Direct and Indirect Measurements of Evapotranspiration from Cropland in western Manitoba, Canada

There are several approaches for estimating evapotranspiration (ET) from cropland, ranging from empirical models based on common climate data to calculating the soil water mass balance between seeding and harvest, to measurements based on micrometeorological techniques. Each approach has benefits and limitations, and more work in different climate regions and soil zones is required to compare and improve methodology. This is the case on the Canadian Prairies, where few micrometeorological measurement campaigns of ET over annual cropland have been reported. The work in the present study is being conducted as part of a larger multi-year study investigating the water, carbon and nitrogen balance components of contrasting cropping systems in southwestern Manitoba. Evapotranspiration is being measured directly at some sites using eddy covariance and indirectly at others using the energy balance residual technique, where ET is estimated as the difference between net radiation (Rn) and the sensible and ground heat flux densities (H and G, respectively). Although it would be ideal to directly measure ET with the eddy covariance technique at all of the sites being studied, in the present research program we wanted to investigate the daily to seasonal water balance at multiple fields on the same farm, which made a cheaper measurement methodology desirable. A mobile, roving eddy covariance system was deployed periodically during campaigns throughout the growing season to evaluate the ET measurements derived from the energy balance residual stations. Careful, duplicated measurements of Rn were made in separate locations in the field flux footprints, and G was evaluated with duplicated arrays of heat flux plates and soil temperature/moisture probes. Previous research from boreal forest sites in Canada (Amiro, 2009) indicated that the techniques can compare well at weekly to seasonal time-scales and we wished to evaluate this for our cropland sites. In addition to the micrometeorological flux campaigns at the sites, ET was also estimated using the simplified water balance (residual of precipitation and soil water variation) and common empirical models and compared to the direct and indirect measurements. Preliminary results from the 2012 and 2013 growing seasons will be presented and discussed in the context of the relative uncertainty of the measured variables (Rn, H and G), the assumptions made with the techniques, energy balance closure, and how the micrometeorological measurements compare with simpler methods of estimating and modelling ET.