Determination of Actual Evapotranspiration and Crop Coefficients of
Tropical Indian Lowland Rice (Oryza sativa) Using Eddy Covariance
Approach
Abstract
Rice (Oryza sativa) is a major staple food crop in India occupying about
44 million ha (Mha) of cropped land in meeting food requirements for
about 65% of the population. As water scarcity has become a major
concern in changing climatic scenarios precise measurements of actual
evapotranspiration (ETa) and crop coefficients (Kc) are needed to better
manage the limited water resources and improve irrigation scheduling.
The eddy covariance (EC) method was used to determine ETa and Kc of
tropical lowland rice in eastern India over two years. Reference
evapotranspiration (ETa) was estimated by four different approaches–
the Food and Agriculture Organization-Penman-Monteith (FAO-PM) method,
the Hargreaves, and Samani (HS) method, the Mahringer (MG) method, and
pan evaporation (Epan) measurements. Measurements of turbulent and
available energy fluxes were taken using EC during two rice growing
seasons: dry season (January-May) and wet season (July-November) and
also in the fallow period where no crop was grown. Results demonstrated
that the magnitude of average ETa during dry seasons (2.86 and 3.32 mm
d-1 in 2015 and 2016, respectively) was higher than the wet seasons (2.3
and 2.2 mm d-1) in both the years of the experiment. The FAO-PM method
best-represented ETa in this lowland rice region of India as compared to
the other three methods. The energy balance was found to be more closed
in the dry seasons (75–84%) and dry fallow periods (73–81%) as
compared to the wet seasons (42–48%) and wet fallows (33-69%) period
of both the years of study, suggesting that lateral heat transport was
an important term in the energy balance calculation. The estimated Kc
values for lowland rice in dry seasons by the FAO-PM method at the four
crop growth stages; namely, initial, crop development, reproductive, and
late-season were 0.23, 0.42, 0.64, and 0.90, respectively, in 2015 and
0.32, 0.52, 0.76 and 0.88, respectively, in 2016. The FAO-PM, HS, and MG
methods produced reliable estimates of Kc values in dry seasons, whereas
Epan; performed better in wet seasons. The results further demonstrated
that the Kc values derived for tropical lowland rice in eastern India
are different from those suggested by the FAO implying revision of Kc
values for regional-scale irrigation planning.