GHAZI ABU RUMMAN, firstname.lastname@example.org
Research Scientist ICT International, PO Box 503 Armidale 2350 NSW, Australia. www.ictinternational.com
Scheduling the irrigation system (frequency and volume) is important to conserve water resources in particular under drought conditions. Several factors affecting the required volume of irrigation such as irrigation method, soil water intake characteristics, actual soil water content and soil type.
Measuring the actual water use of trees has been the concern for scientists, researcher and growers to reduce the impact of drought on farm production, given the reported best horticultural management practices of irrigation scheduling, planting density and pruning, yet there is a gap in the knowing how much a tree uses water. Subsequently water schedules are not meeting the plant water requirements.
Measuring the meteorological data and using this data to estimate the plant water use has a potential to lead to either under or over irrigation. To avoid this problem and to measure directly plant water use through measuring the sap flow in the stem knowing that the flow can be zero or even reverse flow. In this study, I used newly developed instruments that uses the Heat Ratio Method (HRM) and allows to continuously measuring the sap flow and the daily water use of citrus trees and olive trees during two months of the growing season.
Between November and December 2010 average tree water use was 3.96 litres tree-1 day-1. On a plantation scale, the average tree water use was 0.3 ML hectare-1 during the study period whereas the recommended irrigation volume in the same area for the same period 0.59 ML ha-1. Monitoring sap flow and total tree water use has clearly demonstrated that scheduling the irrigation system based on the actual plant water use can save up to 38% when using the direct innovative technology.
Scheduling the irrigation system to deliver the right volume of irrigation water at the right time to meet the plant water requirement is not an easy process and has been investigated previously using weighing lysimeters for grasses and small trees. For large size trees, soil moisture content and soil water potential were used as indicators to reflect on plant water status/stress (Hillel, 1998), however the actual water used by the plant is an accurate indicator to optimise plant growth and productivity and also to minimise inducing any water stress on plants.
As the climate conditions vary significantly, plant water use respond to these changes (Fares and Alva, 1999), therefore the currently used averages of plant water requirement for growing season might fail to address the plant water requirement and consequently, water stress prevails and negatively affects the plant growth and productivity. Scheduling the irrigation system should be based on the actual plant water use; Sap Flow Meters (SFM) is the way forward to achieve the water balance and optimize the plant productivity.
Measurements are based on the Heat Ratio Method (HRM) principle which enables the meter to capture not only the low flow sap flow but also the reverse flow at night (Burgess et al., 2001) in both small and large woody stems and also roots. Heat Ratio Method (HRM) is an improvement of the Compensation Heat Pulse Method (CHPM). Being modified heat pulse technique power consumption is very low using approx 70m Amp per day at a 10 minute temporal sampling interval under average transpiration rate, this in return provide an insight to the proper time to irrigate and also how much to irrigate once we can quantify the real-time plant water use.
Sap flow seems to follow different patterns (Burgess and Dawson, 2004). Nocturnal sap flow was observed during nights of low relative humidity (between 20 and 40%) supporting the notion that trees (redwood) has porous stomata. Maximum transpiration rates around midday, whereas days of fog cease transpiration. Instead, reverse sap flow was observed. The rate of reverse flow was as high as 7% of the previous day’s transpiration. The mechanism by which moisture enters the leaf is via the hyphae or hairs that extend from stomata that act as wicks to draw the water back in.
Previous studies showed how successful was the SFM to quantify the tree water use for species selection to rehabilitate mine sites whereby the shallow groundwater prevail. Three species: Acacia spp, Tamarix spp, and Algorrobo spp. were studied for more than 18 months, results showed that Acacia, Tamarix and Algorrobo transpired approximately 35,000, 16,500 and 6,000 litres of water, respectively. Quantifying total tree water use allowed more precise management decisions to be made in terms of adherence to environmental regulations plus, the data to accurately expand the mines capacity whilst continuing to meet current and potentially future modifications to environmental regulations (Domingo et al., 1996; González-Altozano and Castel, 1999; Goldhamer and Salinas, 2000).
A comparison of sap flux and water relations of leaves of various trees found that Melaleuca trees use up to 17 litres hr-1 whereas Prunus use 8 litres hr-1 (Misra and Sands, 1992). Maple trees of the same species and age showed significant difference between shaded and exposed trees to sunlight; 140 L day-1 (12,500 L year-1) for exposed trees and 65 L day-1 (5,740 L year-1) for shaded ones (Cermak et al., 2000).
Water comprises 85-90% of the fruit mass by weight, so harvesting the fruit can reduce tree water demand; Carrizo citrange (Citrus sinensis [L.] Osb. × Poncirus trifoliata [L.] Raf.) have good drought tolerance, Cleopatra mandarin and Troyer citrange have moderate tolerance and P. trifoliata and Sweet orange have poor tolerance.
A study of mature orange tree over 3-yr period showed that the average evapotranspiration of these trees (Etc) ranges between 1.3 and 5.5 mm d-1 (Castel et al., 1987) under Mediterranean-type climate. In contrast, another study by Martin et al. (1997) reported an ETc range of 1.1 to 10.6 mm d-1 for citrus tree in Arizona under arid conditions, indicating similar minimum but double that of the maximum daily ETc in the Mediterranean climates.
Scheduling the irrigation system (frequency and volume) is important in particular under drought conditions. Several factors affecting the required volume of irrigation such as irrigation method, soil water intake characteristics, actual soil water content and soil type. An innovative technique is used to measure the actual plant water use.
Measuring the water balance enables us to schedule the surface, drip and/or popler irrigation systems of trees and also budgeting for exact volume of required water by the tree; adding to that when the soil infiltration rate is known; then the water balance model will respond to the exact volume of water to keep the trees at their optimum growth and health.
Materials and Methods
The HRM needles (SFM, ICT International Pty Ltd., Armidale, NSW, Australia) have two radial measurement points for the characterization of radial sap flow gradients making measurements more accurate. Through microprocessor control, the inner measurement point can be activated or deactivated dependent on the specific wood anatomy of the species being measured (Figure 2). This provides a great flexibility in stem diameter range from >10 mm diameter woody stems or roots of Citrus trifoliata L. trees.
This enables water flows to be monitored in stems and roots of a wide range of different species, sizes and environmental conditions including, drought or water stress.
The HRM probes consists of three 35 mm long needles integrally connected to a 16-bit microprocessor. The top and bottom probes contain two sets of matched and calibrated high precision thermistors located at 7.5 mm and 22.5 mm from the tip of each probe (Figure 1). The third and centrally located needle is a line heater that runs the full length of the needle to deliver a uniform, and exact pulse of heat through the sapwood and also to account for the vast proportion of sap flow in Citrus trifoliata L (Ford et al., 2004; Fiora and Cescatti 2006).
The utility software enables the Sap Flow tool (SFT, ICT International Pty Ltd., Armidale, NSW, Australia) to be used in the manual mode. This provides the ability to evaluate the efficacy of pulse intervals by viewing the raw measured temperatures on screen. Subsequent reports can then be viewed detailing the duration of time the heat pulse required to deliver the exact amount of heat energy in Joules, the temperature rise following the heat pulse, temperature ratios between measurement points, sap velocity or sap flow. Figure 1. outlines sensor design and the location of thermocouples in sapwood.
Figure 1. The heat ratio method (HRM) consists of three needles connected to a stand-alone data logger with a central heater and two temperature sensing needles, each with two measurement depths spaced radially across the sapwood.
Soil water content in the root zones of each tree was maintained within field capacity. Irrigation scheduling was provided using an automated irrigation control system. Duration of irrigation events was adjusted monthly to provide water at the recommended volumes (Figure 4). All irrigation events occurred between 6-8 am to minimize surface evaporation from both wind and radiation.
Results and Discussion
Daily water use.
The sap flow meter detects the daily water use on regular basis and was able to quantify the daily water use, over the course of the study, there was significant difference between the water use of an individual days, the P value was < 0.001. Figure 3 shows that water use of Citrus trifoliata L within a period of 4 days declined in the 3 subsequent days in which the reduction was 9, 25 and 60% for the 2nd, 3rd and 4th day subsequently consequently when compared to the water use of the 1st day.
Tree water requirement is well document in the literature for the different tree species (Appendix 1) which takes into account the environmental conditions and crop factor; the Penman–Monteith equation (Monteith and Unsworth, 1990; Allen et al., 1998) is the established method for determining the ET of the major herbaceous crops with sufficient precision for management purposes. However, potential increase of uncertainty might occur when using the same approach to determine the ET requirements of tree crops (Fereres and Goldhamer, 1990; Dragoni et al., 2004; Testi et al., 2006), however the recommended water use is based on monthly averages, however Figure 1 proved that the calculated daily water use during the course of the study of Citrus trifoliata L was 3.96 L tree-1 whereas the actual measured water use ranges from 2.38-6.01 L tree-1.
Using the information from Figure 1, it was found that 38% of the daily water use happens in 4 hours (11 am – 3 pm) (Figure 5). Water use normally falls during late autumn and winter with the onset of cooler temperatures and the slowing of tree growth (Donovan et al., 2001; Dawson et al., 2007).
Previous studies for the same species showed that water stress during late spring and summer (March – August) at the time of late cell division and cell expansion, will have a big impact on fruit size (data not shown). Water stress closer to harvest can influence internal fruit quality characteristics such as acidity, % juice, TSS and fruit maturity. Water stress can also restrict vegetative growth and reduce canopy development, which is especially important in young trees and for next seasons flowering sites.
Figure 3. Tree water use of Citrus trifoliata L grown under field conditions in Mediterranean- type climate during the hottest period of the year. Data presented are for 4 subsequent days were tree water use dropped significantly. Dotted line signifies the recommended tree water use by citrus trees.
Figure 4. The recommended citrus tree water use based on averages over 10 years.
Figure 5. A 3-D schemetic graph showing the water use of Citrus trifoliata L every 30 minutes. Illustration of depth of sap wood is given on Z axis and the sap flow rate is given on Y axis.
Sap flow provides important information on tree behaviour, impact of stresses (e.g. drought) and can be applied in many studies related to eco-physiology, ecology, hydrology and environment. Sap flow measured on individual sample trees can be up-scaled for stands or certain areas (e.g. streets, parks, etc.), when based on the biometric or remote sensing data, i.e., such data can be calibrated in terms of transpiration
Sap flow can be automatically measured for long periods of time in trees of any size and species (or their parts) under any terrain and environmental conditions. No infra structure or constructions are needed.
Sap flow velocities appear to vary somewhat between species but overall trends are similar. Sap flow meter is able to quantify the water budget of various location, stress patterns, cultivar, planting dates, and other factors. In particular, cultivars with high tolerance to drought stress at various stages of development.
Using the real-time measurement of sap flow provides insight to irrigation managers to decide on irrigation timing and volume to meet the tree water requirement.
Table 1. The estimated sap wood area and the calculated sap flow for fruit and forest trees.
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