Using FAO-56 model to estimate soil and crop water status: Application to a citrus orchard under Regulated Deficit Irrigation Giuseppe Provenzano1, Pablo Gonzàles-Altozano2, Juan Manzano-Juàrez2, and Giovanni Rallo3 Università degli Studi di Palermo, Dipartimento Scienze Agrarie e Forestali. Viale delle Scienze 13, 90128 Palermo, Italy (giuseppe.provenzano@unipa.it) Universitat Politècnica de València, Departamento de Ingenieria Rural y Agroalimentària. Camino de Vera s/n, 46022 Valencia, Spain Università di Pisa, Dipartimento Scienze Agrarie, Alimentari e Agro-ambientali. Via del Borghetto 80, 56124 Pisa, Italy EGU2015-2884 -----------------------------------------------------------------------T-100%-------------------------------------------------------------------------- Introduction and Objectives Agro-hydrological models are considered an economic and simple tool to quantify crop water requirements, whereas Regulated Deficit irrigation (RDI) is potential strategy for sustainable crop production in regions characterized by water scarcity. Accurate evaluations of crop water status and actual transpiration fluxes play a key role in irrigation scheduling under water saving strategies, in order to avoid that too severe crop water stress can become detrimental to yield and fruit quality. Objective of the research was to assess the suitability of FAO56 agro-hydrological model (Allen et al., 1998) to estimate soil and crop water status on citrus orchards under different water deficit conditions. The ability of the model to predict actual crop water stress was evaluated on the basis of similarities between the temporal dynamic of simulated relative crop transpirations, RT, and measured midday stem water potentials, MSWP. During dry periods, relative crop transpirations were then correlated to MSWPs, with the aim to assess the model ability to predict crop water stress and to identify “plant-based” irrigation scheduling parameters. Material and Methods Experiments were carried out during three years from 2009-2011 in Senyera, Spain (39º 3’ 35.4” N, 0º 30’ 28.2” W) in a commercial orchard planted with Navelina/Cleopatra citrus trees. Three RDI treatments were considered: in the first (control, T-100%), irrigation doses (Id) were determined according to crop evapotranspiration and precipitation data obtained from a meteorological station installed nearby the plot, whereas in the other two, water application was reduced to 40%Id (T- 40%) and 60%Id (T-60%) only during the initial fruit enlargement phase (July-August), being the plots irrigated at 100%Id during the remaining periods. In all the treatments, soil water content was monitored along a soil profile with a FDR probe (Enviroscan, Sentek) and crop water status by means of MSWPs, measured with a Scholander chamber (Solfranc SF-Pres-35) on leaves wrapped in bags at least 2 hours before measurements. Soil and crop parameters (available water, tree height, root depth and fraction cover) were measured in each treatment, whereas crop coefficients were obtained from the FAO56 paper (Allen et al., 1996). Simulations were run during the three investigated years (DOY 1 - DOY 365), by assuming a soil water content of 0.27 m3 m−3 (field capacity) at the begin of simulations, as consequence of the copious rainfall occurred during each antecedent period. Results and Discussion Fig. 3 shows the results of model validation (from 2009 to 2011) obtained for the considered treatments (T-40%, T-60% and T-100%). For each of them, water supplies (precipitation and irrigation), temporal dynamics of measured and simulated soil water contents (SWC), relative transpiration (RT), midday stem water potential (LWP), as well as solar radiation (Rg) and reference evapotranspiration (ET0) are shown by proceeding from up to down. It was observed that FAO56 model simulates with a reasonably good accuracy (errors lower than 0.2 m3 m-3), acceptable for practical applications, the average soil water content in the root zone. In treatments T-40% and T-60%, in which RDI strategies were adopted, the relative transpiration simulated by the model, representing an index of crop water stress, generally follows the seasonal trend of midday stem water potential and allows to identify the trend of actual crop water status as recognized in the field. This latter result confirms that FAO56 model is able to properly reproduce, for the investigated crop, even the crop water stress conditions recognized in the field. Fig. 4 shows, for each monitored soil layer, experimental MSWPs as a function of the corresponding soil water status expressed in terms of soil water content (SWC) and Depletion (De). The latter represents the water height required to bring soil water content in the root zone at field capacity. Even though it was quite difficult to identify unambiguous thresholds of soil water content, due to the evident dispersion in the upper soil layers, a SWC≈20% (De≈85 mm) can be considered a critical threshold separating two different plant behaviors: for SWC>20%, MSWPs are approximately constant and equal to 10 bar (absolute value), identifying a condition of negligible water stress (González-Altozano and Castel, 1999). On the other hands, for SWC<20%, the lower the soil water content the smaller the MLWP, as consequence of the progressively increasing crop water stress. -------------------------------------------------------------------------T-60%--------------------------------------------------------------------------- Fig. 4 -Experimental values of Midday Stem Water Potential (MSWP) and corresponding soil water status expressed in terms of soil water content, SWC, and Depletion, De. --------------------------------------------------------------------------T-40%-------------------------------------------------------------------------- Conclusions The suitability of FAO56 agro-hydrological model to estimate citrus orchard soil and crop water status under regulated deficit irrigation was investigated. Model performance was assessed by means a long series of soil water content measurements continuously acquired during three years of field observations. The model simulates with a good accuracy, acceptable for practical applications, the average of soil water contents in the root zone, with errors generally lower than 0.2 m3 m-3. Values of SWC higher than about 20% (De≈85 mm) identify conditions of negligible water stress. On the other hands, for SWC lower than 20%, midday stem water potentials decrease, as consequence of the progressively increasing crop water stress. Fig. 1 - Midday Stem Water Potential measured on wrapped leaves with Scholander Chamber (SF-Pres-35, Solfranc) Fig. 2 - Soil water content measured with Envirocan SM probe (Sentek) References Allen, R.G, Pereira, L.S., Raes, D. and Smith,M., 1998: Crop evapotranspiration - Guidelines for computing crop water requirements - FAO Irrigation and drainage paper 56, FAO, Rome. González-Altozano, P., Castel, J.R. (1999). Regulated Deficit Irrigation in „Clementina de Nules‟ citrus trees. I. Yield and fruit quality effects. Journal of Horticultural Science & Biotechnology 74(6): 706-713. Fig. 3 - Validation of FAO56 model