Computers on the Farm Conference

Using Data From The University Of Missouri
Commercial Ag Weather Station

There are primarily three uses I have found for the information provided by the weather station located at Lamar, Missouri. Access has really been convenient with the data presented on the web at the AgEbb site. The events recorded by the weather station are air temperature, relative humidity, wind direction and speed, soil temperature at the two-inch depth and solar radiation. Data can be found for hourly and daily conditions. Reports provided on the web site include a daily weather report, hourly weather data, calculated weather indices and accumulated weather summaries, year to date. The various locations of weather stations can be found on the web at http://agebb.missouri.edu/weather/stations/index.htm. By clicking on the county of choice, the data can be found.

The first piece of information that I found useful was the soil temperature. In the spring, gardeners would call asking when the soil was warm enough to plant tomatoes. We were able to provide that information by accessing the web and checking. In many cases we gave the web address to those that had Internet access so they could track it themselves.

The second use has been a great deal more extensive. While southwest Missouri isn't a huge irrigation area, seven counties irrigated 20,560 acres of farmland according to the 1997 Census of Agriculture. Three of these counties irrigated 90 % of this. When I began working in southwest Missouri in 1990, it was pointed out to me that this area has wet springs and falls and dry summers. A rough estimate in the past 10 years is that's been true about 75 % to 80 % of the time. When the summers have been extremely dry, neighboring producers complain about irrigators lowering the water table below their well's ability to pump water. Whether this is the case or not, it's becoming increasingly important to use irrigation water diligently. Irrigation scheduling has been in use for a long time. My first exposure to scheduling was a 1968 MU Guide entitled "Irrigating Corn on Claypan Soils in Missouri" by Dr. C. M. Woodruff.

Dr. Woodruff had developed a unique concept in plotting rainfall, runoff and crop stress. Crop use was assumed to be 0.16" of water per day. Since that time others have suggested that water needs vary depending on environmental conditions as well as stage of plant growth.

In the 1970s, I believe, a study was conducted at the University of Missouri Southwest Research Center regarding evapotranspiration. It can be seen from this data that evapotranspiration during June, July and August can average 0.20" to 0.22" per day with a maximum of as much as 0.37" per day.

I have used these two pieces of data, soil moisture capacity by soil type and the environmental observations recorded on the web by the weather station to develop a spreadsheet that predicts the need for irrigation. The weather data used includes maximum temperature, wind speed and precipitation.

Some of my water needs calculations are a bit crude, but valid I think. For example, how does wind affect evaporation? There is a complicated formula that determines this relationship, but it contains variables of which I've never heard. So, here's my rationale. In a grain bin, the evaporation of moisture from the kernel can be estimated by Btu/hr = cfm x change in temperature x 1.1. Then it takes an estimated 1250 Btu to evaporate a pound of water. Wind speed can be converted from mph to fpm (feet per minute). Total plant leaf surface area can be converted to square feet. So multiplying leaf surface area by feet per minute air velocity yields airflow in cfm (cubic feet per minute). Then if you assume only one-degree temperature change across the leaf surface, you can estimate moisture evaporation. Or so I think. At least that's what I've done. In the spreadsheet, ranges of air speed have been used to increase the daily water needs of the crop as follows:

Additionally, greater ambient air temperature increases evaporation, hence the higher maximum evapotranspiration rates recorded at the Southwest Center. In the spreadsheet, I've assumed that a one day event of higher temperature doesn't affect the crop that much, so I average a three day event and increase or decrease the water need according to the following parameters:

Imposing the effect of wind and temperature on crop use then varies the need from a low of 0.16" per day up to a maximum of 0.36" per day. The point to all this is not the numbers of my assumptions, but rather the process. The numbers can be changed in the formulas, but I believe the process is valid.

The third use of the weather station data has been to create windroses. A windrose is a graphical presentation of the frequency of wind direction and/or the velocity of wind by direction. The Midwest Plan Service has included windroses for various locations in the United States in some of their handbooks based on data collected from 1951 to 1960. The nearest these are to southwest Missouri are Kansas City, MO, St. Louis, MO, Little Rock, AR, Wichita, KS and Oklahoma City, OK. The weather station collects data for wind speed and direction, but it's only displayed on the web hourly for a 24- hour period then it resets. However, I've been able to obtain the annual records from AgEbb. Consequently, I've created a spreadsheet that tabulates the frequency of direction from 16 compass points as follows:

The same data can be also be used in the spreadsheet to create a windrose showing the maximum and/or the average velocity by direction.

My primary intent for the use of windroses was to locate livestock facilities in relation to the farmstead, neighboring producers and neighboring landowners and orienting facilities in relation to environmental conditions. Another use of the wind information from the web might include determining optimum pesticide applications to prevent drift.

I have yet to use the windroses because 1994 and 2000 data are all that have been tabulated in the spreadsheet. The irrigation scheduling has been used to report to producers the amount of water shortage in 1999 and 2000. It was also used as a comparison with the Arkansas Scheduler in a study conducted in Jasper County in 2000.