Arboriculture & Urban Forestry 39(3): May 2013 fall events were greater than 4–7 mm as measured by the RBG Automatic Weather Station. From July 2009 to June 2011, average canopy interception of total rainfall over the soil moisture sensor sites was 34% compared to AWS data. (Note: to reduce excessive labor in data collection, these measurements did not include daily amounts less that 2 mm, so it is likely this is an underestimation of actual inter- ception values due to the higher proportion of interception for smaller rainfall amounts.) Canopy interception values approaching 67% were measured for some important urban forest sites in the RBG. The variation of rainfall effectiveness for respective events and sites was also readily monitored and observed through soil moisture sensing. This reinforc- es the importance of applying an ‘effective rainfall’ factor in irrigation scheduling methodology (Symes et al 2008). IRRIGATION EFFICIENCY There are a multitude of techniques employed in irrigat- ing trees, including sprinklers, sprays, bubblers, drip emit- ters, driplines, wells, and various perforated pipe distri- bution systems. The key issues are the area of root plate watered, depth of watering, infiltration effectiveness, soil water storage capacity, and total amount of water applied. In the design and management of tree watering systems there are some key characteristics that should be considered. Effective Delivery – Deep Watering Deep watering, for example 200 to 500 mm, is recommended for mature trees as the recharging of deeper soil layers can enhance tree resilience, particularly during periods of drought. This generally requires long run times—hours not minutes— and slow application rates, if drip emitters are being used. Water will only move down the soil profile under satu- rated conditions. This requires the wetting of the shal- lower soil layers prior to the deeper layers being wet. In some situations, the placement of the delivery out- lets (e.g., subsurface drip, wells) deep into the soil pro- file can be used to overcome the need for watering of the top soil layers. This strategy reduces the competition for water between shallow rooted vegetation and the tree. Dripline Systems Many dripline systems, as well as sprays, are currently only applying water in the top 100 mm to 150 mm of the soil. Delivery using close emitter spacing interval, for example 300 mm apart, low-flow-rate drippers (e.g., 1.5 L/h), for rela- tively short periods, is not ideal for trees. The ideal drip deliv- ery would be wide spacing, for example 0.5 m or more, with higher flow rates, providing soil infiltration and percolation capacity is adequate, so that deep soil wetting can be achieved. Zoning of Irrigation The ability to control the application of water to areas of vegetation or single large plants (trees) is essential, in terms of achieving effective watering and efficiency. In the de- sign of irrigation systems, the areas containing tree roots should be identified and the water delivery control arranged so that the specific water requirements of that area can be 121 satisfied, without necessarily watering adjacent vegetation or areas. Zoning of tree watering is essential and is some- times required to comply with water restriction conditions. Strategies to achieve high water-use efficiency in the irrigation of urban trees are outlined in Connellan (2013). ROOT ZONE SOIL MOISTURE SENSING Soil moisture sensing is one complementary technological tool that can be used to provide a greater understanding of plant water use, and assess irrigation and rainfall effectiveness. Knowledge of the soil moisture content, and the response of plants to soil moisture conditions, is essential for precision scheduling of irrigation (Symes et al. 2008). The technol- ogy ranges from cost effective but simple equipment to high- ly sophisticated and expensive systems that are used more for research purposes or large-scale agricultural enterprises (Charlesworth 2000). Nevertheless, the information provides a useful insight into the physical (soil hydrology) and biological (plant water use) patterns under the soil surface and helps close the loop in landscape water management (Symes et al. 2008). It is improved when combined with meteorological measurement and professional judgment to help compensate for the high lev- els of landscape variability. RBG is currently in a partnership research project to quantify plant water use, including weather data and horticultural expertise (Symes et al. 2008). Apart from the immediate application to improve irrigation management, it is also anticipated that this research will assist in establishing baselines for understanding the influence of the current climate on plant water use, and assessing future trends that may develop. Soil Moisture Sensor Applications in Scheduling Knowledge of soil moisture content of plant response to soil moisture conditions is essential for precision schedul- ing of irrigation. The soil moisture level is typically deter- mined using a predictive technique through ET estimation and conducting a soil water balance. Soil moisture sens- ing allows the actual value of soil moisture to be an input into the scheduling decision making process. The incor- poration of soil moisture sensing in the control process as feedback makes this a true, closed-loop type of control. Access to soil moisture data significantly expands knowl- edge of plant and soil water behavior. Identification of the time the soil moisture levels reach a set-point value, to ini- tiate irrigation, is only one application of the technology. The nature of the soil moisture data that can be obtained determines how it can be used. The number, loca- tion, and precision of sensors and frequency of readings are all important. Although a single sensor, positioned within the root zone and monitored on a daily basis, pro- vides valuable information, the installation of multiple sensors greatly expands the knowledge base. The instal- lation of multiple sensors at selected positions down the soil profile allows soil moisture in the different soil zones to be monitored and changes between zones to be analyzed. Continuous monitoring of sensors with ac- cess through the internet, in real time, provides the op- portunity for enhanced analysis of the plant soil system. Portable probes provide for assessment of variations in plant water use (ETc) rates across the various hydrozones of the landscape. ©2013 International Society of Arboriculture
May 2013
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