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General Description

The ITRC-METRIC process is based on a surface energy balance and includes corrections for aerodynamic resistance. It depends upon both accurate and frequent LandSAT satellite thermal images and understanding of the cropping systems within a region. The METRIC programs have gradually evolved from research in the US and other countries with the objective of being able to directly estimate actual ET over large areas with limited data availability (such as crop type, irrigation method, irrigation practices, etc.).
http://www.itrc.org/projects/metric.htm

Model Domain

Evapotranspiration of land uses for Californian crops.

Developer

The Irrigation Training & Research Center (ITRC) at California Polytechnic State University, San Luis Obispo

Hardware computing requirements

None specified

Code language

None specified

Original application

Contracted by the California Department of Water Resources (DWR) to compute actual evapotranspiration (ETc) from vegetation throughout the California Delta for 2015 and 2016.
Original METRIC developed by Dr. Richard Allen (University of Idaho)

Public/proprietary and cost

All equations publically available.
Corrections require either advanced coding or access to REF-ET software to calculate ETo reference value for the study area.

Physically or empirically based

Empirical

Mathematical methods used

ITRC utilizes a grass reference ETo basis for all aspects of METRIC as well as integration of raster based ETo for interpolation, in contrast with original METRIC methods which used alfalfa.
METRIC modeling process relies on surface temperature data from the LandSAT thermal band. Actual ETc cannot be computed for the regions covered by clouds or fog.

Input data requirements

• LandSAT imagery (cloud-free)
  
• Digital elevation maps
  
• NASS CropScape data or other land cover imagery with the requisite crop information, corrected/smoothed
  
• CIMIS Corrected weather station data (hourly and daily) - Solar radiation (W/m2), vapor pressure (kPa), air temperature (ºC), wind speed (m/s), precipitation (mm), relative humidity (%), dew point temperature (ºC), PM ETo (mm)
  
• Corrected spatial grass reference evapotranspiration (ETo) maps (daily)
  
• Spreadsheet calculated values
  
• Tabulated constants

Outputs

Raster imagery of ET values (12 band, one for each month)
Crop ETc tables for California zones available online as pdf and .xls for wet-dry-typical years

Pre-processing and post-processing tools

REF-ET software or other methods needed to calculate ETo values (Spatial CIMIS may be effective).
GIS or other

Representation of uncertainty

No specific representation

Prevalence

Commonly accepted method of ET assessment – part of OpenET

Ease of use for public entities

Requires user with a strong knowledge of agricultural practices in the study area and capability with GIS or access to high quality, corrected data sources (specifically Spatial CIMIS, land cover data).

Ease of obtaining information and availability of technical support

Scientific basis widely available, manuals and help on applications are scarce.

No ITRC contacts for implementation.

Source code availability

None

Status of model development

No further direction specified, most recent report dated 2016.

Challenges for integration

ET methods require cloud-free satellite imagery, which may not be available for desired modeling period or at frequency needed for other models. Raster imagery may need to be scaled for other model output boundaries.