Exercise 1 Deriving
Essential Climatic Variables for Driving 3-PG Model

Problem: 3-PG requires meteorological data not available at standard weather stations.

Solution: Use techniques from meteorology to derive variables other than precipitation and temperature extremes.

Procedures for estimating vapor pressure deficit, frost frequency, and solar radiation:

A. Open sheet labeled’ Exercise 1’ and copy monthly __max__
and __min__ temperatures from “Climate data” block.

B.Open sheet labeled ‘Tools’ and go to bottom of page and
find module for estimating daytime average vapor pressure deficits (green box).
Paste Tmax and Tmin along side the green VPD box—then paste both columns
separately over the existing information in the VPD box (*Past special,
transpose*).

C. Copy monthly mean VPD values generated, to Exercise 1 in
the blue Climate data block *(Past special, values, transpose)*

D. Return to ‘Tools’ and copy __minimum__ temperatures
for Exercise 1 into the green frost days box at bottom
of page. Copy frost days/month into Climatic Data box in
Exercise 1. Then delete the
pasted material that you placed alongside the green box.

E. Open sheet’ Radiation Calculation’ (solar) and paste in mean monthly (daily averages of) max and min temperatures on the columns with red headings (TMAX24H, TMIN24H). Using the following formula, the sheet will generate estimates of mean solar radiation when latitude (-41 degrees) and elevation (750 m) are inserted where labeled. Do calculations for each of the 12 months and list in a column at the left side of the page.

F. Copy column of modeled estimates of solar radiation and
return to Exercise 1 sheet to paste values (special paste as values (M Joules m^{2})
in column labeled ‘predicted’. Transfer values from Climatic data of solar
radiation into the column labeled ‘observed’.
Compare predicted with observed by making a graph and determining the
equation and r^{2}_{.}

_{ }

G. Return to’Tool’ page and note that the equation for
predicting solar radiation at SETRES site in

H. Although the day-time maximum vapor pressure deficit is relatively easy to estimate, based on the assumption that the water vapor concentration is the air is a function of the mean minimum temperature, the day-time average appears to be somewhat variable, usually falling between 0.6 and 0.7 of the mean maximum vapor pressure deficit.