Water use efficiency and its drivers of two typical cash crops in an arid area of Northwest China

Ecosystem water use efficiency (WUE), an important indicator to evaluate the interaction between the carbon cycle and the water cycle, plays a critical role in water resources management. In recent years, the cultivation of cash crops is becoming increasingly widespread in arid areas of Northwest China. However, the magnitudes and drivers of their WUEs remain unclear. In this study, we conducted continuous observations of water-carbon fluxes and related environmental factors in the field with the four rotations of cash crops in Northwest China. The results showed that: (1) The mean WUEs of cabbages and oats throughout their whole growth period are 2.24 ± 0.13 and 3.78 ± 1.68 g C kg −1 H 2 O, respectively. The WUE of oats is 1.56–1.77 times higher than that of cabbages, mainly due to their larger differences in gross primary productivity (621.8 and 1025.8 g C m −2 ). (2) The correlation analysis and the principal component analysis indicate that the WUEs of cabbages and oats are mainly affected by the physiological factors (leaf area) and environmental factors (air temperature), respectively. (3) The WUE of cabbages and oats are respectively 1.16–4.82 times and 2.33–9.69 times higher than those of main grain crops, such as maize, wheat, rice, etc. There is a more significant water-saving potential in the two cash crops than main grain crops in arid areas. These findings can provide a reference for the scientific development of typical cash crops and the water-saving practice of agriculture in Northwest China.