The models' 1- to 5-day quantitative precipitation forecasts are routinely verified against gridded analyses of daily rain gauge observations in order to assess the quality of the forecasts. Results show that the models do indeed show skill at predicting the occurrence and amount of rainfall. This skill is highest early in the forecast period and decreases with time. Both models show regionally and seasonally dependent biases, with the global model tending to overestimate wintertime rain and underestimate summertime rain, and the regional model showing the opposite tendency. Lower skill was shown in the tropics than in mid-latitudes. In general, the greater the large-scale organisation of the rain-bearing system, the better the forecast will be.
Several developments are underway to improve the quality of the Bureau's model-based forecasts. There is ongoing research into making better use of observations, as well as introducing improved physics and numerics into the NWP models. Improved methods for statistical forecast modification show promise in reducing both the bias and random error in temperature forecasts. Finally, ensemble prediction techniques will enable more accurate deterministic and probabilistic forecasts to be made, which will impact on the risk assessment in irrigation and other decision processes.
david_white@acslink.net.au ; http://www.acslink.aone.net.au/asit
A substantial number of crop, pasture, horticulture and agroforestry models were assessed for their ability to simulate changes in soil moisture over time, and the impact of periodic water stress and waterlogging on crop growth and yield. Emphasis was on the ability of relevant models to respond in a physiologically realistic way to external variability and shocks.
It was concluded that even relatively simple models using information on rainfall, irrigation and evapotranspiration, when applied to the management of irrigated crops on well-structured soils, can significantly improve the water use efficiency of these irrigated systems. Further improvements of both dryland and irrigated systems can be achieved through the use of models in which the basic physiology of the plant is expressed mathematically so as to convert intercepted radiation and available soil moisture to increased biomass and harvestable yield. Areas where existing models might be further improved are discussed.
The effect of waterlogging on crops has only recently been addressed. Progress to date has been encouraging in terms of relating crop growth and yield to lack of soil aeration and root growth as a result of flooded macropores in the soil.
Models provide a logical framework for improving our knowledge and management of agricultural and other natural resource systems. They are proving to be an often essential adjunct to field work in developing more productive, sustainable and financially viable farming systems. In this way the value of management inputs, including seasonal forecasts, can be determined. Opportunities also exist in terms of further developing and using the models to assist in improving product quality.
In Northern Australia rainfall directly after irrigation can result in water lost through runoff and drainage.This rainfall can also result in waterlogging of the crop and offsite flow of contaminants.We are proposing and idea to use 3-7 day weather forecasts to increase effective rainfall.
To scope this idea we used historic rainfall and the PERFECT water balance model to schedule irrigation.We primarily were interested in the water balance of the cropping system and what effect the new management option had.The PERFECT model allowed us to easily modify the irrigation scheduling using short term weather forecasts.Using the perfect knowledge of ‘future’ rainfall when using historic records we ran simulations of different cropping systems and locations.
The effectiveness of using short term weather forecasts varied between geographic locations and cropping systems.The management option is more successful when there are more opportunities to apply it.The opportunities increase when there are more rain days and higher rainfall, when the crop has a longer growing season, when the crop uses more water and with soils that have a smaller plant available water capacity.
If the management option is more successful with situations that have more opportunities crops that grow in wetter areas are more likely to benefit from this option. Crops such as sugar cane, irrigated pasture and horticulture.
Irrigation accounts for approximately 75% of all water consumed in Queensland. This new century is seeing the limits to water resource development being defined.Water is increasingly being seen as a precious commodity which needs to be managed accordingly. Queensland’s water resources are limited and we need to make the best use of them.
The community’s water use needs are met through three approaches:
All three methods are being used, although in the past most emphasis has been placed on the third option. While the construction of new, economically viable and ecologically sustainable water supply infrastructure will continue where water is available for this purpose, more emphasis will be placed in future on water use efficiency and wastewater reuse.
A major Queensland Government initiative to address the first option is the Rural Water Use Efficiency Initiative.This Initiative to improve irrigation water use efficiency in the State’s rural sector is managed by the Department of Natural Resources and Mines.
Rod Welford, Minister for Natural Resources, launched the Rural Water Use Efficiency Initiative (RWUEI) in Rockhampton in May 1999.NR&M is providing up to$41 million to fund the Initiative, which is a partnership between the Department of Natural Resources and Mines and the following four industries:
The partnership aims to improve the use and management of available irrigation water, thereby improving the competitiveness, profitability and environmental sustainability of Queensland’s rural industries.The key benefits of the Initiative are:
Over four years, up to $41 million will be spent on the Initiative. The additional water which will become available to farmers as a result of implementing water use efficiency. The four major elements of the Initiative are:
The climate forecasting project is a “blue sky” R&D project under the Initiative. (or should that be “grey sky”?) Should this project be successful, it could revolutionize irrigation management in Australia.Another area that could provide a breakthrough in coming years is evaporation control from storages using floating plastic materials in various configurations.
The RWUEI anticipates that the climate forecasting project will predict rainfall events to a degree of reliability such that it will be possible to manipulate irrigation applications to take advantage of rainfall.Consideration will need to be given to the effect of delaying irrigation on possible scheduling and crop stress in the event that rainfall does not occur.If all this can be achieved, substantial savings of water will be result as well as benefits to the environment by limiting runoff associated with rainfall events following recent irrigation.Strategies include simply withholding irrigation for a short time – perhaps a day or a few days, to allow for the possibility of rainfall as a substitute for irrigation if forecasts are suitable.
Obviously the irrigation system and the way it is operated and how it performs are crucial if we are to be capable of taking advantage of such rainfall “windfalls”.
Systems that are capable of accurately bringing the soil to a predetermined soil moisture content at a selected moisture level suitably below field capacity eg. centre pivot and trickle irrigation, offer the greatest opportunity of manipulation to accommodate rainfall. However, even with these systems there needs to be sufficient flexibility in design to allow for catch up in the irrigation cycle. Adopting climate predictions to benefit surface irrigation, which typically is not capable of irrigating to less than field capacity and typically with little capacity for catch up, would present the greatest challenge.
The project needs to take account of system characteristics and management to identify what opportunities might realistically be offered. Farmers will need to understand the risks within, both the forecasting systems and their irrigation systems and management, in the context of incorporating more rainfall into their irrigation strategy.
Our vision for the future is that the principles of WUE, recognising water as a precious resource, will be incorporated into water planning and management throughout all sectors – rural, urban and government (including SunWater) with the whole community understanding the value of water.
The RWUEI is currently examining its role beyond June 2003, which is the end of the current program. The irrigation industry will be well poised after June 2003 to enter a new phase focusing on implementing the findings from the program. These will only become truly apparent at the end of the current project when the findings of the adoption programs, economic analyses and all the R&D work, including the climate project, will be available for integration into the development of new directions for irrigation crop production in Queensland.
Inherent within any system of managing a resource more efficiently (e.g. human resource, fuel consumption etc) is the need to understand the production of outputs relative to the consumption of inputs (the resource). The old adage “if you don’t measure it you can’t manage it” has never been more appropriate than in the current challenge of natural resource and water resource management.The measurement of the water resource on farm might include:
The measurements must realistically translate to a management system that improves the efficiency of the use of the water resource.With all of this information the questions must be asked.
The presentation aims to discuss some of these questions (and answers) from experience in measuring whole farm water use efficiency on several farms in SE Queensland.
An example of validating the PERFECT water balance model with irrigation was performed on a cotton irrigation scheduling experiment at Narrabri. Brian Hearn ran a irrigation scheduling experiment at the Australian Cotton Research Institute in 1985. Treatments involved applying either different amounts of water or irrigating at different times. Each treatment had three replicates, with each replicate being a different variety of cotton. The soil moisture data was of high quality but climate data, harvest figures, and water and water off were either not measured or contained errors.
We found PERFECT to have a fairly high correlation between observed and predicted soil moistures. Using crop parameters calibrated for a fully irrigated crop, PERFECT accurately predicted soil moisture for crops under water stress. PERFECT, because of lack of prediction of soil moisture in lower layers of soil, does not have an accurate simulation of the root dynamics of cotton. We also found that PERFECT does not simulate leaf area index accurately for crops under water stress.
To validate a crop accurately we need, in order of importance, water on and off, local meteorological data, good soil moisture data, leaf area index and crop cover measurements, the soil upper and lower limits, biomass and economic yields and if possible root activity measurements.