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Irrigation Scheduling

Conventional Approach

In theory, the scheduling of irrigation events amounts to nothing more than making two simple decisions – how often and how much.  However, to make those decisions, the producer needs to have crop water use and soil water information provided to him/her on a timely basis.  SoyWater was designed was designed to be a “decision-aid” tool the producer could access whenever he/she needed information to make when and amount irrigation decisions with respect to his/her soybean crop.

Irrigation scheduling methods fall into two general categories relative to the information used for making the when and how much water application decisions.  These include (1) using daily Crop ET estimates as a proxy for daily crop water use, and (2) using soil water sensors placed at various crop root zone depths to quantify daily soil water content.  SoyWater can be used for either or both methods – see the Applications button on the SoyWater Main Menu Bar. 

For irrigation scheduling method (1), a reference ET value for a standard canopy is calculated from solar radiation, temperature, humidity, and wind speed, and then multiplied by a soybean crop coefficient (Kc) specific for a given soybean development stage to provide an estimate of soybean ET.  SoyWater uses published soybean Kc values, though we will note here that Dr. Suat Irmak (a member of the SoyWater Team) has received funding from the Nebraska Soybean Board to conduct research aimed at further refining those published soybean crop coefficients to ensure a better fit of the soybean Kc values for Nebraska soybean growing conditions.  A soybean ET-based irrigation scheduling system uses cumulative daily soybean ET to estimate how much water is being used by the crop on a daily basis (and thus when irrigation might be needed).  The producer chooses the amount of water to apply – less than the amount needed to bring the soil to full field capacity – to allow for some storage of rainfall should it occur immediately after irrigation, and applies that water at a rate suitable for infiltration in the soil to preclude loss of water via runoff.  The maximum amount of water that could be applied in a given irrigation will be a function of the producer’s irrigation system capacity, current crop root depth, and the field’s soil texture (i.e., soil water-holding capacity).  The how often parameter of irrigation scheduling will be determined by rate of soil withdrawal that arises from fluctuating amounts of daily crop ET (more with sunny, windy, hot, low humidity weather; less cloudy, calm, cool, high humidity conditions). 

For irrigation scheduling method (2), soil water sensors placed at differing depths in the crop root zone can provide the producer with a direct measure of changes in soil water content, thus providing the producer with the means of determining when irrigation might be needed (assuming that the location of the water sensors is representative of the soil water conditions prevailing in the rest of the field).  The factors listed in method (1) also apply to method (2), but again, the producer’s irrigation system capacity, current crop root depth, and the field’s soil texture (i.e., soil water-holding capacity) will govern the producer’s decision on how much water can be applied per irrigation, and thus how often irrigation will be needed to replenish the soil water that is continuously being withdrawn by crop ET. 

Stage R3 Approach

SoyWater can also be used for an irrigation scheduling strategy that has been extensively researched by Dr. James Specht (a member of the SoyWater team). He has documented that applying NO irrigation prior to soybean stage R3 (begin pod elongation stage – see photo) on high water-holding capacity soils (i.e., silty clay loam soils) that were at field capacity when the soybean crop was planted (or emerged) r3ultimately had NO IMPACT on final soybean yield, so long as the soil water content was brought back to normal levels on, or soon after R3. More will be said about the italicized caveat in a moment. Dr. Specht notes that this Defer All Irrigation to R3 irrigation scheduling strategy works best when the soybean crop is planted as early as possible, because (1) the yield potential is greater with earlier planting, and (2) moving more of the vegetative stage of crop development to earlier in the season (when temperatures are cooler) lessens the amount of water lost to soil evaporation and allows more of the stored soil water to be used for crop transpiration, which has a higher correlation with crop yield than does crop ET.

The reason the Defer All Irrigation to R3 irrigation strategy works on the heavier soil texture types is that if the field is at field capacity at planting or emergence, there will be six inches of stored soil water in (for example) a silty clay loam soil for the crop to use for ET purposes from emergence to R3. Keep in mind that six inches of water is actually just little less than one-third of the total 20 inches of water a soybean crop will likely have for ET when producing an 85 bu/ac from a 1 May planting date to its stage R7 date of physiological maturity. Producers also need to remember that in most years, the amount of rainfall between emergence and R3 is going to be greater than the amount of crop ET, but even in those few years when rainfall is sparse in that emergence to R3 timeframe, Dr. Specht has shown that withholding irrigation until R3 still does not negatively impact yield. If foregoing double negative sentence is not clear, try this inverse statement: Dr. Specht has shown that applying any irrigation before R3 does not positively impact yield!

Now, let’s go back to the italicized caveat stated above. The Defer All Irrigation to R3 strategy is NOT designed to be a deficit type of irrigation scheduling method – which are designed to improve irrigation water use efficiency despite reducing the final yield that results (which may be an acceptable outcome in areas where water conservation is the overriding concern). The producer using the R3 strategy must bring the soil water content back to normal levels on or soon after stage R3. Accomplishing this goal may simply require shortening the normal cycle time between irrigation events during a 7 to 10 day period immediately after the date of R3 and applying somewhat more water. Thus, producers with limited capacity irrigation systems may not be able to use this strategy. However, the need to apply large irrigation amounts at R3 is expected to be rare, given the rarity of spring May-June droughts.

The producer may be wondering: What is the advantage of deferring irrigation to R3, if “catch-up” irrigation events then have to be applied anyway at soybean stage R3? The advantage is this. Your neighbor, using the conventional irrigation strategy, may experience a year in which an irrigation event of say one-inch is triggered and applied to his field well before soybean stage R3. And then an isolated June rainstorm puts down 1.5 inches on both your field and your neighbors field the next day. Since you never apply irrigation prior to R3, the entire 1.5 inches of rainfall became stored soil water for your crop. If there was only 0.5 inch of soil water storage “room” in the neighbors soil after the one-inch irrigation event, then only 0.5 inch of rain became part of the stored soil water, with the other 1.0 inches of rain water lost as run-off the field. This one-inch difference in rainfall stored in your field and not the neighbors will, as Dr. Specht has shown in past research, not likely make one bit of difference in soybean yield. That’s correct! However, the 1.5” rainfall your soil captured was all rain (and free), while the 1.5’ water your neighbor’s soil captured was only 0.5” rain (free) and 1.0” irrigation (expensive). Thus, while you both will likely have the same yield, your neighbor put on one more inch of irrigation water that had no yield impact. True, he was unlucky, but for you, luck was not a factor in any case.

For more information and advice about using the defer irrigation to R3 stage method of soybean irrigation scheduling, please contact Jim Specht at 402-472-1536 or jspecht1@unl.edu. By the way, you can click on the Charts button in SoyWater to see the date when cumulative historical rainfall trend line and the projected historical crop water use trend line cross over in the displayed graph, then click on R3 to see the vertical line that projects when your crop will attain R3.


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