Comparing Eddy Covariance and Penman-Monteith Methods to Estimate Evapotranspiration from Mid-South Rice Production

In Arkansas, where a majority of US rice is grown, irrigation during the growing season has contributed to a significant decline in regional groundwater sources. To conserve local water resources, different rice irrigation methods were examined and compared to the continuous-flood (CF) practice, which is the most common in Arkansas. Alternate wetting and drying (AWD) is a rice irrigation practice where the flood water is allowed to decline below the soil surface for short periods to better capture precipitation, which offsets irrigation needs within the field. However, drying may also result yield loss due to drought stress.

Evapotranspiration (ET), a key term in both the hydrological cycle and the surface energy balance within a rice field, represents 90% of the available energy consumption and is the largest outflow of irrigation water in the field water balance. Because ET is tied to the hydrological and physiological conditions within the rice field, it may reflect changes during drying events, including reduction of atmospheric exchange between the canopy and atmosphere due to drought stress.

In our study, ET was measured using the eddy covariance method and calculated using a variation of the Penman-Monteith equation for two adjacent, commercial scale rice fields in Arkansas between 2015 and 2018. The field irrigation treatments were unique combinations of CF and AWD for each year. Our results indicate that AWD drying events have no significant effects on ET, which indicates that the rice plants are not under stress. Yields were comparable between both fields in all years across all treatments.

The coupling of the eddy covariance and Penman-Monteith methods produced comparable estimates of ET across all growing seasons and irrigation regimes at the half hourly time scale. The Penman-Monteith method was also able to link meteorological drivers to changes in ET to better understand what key factors elicit change in ET throughout the growing season. During drying events, available energy and atmospheric demand for water explained a majority of the variance in ET with small contributions from the water table and soil moisture. These ET estimates and methods will be essential in constructing a complete field water balance, which is necessary to accurately estimate water savings associated with AWD.