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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Soil, Water & Air Resources Research » Research » Publications at this Location » Publication #302301
Title: Partitioning evaporation and transpiration in a maize field using heat pulse sensors for evaporation measurement

Author
item XIAO, XINHUA - North Carolina State University
item Sauer, Thomas
item SINGER, JEREMY - Basf Corporation North America
item HORTON, ROBERT - Iowa State University
item REN, TUSHENG - China Agricultural University
item HEITMAN, JOSHUA - North Carolina State University

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/18/2015
Publication Date: 2/27/2016
Citation: Xiao, X., Sauer, T.J., Singer, J.W., Horton, R., Ren, T., Heitman, J.L. 2016. Partitioning evaporation and transpiration in a maize field using heat pulse sensors for evaporation measurement. Transactions of the ASABE. 59(2):591-599. doi: 10.13031/trans.59.11059.

Interpretive Summary: Rain that falls onto a crop field may either run off the soil surface to streams and rivers or soak into the ground. The water that enters the ground can then either keep moving down to groundwater or evaporate back into the atmosphere. There are two ways that the water can evaporate - directly from the soil surface or from the plant leaves after being taken up by the roots. Agricultural scientists have long sought for ways to control evaporate because evaporation from the soil is considered lost water, water that was not available for use the by plants. For over a century it has been known that plant growth and yield is directly related to the amount of water that evaporates from the plant's leaves. This study concerns new, continuous methods for measuring evaporation from the soil and from plants in the same field at the same time. The results indicate that most (85%) of the evaporation in a corn field in the middle of the summer occurs from the plants. This is because very little energy in the form of sunlight can reach the soil beneath the corn plants. Errors of the different measurement techniques were discussed and compared with a third technique used to measure both soil water and plant evaporation together. This research is important to scientists interested in separating the components of evaporation and for agency personnel and growers interested in improving water use efficiency.

Technical Abstract: 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.