. The
cells in the Theme "nigbu.shp" are the computational cells for this
exercise.
The purpose of this exercise is to demonstrate how simple soil-water budget calculations can be made within Arcview GIS using global monthly data sets. The study area is the Niger Basin in West Africa. The input data for the soil-water budget model are precipitation, potential evaporation, and water-holding capacity. In this exercise, potential evaporation is calculated using the Priestley-Taylor method; therefore, coverages of temperature and net radiation are required. A description of the soil-water budget methodology and data sources can be found at http://www.ce.utexas.edu/prof/maidment/GISHydro/seann/explsoil/method.htm. All input data has been pre-processed into a usable format. Potential evaporation and water budget calculations are made using the Avenue script language. The soil-water budgeting script could probably be made more efficient by reducing the number of times data are read and written from/to disk, but this modification has not been completed at this time. An Avenue script to make column charts of the components of the soil water budget is also demonstrated.
Before jumping into calculations, a brief tour of the input data sets in West Africa will allow the student to get a feel for the input data: temperature, precipitation, net radiation, and soil-water holding capacity.
Running this exercise requires ArcView 2.x with Avenue. If you do not run the exercise, you can still view graphics of the input data in the form of GIF files that are linked to this document. Some data preparation for this exercise was made using Arc/Info 7.0, but this software is not required to run the exercise.
Files required to complete this exercise may be obtained via anonymous ftp from ftp.crwr.utexas.edu. Once you have successfully logged into this ftp site, type cd pub/hydclass/sbudget and get all of the files in this directory. It's easiest just to use binary mode to transfer all files even though many of them are plain ASCII.
The following is a list of files that you need to obtain:
Project Files:
sbudget.apr
ArcView Shape Files:
afbord.shp,afbord.shx,afbord.dbf
ncovshd.shp,ncovshd.shx,ncovshd.dbf
nigbu.shp,nigbu.shx,nigbu.dbf
nigbupt.shp,nigbupt.shx,nigbupt.dbf
climate.shp,climate.shx,climate.dbf,climate.sbn,climate.sbx
whold.shp,whold.shx,whold.dbf
netrad.shp,netrad.shx,netrad.dbf,netrad.sbn,netrad.sbx
Avenue Scripts: (NOT NEEDED IF YOU USE SBUDGET.APR)
autoplot.ave
balance7.ave
pevap.ave
Text Files:
exer.ctl
pevap1.ctl
pevap.txt
precip.txt
"Sbudget.apr" is an ArcView project file that will load the scripts and
data required to complete this exercise. In order to use "sbudget.apr"
you must make sure that the pathnames in sbudget.apr are set so that Arcview can find the appropriate files. The version of "sbudget.apr" available at our ftp site looks for all files in a directory called "c:/sbudget". If you have a drive "c:" with write permission, the easiest way to run this exercise is to create a directory on your hard disk called "c:/sbudget" and copy all of the files listed above to that directory. If this is not convenient, you need to replace all of the instances of "c:/sbudget" in the text file "sbudget.apr" with a string that names the path to your working directory. The programs WordPad and Microsoft Word offer string search and replace algorithms if you are working in the PC or Macintosh environment. On a UNIX platform you can use the SED command to do a global search and replace. For example, to run the exercise in a directory called /home/seann/test, you can issue the command:
% sed 's/c:\/sbudget/\/home\/seann\/test/g' sbudget.apr > sbudget2.apr
to replace all the instances of "c:/sbudget" with "/home/seann/test". In this example, the output was sent to a new project file called sbudget2.apr.
Once you have a project with correct pathnames, start ArcView and open this project file. You may need to resize the window to fit your monitor.
Once you have successfully opened the project file, activate
the View called "wbudget". The boxes next to the Themes "Africa Border,"
"Niger Basin," and "nigbu.shp" should have a . The
cells in the Theme "nigbu.shp" are the computational cells for this
exercise.
This part of the exercise is intended for you to familiarize yourself with the input data: climate, soils, and net radiation.
Mean monthly temperature and precipitation estimates interpolated
to a 0.5 by 0.5 mesh were obtained from Cort Willmott at the University
of Delaware. These data are from the "Global Air Temperature
and Precipitation Data Archive" compiled by D. Legates and
C. Willmott. The monthly precipitation estimates used in this
study were corrected for gage bias. Data from 24,635 terrestrial
stations and 2,223 oceanic grid points were used to estimate the
precipitation field. The climatology is largely representative
of the years 1920 to 1980 with more weight given to recent ("data-rich")
years (Legates and Willmott, 1990).
The raw data obtained from the University of Delaware were downloaded via ftp in formatted text files. FORTRAN and AML were used to generate a polygon coverage (converted to a shapefile to simplify ftp transfer) of these data for West Africa. Mean monthly and annual climate data are attributes of the shapefile climate.shp. The Themes that you see called "Average Temp" and "Mean Annual Rainfall" both reference the file climate.shp.
To view the climate data in the project sbudget.apr:
next to the Theme "nigbu.shp".
tool.
next to the Theme "Average Temperature"
and add a next to the Theme "Mean Annual Rainfall."
If you are not running ArcView, you can take a look at the distribution of annual temperatures for this region of the globe by clicking here: ViewTemperature and the annual rainfall by clicking here: View Precipitation
A grid of water-holding capacity estimates was provided by FAO for the Niger Basin project. Values in this grid were compiled by an expert at FAO using information from the CD-ROM Digitized Soil Map of the World (FAO, UNESCO, 1974-1981). There are 6 unique values of water-holding capacity in this grid: 0, 10, 30, 75, 125, and 200. For use in ArcView 2.x, the grid of water-holding capacity estimates provided by FAO was converted to a polygon coverage.
To view this data remove the check mark next to the Theme "Mean Annual Rainfall" and check the box next to "Water Holding Capacity".
You may also see the distribution of water-holding capacity by clicking here: View Water-holding capacity.
Global mean monthly net radiation data generated as part of the International Satellite Cloud Climatology Project (ISCCP) are now available for a 96 month period extending from July 1983 to June 1991. The data are given on the ISSCP equal-area grid which has a spatial resolution of 2.5 degrees at the equator. A Theme with this data attached is available in your project ("sbudget.apr"). In the attribute table for the Theme "Mean Monthly Net Rad" there are 96 fields which contain estimates of net radiation (W/m^2) for each of the months between July 1983 and June 1991. There are also 13 fields that contain the eight-year monthly and annual mean values for this period of record.
To view a map of net radiation averaged over all 96 months (W/m2), remove the check mark next to the Theme "Water Holding Capacity" and check the box next to "Mean Monthly Net Rad."
You may also see the radiation map by
clicking here: View Netrad
You can see that net radiation has low values in the dry desert regions (inland) and higher values along the coast and in the ocean. Differences are caused by the higher albedo (reflection of incoming short-wave (solar) radiation) in the desert and the fact that much of the incoming energy in the wetter regions is used to transform water into water vapor and thus can't be re-emitted as long-wave radiation. Lower water vapor and fewer clouds in the desert regions allow more long-wave radiation emitted by the surface to pass freely back out of the atmosphere.
The script provided here uses the Priestley-Taylor method to calculate potential evapotranspiration (PE). For more discussion of methods for estimating PE, see http://www.ce.utexas.edu/prof/maidment/GISHydro/seann/explsoil/method.htm.
), and use the selection
tool 
, select some computational points. I would suggest using 10 points
or less.
button
in the Project window to add the textfile "pevap1.txt" to the project. These are
the calculated results. View Add Menu
to select the cells in "nigbu.shp" where you
wish to compute the soil-water budget.
This computation takes some time, so selecting less than 10 cells is wise.
You can select any cells (not restricted to points where you have
calculated potential evaporation because the results for all
cells have been pre-computed for you).
Four dBase files have been created by the script "balance7.ave" that was
initiated when you selected "Run Soil Budget." "Surp1.dbf" contains
computed surplus. "St1.dbf" contains soil moisture storage.
"Dst1.dbf" contains change in storage, and "evap1.dbf" contains
computed evaporation.
A script has been written which makes it easy to view the charts of the
precipitation, potential evaporation, soil moisture, evaporation,
and surplus. To plot these charts, click on the 
and then click on a cell where the soil water budget has been computed.
You should see 5 column charts pop up.
O.K. That's all for now. Have Fun!!
