School of Agriculture, Food and Ecosystem Sciences - Theses
Permanent URI for this collection
Search Results
1 - 2 of 2
-
ItemPeach tree water use( 2004)Peach tree water use (TWU) was measured and simulated in order to improve both current understanding of water use and to devise a practical irrigation scheduling method for orchards in northern Victoria. TWU was appraised in terms of a basal crop coefficient (Kcb) and reference crop evapotranspiration (ETo). Weighting of ETo by measures of vegetative ground cover and solar radiation interception were explored as techniques to account for changes in TWU associated with tree size, row orientation and tree training method. Sap flow (SF), using the compensation heat pulse technique, was evaluated as a method to measure TWU. Field experiments explored the variation in sap velocity (SV) in the trunk of peach trees and the accuracy of the technique to measure TWU. Sap velocity varied radially across the sapwood so a sap velocity profile was used to calculate sap flow (SF) from measurements made at specific depths in the sapwood. Analysis of the variation in SF around the circumference of a tree revealed the need for at least four sensors installed at different positions in the trunk. Calibration using the cut-tree method established the relationship: TWU = 1.44 (� 0.04) SF. A short-term experiment was conducted to determine effects of reducing tree size on TWU. Tree size was progressively reduced by de-branching an individual isolated tree over a 15-day period. Measures of TWU by sap flow were compared with estimates derived from ETo, and either canopy cover (CC; estimated from the horizontal extent of the canopy) or the area of shade cast by the tree on the soil surface (ASH). ASH was estimated prior to each de-branching event using a combination of photographs of the tree taken from the direction of the sun together with measures of fractional radiation interception in the area of shade cast by the tree. TWU and ETo averaged 42.1 litre/d and 4.7 mm/d, respectively, in the 6-day period prior to de-branching. CC and effective canopy cover (ECC; estimated as ASH measured at solar noon) were 7.8 and 5.8 m2 respectively, in that period. Five de-branching events reduced TWU, CC and ECC by > 95 %. To account for the daytime variation in ASH, effective area of shade (EAS) was calculated from estimates of ASH at solar noon and 3 h each side of solar noon. Kcb, the basal crop coefficient defined by Allen et al. (1998), was related to EAS by Kcb = 1.12 EAS. The effects of row orientation on TWU were investigated by a combination of field and modelling studies on isolated trees and hypothetical hedgerow orchards. TWU was measured by sap flow in six isolated field-grown trees pruned in a hedge shape and orientated north-south (NS) and east-west (EW). TWU was simulated by weighting ETo by the area of shade cast by the tree on the horizontal soil surface (ASH). The model effectively weighted ETo (`Big Leaf') by the fraction of direct beam radiation intercepted by tree foliage in orchards. ASH was estimated using a radiation interception model. Maximum rates of TWU in NS trees were attained in the morning when ETo was approximately half its maximum, and these rates were maintained for much of the day, whereas TWU for EW trees increased linearly with increase in ETo. Leaf area density (p) was estimated as 1 m2/m3 by comparison of observed and simulated TWU and was subsequently used in simulation of ASH in hedgerow orchards. The maximum rate of simulated TWU (TWU') occurred approximately 3 h each side of solar noon in a NS orientated hedgerow but declined in the middle of the day. Shaded leaf transpiration was suggested as a possible contributor to TWU that was not considered in the radiation interception model. The study demonstrated that EAS-weighted ET0 provided a reliable estimate of daily TWU irrespective of row orientation, leaf area density and hedgerow width. TWU and ASH were compared in Tatura trellis (TT) and central leader (CL) orchards. Field observations indicated that TWU in TT orchards attained a maximum near solar noon (matching ET0) whereas in CL it reached a maximum in the morning then remained constant during the middle of the day. Measured ASH revealed a substantial depression at solar noon in CL compared with TT. ECC was 2.3 and 1.7 m2 in the TT and CL trees, respectively. By comparison, EAS was 14 and 30 % greater than ECC for TT and CL trees. These differences between ECC and EAS were even greater in simulated hedgerows taller than used in the field experiment, leading to considerable variation in the relationship between simulated ECC-weighted ETo and TWU. On the other hand, the relationship established between simulated EAS-weighted ET0 and TWU for hypothetical TT trees (TWU = 1.17 EAS ETo) was the same as that established for isolated trees. The results of this thesis suggest that TWU of well-watered trees is slightly above the unit rate of water use described by ET0 (Allen et al. 1998) over the area of shade cast by the tree on the soil surface. Assuming negligible understorey water use, irrigation amount can be simply estimated from 1.12 EAS ETo where EAS accounts for daytime changes in ASH associated tree size, row orientation, training system, and leaf area density. Given this, irrigation requirement can be calculated from ETo and routine measures of orchard EAS. This is a simpler procedure than current FAO recommendations based on ET0 that require a look-up table of Kcb values for different growing periods and further adjustments for vegetation cover. In contrast, the EAS-weighted ETo proposed here provides an objective estimate of TWU for which EAS can be readily established by routine measurements of the fraction of shade in an orchard by irrigation consultants, extension officers, or growers.
-
ItemRoot-shoot interactions in the growth of irrigated white clover( 1993)White clover pastures support the dairy industry in the irrigated area of northern Victoria. However, pasture production is low because conditions for root growth are sub-optimal, particularly under flood irrigation. This thesis investigated the possibility that the growth of white clover can be increased by reducing the limitations to root growth. A series of experiments examined the response of white clover plants to various soil-based treatments and quantified relationships between root and shoot growth. Plants were grown in intact soil cores in the greenhouse with shoot and root growth measured by destructive harvest. The cores were collected from a range of field sites that were characterised by their different soil physical properties and the variation in pasture yield they supported. Other cores contained a sand-based potting mix in which the conditions for root growth were superior to the most productive field soil. Despite the large effects of soil treatment on white clover production, the growth of shoots and roots was highly correlated (R2>0.95). A prerequisite of high shoot yield is, therefore, a large root system. In one experiment, soil drying or defoliation perturbed the correlation but this disruption was only temporary. In another,experiment, the repeated cycles of drought stress that accompanied a series of extended irrigation intervals had no effect on the relationships between shoot and root growth. In field soils, the restrictions to root growth could not be overcome by intensive irrigation and fertiliser management. However, plants in the treatments in which the soil physical properties had been modified produced 4.0 - 6.5 times as much shoot DM compared with the least productive treatment. This suggests that the potential to improve pasture yield by amelioration of the soil physical properties is very large. Two further experiments were conducted in which either the soil texture or the frequency of irrigation varied between the upper and lower sections of the soil cores. In both cases the production of shoots was correlated with total root production. However, when `unfavourable' conditions restricted the growth of roots in one layer, extra growth of roots in the `favourable' layer was not sufficient to compensate. As a consequence, both total root and shoot growth were reduced. Taken together, these results suggest that there is a large scope to improve the yield of white clover by removing the restrictions to root growth that currently exist in field soils. This will probably entail both amelioration of the soil physical properties and careful management with respect to water and fertiliser applications. However, if the experiments reported here accurately reflect the field situation, then the growth of white clover pastures can only be maximised if the entire root zone is modified.