Water use and salinity dynamics in the soils are the crucial management factors influencing the productivity and long-term sustainability of almond and associated environment. In this study, HYDRUS-2D was calibrated and validated on measured spatial and temporal water contents and soil salinities (ECe) distributions under almond irrigated with different water qualities (ECiw) at different physiological stages. During two irrigation seasons (2014–15 and 2015–16), less saline irrigation water (average ECiw 0.78 dS/m) was substituted for recycled irrigation water (average ECiw 1.9 dS/m) in three phenologically different growth stages; pre-pit hardening, kernel growth, and post-harvest, along with no and full substitution during the entire season. Graphical and statistical comparisons (RMSE, MAE, ME, the Nash and Sutcliffe model efficiency, and the coefficient of determination) between measured and simulated values of water contents and ECe in the soil showed a close agreement in all treatments. The water balance data revealed that the seasonal crop evapotranspiration of almond (ETc) varied from 850 to 955 mm in different treatments over the two seasons which represented 68–79% of the water application. Trees irrigated with only less saline water through the two seasons (average ECiw 0.78 dS/m) showed 10% higher plant water uptake as compared to those irrigated with recycled water only (average ECiw 1.9 dS/m). Substituting less saline irrigation during the kernel growth phase, between pit-hardening and harvest, showed greater water uptake by almond and lower salinity buildup in the soil as compared to treatments that substituted less saline irrigation early or late in the season. For all treatments, the average daily root zone ECe (2.4–3.7 dS/m) remained above the level of the almond salinity tolerance threshold (ECe = 1.5 dS/m) throughout the period of investigation. Water use efficiency of almonds varied in a narrow range (0.21–0.25 kg m−3) for different treatments. Deep drainage below the root zone (2 m) varied from 22.4–31.1% of the total water application (Rainfall + Irrigation), which was episodic and insufficient to contain the salinity below the almond threshold. This study provided a greater understanding of soil water and salinity dynamics under almond irrigated with waters of varying qualities.

Partitioning of evapotranspiration and estimating of irrigation contribution to evaporation play a crucial role in managing scarce water resources and help in increasing the water productivity of crops, especially of sparsely vegetated plants. In this study, the FAO-56 dual crop coefficient (DCC) approach for estimating evaporation from soil under cropped conditions is adapted for subsurface drip irrigation (SDI). This new approach involves one additional variable, the fraction of the irrigation depth contributing to evaporation (fI,Es), which was defined and integrated into the FAO-56 equations for estimating daily water balance from the evaporation layer (0–15 cm). Impacts of soil texture, heterogeneity, irrigation depth, design parameters of the irrigation system on fI,Es, and the fraction of the soil surface wetted by irrigation (fw) (and consequently the exposed and wetted fraction (few)), were evaluated through HYDRUS-2D simulations. The modified procedure was compared with the existing FAO-56 method for estimating components of annual ET for wine grape under SDI. The model simulations showed that the fI,Es fraction in a homogeneous, isotropic light-textured soil was minimal (0.04) when SDI was placed at a depth of 25 cm. However, in medium and heavy textured soils fI,Es was 4 times larger than in light-textured soils. The value of fw was slightly higher in fine-textured (0.09) than in medium-textured soils (0.07). In Duplex soils with two contrasting textural layers, fw (0.12–0.16) was higher due to the presence of a heavy-textured soil layer just below the drip line. Similarly, in Triplex soils (3 different textural layers), placing the drip line in the middle layer effectively reduced both fI,Es and fw close to zero. In contrast, fw (0.18–0.30) and fI,Es (0.28–0.42) both increased considerably in heterogeneous soils. Both fractions (fw and fI,Es) increased with an increase in irrigation depths, except for fI,Es in loamy sand. The fractions were slightly lower when a drip line was placed at a depth of 10 cm (an evaporation zone) than when it was placed on the soil surface. Applying the same amount of water with different discharge rates had little impact on fI,Es and fw fractions. An increase in the drip line spacing proportionally decreased the wetted fraction on the soil surface. Annual evaporation for SDI irrigated wine grapes at the field study site, estimated using the existing FAO-56 procedure, was overestimated by about 5–6% compared to using the modified procedure. However, this deviation between the two approaches increased (18%) for heavier soil textures. It is concluded that the existing FAO-56 procedure needs to be adjusted when used to estimate evaporation under subsurface drip irrigation. However, the impact of the proposed modification on evaporation needs further evaluation under other crops, soils, and climatic conditions.

Estimation of all water fluxes temporally and spatially within and out of the crop root zone, and evaluation of issues like salinity and nutrient leaching, are necessary to fully appraise the efficiency of irrigation systems. Simulation models can be used to investigate these issues over several seasons when the cost of long term monitoring is prohibitive. Model results can be used to advise growers if improvements are required to various aspects of irrigation system operations. In this study, HYDRUS-2D was used to evaluate data measured during one season in a young mandarin ( Citrus reticulata) orchard, irrigated with an intensive surface drip fertigation system. Water contents, salinities, and nitrate concentrations measured weekly in the field were compared with model predictions.The temporal mean absolute error (MAE) values between weekly measured and simulated water contents ranged from 0.01 to 0.04cm3cm-3. However, modelling error (MAE) was slightly larger at 10cm depth (0.04cm3cm-3), as compared to greater depths (0.02-0.03cm3cm-3). Similarly, the errors were larger in the surface soil layer (25cmdepth) for nitrate-nitrogen, NO3--N (1.52mmol(c)L-1), as compared to greater depths. The spatial and temporal soil solution salinity (ECsw) and NO3--N data showed accumulation of salts and nitrate within the soil up until day 150 of the simulation (December, 2006), followed by leaching due to high precipitation and over irrigation at later times. Only 49% of applied water was used by the mandarin trees, while 33.5% was leached. On the other hand, the simulation revealed that a significant amount of applied nitrogen (85%) was taken up by the mandarin trees, and the remaining 15% was leached. The results indicate that the irrigation and fertigation schedule needs modifying as there was overwatering from December onwards.Different permutations and combinations of irrigation and fertigation scheduling were evaluated to optimise the water and nitrogen uptake and to reduce their leaching out of the crop root zone. Slightly higher nitrogen uptake (1.73kgha-1) was recorded when fertigation was applied second to last hour in an irrigation event, as compared to applying it earlier during an irrigation event. Similarly, a 20% reduction in irrigation and N application produced a pronounced reduction in drainage (28%) and N leaching (46.4%), but it also decreased plant N uptake by 15.8% and water uptake by 4.8%, and increased salinity by 25.8%, as compared to the normal practice. This management would adversely impact the sustainability of this expensive irrigation system. However, reducing only irrigation by 30% during the 2nd half of the crop season (January to August) reduced drainage and N leaching by 37.2% and 50.5%, respectively, and increased N uptake by 6.9%. Such management of irrigation would be quite promising for the sustainability of the entire system. It is concluded that judicious manipulations of irrigation and fertilizer applications can be helpful in designing drip irrigation schedules for perennial horticultural crops to achieve improved efficiency of irrigation and fertigation applications and reduced contamination of receiving water bodies. © 2014 Elsevier B.V.

Adoption of high input irrigation management systems for South Australian horticultural crops seeks to provide greater control over timing of irrigation and fertilizer applications. The HYDRUS 2D/3D model was used to simulate water movement in the soil under an orange tree planted in a field lysimeter supplied with 68.6mm of irrigation water over 29 days. Simulated volumetric water contents statistically matched those measured using a capacitance soil water probe. Statistical measures (MAE, RMSE, tcal) indicating the correspondence between measured and simulated moisture content were within the acceptable range. The modelling efficiency (E) and the relative efficiency (RE) were in the satisfactory range, except RE at day 19. Simulated daily and cumulative drainage fluxes also matched measured values well. Cumulative drainage flux was 48.9% of applied water, indicating large water losses even under controlled water applications. High drainage losses were due to light texture of the soil and high rainfall (70mm) during the experimental period. Simulated root water uptake was 40% of applied water.The calibrated HYDRUS model was also used to evaluate several scenarios involving nitrate fertigation. The numerical analysis of NO3-N dynamics showed that 25.5% of applied fertilizer was taken up by the orange tree within 15 days of fertigation commencement. The rest of the applied NO3-N (74.5%) remained in the soil, available for uptake, but was also vulnerable to leaching later in the growing season. The seasonal simulation revealed that NO3-N leaching accounted for 50.2% of nitrogen applied as fertilizer, and plant N recovery amounted to 42.1%. The scenario analysis further revealed that timing of a nitrogen application in an irrigation event had little impact on its uptake by citrus in the lysimeter. However, slightly higher NO3-N uptake efficiency occurred when fertigation was applied late in the daily irrigation schedule, or was spread out across all irrigation pulses, rather than being applied early or in the middle. Modelling also revealed that pulsing of irrigation had little impact on nitrate leaching and plant uptake. Applying less irrigation (50% or 75% of ETC) resulted in higher nitrate uptake efficiency. This study showed that timing of water and fertilizer applications to an orange crop can be better regulated to enhance the efficiency of applied inputs under lysimeter conditions. © 2013 Elsevier B.V.

Accurate estimation of evapotranspiration (ET) and its partitioning into transpiration and evaporation is fundamental for improving water management practices in water-limited environments and under deficit irrigation conditions. This investigation was conducted to estimate the water balance and ET components of subsurface drip (SDI) irrigated Chardonnay wine grapes for two seasons (2010–2011 and 2011–2012) using a numerical model (HYDRUS-2D). Treatments involved the application of different volumes [51% (I1), 64% (I2), 77% (I3), and 92% (I4) of normal application] of water for irrigation. A modified version of the FAO-56 dual crop coefficient approach was used to generate daily transpiration and evaporation as inputs to the HYDRUS-2D model. The calibrated and validated model produced estimates of actual evapotranspiration (ETCact), actual transpiration (Tpact), and actual evaporation (Esact), and deep percolation under varied irrigation applications. The model-simulated values were then used to estimate actual crop coefficients (Kcact and Kcbact), and water productivity of wine grape under different deficit irrigation conditions. Seasonal ETCact simulated by HYDRUS-2D for different treatments varied between 239 and 382 mm. However, seasonal evaporation accounted for 44–59% of seasonal ETCact losses in different treatments. The modelled daily transpiration rate in I4 treatment (Tp4act) varied from 0.11–2.74 mm/day. Deep percolation accounted for 35–40% of the total water applied by rainfall and irrigation. The mean value of actual crop coefficient (Kcact) estimated by HYDRUS-2D simulated ETC act over the two seasons was 0.27, which matched with other investigations. Similarly, values of Kcbact for initial, mid and end stages were 0.13, 0.27 and 0.14, respectively. Monthly values of evaporation coefficient (Ke) ranged from 0.1 to 0.32, with a mean value of 0.18. Water productivity with respect to ET losses (WPETC) ranged from 5.9 to 6.2 kg/m3 of water use. However, water productivity for transpiration (WPTC) almost doubled as compared to WPETC in all treatments. The impact of deficit irrigation on berry juice composition (Brix, pH and titratable acidity) was lower than the inter-seasonal variability. These results can help develop better irrigation management strategies for SDI irrigated wine grapes under water scarce conditions.