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

Abstract

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 −0.13mmd−1 (R2=0.81) for ET, 0.92 and −0.07mmd−1 (R2=0.76) for E, 0.93 and 0.16mm 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 0.28mmd−1 (R2=0.79) for ET, 0.49 and 0.42mmd−1 (R2=0.46) for E, 1.10 and 0.38mmd−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.