Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2021
Publication Date: 2/26/2021
Citation: Wagle, P., Gowda, P.H., Northup, B.K., Neel, J.P. 2021. Ecosystem-level water use efficiency and evapotranspiration partitioning in conventional till and no-till rainfed canola. Agricultural Water Management. 250:106825. https://doi.org/10.1016/j.agwat.2021.106825.
Interpretive Summary: The Flux Variance Similarity method was used to partition evapotranspiration (ET) into evaporation (E) and transpiration (T) using high frequency (10 Hz) eddy covariance data collected for the 2016-2017 growing season (November 2016 – May 2017) from co-located canola fields managed under two tillage systems (conventional till, CT and no-till, NT). Ecosystem water use efficiency (EWUE, ratio of carbon gain to water loss) was determined based on different metrics of carbon (gross primary production, GPP and net ecosystem production, NEP) and water vapor (ET and T) fluxes. Canopy stands and fluxes were similar until winter in both fields, but were substantially lower under NT during spring 2017 due to poor recovery of crop stands after winter dormancy. During wet spring periods, larger differences were observed for T than ET between CT and NT canola fields due to more evaporative loss of water in NT field caused by sparse canopy. The 7-day average T:ET ratios, which were driven by crop growth and rainfall events, ranged from =0.40 to >0.80 during the growing season. The EWUEGPP_ET and EWUEGPP_T were similar in both fields until winter, as were EWUENEP_ET and EWUENEP_T. Large differences in EWUE were observed from February to April. Large differences in cumulative NEP than GPP caused substantially large difference in EWUENEP (~79% times higher in CT) than in EWUEGPP (~13% times higher in CT) between the two canola fields at the seasonal scale. Different magnitudes of differences in EWUE based on selected metrics of EWUE highlight the necessity of using multiple EWUE metrics to systematically compare EWUE across sites.
Technical Abstract: Accurately determining productive water loss (i.e., transpiration, T, fraction of evapotranspiration, ET) is crucial to improve ecosystem water use efficiency (EWUE) and develop water-saving management practices. High frequency (10 Hz) time series eddy covariance (EC) data for the 2016-2017 growing season (November 2016 – May 2017) for conventional till (CT) and no-till (NT) canola (Brassica napus L.) fields were analyzed to partition ET into T and evaporation (E) using the Flux Variance Similarity (FVS) method. The objectives of this study were to determine and compare seasonality of T and T:ET ratios, and compare EWUE based on different metrics of carbon (gross primary production, GPP and net ecosystem production, NEP) and water vapor (ET and T) fluxes in CT and NT canola fields. The magnitude (7-day average) of T reached 3.06 ± 0.3 and 3.03 ± 0.23 mm d-1 in CT and NT fields, respectively. The T:ET ratios (7-day average) ranged from =0.40 to >0.80 as the ratios were driven by crop growth and rainfall events. Growing season average T:ET ratio was identical (~0.70) for both fields. Canopy stands, fluxes, and EWUE were similar until winter in both fields, but were substantially lower in NT field during spring 2017 due to differences in recovery of canola stands after winter dormancy. For the February-April period, EWUEGPP_ET and EWUEGPP_T were 34% and 27% higher in CT than NT field, respectively. In comparison, EWUENEP_ET and EWUENEP_T were 84% and 75% higher in CT than NT field, respectively. The results indicate that EWUE can be overestimated based on ET than T for the period of higher E. Different magnitudes of differences in EWUE based on selected metrics of EWUE between two canola fields suggest that a number of EWUE metrics are needed to systematically compare EWUE across sites.