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Soil water content measured by FDR probes and thresholds for drip irrigation management in peach trees 317In situ determination of apparent soil water saturation(aSWsat)While irrigation continued, the water content at each soillayer increased progressively up to a constant value whichwas maintained during a long time (more than 24 h) untilirrigation was switch off (Figure 1). At this constant soil watercontent, the soil pores within the volume of influence of thesensors were quasi saturated and the water flow had reacheda steady state (Jury et al., 1991). The repeating pattern ofnearly constant maximal soil water content following severalabundant irrigation events indicated an in situ apparentsoil water saturation (aSWsat). Under our experimentalconditions the aSWsat for each sensor was about 0.32, 0.35,0.35 and 0.29 cm 3 cm -3 at 0.10, 0.20, 0.30 and 0.50 m depths,respectively. Accordingly, the cumulative soil water contentin the 0-0.50 m soil depth, called soil water store (SWS), was162 mm 0.5 m -1 . The determination of aSWsat helps for theestablishment of irrigation strategies that minimize drainageto deeper soil layers (Starr and Paltineanu, 1998).In situ determination of field capacity within the wettedbulb (FC b)After the cessation of irrigation (Figure 1), at midnight of thesecond day, the soil water content decreased markedly during2-3 hours due mainly to the redistribution of the gravitationalwater, then soil water content at each sensor depth showeda tendency to stabilize at a constant value because the waterredistribution was decreasing significantly. The soil watercontent under these conditions fit well with the conceptof “field capacity” defined as “the amount of water heldin soil after excess water has drained away and the rate ofdownward movement has materially decreased” (Veihmeyerand Hendrickson, 1949). Thus, the field capacity valuesdetermined for each sensor were 0.26, 0.27, 0.30 and 0.25cm 3 cm -3 at 0.10, 0.20, 0.30 and 0.50 m depth, respectively.The SWS at the 0-0.5 m soil depth at field capacity was 136mm 0.5 m -1 .Plant water uptakeOn the third day, from sunrise until mid afternoon, a rapiddecrease in the soil water content was detected by the differentcapacitance sensors (Figure 1). However, the magnitude ofthe diurnal soil water loss was more evident in the first layer(0.10 m depth), less important in the subsequent soil layersand negligible at the deepest one (0.50 m depth), whichindicates the depth of the active root zone as well as theintensity of root water uptake at each soil layer. In general,root development under drip irrigation is constrained to thesoil volume wetted by the emitter, near the soil surface withroot length density decreasing with depth (Michelakis et al.,1993; Stevens and Douglas, 1994).Thresholds for irrigation schedulingThresholds for irrigation scheduling are practical guide linesto determine the quantity of water and irrigation frequencythat supply the plant needs without producing undesiredwater losses. In agreement with the observations abovedescribed the continuous real time measurements of the soilwater dynamics provided useful information for irrigationmanagement such as: the advance of the wetted front, theapparent soil water saturation and the depth of the plant activeroot zone. Accordingly, two limits could be established toprecisely determine the dose and frequency of irrigation:- Lower limit or “Refillpoint”; it corresponds to the SWSat which irrigation should be resumed before the plantsundergo water stress. The amount of water allowed to bedepleted from the soil profile before resuming irrigationdepends, among others aspects, on the crop, the cultivar, thephenological stage and on the intensity of the evaporativedemand. For these reasons, exhaustive local studies shouldbe carried out to determine the manageable allowable deficitfor each crop. A refillpoint” equal to 90% of the field capacitymight be practical for nearly all crops as it is equivalent to amanageable allowable deficit lower than most of the valuesrecommended by FAO (Allen et al., 1998).- Upper limit or “fullpoint”, which corresponds to the SWSvalue recorded when the wetted front reaches the bottom ofthe active root zone during a nightly irrigation event. Thatcorresponds roughly to water content between aSWsat andFC b. This may lead to decreasing percolation of water beneaththe active root zone in excess of any required leaching forsalinity management.Therefore, the irrigation dose is related to the time neededto raise the SWS from the refillpoint up to the fullpoint,whereas the irrigation frequency is determined by the effectof the evaporative demand (ETc) to deplete the SWS fromthe fullpoint down to the refillpoint.The dynamic of the SWS considering several irrigation eventsis shown on Figure 2. The SWS represents the cumulativewater content within the active root zone (0-0.50 m depth)
318 Agric. Téc. Méx. Vol. 34 Núm. 3 Julio - Septiembre 2008 Oussama Hussein-Mounzer et al.which is the sum of the sensors’s values at 0.10, 0.20, 0.30and 0.50 m depths plus interpolated values at 0.40 m depth.The predetermined irrigation scheduling limits and thedynamic of the soil water content at the bottom of the activeroot zone are also shown. Each rapid rise in the soil waterstored corresponded to an irrigation event. The first abundantirrigation induced saturated conditions down to the deepestsensor and produced percolation beneath the root zone.Soil water content (cm 3 cm -3 ) Soil water store (mm 0.5) m -11701601501401301201100,320,300,280,260,240,221 2 3 4 5 6 7 8 9 10 11DaysFigure 2. Changes in: A) the soil water store (SWS)within 0-50 cm depth, and B) in the soil watercontent at the bottom of the active root zone at50 cm depth). The horizontal lines correspondto the upper limit or “fullpoint” (thick line FP),field capacity (thin line FC) and lower limit or“refillpoint” (dash-dot line RP). The dashedarrows represent the tendency of the soil waterstore toward field capacity.The subsequent irrigation events were guided by the irrigationscheduling limits. On days 4, 5, 6 and 7 the quantity of waterapplied refilled the store up to its upper limit without increasingthe water content at the lowest layer. On days 8, 9 and 10 thesoil water content at 50 cm depth started to increase graduallyas irrigation doses raised the store over its upper limit. Afterevery irrigation event and the following water distributionwithin the soil layers the soil water content showed a tendencytoward the FC (Figure 2, dashed arrows).Accordingly, any irrigation event that did not increase the soilwater store up to its upper limit can be considered deficientFPFCRP4321Emitter discharge rate (L 15 min -1 )since it did not wet the profile of the active root zone, whereasthe soil water store above the “fullpoint” can be interpretedaccording to the moment of the irrigation event:- During a nightly irrigation event, it indicated an excess ofapplied water.- During a daily irrigation event, under high evaporativedemand ETc, it indicated that the soil pores within the sensors’volume of influence were nearly saturated even before thewetted front reached the deepest sensor. A low unsaturatedhydraulic conductivity on the limits of the wetted bulb, witha high rate of water absorption by the root system slows downthe advance of the wetted front.On the other hand, it is important to note that graphicaldetermination of irrigation thresholds minimizes theimportance of small fluctuations in soil water content.Independently of the value of soil water content, the soilsaturation and deep percolation events can be easily detectedand corrections made on time to maintain the crop at theoptimum irrigation condition and water use efficiency at itsmaximum.Soil and plant water statusThe meteorological conditions (ETo and rain), the plant waterstatus (stem water potential, Ψ x) and the irrigation eventstogether with the variation of the daily average of soil waterstore (SWS), measured with the multisensor capacitanceprobes, across the growing season, are shown in Figure 3.Each sudden increase in the SWS corresponds to an irrigationor rain event and the subsequent daily decreases are mainlydue to crop evapotranspiration.Under low climatic demand and frequent rain events, theΨ xwas similar (-0.5 MPa) for both treatments from April tolate May even the SWS was close or below the refillpoint.From June to late August the high evapotranspiration values(ETo > 6 mm d -1 ) induced a gradual decrease of the Ψ xwhilethe leaf area index was increasing (Mounzer, 2005). In July,the Ψ xdecreased down to -1.0 and -1.2 MPa for T1 and T2,respectively, pointing out how high evapotranspiration caninduce stress effects on the plant even to well watered trees(Figure 3). These observations are in accordance with thestudies of Lampinen et al. (1995), Intrigliolo and Castel(2004) and with the results of McCutchan and Shackel (1992),which underlined that under non limiting SWS, the valuesof stem water potential higher than -1.0 MPa are due to high
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