Optimal applied water and nitrogen for corn Sh. Zand-Parsa, A.R. Sepaskhah * Department of Irrigation, Shiraz University, Shiraz, Iran Accepted 28 February 2001 Abstract Water and fertilizer applications should be limited due to scarce resources and environmental protection aspects. An analysis of crop yield production and pro®t maximization was conducted to determine the optimal water and nitrogen allocation. In this analysis based on crop production and cost functions, a series of equations for determination of applied water and nitrogen for three conditions of maximum yield w m and N m , respectively), maximum pro®t under limited land w l and N l , respectively) and maximum pro®t under limited water w w and N w , respectively) were derived. The associated crop production function was determined from the results of a corn experiment with four levels of nitrogen fertilization and varying amounts of applied water via a line source of sprinkler irrigation. The previously derived equations were also applied to the experimental ®eld data and ®nally the optimum amounts of applied water and nitrogen were determined at different conditions w m , w l , and w w for water and N m , N l , and N w for nitrogen, respectively). At present market value 15.55 Rls/m 3 for water and 652 Rls/kg for nitrogen) the amounts of w m , w l , and w w were 1.0, 0.99, and 0.74 m, respectively, and the amounts of N m , N l , and N w were 212, 212, and 206 kg N/ha, respectively. Because of the low price of nitrogen, the optimum amounts of nitrogen at three mentioned conditions were similar. But if the price of nitrogen and water are increased i.e. 50000 Rls/kg N and 100 Rls/m 3 water), then the amounts of applied nitrogen and water at the mentioned three conditions would be 212, 67, and 61 kg N/ha, and 1.00, 0.93, 0.84 m, respectively. When water is limiting, the optimum amount of applied water would not be different by changing the water price, however, it may be increased by a little amount when the nitrogen price is increased. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Water; Nitrogen; Corn 1. Introduction Optimal allocation of irrigation water and nitrogen fertilizer is very important for agricultural purpose, especially in arid regions or regions where contaminated water is a Agricultural Water Management 52 2001) 73±85 * Corresponding author. Tel.: 98-71-201012; fax: 98-71-48093. E-mail address: sepas@hafez.shirazu.ac.ir A.R. Sepaskhah). 0378-3774/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0378-377401)00106-8