Comparison of dual crop coefficient method and Shuttleworth–Wallace model in evapotranspiration partitioning in a vineyard of northwest China

• We compared the FAO dual-Kc method with the Shuttleworth–Wallce (S–W) model in a vineyard of northwest China.
• Both approaches can estimate ET with good accuracy.
• The FAO dual-Kc method had higher accuracy in partitioning E and T, while the S–W model overestimated T and underestimated E.
• Both approaches had obvious discrepancies of E after rainfall and irrigation.
• Estimated ET and its components were sensitive to soil field capacity and wilting point in both approaches.

The objective of this study was to evaluate the potential use of dual crop coefficient method in FAO-56 (FAO dual-Kc) and Shuttleworth–Wallace (S–W) model in estimating evapotranspiration (ET) and its components (plant transpiration and soil evaporation) of a vineyard in arid region of northwest China. Continuous measurements of ET with eddy covariance, plant transpiration (T) with sap flow system and soil evaporation (E) with micro-lysimeter in 2013 and 2014, were used to validate the performance of two approaches. Results indicate that sap flow system and micro-lysimeter can provide accurate measurements of T and E at hourly and daily scales if compared to eddy covariance, respectively. The FAO dual-Kc method in partitioning ET was acceptable when using the site-specific basal crop coefficient obtained from sap flow, with the slope and intercept of linear regression of 0.96 and −0.13 mm d−1 (R2 = 0.81) for ET, 0.92 and −0.07 mm d−1 (R2 = 0.76) for E, 0.93 and 0.16 mm d−1 (R2 = 0.80) for T, respectively. The S–W model can better estimate ET, but overestimated T and underestimated E when using site-specific soil surface resistance, with the slope and intercept of linear regression of 0.98 and 0.28 mm d−1 (R2 = 0.79) for ET, 0.49 and 0.42 mm d−1 (R2 = 0.46) for E, 1.10 and 0.38 mm d−1 (R2 = 0.81) for T, respectively. Both approaches had obvious discrepancies of E after rainfall and irrigation, especially the S–W model, and overestimated T after a snowfall. Sensitivity analysis indicates that estimated ET and its components were sensitive to soil field capacity and wilting point in both approaches, and in the S–W model, predicted T was also sensitive to leaf area index (LAI) and minimum stomatal resistance and predicted E sensitive to soil surface resistance and LAI. Thus two approaches can estimate ET with good accuracy, but the FAO dual-Kc method had higher accuracy in estimating E and T.