Comparing Methods for Estimating Crop Consumptive Water Use in the Delta: Background and Preliminary Results
Eric Kent, Kyaw Tha Paw U, Jenae’ Clay, Michelle M. Leinfelder-Miles, Josué Medellín-Azuara, Yufang Jin, Quinn Hart, Andy Wong, Nadya Alexander,
Andrew Bell, Martha Anderson, Daniel Howes, Forrest Melton, Tariq Kadir, Morteza Orang, Jay R. Lund
SJC and Delta Field Crops Meeting
January 6th, 2017
Background
• SB 88: measurement of diversions
• Challenges to this: Measurements difficult because of hydrology—shallow ground water, channel waters higher than land levels, so seepage important
• ET/consumptive water use measurements can be more accurate way to get at agricultural water use
Background: Measuring and Estimating Consumptive
Water Use Field Measurements 7 Model-Based ET Estimates
https://watershed.ucdavis.edu/project/delta-et
Background: Estimating Consumptive Water Use
• CalSIMETAW (DWR) • DETAW (DWR) • SIMS (NASA)
Crop coefficient methods
𝐸𝑇𝑎 = 𝐸𝑇𝑜 ∗ 𝐾𝑐
Snyder, R.L. 2014
Crop coefficient
Reference ET Accounts for weather
Background: Estimating Consumptive Water Use
Satellite Remote Sensing
Advantages • Complete spatial coverage • Less expensive than field measurements (for large
numbers of fields) Disadvantages • Satellite overpasses only about every two weeks –
ET must be interpolated in between • Additional uncertainties
Background: Estimating Consumptive Water Use
Direct measurement of water vapor flux with Eddy Covariance
Energy Balance Residual with Eddy Covariance
Energy Balance Residual with Surface renewal
Field Measurements
Field Measurements Energy Balance Residual Method-for Consumptive Water Use = actual Evapotranspiration
𝐸𝑇𝑎 =𝐿𝐸
𝐿
Water Evaporated and Transpired (mm)
=
Total amount of energy used to change water from liquid to gas
L=the amount of energy needed to convert 1 mm of water from liquid to gas
Field Measurements
Energy Balance Residual Method
𝐸𝑇𝑎 =𝐿𝐸
𝐿
Water Evaporated and Transpired (mm)
=
Total amount of energy used to change water from liquid to gas
the amount of energy needed to convert 1 mm of water from liquid to gas
𝑅𝑛 = 𝐻 + 𝐺 + 𝐿𝐸
So we will obtain ETa by measuring LE
Field Measurements
Energy Balance Residual Method
𝐸𝑇𝑎 =𝐿𝐸
𝐿
Water Evaporated and Transpired (mm)
=
Total amount of energy used to change water from liquid to gas
the amount of energy needed to convert 1 mm of water from liquid to gas
𝑅𝑛 = 𝐻 + 𝐺 + 𝐿𝐸
LE = 𝑅𝑛 −𝐺 − 𝐻
Rearranging to solve for LE
So we will obtain ETa by measuring LE
Field Measurements
Net Radiation
LE = 𝑅𝑛 −𝐺 − 𝐻
• Shortwave radiation from the sun + thermal radiation from Earth
• The main source of energy at the crop surface
Field Measurements
Ground Heat Flux
Assumed negligible at the daily scale where measurements are not available
LE = 𝑅𝑛 −𝐺 − 𝐻
Field Measurements
Sensible Heat Flux
Energy from the surface used to heat the air (or vice versa)
Eddy Covariance Surface Renewal
LE = 𝑅𝑛 −𝐺 − 𝐻
2016: Alfalfa Surface Renewal
LE = 𝑅𝑛 −𝐺 − 𝐻
Energy Balance Components
12:00 00:00 06:00 18:00 00:00
July 28 July 29
(W/m2)
0
200
400
600
Preliminary Results: 2015 • One month of field measurements in fallow fields (4 fallow fields-all above
sea level)
Reference Evapotranspiration CIMIS
Bare Soil Measured Evapotranspiration
Preliminary Results: 2015 • One month of field measurements in fallow fields (4 fallow fields-all above
sea level)
• Comparison of 7 models for all Delta Crops in 2015
• 2015 preliminary report: https://watershed.ucdavis.edu/project/delta-et
Reference Evapotranspiration CIMIS
Bare Soil Measured Evapotranspiration
2016 Field Measurements
• 14 Fields; 20 ETa stations
• Alfalfa, Corn, Pasture
• Different Irrigation System Types
• Different Groundwater Table Depths
Before Alfalfa Cutting
(W/m2)
(W/m2)
Daily Total ET = 0.16 in (4.15 mm)
Daily Total ET = 0.05 in (1.25mm) After Alfalfa Cutting
Ongoing Work
• Processing and analysis of 2016 field data
• Comparison with modeled ET from 7 different methods
• Final peer-reviewed report Spring 2017
• Additional fallow field measurements
Acknowledgments
Delta Growers and Land Owners, State Water Resources Control Board, California Department of Water Resources, Delta Protection Commission, Delta Stewardship Council, North Delta Water Agency, Central Delta Water Agency, and South Delta Water Agency
Michael George, John Collins, Christine Rico, Juan Mercado, Jean-Jacques Lambert, David Edgar, Sebastian Häusler, Shirley Alvarez, Megan McAuliffe, Berline Francis, Nicolas Bambach, Nadia Vendrame, Yujiu Xiong, Phumudzo Tharaga, Rodrigo Monteiro, Matthew Gilbert, Hananeh Jafary, and Matthew Read, J. Andrés Morandé, Barbara Bellieu, and Cathryn Lawrence
This work would not have been possible without the collaboration and support from the following: