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Water Budget Data

Methods for Collecting Water Budget Data

Description of data files for station P33 in Shark Valley Slough
Lat 25 36 55 long 080 42 11

There are 12 ASCII data files in this directory that contain the data for site P33. Some of these files are not necessary for ET computation but may be of interest anyway. All files contain data taken at 15-min intervals from 95-12-01 to 97-09-02. Format of all files is identical, with date (yy-mm-dd) in columns 1-8, time (Hr:min) in columns 10-14, and the data value in columns 15-27 (right justified). The data value is the average for the 15-min period.

These data have been reviewed and edited, and obviously erronous data have been removed (I hope). There probably are still some more subtle problems with the temperature gradient and vapor-pressure gradient data. These gradients are small in magnitude and may be perturbed at times by insect or moisture contact with the sensors. These types of perturbations are relatively infrequent and probably have little effect on the daily ET sum, though some 15-minute values of computed ET could be off. PLEASE DO NOT CONSIDER ANY OF THESE DATA TO BE FINAL!! THE DATA ARE CONSIDERED TO BE PROVISIONAL AND UNDER REVIEW UNTIL APPROVAL OF THE FINAL REPORT IN SEPTEMBER 1999. If you should find problems, or just have questions about any of these data, I would very much like to hear from you.

A summary of the energy-budget method of ET calculation may help to describe usage of the data files. The basic energy-flow equation is:

Q + G + W + Le + H = 0, (eq. 1)

where Q is net solar radiation, G is soil heat flux, W is heat storage in water (above land surface), Le is the latent heat (ET), and H is the sensible heat (convection). Values for the "W" term are not included in the data files I have supplied, but are a function of water depth, heat capacity of water, and the mean change in water temperature from one 15-min interval to the next.

The sign convention used with these data is that energy flow is "+" for energy into the ET unit, and "-" for energy out of the unit. Thus, net radiation (Q) is "+" during the day, and "-" at night, and W is "-" when the surface water is heating up during the day and "+" when the water cools. Latent heat (Le), or ET, is "-" while ET is occurring and (theoretically) "+" while condensation, or dew, is forming (although I don't think we can actually measure dew formation with our equipment).

The data files for the P33 station are:

File name Attribute Units Comments
Need for ET calculation
VAP.PRES.GRAD vapor press. grad kP/m vertical dist. 1 meter
AIR.TEMP.GRAD air temp. grad Deg. C/m vertical dist. 1 meter
SOIL.HEAT.FLUX soil heat flux watts/m**2 "G" term in eq. 1
TOP.WAT.TEMP surface water temp Celsius for calculating "W" term
BOT.WAT.TEMP bottom water temp Celsius for calculating "W" term
NET.RAD net solar radiation watts/m**2 "Q" term
WATER.LEVEL water level feet "W" term (LSD is 5.20)
AIR.TEMP Air temp. Celsius For Bowen ratio calc.
Not necessary for ET calculation
WIND.DIR wind direction deg from N direction wind is FROM
WIND.SPEED wind speed mi/h
REL.HUMID relative humidity % of saturation
PYRONOMETER incoming solar watts/m**2 LX-200 sensor

The following is a listing of a simple SAS routine I have used for preliminary ET calculation. It should illustrate most of the details of using the data in the above-listed files. In this routine, the vapor-pressure gradients (de) are automatically screened to exclude values < -0.05 Kpa or > 0.05 Kpa. When values outside this range occur, the last value within the acceptable range is used. This is a rather crude data screening procedure, and I am presently looking at other ways for filtering out questionable data.


     data atemp; infile 'AIR.TEMP';
input @1 date yymmdd8. hr 10-11 min 13-14 @15 atemp;
data de; infile 'VAP.PRES.GRAD';
input @1 date yymmdd8. hr 10-11 min 13-14 @15 de;
if de < -9998 then delete;
data dt; infile 'AIR.TEMP.GRAD';
input @1 date yymmdd8. hr 10-11 min 13-14 @15 dt;
if dt < -9998 then delete;
data  q; infile 'NET.RAD';
input @1 date yymmdd8. hr 10-11 min 13-14 @15 q;
data g ; infile 'SOIL.HEAT.FLUX';
input @1 date yymmdd8. hr 10-11 min 13-14 @15 g;
data wts ; infile 'TOP.WAT.TEMP';
input @1 date yymmdd8. hr 10-11 min 13-14 @15 wts;
data wtb ; infile 'BOT.WAT.TEMP';
input @1 date yymmdd8. hr 10-11 min 13-14 @15 wtb;
data stage ; infile 'WATER.LEVEL';
input @1 date yymmdd8. hr 10-11 min 13-14 @15 stage;

data all; merge  atemp de dt q g wts wtb stage; by date hr min;
gamma = 0.000646*101.3*(1.0+ 0.000946*atemp);
avewt = mean (of wts wtb);

data fin; set all;
y=year(date); m=month(date);
lgamma = lag(gamma);
lavewt = lag(avewt);  delwt = avewt-lavewt;
depth = stage-5.2;
wheat = -delwt*depth*0.3048*1000000/(14.34*15);
avegam = mean (of gamma lgamma);
if de < -50 | de > 50 then de = lag(de);
bowen = avegam*dt/de;
if bowen > -1.4 & bowen <= -1. then bowen = -1.4;
if bowen  <   -0.6 & bowen >= -1. then bowen = -0.6;
avail = q + g + wheat;
latent = -avail/(bowen+1.);  sense = bowen*latent;
lamda = 2500.25-2.365*atemp;
et = -0.002362*15*latent/lamda;

proc means sum; by y; var et; run;

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Last updated: 15 January, 2013 @ 12:42 PM (KP)