A Spreadsheet Macro for Post- Processing Data from a Weather Station/Energy Balance System Antonio Gonzalez 1 and Eric Harmsen 2, 1 Dept. of Mechanical Engineering 2 Dept. of Agricultural Engineering

Acknowledgments This material is based on research with financial support from NASA-URC, UPRM-TCESS and NASA-EPSCoR.

Water Conservation Agriculture is the greatest consumer of water in society. It is estimated that 69% of all water withdrawn on a global basis is used for agriculture. Conservation of water requires estimating the crop evapotranspiration

Evapotranspiration Equations Penman-Monteith Energy Balance Method Vapor Gradient Method The above methods requires meteorological data!!

Weather Station Measured Data Air Temperature Relative Humidity Net Radiation Wind Velocity and Direction Barometric Pressure Soil Moisture Soil Temperature Soil Heat Flux

The vapor gradient method requires that the relative humidity and temperature be measured within and above the crop canopy

Initially the Temp/RH sensor was raised and lowered manually

Weather Station The weather station was modified to automatically obtain the vertical temperature and relative humidity (RH) at two heights (0.1 and 2 m). Data is saved to the data logger every 10 seconds. Temp/RH data are collected in each vertical position throughout the day every two minutes.

Elevator DeviceProgrammable Logic Control Device for automatically raising and lowering the Temp/RH sensor

Problem To obtain the vapor density gradient it is necessary to separate the humidity data into the “up” data and the “down” data. Since data is collected every 10 seconds, manually processing the data is tedious and time consuming.

The Spreadsheet Macro Separates automatically the data into up and down positions. Produces graphs of RH, Air Temperature, Net Radiation, Wind Speed, Soil Temperature as a function of time. Calculates the vapor density differences between the up and down positions. Estimates evapotranspiration using a vapor gradient approach and the Penman-Monteith energy equation.

Visual Basic Code

Tables

Some Graphs Produced by the Spreadsheet Macro

Relative Humidity and Sensor Position

Evapotranspiration - Sugar Cane Lajas Nov. 9 th, 2004 r a = 66/u, r s = 300 sec/m, ET = 1.6 mm

Benefits of Spreadsheet Macro Manually processing the data takes many hours, whereas the spreadsheet macro can post-process the data in a few minutes. Using the spreadsheet macro eliminates errors that may be introduced when the data is processed manually. It is possible for anyone to process the data with very little training.