Improving Water Use Efficiency by Reducing Groundwater Recharge Under Irrigated Pastures
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IMPROVING WATER USE EFFICIENCY BY REDUCING GROUNDWATER RECHARGE UNDER IRRIGATED PASTURES Final Report December 2001 Hayden Kingston, Matthew Bethune, Jahangir Alam Improving Water Use Efficiency by Reducing Groundwater Recharge under Irrigated Pastures FINAL REPORT Project DAN 11 December 2001 Hayden Kingston NSW Agriculture, Finley Matthew Bethune Institute of Sustainable Irrigated Agriculture, Tatura Jahangir Alam Formerly NSW Agriculture, Deniliquin This report summarises work conducted between July 1997 and December 2001. This project was funded by, Land and Water Australia, the Dairy Research and Development Corporation (DRDC) through the Murray Dairy Regional Development Program, Murray Community Land and Water Management Plans, NSW Agriculture and the Victorian Department of Natural Resources and Environment. For more information contact Hayden Kingston Matthew Bethune NSW Agriculture Institute of Sustainable Irrigated Agriculture, DNRE 241 Murray Street Private Bag 1, Ferguson Road Finley, NSW, 2713 Tatura, Vic, 3616 Phone 03 5883 1644 Phone 03 5833 5222 Fax 03 5883 1570 Fax 03 5833 5299 Email hayden.kingston@agric.nsw.gov.au Email Matthew.Bethune@nre.vic.gov.au Executive summary The project, ‘Improving Water Use Efficiency by Reducing Groundwater Recharge Under Irrigated Pastures’ (DAN11), was established to quantify recharge under well-managed flood irrigated pasture. A range of approaches, from field based experimental programs to lysimeters, was used to quantify recharge under flood irrigated pasture. The project focussed on the Berriquin and Shepparton irrigation areas. Recharge was estimated to be less than 0.2 ML/ha/y on most of the soils examined. The results indicate that recharge on the lighter soils was higher in 1996 than during 1998, 1999 or 2000 on the lighter soils in the Berriquin and Wakool areas. The reduced recharge is associated with drier than average climatic conditions over the study period, and is mirrored by the reduction in regional groundwater levels in both the Berriquin and Shepparton irrigation areas. A review of previous studies identified that recharge in excess of 3 ML/ha/y had been measured on one fine sandy loam soil. However, there is very little supporting information describing recharge under such sandy soils. Irrigation water quality had a large impact on recharge. Models describing infiltration under flood irrigated pastures needed to account for soil cracks to quantify the rapid initial wetting to depth following irrigation. The SWAP model, which accounts for infiltration through cracks, provided a good description of the soil water dynamics when tested against lysimeter and field data. However, uncertainties in parameters that describe soil hydraulic properties were the major limitation to the practical application of the model at a wider scale. This limitation was also identified at a national workshop, ‘Modelling water movement in cracking soils,’ organised by the project team. The model was used to assess the impact of a range of irrigation schedules on recharge. Recharge levels were found to be relatively insensitive to irrigation schedule on the heavier clay loam soils. However, irrigation schedule had more of an impact on recharge on a fine sandy loam. An appropriate framework is required that delineates soils that are suitable for flood irrigation, soils where we can reduce recharge by improving irrigation management, and soils unsuitable for flood irrigation. The main impediment to developing such a framework is the lack of good data describing soil hydraulic properties. Policy implications Good management of flood irrigated perennial pastures on heavy clay loam soils in Northern Victoria and southern NSW will not result in excessive recharge. Recharge under flood irrigated sandy soils is likely to be excessive in many areas. Improved scheduling of flood irrigation had only limited scope to reduce recharge. Alternative irrigation systems, use of deep-rooted perennials such as lucerne, modified layouts or land use change may be options to reduce recharge in these areas. Increasing water salinity will increase recharge under irrigated pasture. Water managers need to assess the risk of increased water salinity resulting from regional salt disposal and reuse schemes and dryland salinity. Increased irrigation water salinity resulting from any of these factors will inevitably result in increased recharge. Improving Water Use Efficiency by Reducing Groundwater Recharge Under Irrigated Pastures Pasture water use was well defined by the reference crop evapotranspiration (ETo- FAO-56). Therefore, ETo provides an upper limit on pasture irrigation requirements and should be adopted for setting maximum annual irrigation intensities. The setting of maximum irrigation intensity can be refined further if the amount of rainfall that contributes to plant water use is quantified. There is some scope to reduce recharge during winter by drying the soil profile in autumn. However, the pasture rootzone is too shallow to prevent recharge every winter. Limiting the time for infiltration through good surface drainage still offers the best method for reducing recharge during winter. Recharge occurred over the spring period as a result of increased soil water storage over winter. Improved irrigation scheduling during spring may encourage plant water extraction of this excess stored water and reduce recharge. The SWAP model could be used to inform policy decisions. It described the key components of the water balance, including infiltration through soil cracks. Small conceptual changes to the SWAP model were required to enable its use in the irrigation region, and these should be formalised to provide a robust tool for assessing policy options. A major limitation to the use of models for informing policy and management is the lack of good data describing the soil hydraulic properties affecting water movement. A methodology to transfer soil properties across spatially varying soils is also required. Farm management practices (such as nutrient management) have a large impact on the amount of dry matter produced per unit of water. Recommendations Evapotranspiration and effective rainfall data should be used to set maximum irrigation intensities for irrigated perennial pastures. A practical process to measure actual irrigation intensity on irrigated dairy farms should be developed. This requires measurement of the irrigated area of perennial pasture and water applied to this area. The setting of this maximum irrigation intensity is the first step in identifying which farms are over irrigating and potentially have high recharge. A maximum acceptable level of recharge needs to be defined for irrigated pastures. This level would provide a basis for targeting farms to change management where unacceptable recharge levels occur. The extent of light sandy soils with potential for high recharge needs to be appraised. This will allow management on these areas to be assessed and modified if necessary to reduce recharge. It will also be useful information for long term planning and consideration of future irrigation development. Changes made to the SWAP model, so that it could be applied to flood irrigation, should be clearly documented and formalized with the original model developers for incorporation in future releases of the model. This will benefit future studies looking at the water balance of cracking soils. Funding for a visiting scientist may be an effective method to achieve this. Further investigation of the hydraulic properties of soils in this region should be undertaken. The appropriate scale for such investigations should be considered, as data describing hydraulic Improving Water Use Efficiency by Reducing Groundwater Recharge Under Irrigated Pastures properties is also limiting in many other irrigation areas. Improved understanding of hydraulic properties of soils would enable the application of models such as SWAP for use in assessing the impacts of management and policy decisions on recharge. Continued promotion of Best Management Practices for irrigation management and pasture production is required. These practices will increase the production per unit of water. Acknowledgments We would like to thank the following organisations for their support for this project: Land & Water Australia, Dairy Research and Development Corporation (DRDC), Murray Dairy (MD), Murray Irrigation Land & Water Management Plans, Department of Natural Resources and Environment, Victoria and NSW Agriculture. There are many people who have had input into or assisted during the course of the project: Firstly, the steering committee who provided valuable insight and direction, from the initial project development through to submission of the final report. A special mention must go to the farmers whose co-operation was invaluable to the running of the field sites and to Tony Cook whose hard and tireless work was largely responsible for the successful running of the lysimeter experiment. We also acknowledge the input of Iain Hume and David Mitchell who assisted in setting up and developing methodology for running and interpreting the field sites, Garry Bond for assistance with running the field sites, Barbara McAllister for clerical support and Dr Leigh Callinan for biometric support. Steering committee members and affiliation Member Affiliation Nick Austin NSW Agriculture Martin Blumenthal NSW Agriculture Neil Campbell Dairy farmer Keith Nicol Dairy farmer Alfred Heuperman, replaced by QJ Wang DNRE Maurice