plots (pears and apples), and the lysimeters are located in the center of each plot. .... The lysimetric station was partially settled with founding from an infrastructure ... 1.20. 1-Jan 31-Jan 2-Mar 1-Apr 2-May 1-Jun. 2-Jul 1-Aug 1-Sep 1-Oct. Date.
Pear Crop Coefficients Obtained in a Large Weighing Lysimeter J. Girona, J. Marsal, M. Mata, and J. del Campo. Institut de Recerca i Tecnologia Agroalimentàries (IRTA) Àrea de Tecnologia Frutícola, Centre UdL-IRTA Keywords: Evapotranspiration, light interception, palmette, canopy cover, ETc, ETo. Abstract Hourly pear water consumption (ETc) was determined with a large weighing lysimeter during 2002. The lysimeter was located in the middle of a 4000 m2 pear plot and it contained 3 adult pear trees (c.v. ‘Conference’), spaced at 4 m x 1.6 m. The Lysimeter container had a cross sectional area of 9.5 m2 and a depth of 1.65 m (minimum). Weight was determined by means of 4 load cells with an individual capacity of 15 t, which gave to the lysimeter a total weight capacity of 60 t. The weighing sensitivity was 0.5 kg what produced a lysimeter sensitivity of 0.053 mm. ETo was determined from an automatic weather station adjacent to the lysimeter, and pear crop coefficients were determined as a ETc/ETo ratio. Tree intercepted radiation (LI) was weekly determined at solar non by using a Ceptometer and correlated to the determined Kc. LI and Kc resulted strongly correlated, indicating the potential capacity of LI to be used for estimating Kc values in pear orchards. INTRODUCTION Orchard water requirements or consumption is a critical issue for irrigation scheduling. The water budget method (Doorenbos and Pruitt, 1977) propose to evaluate crop water requirements from reference evapotranspiration data (ETo) which can be obtained from weather data and represents the evapotranspiration (ET) of a 4 to 10 cm high grass cover. Crop evapotranspiration is the result of multiplying ETo by Kc, being Kc the parameter that adjusts ETo to ETc for any particular growing conditions. ETo determinations are not problematic if the weather stations from which ETo is calculated are appropriately located and their sensors work correctly. However Kc’s, especially in tree orchards, are difficult to obtain, because Kc’s should account for orchard specifications such as, cultivar, tree spacing, orchard orientation, tree size and shape, soil management, fruit load, among others. The complexity in determining the appropriate Kc for each case prompted a specific FAO technical paper (Allen et al., 1998). Non-specific Kc values for tree orchards provided in the technical paper FAO24 (Doorenbos and Pruitt, 1977) differentiated only between two fruit tree groups (decidous and non-decidous). In a field experiment using these Kc’s for pear orchard with trees trained to a palmette system indicated that more fruit yield was obtained by supplying only 70% instead of 100% of full irrigation requirements (Marsal et al., 2002). The latter study did not provide evidence of whether discrepancy between maximum yield and full irrigation requirements was due to a miscalculation of orchard water consumption or to the fact that for pears maximum water consumption does not imply the highest fruit yield. After this discrepancy a large weighing lysimeter was built so that orchard water consumption for pear trees trained to a palmette can be determined. The results reported in this manuscript represent the first significant data obtained for pear.
1
MATERIAL AND METHODS Lysimeter description The IRTA lysimetric station, located within the Mollerussa EEL fields, has two weighing lysimeters and one automated weather station. The equipment is located in two plots (pears and apples), and the lysimeters are located in the center of each plot. The pear orchard was planted in 1999 with cv. “Conference” pear trees on a M-A rootstock. Trees were spaced at 4m x 1.6 m. The weather station is located over grass nearby the lysimeter site 20 m. All this equipment comprised a total area of almost 2 ha (figure 1). The crop coefficients values (Kc) were determined as: Kc = ETc / ETo, where ETc is obtained directly from the lysimeters, and ETo from the weather station according to Penman-Monteith equation. Each weighing lysimeter has a 17 m3 container, 1.70 m depth and a real area of 9.5 m2 (2 x 4.8 m). Depth of inhabitable space is 2.40 m. The weighting system is composed by 4 load cells with a unit capacity of 15 t, so in total the system can weight up to 60 t. The system sensitivity is 0.5 kg which allow us to detected water consumption of 0.053 mm. The system is composed by a cement construction with a hole where to locate the lysimetric container and another, smaller, that allows the access to the inhabitable zone. The container was filled with the same soil displaced to create the cavity for the lysimeter. The original soil layers were preserved during the procedure of filling the container but soil bulk density was not attempted to reproduce in order to avoid problems of compactation. Three trees were planted on top of the container at the same plant distance as the surrounding orchards (figure 2B). Load cells are connected to a logger that continuously records the weight of each container. The weight difference between hours indicate the water used by the system within one hour (which includes soil evaporation and plant transpiration). Hourly weight and water consumption values were both recorded. One drainage (200 L) and two irrigation (100 L each) recipients hanged from the lysimetric containers. The irrigation recipients were daily refilled at night with a similar amount of water lost the day before, and this water was used to irrigate the lysimeter (figure 2A). In this way, the system allowed to determine of crop evapotranspiration, without any interference from drainage water or irrigation. Control procedures Recorded data were daily verified and the entire system was checked weekly to ensure accurate measurements. In addition, light interception (LI) was determined weekly using a ceptometer (Accupar, Decagon Devices Inc., Pullman, Wash., USA). Ceptometer data were collected both under the tree canopy (32 readings at fixed positions) and above the canopy (2 measurements). RESULTS AND DISCUSSION Diurnal patterns of ETc and ETo in pear trees were very similar (figure 3) especially when some variability in hourly water consumption was corrected by adjusting the data to a quadratic line. On a seasonal basis, Kc values (caluclated from daily observed data in water consumption) had a well defined pattern (figure 4). This pattern was obtained by following the philosophy of the procedure proposed by Allen et al.(1998)
2
in which only the lowest Kc are included in the adjustment so that the evaporation component from orchard water consumption is minimized. In irrigation days or with rainfall, soil evaporation typically increases and so does Kc. Therefore, the procedure used in this study represent an approach closer to what a Kcb means as defined by Allen et al. (1998). Kc’s values corresponding to the dormant period were of 0.22 and increased proportionally with leaf appraisal and shoot growth phase until reaching values at midseason of 0.85. During the period of highest water consumption Kc’s were 30% lower than the value of 1.1 proposed by FAO for the same period (Doorenbos and Pruitt, 1977). This low mid-season Kc values could be due for one side to the reduced ground cover related to the training system used, and perhaps to a certain role in canopy architecture features being different in the palmette system as compared to other training systems such as the vase. Corrections for insufficient shaded area (
