Analysis of rainfed alfalfa evapotranspiration measured by an eddy covariance system

The aim of this study was to quantify the evapotranspiration (ETec) of a rainfed alfalfa crop using the eddy covariance technique. The study was carried out during the alfalfa growing seasons (April- August 2009, April-August 2010) at the experimental farm of the University of Perugia. In central Italy alfalfa is grown for 3 to 4 years continuously, with at least 3 cutting cycles for year (usually between April and August) and a dormant period in winter. For the quantification of ETec an open-path eddy covariance system (EC) was used. The derivation of water and energy fluxes starting from raw wind, temperature and gas concentration data by means of the EC technique implies a remarkably long sequence of operations including calibration, corrections and statistical tests for assessing data quality. These operations were carried out by the EddyPro® software. After that, the output data were used for the flux-partitioning and all original data, flagged with a quality indicator with non-turbulent conditions, were dismissed. Then the gap-filling of the EC and meteorological data was performed to obtain reliable values. Furthermore the test of the energy balance closure gave satisfactory results. The ETec dynamics were consistent with the growth stages and the cuttings during both 2009 and 2010. Furthermore the comparison between the tabulated crop coefficients (Kc) and the ratio of ETec to reference evapotranspiration (ET0) was performed. This analysis showed a good agreement during the 2nd cutting cycle (May-June) for both 2009 and 2010, whilst during the 3rd cutting cycle (July-August) the ratio ETec/ET0 was considerably lower than Kc for both years. The reason of this behavior was found in the presence of water stress conditions during the last cutting cycle. This fact was confirmed by the application of a bucket soil water model, used as an exploratory, not confirmatory, tool to analyze the soil water availability dynamics during the growing season. Additional measurement campaigns will be carried out in order to deepen the knowledge about the Kc dynamics in rainfed crops and to assess the productivity of water under various meteorological and agricultural conditions.