High frequency irrigation systems involve fastidious planning and complex designs, so that timely and accurate additions of water and fertilizer can result in sustainable irrigation. At the same time these production systems are becoming more intensive, in an effort to optimise the return on expensive and scarce resources such as water and nutrients. Advanced fertigation systems combine drip irrigation and fertilizer application to deliver water and nutrients directly to the roots of crops, with the aim of synchronising the applications with crop demands , and maintaining the desired concentration and distribution of ions and water in the soil . Hence a clear understanding of water dynamics in the soil is important for the design, operation, and management of irrigation and fertigation under drip irrigation . However, there is a need to evaluate the performance of these systems, because considerable localised leaching canoccur near the drip lines, even under deficit irrigation conditions . The loss of nutrients, particularly nitrogen, from irrigation systems can be expensive and pose a serious threat to receiving water bodies . Citrus is one of the important horticultural crops being grown under advanced fertigation systems in Australia. Fertigation delivers nutrients in a soluble form with irrigation water directly into the root-zone, thus providing ideal conditions for rapid uptake of water and nutrients. Scholberg et al. demonstrated that more frequent applications of a dilute N solution to citrus seedlings doubled nitrogen uptake efficiency compared with less frequent applications of a more concentrated nutrient solution. Delivery of N through fertigation reduces N losses in the soil-plant system by ammonia volatilisation and nitrate leaching . However, poor irrigation management, i.e., an application of water in excess of crop requirements,hydroponic nft channel plus the storage capacity of the soil within the rooting depth, can contribute to leaching of water and nutrients below the rootzone.
Therefore, optimal irrigation scheduling is important to maximise the uptake efficiencies of water and nutrients . Most of the citrus production along the Murray River corridor is on sandy soils, which are highly vulnerable to rapid leaching of water and nutrients. Nitrogen is the key limiting nutrient and is therefore a main component of fertigation. An increasing use of nitrogenous fertilizers and their subsequent leaching as nitrate from the root zone of cropping systems is recognised as a potential source of groundwater contamination, because the harvested crop seldom takes up more than 25–70% of the total applied fertilizer . Several researchers have reported substantial leaching of applied N under citrus cultivation in field conditions . Similarly, in lysimeter experiments, Boaretto et al. showed 36% recovery of applied nitrogen by orange trees, while Jiang and Xia reported N leaching of 70% of the initial N value, and found denitrification and leaching to be the main processes for the loss of N. These studies suggest that knowledge of the nitrogen balance in cropping systems is essential for designing and managing drip irrigation systems and achieving high efficiency of N fertilizer use, thereby limiting the export of this nutrient as a pollutant to downstream water systems. Quantifying water and nitrogen losses below the root zone is highly challenging due to uncertainties associated with estimating drainage fluxes and solute concentrations in the leachate, even under well-controlled experimental conditions . Moreover, direct field measurements of simultaneous migration of water and nitrogen under drip irrigation is laborious, time-consuming and expensive . Hence simulation models have become valuable research tools for studying the complex and interactive processes of water and solute transport through the soil profile, as well as the effects of management practices on crop yields and on the environment .
In fact, models have proved to be particularly useful for describing and predicting transport processes, simulating conditions which are economically or technically impossible to carry out in field experiments . Several models have been developed to simulate flow and transport processes, nutrient uptake and biological transformations of nutrients in the soil . HYDRUS 2D/3D has been used extensively for evaluating the effects of soil hydraulic properties, soil layering, dripper discharge rates, irrigation frequency and quality, timing of nutrient applications on wetting patterns and solute distribution because it has the capability to analyse water flow and nutrient transport in multiple spatial dimensions . In the absence of experimental data we can use multidimensional models solving water flow and nutrient transport equations to evaluate the multi-dimensional aspect of nitrate movement under fertigation . However, earlier simulation studies have reported contradictory results on nitrate distribution in soils. For example, Cote et al. reported that nitrate application at the beginning of an irrigation cycle reduced the risk of leaching compared to fertigation at the end of the irrigation cycle. On the other hand, Hanson et al. reported that fertigation at the end of an irrigation cycle resulted in a higher nitrogen use efficiency compared to fertigation at the beginning or middle of an irrigation cycle. These studies very well outlined the importance of numerical modelling in the design and management of irrigation and fertigation systems, especially when there is a lack of experimental data on nutrient transport in soils. However, there is still a need to verify the fate of nitrate in soils with horticultural crops and modern irrigation systems. Therefore, a lysimeter was established to observe water movement and drainage under drip irrigated navel orange, and to calibrate the HYDRUS 2D/3D model against collected experimental data. The model was then used, in the absence of experimental data on nitrate, to develop various modelling scenarios to assess the fate of nitrate for different irrigation and fertigation schemes.The study was conducted on a weighing lysimeter assembled and installed at the Loxton Research Centre of the South Australian Research and Development Institute. The lysimeter consisted of a PVC tank located on 1.2 m × 1.2 m pallet scales fifitted with 4 × 1 tonne load-cells, and connected to a computerised logging system which logged readings hourly.
A specially designed drainage system placed at the bottom of the lysimeter consisted of radially running drainage pipes,nft growing system which were connected to a pair of parallel pipes, which facilitated a rapid exit of drainage water from the lysimeter. These pipes were covered in a drainage sock and buried in a 25-cmlayer of coarse washed river sand at the base of the lysimeter, which ensured easy flushing of water through the drainage pipe. A layer of geo-textile material was placed over the top of the sand layer to prevent roots growing down into it, as this layer was intended to be only a drainage layer. A healthy young citrus tree was excavated from an orchard at the Loxton Research Centre and transplanted into the lysimeter. A soil profile approximately 85 cm deep was transferred to the tank with the tree and saturated to remove air pockets and to facilitate settling. The final soil surface was around 10 cm below the rim of the tank. Soil samples were collected from0 to 20, 20 to 40, 40 to 60, 60 to 85, and 85 to 110 cm depths to measure bulk density and to carry out particle size analysis. Two months after transplanting, the lysimeter was installed amongst existing trees in the orchard. Measurements were initiated after about six months, in order to enable the plant to adjust to the lysimeter conditions. The lysimeter was equipped with Sentek® EnviroSCAN® logging capacitance soil water sensors installed adjacent to the drip line at depths of 10, 20, 40, 60, and 80 cm to measure changes in the volumetric soil water content. Drainage water was directed through flexible piping into a large bin installed below ground level. The experimental site was approximately 240 m from an established weather station, which measured air temperature, relative humidity, wind speed , rainfall, and net radiation.Irrigation was applied using 3 pressure compensated emitters with a discharge rate of 4 L h−1. Emitters were located on a circle 25 cm away from the tree trunk at an equal distance from each other . The irrigation schedule was based on the average reference evapotranspiration during the last 10 years at the site, multiplied by the crop coefficient taken from Sluggett . The cumulative crop evapotranspiration during the 29 day experimental period was equal to 65.3 mm, and daily ETC varied from 1.68 to 3.39 mm. Irrigation was initiated on 16 August 2010 and terminated on 13 September, 2010. Irrigation and rainfall were recorded daily and drainage volume was measured 3 times per week throughout the trial period. Daily irrigation was applied in 5 short pulses using an automated irrigation controller, with 2 h breaks between irrigation pulses. The amount of irrigation water applied was slightly higher than ETC for the period. A total of 70 mm of rainfall fell during the experimental period, including a single event of 52 mm on 3 September 2010.The simulation domain was represented by a 110-cm deep and 100-cm wide cylindrical cross section. Drip irrigation was modelled as a circular line source 25 cm from the centre of the lysimeter with a uniform water flux along the drip line.
This simplification was made to enable HYDRUS to model this problem in a 2D axi-symmetrical mode , rather than in a full 3D mode, which would be computationally much more demanding. Additionally, since the surface wetted area and input flux densities under drippers were dynamic, an option that we would not be able to model with HYDRUS in a 3D mode, we assumed that the simplification of the problem to axi-symmetrical 2D was adequate. Moreover, the drainage system laid out in the lysimeter also supported the use of an axi-symmetrical domain as the drainage pipes run in a circular fashion to collect and flush drainage water out of the lysimeter. The transport domain was discretized into 3294 finite elements, with a very fine grid around the dripper and near the outflow , with gradually increasing element spacing farther from these two locations . Simulations were carried out over a period of 29 days.Since most soils on which citrus is grown in South Australia are coarse textured soils with good drainage, high oxygen levels, low organic matter, and low microbial populations, denitrification and mineralisation was assumed to be negligible in this study. Similarly, the soil adsorption of nitrate was also considered to be negligible since both nitrate and solid surfaces are negatively charged. Plant uptake of non-adsorbing nutrients like nitrate is controlled mainly by mass flow of water uptake . Therefore, it was assumed that nitrate was either passively taken up by the tree with root water uptake or moved downward with soil water. Spatial distribution of nitrate in the transport domain was thus simulated using the convection–dispersion equation for a nonreactive tracer. The longitudinal dispersivity was considered to be 5 cm, with the transverse dispersivity being one-tenth of this . Similar values of these parameters have been used in other studies .Citrus trees in this region are fertilised from early September till March, and in drip systems fertilizers are mostly applied with the second irrigation pulse for the day. All fertigation scenarios reported here are hypothetical. Fertigation was assumed to be supplied with the same quantity of water as in irrigations without fertigation and to conform to the 2D axi-symmetrical domain. For the initial scenario, fertigation pulses were applied from 30 August 2010 at the rate of one fertigation pulse each day. These were followed by 2 days without fertigation and then another daily fertigation pulses. The resultant dose of N for the period from August till September was calculated based on recommended fertilizer application practices for 5–6 year old orange tree. The seasonal recommended dose of nitrogen for an orange tree of this age is 139 g N applied from September to March . Hence for the seasonal simulation, nitrogen was assumed to be applied in equal monthly doses , in similar pulses as described for the experimental period. The simulation was run for 300 days in order to evaluate the fate of seasonally applied nitrogen fertilizer in citrus. Further scenarios examining the impact of timing of nitrogen application on the efficiency of nitrogen uptake simulated a fertilizer application either at the beginning , middle , or end of the daily irrigation scheme. Since the daily irrigation consisted of 5 pulses, fertigation was applied during the 2, 3 and 4 irrigation pulse in the PF1, PF2 and PF3 scenarios, respectively. It is a common practice that the initial and final irrigation pulses are fertilizer free to ensure a uniform fertilizer application and flushing of the drip lines. In addition to these simulations, two continuous fertigation scenarios were also performed to compare pulsed and continuous fertigation.