UC Riverside

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.