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4.2 Kiwi Orchard

4.2.1 Experimental Sites

The ET of the kiwi (Actinidea deliciosa) was studied in a mature kiwifruit orchard irrigated daily with micro-sprinklers, in a loamy soil. The experiment took place during 2003 and 2004, in the region of Guimaraes (41°31' N, 8°27' W, 145 m a.s.l.), NW Portugal. The climate of the site is Mediterranean with strong Atlantic influence. Average annual rainfall is 1500 mm concentrated during the colder months. Mean annual temperature is 14.1°C. The plot was 11 ha and was slopping slightly towards the W. The orchard was planted in 1989, with a E-W row orientation and a spacing of 5 m x 5 m for female plants (var. Hayward) and

5 m x 20 m for male plants (var. Matua and Tomuri), planted in the rows between females, every 20 m (T - bar training system). Management of the orchard ground cover ensured that weeds covered a limited part of the soil surface, particularly between the rows. Below the canopy, the ground was maintained in a relatively bare condition because of shade. The bud break occurred in late March and harvesting was by early November. The orchard was daily irrigated at night (3.5 mm/day) with micro-sprinklers and additionally during the day, for the hotter periods.

4.2.2 Measurements

Evapotranspiration (ET) was measured using the eddy covariance (EC) method. Soil evaporation (Es) plus understory transpiration Tu (Esu = Es+Tu) was measured using a set of eight mini-lysimeters. Transpiration (TEC) was calculated as the difference (ET-Esu) and compared with the results from SF approach.

The EC system included a 3-D sonic anemometer and a krypton hygrometer, models CSAT3 and KH2O, respectively (Campbell Scientific Inc., Logan, USA) mounted at the top of a 6 m tower. The fetch was estimated to be 350 m for the dominant wind direction. The measurements were running from 5th to 15 th and from 18th to 25 th August 2003 and from 3rd to 9th and 13th to 18th July 2004.

Net radiation (Rn) was measured at 6 m above the ground with a net radiometer (S-1 -Swissteco Instruments, Oberriet, Switzerland). Soil heat flux (G) measurements were made with 5 soil heat flux plates (HFT-3.1 - Rebs, Seattle, USA) placed at a depth of 5 cm and copper-constantan thermocouples at a depth of 2.5 cm, in a diagonal line between rows. Surface (0-5 cm) soil water content was measured to calculate soil heat capacity, using theta probe sensors, model ML2x (DELTA-T Devices, Cambridge, UK).

Esu was measured, using a set of 8 mini-lysimeters (ML), 15 cm diameter, 20 cm long, from the loss of mass of an undisturbed volume of soil placed accounting for the spatial variability caused by differences in incoming radiation and soil cover, properties and moisture. The MLs were weighted daily and the soil replaced every two days to avoid divergence from the surrounding soil due to changes in root extraction or transport of water within the subsoil.

SF measurements were made with 15 Granier sensors (UP GmbH, Germany): 12 installed on April 2003 (8 of 1 cm length and 4 of 2 cm length) and 3 on July 2004 (1 cm). The average trunk diameter was 10 cm. The length of the Granier sensors was based on the stem diameters and on observations of the mobility of a dyeing solution (safranin, toluidine blue and fast green) applied in the bottom section of a number of cut trunks, in a separate destructive experiment. According to this, probes were likely entirely in contact with the xylem area. It was considered that the entire trunk section below the bark was active for sap transport, with the exception of a small area with a diameter of 1.6 cm at the centre of the stem (Silva, 2002). To determine mass flow at the insertion level, u (Equation [5]) was multiplied by the area of the conducting xylem section per unit of ground area to obtain T (mm s-1), in the following called Tgr.

In this study, the two sets of data for T were compared and an empirical correction adapted from Clearwater et al. (1999) was applied directly to AT (Equation [4]) obtaining AT corrected (ATc) as described in Silva et al. (2007). The correction empirically accounts (one step) for the effect of radial profile and the possible lack of contact with diffuse vessels in the xylem area.

4.2.3 Results and Discussion

The available energy (Rn-G) exceeds the convective fluxes (H+LE), during the EC measurement period, by 27% and 20% in 2003 and 2004, respectively, being within the limits often found for the closure error. Consequently, the convective fluxes obtained from turbulence measurements in the atmosphere are possibly underestimated (Tanner et al., 1985), the real latent heat flux being likely higher than the measured flux.

The verification of the original calibration equation for the Granier method was made during periods of simultaneous measurements of ET and Esu. DT symbols in Figure 11a represent the relationship between 30 minute values for k (Equation [4]) and sap flow density, obtained from ET-Esu. DT symbols in Figure 11b represent the relationship between daily total T, measured by the two approaches (Granier method and ET-Esu). Both for 30 minute and daily values, the sap flow obtained with the original Granier method was seriously underestimated when comparing to T obtained from ET-Esu. There is, however, a good correlation between the two (on a daily scale, r2 = 0.74, Figure 4b).

Figures 11a and 11b (DTc) also show the same relationships after AT correction (DTc values). During the day, at noon, T still is underestimated (Figure 4a), although this can result from tissue capacitance or errors in T estimates (obtained from ET-Esu). For daily values, r2 = 0.87.

Figure 11: (a) Mean k values (Equation [4]) and sap flux density obtained from ET-Esu before the correction (DT) and after the correction (DTc) in AT (30 min values); (b) comparison between total daily T obtained by the Granier method (Tgr) and T obtained from ET-Esu (Tec), before (DT) and after (DTc) correction of AT (from Silva et al., 2007), kiwi orchard, Guimaraes (Portugal).

Figure 11: (a) Mean k values (Equation [4]) and sap flux density obtained from ET-Esu before the correction (DT) and after the correction (DTc) in AT (30 min values); (b) comparison between total daily T obtained by the Granier method (Tgr) and T obtained from ET-Esu (Tec), before (DT) and after (DTc) correction of AT (from Silva et al., 2007), kiwi orchard, Guimaraes (Portugal).

Figure 12 shows daily totals for the T values obtained by the two approaches, after correction for Tgr, suggesting that the Granier method can provide good estimates of stand transpiration, using this correction.

After correction of AT, stand T was determined for the two vegetative periods (Figure 13). T increased from 1 to 3 mm/day, during Spring and early Summer, decreased progressively till November and ended abruptly after leaf fall, following a period of low air temperatures (late November).

Figure 12: Daily T for kiwi (Portugal) using the Granier method, for the periods when the methods were combined in 2003 and 2004, separated by vertical line (TEC transpiration obtained from ET-Esu, Tgr transpiration obtained by the Granier method but after correction of AT (Silva et al., 2007).
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