Dynamics of evapotranspiration over a non-irrigated alfalfa field in the Southern Great Plains of the United States

Authors: Wagle, Pradeep; Gowda, Prasanna; Northup, Brian

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2019
Publication Date: 7/31/2019
Citation: Wagle, P., Gowda, P.H., Northup, B.K. 2019. Dynamics of evapotranspiration over a non-irrigated alfalfa field in the Southern Great Plains of the United States. Agricultural Water Management. 223:105727. https://doi.org/10.1016/j.agwat.2019.105727.
DOI: https://doi.org/10.1016/j.agwat.2019.105727

Interpretive Summary: Details on the magnitudes and annual dynamics of evapotranspiration (ET) with respect to controlling biophysical factors in non-irrigated alfalfa, a high quality perennial legume forage, are lacking. An eddy covariance system was used to measure ET from April 2016 to May 2018 over a non-irrigated alfalfa field in El Reno, Oklahoma, USA. The alfalfa field was harvested periodically for cumulative forage dry yields of ~7.5 and 10 t ha-1 in 2016 and 2017, respectively. Annual dynamics of ET followed growth patterns and status of alfalfa. Higher than average precipitation and consequent forage production in 2017 caused higher cumulative ET during the main growing season (April–October) in 2017 (734 mm) than in 2016 (652 mm). Annual ET in 2017 (wet year) was ~900 mm, which was ~0.8 times (i.e., 80%) of total annual precipitation. Optimum air temperature (Ta) and vapor pressure deficit (VPD) for ET was approximately 30 ºC and 3 kPa, respectively. The 8-day composite values of ET and ecosystem water use efficiency (EWUE, the ratio of gross primary production to ET) reached about 5.6 mm d-1 and 4 g C mm-1 ET, respectively. The standard Moderate Resolution Imaging Spectroradiometer ET (ETMOD16) product underestimated ET by 36% compared to tower ET. A strong correspondence of ET with remotely sensed vegetation indices offered the potential of upscaling site-level alfalfa ET to larger areas.

Technical Abstract: Accurately quantifying the dynamics of evapotranspiration (ET) is crucial for efficient water management and improved water use efficiency. However, details on the magnitudes and annual dynamics of ET with respect to controlling biophysical factors in non-irrigated alfalfa (Medicago sativa L.) are lacking. Using the eddy covariance (EC) technique, daily magnitudes, annual dynamics, and budgets of ET were quantified from April 2016 to May 2018 over a non-irrigated alfalfa field in central Oklahoma, USA. The field was harvested periodically for hay and cumulative dry forage yields were approximately 7.5 and 10 t ha-1 in 2016 (dry year) and 2017 (wet year), respectively. Daily ET reached up to 6.9 mm d-1 and 8-day average ET reached up to 5.64 mm d-1. Cumulative seasonal (April-October) ET was 652 mm (~1.3 times of precipitation) in 2016 and 734 mm (~0.8 times of precipitation) in 2017. Annual ET in 2017 was ~900 mm (~0.8 times of annual precipitation). Optimum air temperature (Ta) and vapor pressure deficit (VPD) for ET was ~30 °C and ~3 kPa, respectively. More forage production was associated with a greater increase (~22%) in carbon uptake (gross primary production, GPP) than ET (~13%) in 2017 compared to 2016. Consequently, ecosystem water use efficiency (EWUE) at the seasonal scale (seasonal sums of GPP to ET) was 2.38 and 2.57 g C mm-1 ET in 2016 and 2017, respectively. Despite strong correspondence (R2 = 0.73) between measured ET and Moderate Resolution Imaging Spectroradiometer (MODIS) ET (ETMOD16), the standard ETMOD16 product underestimated ET by 36% compared to measured ET. The MODIS-derived enhanced vegetation index (EVI) and photosynthetically active radiation (PAR) explained 83% of variations in alfalfa ET, indicating the potential of integrating remote sensing observations and climate data to extrapolate site-level alfalfa ET at larger areas.