develop improved weather forecasts for districts across the Burdekin sugarcane region . These improved forecasts will be integrated into IrrigWeb , allowing irrigators to make more confident decisions around the risk of rainfall and whether to irrigate or not .
Slope and furrow length can also be important factors in influencing irrigation efficiency . Slopes of less than 1 per cent are most often used , and in the Burdekin , SRA ( 2014 ) reports that most growers prefer slopes between 0.06 per cent and 0.3 per cent . Row lengths typically vary from less than 25m to over 1,000m and tend to be longer in the BRIA ( SRA , 2014 ). Low water-use efficiencies and excessive deep drainage losses may result from the use of very long furrows . Waterlogging problems are likely where long rows and low slopes are used .
Correct irrigation scheduling aims to apply the correct amount of water at the correct frequency to produce the optimum yield . Different soil types store different amounts of Readily Available Water ( RAW ), the water that plants can easily access , and Plant Available Water ( PAW ), the total amount of water a plant can extract from the soil . Crops that are actively growing with a full canopy will require more moisture than those that have been recently planted or are nearing maturity . There are many available scheduling methods and their costs vary widely , chiefly depending on accuracy .
A wide range of soil moisture sensors are available to the sugarcane industry including evaporation minipans , tensiometers , gypsum blocks , and automatic monitoring equipment such as time-domain reflectometry and capacitance probes . If scheduling tools are to be widely adopted , they need to be simple — simple to install , setup , interpret and respond to .
Soil moisture sensors are usually used to assist with the frequency of irrigation events , i . e . when to irrigate , but , unless calibrated , provide no information regarding the correct amount to apply .
G-Dot soil moisture sensors are currently a popular tool among sugarcane growers because they provide a simple visible tool which , when located on a well trafficked headland , allow growers to respond quickly and easily ( SRA , 2016 ). The GDot displays soil moisture tension represented by fluorescent yellow flip dots . The more yellow dots showing , the wetter the soil is ; the fewer lit yellow dots , the drier the soil . The GDot uses a granular matrix sensor , a type of gypsum block . Electrodes embedded into gypsum blocks are used to measure the electrical resistance between them under the presence of moisture and those signals are related to soil moisture tension , that is , how hard it is for the plant to extract water . The cable from the GDot display to the sensor can be extended allowing the sensor to be installed some distance along the sugarcane row ( recommended up to 80m ), while the display can be placed at the end of the row where it is easily accessible and visible . GDots are practical instruments but have limitations in terms of display units and ability to store data .
Choice of irrigation system to minimise runoff . Maximising irrigation efficiency and minimising nutrient losses in sugarcane may require moving from irrigation systems with low efficiency ( i . e . conventional furrow irrigation ) to those with high efficiency ( e . g . drip irrigation or overhead low-pressure irrigation ). With highefficiency irrigation systems , there is the potential to have very little run-off of irrigation water ( Bhattarai and Midmore , 2015 ), a process that is responsible for transporting most nitrogen from furrow-irrigated fields ( Thorburn et al ., 2011b ; DNRM , 2016 ). Appropriate management can ultimately lead to irrigation runoff losses being eliminated , while losses to deep drainage can be reduced to a minimum . Overhead lowpressure irrigation also reduced nitrogen losses compared with furrow irrigation ( Attard et al ., 2013 ).
Worldwide , average application efficiencies of different systems are reported as : surface irrigation 60-90 per cent , sprinkler 65-90 per cent and drip 75-90 per cent ( Fairweather et al ., 2000 ). However , these efficiencies can be misleading and depend on soil type , moisture conditions before irrigation , depth to groundwater , the crop being grown , management practices , and quality of irrigation water . Conceptually , it may be expected that a drip system would be a more efficient way of applying water , but due to variability in these characteristics , can be very site dependent ( Hodgson et al ., 1990 ). It is the management of the system for a particular soil and crop combination that is the important input to improve irrigation efficiency . A technology that can lead to potentially high efficiencies , such as drip irrigation , still has to be well designed and managed to take full advantage of that potential .
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