Two-Dimensional Simulation Model for Contour Basin Layouts in Southeast Australia. I: Rectangular Basins Manoj Khanna 1 ; Hector M. Malano, A.M.ASCE 2 ; John D. Fenton 3 ; and Hugh Turral 4 Abstract: Contour basin irrigation layouts are used to irrigate rice and other cereal crops on heavy cracking soils in Southeast Australia. In this study, a physically based two-dimensional simulation model that incorporates all the features of contour basin irrigation systems is developed. The model’s governing equations are based on a zero-inertia approximation to the two-dimensional shallow water equations of motion. The equations of motion are transformed into a single nonlinear advection–diffusion equation in which the friction force is described by Manning’s formula. The empirical Kostiakov equation and the quasi-analytical Parlange equation are used to model the infiltration process. The governing equations are solved by using a split-operator approach. The numerical procedure described here is capable of modeling rectangular basins; a procedure for irregular shaped basins is presented in Paper II. The model was validated against field data collected on commercial lasered contour layouts. DOI: 10.1061/ASCE0733-94372003129:5305 CE Database subject headings: Basins; Australia; Two-dimensional flow; Simulation models; Irrigation. Introduction 80% of Australia’s national irrigated area is located in the Murray–Darling Basin, which covers parts of New South Wales, Victoria, Queensland, and South Australia. The Murray–Darling Basin located in the southeast of Australia covers 1,061,469 km 2 , equivalent to 14% of the country’s total area. Irrigated agriculture in Australia has resulted in substantial benefits to individual rural communities and the nation as a whole. It accounts for 28% of the total value of all agricultural production in Australia and contrib- utes one third of the national output from rural industries by pro- ducing most of Australia’s dairy products, cotton, rice, fruit, veg- etables, and wine. Contour basin irrigation layouts are used to irrigate rice in heavy soils in the Murray–Darling River basin in southeastern Australia. Most of the rice cultivation in Australia is done on these layouts, which are found in the states of New South Wales, Victoria, and Queensland. Approximately half of the irrigated land of southern New South Wales is developed under contour basin irrigation systems. They are used primarily on soils that have very low infiltration rates. Contour basin layouts in south- east Australia exhibit particular features which distinguish them from similar forms of irrigation elsewhere in the world. In these layouts, water is applied to group of multiple basins divided by check banks constructed across the slope. These are made by borrowing soil from the basin resulting in a toe-furrow along the banks. The basins are filled to the desired depth of water, which is retained until it infiltrates into the soil. The excess water is drained back into the supply channel and into the next basin. A typical layout of a contour basin irrigation system is shown in Fig. 1. The water flow patterns during inflow/advance and recession/outflow are shown in Figs. 2 and 3. Contour layouts are designed and used for cultivation of rice where ponding of water is required. Due to shortage of irrigation water, the recent cap on water allocation in the Murray–Darling basin, and growing concern about the environmental impacts of waterlogging and salinity, the agriculture department in the area promotes crops with low irrigation requirements to be grown in rotation with rice to minimize adverse impacts on the environ- ment and ground water. Crop rotation will also help improve the soil structure and organic matter and will ultimately enhance the water use efficiency without affecting the net income of the farm- ers. Old layouts are inefficient when used with crops other than rice which do not require ponding of water. This is due to poor drainage from the basin leading to overirrigation, groundwater accession, and poor productivity. Similar problems of poor drain- age also occur with rice cultivation when basins must be drained during the maturing stage prior to harvest. Since rice has been traditionally grown on these soils with low permeability it is vital to introduce management flexibility that enables farmers to grow other crops on the same layouts that will yield better returns and will provide an alternate management option. However, existing practices for upgrading contour layouts are guided only by experience and intuitive understanding. Overseas criteria and experience are not applicable to these situations as upland crops are not irrigated on contour irrigation systems. Basin irrigation systems used in other countries are usu- 1 Senior Scientist, Water Technology Centre, Indian Agricultural Re- search Institute, New Delhi 110012, India. E-mail: mkhanna@iari.res.in 2 Associate Professor, Dept. of Civil & Environmental Engineering, University of Melbourne, Victoria 3010, Australia. E-mail: h.malano@devtech.unimelb.edu.au. 3 Professor, Dept. of Civil & Environmental Engineering, Univ. of Melbourne, Victoria 3010, Australia. E-mail: fenton@unimelb.edu.au 4 Senior Researcher, International Water Management Institute, P.O. Box 2075, Colombo, Sri Lanka. E-mail: h.turral@cgiar.org Note. Discussion open until March 1, 2004. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on November 27, 2001; approved on January 8, 2003. This paper is part of the Journal of Irrigation and Drainage Engineering, Vol. 129, No. 5, October 1, 2003. ©ASCE, ISSN 0733-9437/2003/5- 305–316/$18.00. JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING © ASCE / SEPTEMBER/OCTOBER 2003 / 305