EGU2014‐3151 USING HYDRUS 2‐D TO ASSESS EMITTERS OPTIMAL POSITION FOR EGGPLANTS UNDER SURFACE AND SUBSURFACE DRIP IRRIGATION Hiba Ghazouani 1,2 , Dario Autovino 1 , Boutheina Douh 2 , Abdel Hamid Boujelben 2 , Giuseppe Provenzano 1 , and Giovanni Rallo 1 1)Dipartimento Scienze Agrarie e Forestali, Università degli Studi, viale delle Scienze 13, 90128 Palermo, Italy. 2)Département du Génie des Systèmes Horticoles et du Milieu Naturel, Institut Supérieur Agronomique de Chott Mériem (ISA‐CM), BP 47, Sousse 4042, Tunisie SWRC for the four investigated layers and average curve for the entire soil profile. Comparison between SWCs measured by TDR and corresponding simulated at 15, 45 and 75 cm depths, and 0, 20 and 40 cm far from the emitter for DI (left side) and SDI (right side). Irrigation water use efficiency 0 1 10 100 1000 0.0 0.1 0.2 0.3 0.4 0.5 Soil water content [m 3 m -3 ] Soil matric potential [m c. a.] 0-20 cm 30-50 cm 60-80 cm van Genichten model θr θs α n n m 1 1− = R 2 0.08 0.40 0.76 1.60 0.37 0.95 Fig. 1 TRIME-FM TDR used for soil water content measurements. In Tunisia, the rapid increase of irrigated area and agricultural intensification are contributing to the persistent rising of water demand and, due to the arid or semi- arid climate and the frequent periods of drought, the country needs to face with severe water shortage. Micro-irrigation systems are increasing their popularity in the country because of the high water use efficiency. Subsurface drip irrigation, with laterals installed below the soil surface and in proximity of plant roots, represents one of the most advanced micro-irrigation method, used to supply water directly to the roots, while preserving a relatively dry soil surface, so to reduce evaporation losses. Agro-hydrological models represent an attractive tool to predict soil water dynamic and to provide guidelines for plant design and for optimizing irrigation water use (Rallo et al., 2012). However, due to the simplifying assumption in their theoretical development, as well as the high number of required variables related to soil, plant and external environment, such models need to be validated, before any other successive use. INTRODUCTION OBJECTIVES Objective of the work is to assess the performance of Hydrus-2D model to predict soil water contents in the root zone, under traditional and subsurface drip irrigation (DI) systems, for Eggplant crop (Solanum melongena L.). The performance was initially evaluated on the basis of the comparison between measured and predicted soil water contents. Then, the model was applied in order to analyze the effects of different drip line depths on the terms of water balance and to choose the best position of the lateral aimed to optimize water use efficiency. MATERIALS AND METHODS Experiments were carried out, from April to June 2007, at Institut Supérieur Agronomique de Chott Mériem (Sousse, Tunisia). Plants were spaced 0.40 m along the row and 1.2 m between the rows and irrigated with traditional and subsurface DI, by means of laterals with 0.40m spaced coextruded emitters, discharging a flow rate equal to 4.0 l h -1 at 100 KPa. Spatial and temporal variability of SWCs were acquired with a Time Domain Reflectometry (TRIME-FM TDR) on four 80 cm long access tubes, installed along the directions perpendicular to the plant row, at distances of 0, 20, 40 and 60 cm from the lateral. Irrigation water was supplied, accounting for the rainfall, every 7-10 days at the beginning of the crop cycle (March-April) and approximately once a week during the following stages till the harvesting (May- June), for a total of 10 one-hour watering (40 l/plant). For model application, soil evaporation, Ep, and crop transpiration, Tp, were determined according to the modified FAO Penman-Monteith equation and the dual crop coefficient approach (Allen et al., 1998). Soil Water Retention Curves (SWRC) were determined by hanging water column apparatus for matric potentials h ranging from -0.05 to -1.5 m and by pressure plate apparatus for h values of -3.37 m, -10.2 m, -30.6 m, and -153.0 m, by using respectively undisturbed soil samples, 0.08 m diameter and 0.05 m height collected in the layers 0-20, 30-50 and 60-80 cm and sieved soil samples 0.05 m diameter and 0.01 m height. RESULTS AND DISCUSSION ”TAKE HOME” MESSAGES The van Genuchten model (van Genuchten, 1980) was used to fit experimental data, with function parameters estimated by SWRC Fit (Seki et al., 2007). Saturated hydraulic conductivity was measured with a constant head permeameter on the same undisturbed soil samples, by following the Darcy assumptions. Water stress function for eggplant was represented by means of Feddes linear model, whose parameters were defined according to Taylor and Ashcroft (1972). Rooting system parameters were experimentally determined on the basis of field observations and in particular of the maximum depths, the depth at which root density is maximum and the maximum root horizontal extension. With reference to the SDI system, simulation domain was assumed 80 cm depth and 40 cm wide, with a single emitter characterized by a radius of 1.0 cm, located to a depth of 25 cm. A time-invariant flux density of 318 cm h -1 , corresponding to the emitter discharge of 4.0 l h -1 , was assumed at the emitter boundary surface during irrigation, whereas the absence of flux was considered during each redistribution process following irrigation. Atmospheric boundary condition was considered on the soil surface, as well as the absence of flux along the lateral and the lower borders of the simulation domain. Fig. 2 Position of access tubes for TDR sensor, as equipped in both DI and SDI systems In the upper right figure, the comparison between measured and simulated SWCs is showed for both treatments (DI and SDI). As can be observed, Hydrus-2d allows to well simulate the dynamic of punctual SWCs around an emitter during irrigation season. In terms of average values, it is possible to notice that simulated values are located in the range of variability of the corresponding measured. The value of Root Mean Square Error (RMSE) shows that for both treatments, the model simulates soil water contents, for the different lateral positions, with errors always lower than 4%. DI SDI_5 SDI_15 SDI_20 SDI_45 Before irrigation Irrigation 24 h after irrigation 226 h after irrigation 266 h after irrigation Distribution of SWCs corresponding to different emitter depths, before and after irrigation The optimal emitter depth was determined by simulating five different scenarios, with the emitter laid on the soil surface (DI) and buried at 5, 15, 20 and 45 cm depths (SDI_5-SDI_45). For each scenario, water use efficiency (WUE), defined as the ratio between actual transpiration and total amount of water provided during the entire growing season, was determined. Figure on the left side shows the distribution of SWC corresponding to the different emitter depths before and after irrigation, whereas on the right side WUE is indicated for the different scenarios. As can be observed, WUE is maximum when emitter depth is between 5 and 20 cm (optimal installation depth). On the other side, when emitter is laid on the soil surface WUE is limited by soil evaporation, whereas for the higher depths is affected by the deep percolation. van Genuchten model Drip Irrigation Subsurface Drip Irrigation Under the investigated conditions, Hydrus-2d allows to well simulate the dynamic of punctual SWCs around an emitter during the irrigation season. Water Use Efficiency significantly increases when the emitter is buried at depths between 5 and 20 cm, as a consequence of the reduction of soil evaporation. Installation depths equal to 45 cm or higher increase of deep percolation and limit root water uptake, REFERENCES Rallo, G., C. Agnese, M. Minacapilli, G. Provenzano. 2012. Comparison of SWAP and FAO Agro- Hydrological Models to Schedule Irrigation of Wine Grapes. J. Irrig. and Drain. Eng. 138(7), 581- 702. Taylor, S.A., Ashcroft, G.M. 1972. Physical Edaphology. Freeman, San Francisco, 434-435. van Genuchten, M.Th. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892–898. Šimůnek, J., M. Th. van Genuchten, M. Šejna. 2011. The HYDRUS Software Package for Simulating Two- and Three-Dimensional Movement of Water, Heat, and Multiple Solutes in Variably-Saturated Media. Technical Manual, Version 2.0, PC Progress, Prague, Czech Republic, pp. 258. I.S.A CHOTT-MARIEM