SOIL MOISTURE


May 2


May 9


May 16


May 23


May 30


Jun 6


Jun 13


Jun 20


Aug 3


May 2


May 9


May 16


May 23


May 30


Jun 6


Jun 13


Jun 20


Aug 3


Figure 1. 20 kPa trigger May 2


May 9


Figure 3. 40 kPa trigger May 16 May 23 May 30 Jun 6 Jun 13 Jun 20 Aug 3 May 2 May 9 May 16 May 23 May 30 Jun 6 Jun 13 Jun 20 Aug 3


Figure 2. 30 kPa trigger


between that date and June 21, there were only 9.5 inches of rainfall received at SIRP. Three significant rainfall events affected the SWT on May 24, June 1 and 14. It is important to note these dates, as these events affected the SWT in some treatments. For irrigation treatments, the soil water tension data was weighted by rooting depth, meaning in early season the deeper sensors were not utilized. As the season progressed, the data from the deeper sensors were integrated into irrigation decisions.


As shown in figure 1, overall, deep moisture was retained until the end of June. However, at that time, it can be seen by the drop in the 16-inch sensor depth (orange line) that water use was occurring at a more rapid rate. In this treatment, drastic changes only occurred in the 8-inch sensors (blue line), but soil moisture was recovered with irrigation relatively easily. Past studies have shown that this trigger level, while not yield-limiting, causes profit loss due to the cost of overirrigating without associated yield returns. Figure 2 represents more drastic changes in all three depths, showing that not maintaining soil moisture at a higher level caused the crop to utilize moisture from deeper levels faster than the 20 kPa treatment. Except for the 24-inch sensor (gray line), soil moisture was able to be recovered at the 8- and 16-inch depths with irrigation.


As the SWT was allowed to increase to a higher average level in the 40 kPa treatment (figure 3), the changes in the 8-inch sensor were more drastic, and we could not recover moisture at the


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Figure 4. 50 kPa trigger May 2 May 9 May 16 May 23 May 30 Jun 6 Jun 13 Jun 20 Aug 3


Figure 5. 60 kPa trigger


16-inch depth as easily with irrigation. However, moisture was not utilized at the 24-inch depth providing the assumption that there was still adequate shallow moisture in the profile.


Similarly, figure 4 represents the 50 kPa treatment, where more drastic changes were observed in all three depths and there was no recovery of water use at the 16-inch sensor without a significant rainfall event. While past studies have shown this SWT treatment was not yield-limiting and did not maximize profit, any mistakes at this soil moisture level will lead to yield reductions. Figures 4 and 5 are very similar, except the scale in figure 5 reached a higher level and it was more difficult to recover deep (16-and 24-inch) moisture in the 60 kPa treatment (figure 5). The rainfall events were able to lower the SWT levels, but it should be noted that there was a delay as the moisture took time to infiltrate to the deeper levels.


Winter 2023 | Irrigation TODAY 17


Soil water tension (SWT kPa)


Soil water tension (SWT kPa)


Soil water tension (SWT kPa)


Soil water tension (SWT kPa)


Soil water tension (SWT kPa)


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