Portable canopy chamber measurements of evapotranspiration in corn, soybean and reconstructed prairie
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Evapotranspiration (ET) is the combination of soil water evaporation and plant transpiration. ET is a vital component of a field water balance; however, accurate determination of ET is difficult. Indirect methods and direct methods are used to measure ET at the field scale. Typical indirect methods estimate ET with energy balance or field water balance measurement. One possible direct method for determining ET involves the use of canopy chambers. The objectives of this study are to construct a portable canopy chamber; to quantify the diurnal and seasonal trends of ET in three cropping systems; and to compare reference evaporation (Priestley-Taylor method), ET estimated from field water balance components (i.e., rainfall, soil water storage and drainage), and portable canopy chamber measured ET. Three cropping systems, including corn and soybean in a corn-soybean rotation and reconstructed prairie, were studied at the Comparison of Bio-fuel Systems (COBS) research site in central Iowa. Portable canopy chambers were used to measure ET in the different cropping systems during the part of growing season in 2013, and the whole growing season in 2014. Three different chamber sizes were used to match different crop growth stages. Data were collected on 18 days (i.e., maximum ET flux measurements, average ET flux measurements, and diurnal measurements) during the part of 2013 crop growing season and 15 days (diurnal measurements) during the 2014 crop growing season. In 2013 (DOY 164 - 206 for corn and DOY 164 - 255 for prairie and soybean) the cumulative chamber measured ET values were less than half of the reference evaporation values, due to relatively dry soil conditions. In 2014 (DOY 156 - 261 for corn and DOY 156 - 277 for prairie and soybean), because of wet weather condition, cumulative chamber measured ET fluxes were 80% (corn), 70% (prairie), and 67% (soybean) of the reference evaporation, respectively. The cumulative chamber measured ET values were similar to the cumulative water balance estimated ET values for both growing seasons. The differences between chamber measured ET and reference evaporation were expected. However, the agreements between chamber measured ET and water balance estimated ET in both years provided strong evidence that the portable canopy chamber accurately measured field ET at the plot (m^2) scale.