April 30, 2010

A new UNL website offers soybean producers an easy-to-use decision aid for optimizing irrigation efficiency and energy use.

SoyWater, available at hprcc-agron0.unl.edu/soywater/, asks a few, simple questions specific to the field and then provides daily estimates of crop water use and soil water status.

Users log on and after just a few mouse-clicks, SoyWater provides a table of real-time, actual daily weather data for computing daily crop water use from planting (or emergence) to the day before producer log-in. It then uses historical weather data to compute daily crop water use from the day of log-in to the day of expected crop maturity.

The daily weather data is from a weather station of the Automated Weather Data Network (AWDN) maintained by the High Plains Regional Climate Center (HPRCC). (SoyWater automatically identifies the weather station nearest the producer’s field.) During the season, the producer will log in to SoyWater to input the date and amount of local rainfall and any irrigation applications.

Based on this information, SoyWater will highlight the projected date when the first (or next) irrigation should be scheduled and projects the dates of specific soybean vegetative (V-) and reproductive (R-) growth stages. SoyWater can be useful to both rainfed and irrigated soybean producers interested in predicting the date of a V- or R-stage. This information also can be useful when making other crop management decisions for a particular soybean field, such as when the effectiveness of a herbicide or fungicide depends on application at a specific growth stage.

Adding Field Specific Data

SoyWater requires minimal user input, including:

1. The GPS Coordinates for the Field. If these are not known, the producer can use SoyWater’s Google-based, aerial view to identify the field and SoyWater will then provide the GPS coordinates. The program will automatically identify an Automated Weather Data Network (AWDN) weather station closest to the field by using its weather search interface (Figure 1).
2. Soybean Planting or Emergence Date. (The latter is strongly preferred.) The user can click on the word "emergence" on the SoyWater page to view an illustration of  the emergence stage (Figure 2).
3. The Maturity Group (MG) of the Soybean Variety. This number is usually evident as part of the seed company’s variety brand number. (For example, 2.0, 2.3, 2.6, 2.9, 3.1, 3.3, 3.6 or 3.9 represent a range of early to late maturity groups of some of the soybean varieties commonly grown from northern to southern Nebraska.) A seed dealer also can identify a variety’s MG. A soybean simulation model, known as SOYSIM, is embedded in SoyWater and uses the MG value to project the calendar dates of the critical V- and R- stages of that variety, including the calendar date of R7 (physiological maturity). Knowing the projected date of R7 and the projected amount of soil water present on that date are critical to scheduling the final irrigation event (date and amount) well before the crop reaches maturity. This can help producers avoid applying irrigation water late in the growing season when it is not needed.
4. Soil Texture of the Field. To help producers with this determination, SoyWater includes a Google-NRCS map interface that shows what soil textures are present in the selected field (Figure 3). The user then selects the soil texture that is most “representative” of the field from a menu of choices. SoyWater will use this soil texture information in its soil water balance calculations.

Based on this information, SoyWater generates a table of data, including crop water use, daily soil water status, crop growth stage, and suggested irrigation dates (Figure 4).

Using the SoyWater Data

With information provided by the user and the High Plains Regional Climate Center, SoyWater generates a  field-specific table of daily crop water use and soil water status, crop growth stage, and suggested irrigation dates that can be used with any irrigation management stragegy. (See story, Using SoyWater to Schedule Irrigations.)

After logging in to SoyWater, click on any date when a rainfall or irrigation event occurred. SoyWater will open a new page so you can enter the amount of rainfall or irrigation on that date or update previous entries. SoyWater will then display a table showing all rainfall and irrigation inputs in red. For SoyWater to provide reliable, up-to-date crop water use data, it is important that the user input all rainfall and irrigation amounts.

The SoyWater table generated for each field contains four types of information:

• Date. The first column indicates the date so you can clearly input and track daily crop water use throughout the season.
• Growth Stage. The second and third columns display critical soybean V- and R- stages, based on emergence date and the relative maturity of the variety. Click on any V- and R- stage displayed in blue to bring up a drawing or photograph of the growth stage. An estimated (approximate) taproot tip depth at that stage is provided at the bottom of the illustration.
• Crop Water Data. Daily and cumulative crop water use numbers are provided in the next columns. (These will either be actual numbers for the 2010 season up to the date of producer log-in, or historical numbers for those dates after user log-in.) Columns showing the cumulative amount of rainfall or irrigation received to date during the season also presented.
• Suggested Irrigation Dates. The last column in the table shows the cumulative soil water depletion in each (daily) cell. The first date highlighted in yellow is the date when the soil root zone is estimated to have reached 35% depletion of a field capacity value. This is when the first irrigation should be applied. The next highlighted cell indicates when 35% depletion is expected to occur next and when the next irrigation should be applied. Note that the 35% depletion value is the default setting. The producer can select a different depletion percentage at the start of the season or later, although the latter is recommended only for advanced or experienced users.

Additional Benefits with SoyWater

Other irrigation-related applications are also available on the SoyWater website. Producers can input readings from hand-held meters or data loggers connected to Watermark soil water sensor probes that have been installed in the field at various soil depths. The producer must input the soil texture type, along with the readings, and SoyWater will translate the readings into inches of water per foot of soil depth. Currently, SoyWater can not use soil water sensor data in the soil water balance computation, but this utility may be added for next year.

Meeting the Information Needs of Soybean Producers

The UNL SoyWater research team is pleased to offer this website service so soybean producers no longer have to face the drudgery of hand-calculating daily crop water use and soil water balance. This website does all the calculations so a user can get real-time estimates of crop water use.

SoyWater can help producers make better irrigation scheduling decisions, minimizing the amount of irrigation water needed for optimal crop production. In turn, this will reduce seasonal water withdrawals from the Ogallala Aquifer and lessen energy costs for pumping. While SoyWater is not an absolute indicator of when to irrigate, it’s a decision-aid tool that producers can use along with their skills and expertise to decide when to irrigate and how much to apply.

Jessica Torrion
UNL Research Associate, Lincoln
James Specht
Charles E. Bessey Professor of Agronomy and Horticulture, Lincoln