Soil dissipation studies are designed to show degradation and mobility of a compound and its degradates under actual field or use conditions. The objectives of these studies are three-fold: determine a half life of the test compound; determine the concentration, formation and decline of the compound’s degradates; and determine the potential for the test compound and its degradates to leach under site specific conditons. This last objective is where the focus of this paper will be.

How do you determine the "correct" amount of water to apply to a soil dissipation study? How does a sponsor show a leaching event has occurred at a soil dissipation site? Past experience has been to use 110 - 120 % of the historical monthly average to determine a sites water needs. However, in certain areas of the US, during the summer month’s, this would be 0. To show leaching events, some sponsors use tracers such as Bromide, and analyze for the tracer. However, this method creates additional samples to analyze and adds significantly to the cost of the study. With new guidelines being drafted for soil dissipation studies that suggest the inclusion of cropped plots in addition to the traditional bareground plots, there will be a need for a more accurate method of determining water needs for soil dissipation test sites. One method is including the use of Evapotranspiration data in calculating water requirements.

BASF has been incorporating the use of Evapotranspiration or ETo data for several years in determining water needs for soil dissipation studies. The term ETo is used to quantify the net removal of water out of the soil caused by air and plants. Evaporation by air and transpiration by plants thus ETo. This method of determining water needs of crops has been used extensively in irrigated areas.

Problems in the past with calculating ETo was that you had to rely on field methods that were not very accurate and in most cases involved systems that were expensive to establish and maintain, such as the direct measurements from lysimeters or evaporation pans. However, there are also indirect methods such as calculating equations. The most widely used is the Penman - Monteith equation. The main drawback for using these equations in the past has been the complicated mathematical calculations that required computers for processing. However, with the new, more advanced weather stations, these calculations can be performed by the weather station datalogger using data collected from on-site. The processing of data collected throughout the day has improved the accuracy of the daily ETo data.

The Penman - Monteith equation uses several environmental measurements to calculate ETo: Air temperature, relative humidity, incident solar radiation and wind speed. By collecting and processing this data at closely spaced intervals in the field, the weather stations ETo accuracy is improved.

ETo data is expressed as the water needs of a reference crop, typically a pasture grass growing in deep soil with no limiting factors. This number has to be adjusted for different crops. Research has lead to the development of crop coefficients or Kc’s for most agricultural crops. The Kc’s for a certain crop can vary depending on the crop’s growth stage. By multiplying the ETo number by the Kc, you arrive at the ETc, a number which estimates a specific crops’ daily water needs.

In addition to using ETo in calculating the amount of water to apply to a soil dissipation study, BASF has used this data in another way. When used in combination with soil characterization data, on-site rainfall and irrigation, ETo can also be used to calculate volumetric soil water changes and determine leaching events. BASF has now established a second method at each soil dissipation site by which to show the effectiveness of using ETo data to determine soil dissipation site water needs and to show changes in soil water content and calculate soil leaching events. Through the use of time domain reflectometry or TDR technology, we can show a physical measure of changes in volumetric soil water.

Whether using ETo or TDR, accurate on-site rainfall and irrigation records are critical for these methods to work. To show the accuracy in estimating water needs using ETc in our soil dissipation studies, I have included TDR data from several soil dissipation sites we established in 1999.

The first site we will look at is a turf study established in Texas with both a bare ground and a turf plot. I include these three water measurements (Figure 1) for each site for 2 reasons: 1) to show the difference between the two methods of determining the amount of water needed and 2) to show how well the site was managed in respect to keeping up with the water needs based on these methods. This area of Texas is not usually considered an "irrigated" area for crop production and it is evident why from the data shown on this slide. Note that the 110% historical rainfall amount is greater than the 110% ETc requirements. However, since most commercial turf is grown under irrigation, we based the water needs at this site on ETc. The ETc data shown here were based on ETo data collected on-site. Based on this method, the site would require 14.5" of rainfall/irrigation for the first 102 days of this study. The site received approximately 2.5" more water than the ETc requirements over the first 102 days after the application of the test substance.

Here we can see how closely water additions followed the site’s needs based on ETc calculations (Figure 2). BASF requires that the water needs of the site be adjusted every 10 days. Notice the points at which ETc is exceeded by additional irrigation and/or rainfall events. We would likely expect that there would be soil water movement on these dates if the soil had been near field capacity immediately prior to these water applications.

Here we have graphed the TDR data in the bare soil plot (Figure 3). The data is collected in 1 foot increments allowing us the capability of following the change in water content throughout the soil profile. Each spike in the 0-1 ft segment represent a water addition, either rainfall or irrigation. Using soil characteristic data along with the TDR data, we can show leaching events. In the bareground plot, we see the first leaching event occurred at 19 days after the initial application of the test substance and several additional events occurred through the first 102 days of the study.

Graphing the TDR data in the turf plot (Figure 4), we can see that the first leaching event occurred at approximately the same time as in the bare soil plot. However we show a smaller amount of leachate and fewer additional leaching events in the turf plot compared to the bare soil plot. This is what we would expect since the bare soil plot has no crop to help utilize the water additions.

A second turf site was established in Illinois. Again, we measured soil water content with TDRs in both the bare ground and turf plots. Based on the ETc for this site, we can see that irrigation was necessary (Figure 5). An additional 9 inches of water was calculated to be needed based on ETc data when compared to the historical rainfall average. Water additions amounted to 2.5 inches more water than what was calculated by the ETc data. Bare ground and turf plots were treated alike in respect to water additions.

(Figure 6) Here we see how well the water additions matched up the sites needs over the first 136 days after the initial application of the test substance. Initially, the water needs of the site were closely matched but the additions fell off after about 40 days and the site did not catch up until approximately 100 days after the first application. This data should help visualize the problem of falling behind in making water additions to a soil dissipation study. It is very difficult to catch up. The limiting factor is usually the infiltration rate of your soil or the amount the site can take before ponding occurs. Notice at approximately day 75 that the daily ETc is being exceeded and we could expect some water movement downward if the soils field capacity has been exceeded.

By graphing the bare ground TDR data (Figure 7), we see some very small leaching events early on followed by no detectable events until approximately day 80. At this time, even though the site is behind in the overall water additions, the daily ETc requirements are being exceeded, due to a decrease in the daily ETc numbers and an increase in the irrigation intensity.

Here are the TDR results in the turf plot (Figure 8). Pay close attention to the scale on this graph for the accumulated leachate. There are several small leaching events early on then a lull, followed by the increase in leaching amount. Essentially follows the same pattern as in the bare ground soil except in much smaller quantities. This is basically what we would expect, given our knowledge of the crop’s water needs.

Next, we look at a bare soil site in Idaho. The TDR unit is set up to measure soil moisture changes in the bare soil plot. Rainfall is scarce at this site. Over the 110 day period following the initial application of the test substance, the 110% of historical rainfall amounts to only 1.6" (Figure 9). The collection of accurate ETo data is critical for this area. Almost all crops grown in this area are irrigated based on the crop’s water needs or ETc. Using ETc numbers based on the proposed labeled crop, we generate a number which we feel better estimates what water additions to the site would be under normal growing conditions. By 110 days after the first application of the test substance, we can see that the site is behind in meeting the crop’s water requirements by 3.5".

By plotting the acumulative ETc data with the accumulative water additions (Figure 10), we can see that the sites’ water additions began to exceed ETc at approximately 20 days after the first application of test substance but fell back below the ETc level at approximately 45 days after the first application of the test substance. Since the first application of the test substance was based on a crop’s growth stage, and the last application of the test substance was targeting the day of harvest, this first application was late in the year and the irrigation system was shut down for winter before the site could get caught up.

Plotting the TDR data (Figure 11), we can detect the water additions and follow the changes in soil water content through the soil profile. Here we see the first leaching event at 17 days after the first test substance application. No additional events occur and we would not expect any until the irrigation amounts increase enough to match or exceed ET estimates, which will not likely happen until early in the spring.

Next, we look at a site established in a peach orchard in Central Georgia. The site had a bare soil plot but was physically located within the rows of mature peach trees. The planting pattern was a high density method with the in-the-row spacing of these trees being less than 8 feet. As you see here (Figure 12), based on 110% of the historical rainfall data, the site should get 21.1" of water during the first 148 days of the trial. Since this area is in a location not normally considered irrigated, we initially based the site’s water needs on this 110% historical rainfall number. For comparison, I show the 110% of ET requirements for peaches grown in this area. Notice the difference between these two numbers, over 6 inches. By collecting and analyzing the TDR data regularly, we recognized early on in the trial that the amount of water being applied to the site was not enough to generate a leaching event. We increased the frequency and intensity of the soil water additions.

(Figure 13) Even though the inadequacy of the 110% of historical numbers were detected early, the water additions did not catch up to the water needs until approximately 110 days after the first application of the test substance. Even though this was a bare ground plot, it was physically located within the orchard and was influenced by the crops growth.

Plotting the TDR data (Figure 14), we see no increases in soil water content below the 0-1 foot horizon initially. Based on on-site observations of the soil samples, these small negative changes in soil water content were not visually evident. The samples appeared to be moist down to 4 feet. The spray schedule was such that the first application was made to the bare soil when the trees’ water needs were the greatest. We feel this is the reason the water additions were not showing up any deeper than 12". The tree roots were taking up the water as fast as it was applied. Not until approximately 86 days after the initial test substance application do we begin to see recharge to deeper depths. This recharge would have likely been delayed even longer if the trees had not began to go dormant. The importance of keeping up with the water needs of a site on a regular basis cannot be stressed too much. Based on the TDR data, a leaching event occurred at 146 days after the first application of the test substance. If the water needs for this site were based solely on 110% of the historical monthly average, the study would have been seriously compromised.

In summarizing, keep in mind that all the determination methods we use are based on data collected at the test site. The accuracy used in collecting and recording this data is critical in making these calculations work. Based on the data we have collected, the use of the 110% of historical rainfall amounts falls short of accurately predicting a site’s water needs. With the use of the TDR technology, we can show that the use of Evapotranspiration data is a much more accurate way of determining these needs and will be especially useful if or when the new guidelines are accepted and cropped plots become a required aspect of soil dissipation studies.