Reference Evapotranspiration toolbox

Outline of Talk
 Introduction
 Toolbox functionality
 Results
 Conclusions and future development
The ETReference Evapotranspiration (Etr)
MATLAB Toolbox is a computer based framework,
which allows users to compute reference ET values
from available forecast gridded weather data
produced by the National Weather service or from
automated climate stations.
Application of Etr tool box using
Gridded forecast climate data
 Forecast climate data allows calculation of Etr for locations
where automated climate station data are not available.
 Forecast climate data can be used to fill in missing or error
data from automated climate stations.
 Forecast climate data are in a common format needed to
calculate Etr for the entire United State to be used with Et
calculated from satellite data.
Flow Chart of Tool. Model Runs on a
daily time step.
FTP site to download hourly data
Gridded 5kmClimate forecast data at
2 m height including sky cover, air
temp, wind speed, and dew point temp.
Data is downloaded, preprocessed and
stored as flat file in a data base.
 The 5 km gridded data is downloaded every day.
 The National Weather Service over writes the file every day.
 Past data starting in 2005 to current date on a 12 km grid is
available from another fps site.
Reference Et calculated on daily
climate variable.
 On a daily time step calculate:
 Hourly data the using the dew point temperature and max.
and min. air temperature to calculate vapor pressure deficit
for each grid point
 Average 24 hr wind speed for each grid point.
 Clear sky daily solar radiation base on extrestial radiation
calculation for each grid point.
Compute Etr from pre-processed
weather data
Eto = ∆ Rn + γ (6.43(1+0.536*U2) (es-ea)/ (∆+γ) L
 Where: Eto= mm/day
 ∆=slope of the saturated vapor pressure curve (kPa K-1)
 Rn= net radiation (MJ m-2 day-1)
 γ = psychrometric constant (kPa K-1)
 U2= wind speed at 2 m (m s-1)
 es = saturate vapor pressure (kPa)
 ea = actual vapor pressure (kPa)
 L= latent heat of vaporization (MJ kg-1)
Example of Google Earth output
Example of Calculate Daily Etr values.
 The Etr tool box currently runs on a server that downloads
and process the national weather service forecast data on a
daily time step.
 Matlab as a tool used in the Etr tool box was used to process
large gridded forecast data for the United States using limited
computer computation time (15 min).
 Matlab allowed quick development of code for the Etr tool
box compared to writing C++ processing code.
Future Development
 Convert links to data base files from hard wire to a path file.
 Develop a graphical interface to look at output
 Use Google Earth to acquire elevation for a given Lat. Long.

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