Primer on Soywater

This SoyWater web site was designed to deal with several hurdles facing a soybean producer who wants to use irrigation scheduling. One hurdle has been the need to acquire daily crop ET values. Another hurdle has been the drudgery of making the daily calculations needed to update and maintain a soil water balance worksheet for an irrigated field (of which there may be many). SoyWater is an internet-accessible website that will automatically acquire the daily weather data from a weather station closest to the producer’s field. It will estimate the daily soybean crop ET values for the soybean crop in that field. It will then output a table of useful data for the producer including a tabulation of the cumulative amounts of soil water depleted by the crop on each day of the growing season from sowing (or emergence) to crop maturity. By relieving the producer from much of the work, except for the few inputs the producer-user must supply to SoyWater at the start of the season, and the need for the producer to provide local rainfall and irrigation inputs, the producer can now focus on the bottom line – how much soil water has been depleted by my crop as of today (when I login to SoyWater), and on what date should I schedule an irrigation for my crop when the soil water it has withdrawn should be replenished on a “when needed, just-in-time” basis.

The primary goal in creating and refining the SoyWater web site was to provide soybean producers with a convenient means of scheduling irrigations in their soybean fields in a manner that would allow optimization of yields (particularly with earlier planting), while minimizing the amount of water needed for those yields - thus allowing producers to optimize the efficiency of crop water use AND the efficiency of energy use in irrigated agriculture.

First-timer using SoyWater? Read on.

The SoyWater web site will start by first asking the producer to identify the GPS coordinates of the irrigated field, and if the producer does not know these coordinates, the producer can simply show SoyWater where the field is on screen map. Once the producer locates the field for SoyWater, the program will then identify the closest automated weather station maintained by the High Plains Regional Climate Center (HPRCC) that can provide the type of data (i.e., solar radiation, temperature, humidity, and wind speed) needed to estimate crop ET for the producer's soybean field. The producer then must provide SoyWater with some simple inputs.

1. What name does the producer want to use for the field? SoyWater just numbers the fields 1, 2, 3, 4, etc. If the producer wants to use a more recognizable field name (i.e., Northwest Pivot 2), the producer can do so.

2. What is the predominant soil texture of the planted field? If known, the produce simply selects one of the choices in the menu box. Otherwise, the producer can simply page down the screen to see a soil texture map of his planted field. The yellow numbers between the yellow lines in the map can be translated by looking at the box on the right to determine what soil textures are present in the field. When there are multiple soil textures in the field, the producer will need to select a soil texture that is the most predominant one in the field (or else one that will be the most representative for irrigation scheduling purposes), then page back up the screen to select that one in the soil menu texture box.

4. On what date did the producer plant the field, or better yet, on what date did the soybean seedlings emerge? The producer can use the computer mouse to click on the word emergence to see a drawing of a seedling to then know what is meant by the term “emergence”. [Note that SoyWater prefers to use the emergence date because the number of days between sowing and emergence can be quite variable because it is dependent on field soil temperatures which SoyWater cannot reliably estimate from air temperatures, due to field differences with regard to pre-plant surface soil tillage, or the degree of residue cover in min- or no-till fields.]

5. What is the maturity group (MG) number for the soybean variety planted in the field? The MG number for a variety is a genetic predisposition for the variety to mature at a specific time in the fall, though the specific maturity date will depend on the planting date and seasonal temperatures. In Nebraska, the MG could range from 1.9 to 3.9, though numbers like 2.3, 2.7, 3.0, & 3.3 would be typical inputs. SoyWater needs this MG number information to estimate the calendar dates when the crop is likely to attain its successive vegetative stages (Vn, where n = 0 to 16 or more), and its successive reproductive stages (Rn, where n = 1 to 8) of flowering (R1-R2), pod elongation (R3-R4), seed enlargement (R5-R6), physiological maturity (R7), and final maturity (R8). [Scientist-readers of this text will recognize the necessity of knowing the calendar dates of these growth stages for the purposes of invoking the Kc ET parameters in the estimation of daily soybean ET values during the course of the growing season.]

Once the above inputs are made for each (and every) field, the producer simply logs into the SoyWater web site as needed to enter locally relevant rainfall events (dates and amounts) and/or irrigation events (dates and amounts). Of course, the producer can log in at any time to check out SoyWater’s projections of cumulative summation of daily soybean crop water depletion to see what future date SoyWater is suggesting for the first (or next) irrigation that the producer should schedule for the field. Assuming all initial inputs made by the producer are correct and all rainfall and irrigation amounts have been input on the day the producer logs in, SoyWater will provide a tabular listing of the daily crop water use from the planting (or emergence) date, use actual weather data and producer rainfall and irrigation input data up to the date of log-in to calculate soil water depletion up-to-the-day the producer decides to log-in. Moreover, SoyWater will use long-term historical weather data at the nearest weather station to estimate and project forward soil water depletion by the crop during the upcoming days beyond the actual day in the summer that the producer logs in, including all of the way to the end of the growing season (the R7 stage)!

That tabular listing generated on the screen by SoyWater can be examined by the producer. The soil water depletion data cells in the last column of the table are the most relevant in terms of irrigation scheduling. SoyWater will yellow-highlight the first data cell that contains a soil water depletion value greater than that allowable by SoyWater, which by default is 35% of the total plant-available soil water held at field capacity (FC) by the soil (of the specified soil texture type that can be reset). For example, a silty clay loam soil can hold about two inches of water per foot of soil depth at FC, so in a 3-foot crop root zone, there would be six inches of plant-available soil water at FC. If the crop were to deplete 2.1 inches of the 6.0 inches, the soil water depletion percentage would be 2.1 / 6.0 = 0.35 or 35%. On the calendar date when that number or a larger one occurred (or is projected to occur after the log-in day, SoyWater would yellow-highlight the cells in the rightmost table column that had 2.1 or greater. Producers are, of course, allowed to select an “irrigation triggering” depletion percentage different from the SoyWater default value of 35% (see the upper right top portion of the on-screen table). However, producers are advised that they really need to understand the impact on irrigation scheduling dates before changing the 35% default value. In other words, be cautious. Seek out advice if you lack that understanding.

You can e-mail your questions about how to use SoyWater. Please click here.