Global radiation
sensors are used to measure radiation envelopes.
Improved
techniques are needed for accurate quantification of crop evapotranspiration
(ETc) and crop coefficients (Kc) to enhance efficient use of water
resources and protect water quality. Bowen Ratio Energy Balance
System (BREBS) and Eddy Correlation System (ECS) measure surface
energy fluxes to quantify ETc and Kc.
Our overall
research objectives with these high frequency measurement techniques are
to:
Measure
ETc and Kc for corn and soybean for the growing season.
Quantify
evaporative losses during the non-growing (dormant) season.
Investigate
soil profile-soil surface-atmosphere interactions and dynamics during
the dormant season.
Develop
surface coefficients for the dormant season.
Quantify
and analyze energy balance closure parameters by measuring energy balance
equation components (i.e., latent heat, net radiation, soil heat flux).
A BREBS and
an ECS were installed in 2004 over corn crop in a subsurface drip-irrigated
field (34 acres) at UNL's South
Central Agricultural Laboratory near Clay Center, Nebraska. The BREBS
measures energy fluxes every 30 seconds and outputs 60 minute data (average
of four 15 minute datapoints). The ECS samples air every 1/10 sec and
outputs 60 minute average data. Both systems were installed site-by-site
so that direct comparison of energy fluxes with both systems would be
possible if needed. The measurements with both systems include:
Incoming
longwave radiation
Incoming
shortwave radiation
Outgoing
shortwave radiation
Outgoing
longwave radiation
Net radiation
Latent
heat (evapotranspiration)
Sensible
heat
Soil heat
flux
Soil temperature
above and below soil heat flux plates
Soil water
status (hourly) every 12 in. up to 6 ft. Soil water content is also
measured above the soil heat flux plates to correct raw soil heat flux
readings for soil moisture.
Wind speed
and direction
Rainfall
Air temperature
and relative humidity (max and min)
Surface
reflection (albedo)
Actual
vapor pressure
Momentum
flux
Friction
velocity
Water
vapor density
Barometric
pressure
In addition
to the aforementioned measurements, irrigation applications (date and
amount), crop height, stomatal resistance to water vapor transport, leaf
water potential, canopy temperature and yield are being measured.
Impact and Information
Transfer:
The project
will improve our understanding of BREBS and ECS and their application
in agricultural water management. We will apply this knowledge and experience
towards development of improved methods to measure and predict ETc, Kc,
and reference ET (ETo) during the growing seasons, and evaporative losses
during the non-growing seasons in Nebraska. Our overall goals are to develop
and implement methods and tools to address and enhance adaptability, accuracy,
feasibility, and operational characteristics of the BREBS and ECS to measure/model
ETc, Kc, reference ET, and surface energy balance components. We will
work with clientele and transfer our knowledge and experience of this
project through appropriate means to help growers, their advisors, and
decision makers to enhance ET prediction/measurement for efficient use
of water resources. Results of this program will be utilized in preparation
of extension and educational materials and developing curriculum in both
undergraduate and graduate education. This research project will provide
scientifically sound data and knowledge to planners and decision makers
to enable them to develop effective water management policies that will
result in a better utilization of improved crop water use data.
BREBS and ECS
continuously monitor crop evapotranspiration and crop coefficients
of corn during the growing season.
Year-round monitoring
The Bowen Ratio Energy
Balance System over a fully matured corn crop.
1. Exchanger measures humidity and air temperature gradients.
2. Global radiation sensors measure radiation envelopes.
Energy flux measurements
with the BREBS and ECS continue after harvest to quantify
evaporation losses from bare soil or a surface partially covered with
residue.
Measurements
of evaporative losses continue during the winter dormant season to
learn
more about snow melt, soil water recharge, latent heat, albedo, and
latent heat interactions.
During the
winter, the Eddy Correlation System (ECS) monitors latent heat, sensible
heat, and other energy fluxes. (circled on the left.)
1. The KH 20 Krypton Hygrometer measures latent heat
in the gap between the source and detector modules. It is surrounded
by six white, gray-tipped fingers that form the CSAT, a three dimensional
sonic anemometer that measures sensible heat. (close-up on the right.)
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use by the University of Nebraska-Lincoln or the author(s).
