Integrating flux measurements of a single eddy-covariance system and Penman-Monteith modeling to estimate rice evapotranspiration under multiple irrigation regimes

Italy is the leading rice producer in Europe, with over half of total production, almost totally concentrated in a large traditional paddy rice area between the Lombardy and Piedmont regions, in the north-western part of the country, where irrigation is predominantly carried out by continuous flooding. The high water requirements have encouraged the introduction of water saving irrigation practices, as delayed flood irrigation after sowing in dry soil and intermittent irrigation (aerobic rice). In the agricultural season 2013 an intense monitoring activity was conducted on three experimental fields located in the area, characterized by different irrigation regimes (traditional flood irrigation, flood irrigation after sowing in dry soil, intermittent irrigation), with the aim of comparing the water balance components. Actual evapotranspiration (ET) is one of the components which is more pretending in terms of monitoring effort and cost. This work explores the possibility of using a single eddy covariance system in combination with Penman-Monteith (PM) type models for the determination of ET fluxes for the three irrigation regimes. An eddy covariance station was installed on the levee between the traditional flooded and the aerobic rice fields at the experimental site, to contemporaneously monitor the ET fluxes from these two treatments as a function of the wind direction. A detailed footprint analysis was conducted (three different analytical models were applied) to determine the position and the size of the footprint area at each monitoring time step (30-min). Two sets of half-hourly ET values, one for each irrigation regime, were therefore obtained, each one comprising about 10% of the daytime time steps over the whole agricultural season. To confirm the reliability of the measured ET fluxes, the energy balance closure was computed for the two fields and it resulted in an imbalance lower than 10% for both the irrigation treatments. The two eddy covariance data-sets, together with other observational data (i.e., soil and water evaporation measured by micro-lysimeters) were then used to calibrate three Penman–Monteith type models for the estimation of the rice crop transpiration (T), the soil evaporation (ES), and the evaporation from the water covering the soil in the case of flooded regimes (EH20), respectively. The models were implemented using the available agro-meteorological data detected over the rice canopies and the periodically measured values of crop parameters (leaf area index, crop height), obtaining satisfactory calibration results. Then, the calibrated models were used to compute the complete hourly ET data series for the three irrigation regimes.