Partitioning Evaporation and Transpiration in a Maize Field with Heat-Pulse Sensors Used for Evaporation

Evapotranspiration (ET) is the sum of soil water evaporation (E) and plant transpiration (T). E and T occur simultaneously in many systems with varying levels of importance, yet it is often very challenging to distinguish these fluxes separately in the field. Few studies have measured all three terms (ET, E, and T), and in the few cases where such measurements have been obtained, E is typically determined via destructive lysimetery. For 20 consecutive days in a fully-developed maize (Zea mays L.) field, we continuously measured E using heat-pulse sensors and soil sensible heat balance, T using sap flow gauges, and ET using an eddy covariance system. Reference evapotranspiration (ET0) was also calculated from measured weather parameters with the Penman-Monteith equation. During the measurement period, E and T accounted for 15% and 85% of E+T, respectively. E responded to variation in soil moisture, whereas T changed primarily with net radiation. All three ET estimation methods (individually measured E+T, eddy covariance ET, and ET0) demonstrated similar temporal trends and strong correlation (R2 of 0.81 and 0.90 for ET0 and ET vs. E+T, respectively), but ET0 was usually larger than the individually measured E+T and eddy covariance ET, particularly several days after rainfall when the soil was drying. E+T and eddy covariance ET accounted for 83 and 53% of ET0, respectively, during the 20-d period. Disparities in measurements were likely due to variations in measurement scale, which did not reflect the full range of field variability for individually measured E and T, and differences in response to declining soil moisture amongst the three approaches. Overall, results support the need for individual measurement of each term (E, T, and ET) when attempting to interpret ET partitioning, and suggest that soil heat-pulse sensors may provide a viable compliment to previously tested approaches for determining E for ET partitioning.