Estimation Of Evapotranspiration With The Metric Model: Jessie Sagona Student Airborne Research Program August 12, 2009

Estimation of Evapotranspiration with the METRIC Model Jessie Sagona Student Airborne Research Program August 12, 2009

Outline • Introduction – What is ET, why is it important?

• Method – How can we estimate ET? What assumptions go into it?

• Results • Conclusions • Acknowledgements

Introduction • Evapotranspiration (ET) is the rate of water loss from an area due to evaporation and transpiration. • Units: length per time (here, mm/hr). • Important for farmers; related to how healthy crops are

What is METRIC? • Energy balance model designed for ET • Traditionally uses input values from LandSat or MODIS • Not freely available for download!

How does METRIC calculate ET?

Solar radiation • Inputs: – Day of year – Time of day – Latitude – Elevation – Transmissivity of atmosphere – Albedo

Longwave radiation • Inputs: – Surface temperature (from Cassie and James) – Emissivity (both atmosphere and crop)

Ground heat flux • Function of LAI

Sensible heat flux • Difficult to measure directly • In METRIC, calculated via iterations with aerodynamic resistance.

Latent heat flux

Inputs to Code • Once code was ready, used temperature and SAVI estimates as inputs, with other variables set the same for every pixel (i.e. latitude).

• Two types of runs – LAI from MASTER bandmath – LAI from Emily’s analysis – Temperature always from MASTER data

~3.3 km

~0.78 km

Almond irrigation

Drip irrigation

Fanjet irrigation

1.110 1.078 mm/hr mm/hr

1.074 mm/hr

1.081 mm/hr

1.053 mm/hr

Almond ET using Emily’s LAI values • For 2nd field: average LAI = 2.65 • For 3rd field: average LAI = 2.28 • Resulting ET values: – Field 2: 1.064 mm/hr – Field 3: 1.058 mm/hr

ET using LAI from SAVI ET using LAI from ground measurements 1.110 1.078 mm/hr mm/hr

1.074 mm/hr

1.064 mm/hr

1.081 mm/hr

1.056 mm/hr

1.053 mm/hr

ET from CIMIS PenmanMonteith • The Penman-Monteith equation calculates ET based on meteorological parameters. • The CIMIS version has a slightly different treatment of wind speed compared to the “classic” version. – For almond orchard at time of flight: ET = 1.38 mm/hr

Cotton Fields

~2.56 km

~3.15 km

0.929 mm/hr

1.024 mm/hr

1.027 mm/hr

1.059 mm/hr

Cotton ET using Emily’s LAI values • Overall LAI for cotton was 1.49.

ET using LAI from SAVI ET using LAI from ground measurements

0.929 mm/hr

0.875 mm/hr

1.024 mm/hr 0.950 mm/hr 1.027 mm/hr 0.951 mm/hr

1.059 mm/hr

0.965 mm/hr

ET from CIMIS PenmanMonteith • For cotton fields at time of flight: ET = 0.572 mm/hr

Conclusions - Almonds • The fanjet irrigation is more effective than drip irrigation, leading to higher ET. METRIC underestimates ET compared to the CIMIS PM equation.

Conclusions - Cotton • METRIC strongly overestimates ET compared to the CIMIS PM equation. • However, ground LAI measurements and LAI from SAVI are very similar, resulting in similar ET values.

Conclusions - General • METRIC calculates a more accurate ET value for cotton than almonds. • LAI tends to be overestimated when calculated from SAVI, compared to ground measurements of LAI. result: ET is also overestimated

Acknowledgements • Cassie, James, and Emily for direct use of data • Shawn for guiding our project • All of the SARP staff for this great program!

Questions?