the initial 15 days, after which the simulated and observed values matched well. Model evaluation was performed using a number of model performance parameters calculated using measured and model generated soil water contents . The mean absolute error varied from 0.006 to 0.22 cm3 cm−3 and the root mean square error ranged between 0.007 and 0.028 cm3 cm−3, which indicated small deviations between measured and simulated values. However,the maximum values of MAE and RMSE were observed at day 19, confirming the deviations shown in Fig. 5 at this time. However, the values of paired t-test between measured and simulated water contents showed insignificant differences at 5%level of significance at all times.Values of the coefficient of determination varied between 0.68 and 0.96, indicating a reliable generation of water contents by the model at all days of simulations. Similarly, the Nash and Sutcliffe efficiency coefficient values ranged from 0.17 to 0.96, indicating a good performance of the model for the prediction of water contents in this study.However,the relative efficiency value at day 19 reveals unsatisfactory performance of the model at that point according to the criteria suggested by Moriasi et al. . The values of MAE, RMSE, r2, E, and RE for the drainage flux were 2.87, 4.14, 0.97, 0.94, and 0.78 , respectively, which also showed a robust performance of the model for drainage fluxes from the lysimeter. The close match of both water contents and drainage fluxes indicates that the HYDRUS 2D/3D software can be successfully used to predict water movement and drainage fluxes in a lysimeter planted with a citrus tree. Other studies have also reported good performance of this software for various soil, water, and crop conditions under pressurised irrigation systems . Simulated water balance components over the 29 day experimental period are shown in Table 3. It can be seen that simulated drainage, which is similar to the amount measured in the lysimeter, represents 48.9% of the total water balance.
A much higher seasonal drainage has been reported for a lysimeter planted with an orange tree in a fine sandy soil . High drainage is bound to occur in highly permeable, coarse textured soils, such as the sand/loamy soil used in this study, where water drains easily and quickly from the root zone because gravity dominates over capillarity . However, Sluggett estimated deep drainage in the range of 6.1–37.2% under citrus trees growing in light textured soils in the Sunraysia region of Australia. A major contributor to the high drainage measured in this experiment was the high amount of water applied, mostly as a result of large rainfall events. Simulated plant water uptake was estimated to be 40% of the water application, indicating low irrigation efficiency of the drip system. The daily plant uptake varied from 1.2 to 3.14 mm . However, plant uptake is a very complex process, and depends on a number of parameters describing the root and canopy development. Since the HYDRUS model does not support a dynamic behaviour of the root system and considers only the static root parameters, root uptake was optimised on the basis of a changing transpiration rate over time. Additionally, since in the present study we dealt with a tree, for which the root distribution development over time is not as fast as observed for seasonal crops like cereals, the root development was considered relatively constant for the modelling purpose. Hence, a static root distribution and variable atmospheric conditions produced a good approximation of plant uptake, as has been revealed in a number of earlier studies that used HYDRUS for modelling purposes Simulated distribution of nitrate at selected times after commencement of fertigation is shown in Fig. 8. Concentration of NO3-N was maximum at the centre of the plume below the dripper, with a gradual decrease in N concentration towards the outer boundaries of the plume. Subsequent irrigation and fertigation pulses resulted in enlargement of the plume, with a rapid lateral and vertical movement of NO3-N. It is worth noticing that after 15 days of fertigation all nitrate still remained in the lysimeter, reaching a depth of 70 cm. The maximum nitrate concentration at this time was at 20 cm. The simulated NO3-N uptake accounted only for 25.5% of applied nitrogen .
The remaining nitrogen was still available in the soil for plant uptake, provided it was not transformed by soil biological processes. No nitrate leaching was predicted by the model within this initial 15 day period. The total seasonal recovery of applied N amounts to 42.1% by the orange tree, while 7.7% of added NO3-N was retained in the soil atthe end of the season. These results agree with the findings of Paramasivam et al. who reported 40–53% nitrogen uptake in afield experiment on citrus. Similarly, Boaretto et al. showed 36% recovery of applied nitrogen by an orange tree in a lysimeter. The seasonal distribution of nitrate in the soil at 30-day intervals after the fertigation commencement is shown in Fig. 9.It can be seen that nitrate rapidly moved downwards and dispersed in the lysimeter, reaching a depth of 95 cm after 30 days. However, the zone of the maximum concentration remained close to the soil surface. Subsequent fertigation pulses further pushed N near to the leaching outlet at 60 days and N dispersed throughout the lysimeter, beyond which regular N leaching was observed with subsequent fertigations. However, the concentration of N remained much higher in the upper soil depth till 180 days of fertigation, enabling its continued uptake by the orange tree. The nitrogen concentration thereafter reduced drastically in the upper zone as a result of the withdrawal of fertigation after 195 days of simulation . At 210 days after commencement of fertigation ,nft hydroponic system the NO3-N concentration in the domain ranged between 0 and 0.4 mg cm−3, and continued to decline until it completely moved out of the upper 40 cm soil depth at 270 days. At the end of the simulation , only a very small amount of nitrate remained in the lysimeter, with higher concentration occurring at the bottom of the lysimeter , indicating higher vulnerability of this N to leaching. Major leaching of NO3-N took place after 90 days of simulation, amounting to 61%of total N leaching between 90 and 180 days , which corresponds to heavy precipitation of 95 mm on day 115 and 68 mm on day 152 of simulation. Paramasivam et al. and Nakamura et al. also reported that unexpectedly prolonged irrigation or high rainfall following fertilizer applications led to higher NO3-N leaching losses.
Total nitrate leaching amounted to 50.2% of the N applied as fertilizer . Nitrate losses of similar magnitude have also been reported by Syvertsen and Sax and Boman and Battikhi in a lysimeter grown orange tree. On the other hand, low NO3-N leaching losses ranging from 2 to 16% of the applied nitrogen have been reported in some studies on citrus . The migration of nitrate to deeper layers is highly dependent on the amount of irrigation and rainfall, as this is the driving force moving nitrate out of the root zone. Lower nitrate leaching estimated in this study may have been a consequence of improved irrigation and fertilizer management through the drip system. Hence improved water efficiency under drip irrigation, by reducing percolation and evaporationlosses, can contribute considerably towards environmentally safer fertilizer applications . In addition to the factors discussed above, a choice of appropriate source, amount, frequency, and timing of fertilizer applications and the rate of N transformation into NO3 are other important factors that determine the amount of NO3-N leaching out of the vadose zone .Temporal distribution of nitrate for different fertigation scenarios is presented in Fig. 11.Although nitrate movement appears to be similar in all scenarios, small differences can be observed in nitrate distribution in the soil for some scenarios. In scenarios PF and PF3, in which fertilizer was applied with all pulses in low concentrations or towards the end of irrigation, the N concentration after 2, 7, and 14 days was slightly higher in the centre of the plume where root activity was at a maximum. However, the nutrient uptake varied within a narrow range under normal irrigation , indicating an insignificant impact of fertigation timing under conditions experienced in our lysimeter study. Contrary to this, Hanson et al. reported 14% higher nitrate uptake when fertilizer was applied at the end of the irrigation event in a HYDRUS simulation that was based on historical irrigation and fertigation data. A similar observation was also made by Paramasivam et al. and Alva et al. in field experiments. Gärdenäs et al. also concluded that fertigation applied towards the end of the irrigation cycle generally reduces the potential for nitrate leaching under micro-irrigation systems, with the exception of clayey soils.A short fertigation pulse used in our study, as compared to the other studies, may have reduced differences among various scenarios. However, these results imply that fertigation in a short pulse towards the end of the irrigation event or low concentration fertigation with all pulses could increase the efficiency of nitrogen fertigation as compared to other options. Nitrate distribution in the domain after 21 and 28 days were similar in all scenarios , and all differences disappeared by 21 days of simulation. It can be shown that while nitrate distribution varied during one application phase, they were similar for all scenarios at the end of each irrigation cycle. Also, nitrate moved to a similar soil depth after 28 days in all scenarios. These scenarios did not produce any NO3-N leaching because of the short simulation period. A comparison of nitrate uptake between pulsed and continuous irrigations revealed that scenarios with pulsed irrigation had almost alike nitrate uptake as fertigation with continuous irrigation. Similar results were obtained in scenarios with different irrigation quantities. A negligible impact of pulsing on moisture distribution pattern and drainage has been reported in earlier studies for different dripper discharge rates and spacings . This observation further confirms that pulsing has little impact on solute distribution in the soil under optimal irrigation applications as compared to continuous irrigation.Modelling simulations were also performed to evaluate the impact of variable irrigation applications on nitrate movement for scenarios discussed above . It can be seen that plant NO3- N uptake gradually reduced as the amount of irrigation increased. The nitrogen uptake efficiency for the 50% irrigation treatment varied from 55.3 to 56.2% for all scenarios of fertilizer applications, which was about 8.5% higher than uptake recorded for the normal irrigation . On the other hand, a higher amount of irrigation than normal reduced nitrate uptake of an orange tree by further 3.4–3.6%. At the same time, the zone of maximum nitrate concentration moved to a depth of 40–60 cm , where root uptake decreased exponentially due to the reduction in root density.