Assessing FAO-56 dual crop coefficients using eddy covariance flux partitioning

Current approaches to scheduling crop irrigation using reference evapotranspiration (ET0) recommend using a dual-coefficient approach using basal (K-cb) and soil (K-e) coefficients along with a stress coefficient (K-s) to model crop evapotranspiration (ETc), [e.g. ETc = (K-s*K-cb +K-e)*ET0]. However, determining K-s, K-cb and K-e from the combined evapotranspiration (ET) is challenging, particularly for K-e, and a new method is needed to more rapidly determine crop coefficients for novel cultivars and cultivation practices. In this study, we partition eddy covariance ET observations into evaporation (E) and transpiration (T) components using correlation structure analysis of high frequency (10-20 Hz) observations of carbon dioxide and water vapor (Scanlon and Sahu, 2008) at three irrigated agricultural sites. These include a C4 photosynthetic-pathway species (sugarcane-Sacharum officinarum L.) and a C3 pathway species (peach-Prunus persica) under sub-surface drip and furrow irrigation, respectively. Both sites showed high overall K-c consistent with their height (>4 m). The results showed differences in K-e, with the subsurface drip-irrigated sugarcane having a low K-e (0.1). There was no significant relationship (r(2) <0.05) between root zone soil volumetric water content (VWC) in sugarcane and observed K-cb*K-s indicating that there was no stress (K-s =1), while the peach orchard showed mid-season declines, in K-cb*K-s when VWC declined below 0.2. Partitioning of K-c into K-cb and K-e resulted in a better regression (r(2) = 0.43) between the Normalized Differential Vegetation Index (NDVI) and K-cb in sugarcane than between NDVI and K-c (r(2) = 0.11). The results indicate the potential for correlation structure flux partitioning to improve crop ET coefficient determination by improved use of eddy covariance observations compared to traditional approaches of lysimeters and microlysimeters and sap flow observations to determine K-c, K-e, K-s, and K-cb. Published by Elsevier B.V.