Arboriculture & Urban Forestry 39(3): May 2013 Tree Water Demand and Supply Balance To provide some insight into the adequacy of the drip irrigation water supply to meet tree water demand, a tree water balance model was developed for European elms growing in Macarthur Square, Carlton (a small park north of the central business district with a double row of mature elms planted in turf), for the month of January 2009. This modeling exercise was conducted to elucidate why elm tree canopy health had remained poor despite the operation of retrofitted drip line irrigation over the summer period. In Macarthur Square, each elm tree pos- sesses an approximately rectangular canopy of dimensions 15 m E-W and 16 m N-S (240 m2 ). Water was supplied to each tree by two drip lines running parallel E-W, at an irrigation every 0.3 m, which delivered approximately 160 L h-1 tree-1 In January 2009 the system was delivering 450 L tree-1 day-1 Using January 2009 climate data from the Melbourne rate of 1.6 L h-1 per dripper and one dripper . . Regional Office weather station (1 km from the study site) (Bureau of Meteorology 2011), daily and cumulative water use by a single elm tree was estimated using the following relationship: ETL = ETO • KL ence evaporation value, and KL is the landscape coefficient where ETL is landscape evapotranspiration, ETO to model water use in the park, this value reflecting a mid- season value for trees under-planted with turf (Pannkuk et al. 2010). Daily ETO for the planting in question (Pannkuk et al. 2010). Less than 1 mm of rain fell during January 2009, and by this stage of the extended drought elm tree canopy density had thinned, reflecting probable water stress. A KL value of 0.60 was used d-1 to 8.7 mm d-1 each tree ranging from 550 L d-1 to 2,190 L d-1 modeled tree water use for the month was 32,640 L tree-1 during January ranged from 3.8 mm resulting in daily potential water use for . Cumulative . In January 2009, the daily irrigation volume of 450 L for each tree would not have met potential tree water demand on any day in that month. Overall the irrigation met 43% of potential demand. As the soils in Macarthur Square would have been dry leading into spring, these trees would have been subject to continued and increasing water stress, regardless of the retrofitted drip irrigation measures put in place in response to the tree health survey data and extended drought conditions. Drip Irrigation for Winter Soil Water Recharge As the retrofitted drip irrigation lines had been shown to produce a limited zone of wetting in summer (500 mm deep and approximately 500 mm from drip line) and had been shown to not meet summer water use demand, the poten- tial of these retrofitted drip irrigation lines to help recharge soil water contents in late winter, before the onset of sum- mer, was investigated. By recharging soil water profiles in winter, these drip irrigation lines may provide drought- affected trees with respite from continued physiological stress and may encourage fine root growth in springtime in areas to be supplementary irrigated through summer. Eight sites in parklands across Melbourne were chosen, with a range of soil types and conditions. At each park- land location, the drip irrigation system was operated for an is the refer- 141 estimated 14-day period in August 2009 (late winter). As in March 2009, trenches were dug with a backhoe, at right angles to the drip line, at approximately the canopy edge. The depth of the trench was determined by site conditions and direct observations and soil moisture conditions were assessed by i) visual assessment of the extent of the wetting pattern, ii) volumetric soil moisture content using a Theta Probe, and iii) the use of a metal spike to test soil softness (which is directly related to soil moisture content). Figure 6 shows soil moisture profiles at two locations in The Domain, one under a retrofitted drip line and the other an adjacent un-irrigated area. Figure 6. Soil moisture data (% soil water v/v) after winter irri- gation in Domain Park (south) in August (winter) 2009 (A is the irrigated site, B is a neighboring site without irrigation). At this location in The Domain, the soil has a deep sandy loam A horizon (bulk density 1.1 Mg m-3, field capacity 27% by volume, wilting point 9% by volume), with a clay B horizon at 500 mm. The soil in the trench was wet directly below the drip line, and this irrigated wet zone extended into the clay subsoil to a total depth of 630 mm (Figure 6A). In the upper, coarse sandy loam, the irrigated wet zone extended approximately 500 mm on either side of the drip line and was at, or above, field capacity. A comparative trench dug a few meters from the drip line exposed soil that was very dry to the touch except for a layer at the surface wetted by recent rainfall (Figure 6B). At depths of 300 mm the soil was dryer than wilting point, indicating the deficiency of winter recharge rainfall that season. These differ- ences show clearly that the late winter irrigation was responsible ©2013 International Society of Arboriculture
May 2013
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