Evapotranspiration is a crucial component of the water balance of ecosystems and landscapes, especially under arid climates. In the Tarim Basin, China, there is an increasing competition for water between irrigated agriculture, mainly cotton, and natural ecosystems, which results in periods of water shortage. Such water shortages also impact on cotton. Therefore, alternative crops have been searched for, which eventually withstand such periodical water shortages better than irrigated cotton, notably Zyzyphus jujuba and Apocynum pictum. The fruit tree Z. jujuba has been promoted from the previous decade onward in parts of the Tarim Basin. A. pictum is used as medicinal plant and in a small scale as fiber crop. A. pictum is a perennial herb, which is part of the natural riparian vegetation along the rivers of the Tarim Basin and which grows without irrigation. Therefore, the objective of this paper is to investigate the crop evapotranspiration (ETc) of those three plant species over the growing season. In this paper, the Penman-Monteith approach was employed. Daily means of stomatal resistance in June was 118 s/m, 222 s/m, and 927 s/m for cotton, A. pictum leafs, and Z. jujuba, respectively. In October, those daily mean stomatal resistance climbed to 885 s/m and 742 s/m for cotton and A. pictum leafs, respectively. ETc over the growing season was 514.7 mm for cotton, 217.2 mm for A. pictum, and 339 mm for Z. jujuba. The Kc value of Z. jujuba was in the range of other fruit trees. In this study cotton attained high yields compared to world average yields at a low ETc. This high water use efficiency was achieved through a shift from flood to drip irrigation, the utilization of plastic mulch, and breeding of cotton varieties.
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