3.2. Production function
Total amount of applied water relative to total tuber yield in ton per hectare was selected to show production functions through linear regression analysis. As expected, the relationships between tuber yield and crop ET were linear, where single line represented all four irrigation treatments across different nitrogen levels. Over the range of water inputs from 328 to 131 mm between planting and harvest, potato yield increased by about 92 kg ha−1 for each mm of water applied (Fig. 2a). This function had a high coefficient of determination (R2 = 0.973). Previous studies have also shown that potato yield responded linearly to the amount of applied water ( Martin et al., 1992, Fabeiro et al., 2001 and Yuan et al., 2003). The response to nitrogen was also linear over the applied range from 160 to 340 kg N ha−1 across irrigation treatments, where potato yield increased by about 62 kg ha−1 for each kg of N applied ( Fig. 2b). This function had a high coefficient of determination (R2 = 0.882). Ferreira and Goncalves (2007) in potato, Kirnak et al. (2005) in muskmelon and Aujla et al. (2007) in eggplant have reported a similar association-linking yield to water and N application levels.