Relationship Between Crop Phenology and Exchanges of Carbon Dioxide, Water Vapor, and Energy from Croplands.

Croplands affect atmosphere-ecosystem exchanges of carbon, water, and energy in field, regional, and global scales.  These exchanges are of great importance for investigating agricultural nutrient cycling and climate change related topics.  Within this context, studying crop phenology provides a good insight of understanding the temporal patterns of crop production in fine resolution and its relationships with the fluxes of CO₂, H₂O, and energy.  In this study, we monitored crop phenology (15-min interval) using time-lapse cameras for winter wheat, spring garbanzo, spring barley, spring canola, peas, and potatoes at five different agricultural sites in the inland Pacific Northwest (iPNW) region.  Fluxes of CO₂, H₂O, and energy were also measured continuously at these sites using the eddy covariance method.  Results showed that crop growth stages correlated very well with the carbon and water fluxes for each crop species during the growing season.  For winter wheat, the peaks of CO₂ and H₂O fluxes occurred between the growth stages of stem elongation and heading in both low- and high-rainfall areas in the iPNW region.  Spring barley has similar growth stages with winter wheat, but the peak fluxes were found around the flowering stage.  For spring garbanzo and peas, the peak values were observed between pod yellowing and pod browning.  We also found that winter wheat grows rapidly after tillering with CO₂ fluxes increasing from -5 to -20 g C m^-2 d^-1 within a month and the carbon uptake rates gradually slowed down after flowering.  While for spring crops, they start growing quickly a few days after emergence and slowing down after pod browning or tuber bulking.  These results also provided baseline measurements of carbon and water flux data and crop growth stages for the croplands in the iPNW region that will benefit both monitoring works and crop dynamic modeling studies in the future.