How to Schedule Irrigation with Watermark Sensors

Drew Gholson, Irrigation Specialist
By Drew Gholson, Irrigation Specialist and Dan Roach, Ext. Associate June 25, 2021 19:51

How to Schedule Irrigation with Watermark Sensors

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Irrometer Watermark Series: Irrigation Triggers 


An irrigation trigger is the point at which an irrigation cycle starts. Starting too wet wastes water and energy, while starting too dry reduces yield. In this publication, we give guidance on how to select an appropriate trigger for each irrigation system and how to schedule irrigation using Watermark data.

Interpreting Watermark Data

Watermark data can serve as a gauge for the soil water “fuel tank” of the crop. Figure 1 illustrates how to interpret the weighted average centibars (cb) within the active root zone. Centibars are low when wet and high when dry.

Figure 1.
FIGURE 1. The irrigation trigger for a soil water “fuel tank.”

Calculating the Weighted Average

The number of Watermark sensors within the active root zone can depend on crop growth and soil properties. To obtain the weighted average:

  1. Find the column in Table 1 that corresponds to the number of sensors currently included within the active root zone. Once the centibars from a particular sensor have been increasing progressively for some time, this sensor is included for the rest of the season.
  2. Perform the multiplication in each cell of that column.
  3. Add up the result from each cell of that column.

TABLE 1. Template for weighted average calculations.

Sensor depth Two sensors Three sensors Four sensors
6″ 0.5 x ___ cb 0.25 x ___ cb 0.17 x ___ cb
12″ 0.5 x ___ cb 0.25 x ___ cb 0.17 x ___ cb
24″ 0.50 x ___ cb 0.33 x ___ cb
36″ 0.33 x ___ cb

These calculations may be automated by the Watermark average calculator or other web tools/services.

Choosing an Irrigation Trigger

Previous research indicates that yield-reducing water stress tends to occur when the weighted average exceeds 100 centibars. The longer the cycle time for an irrigation system, the farther below 100 centibars the weighted average should be when triggering the start of a new irrigation cycle. Table 2 suggests general triggers for irrigation cycles of various durations.

TABLE 2. Irrigation triggers for different irrigation cycle times.

Irrigation cycle (days) Trigger (cb)
1 100
2 92
3 84
4 76
5 68
6 60
7 52
8 44


Take as an example an irrigation system with one well supplying water to four fields that are irrigated one after another. The time it takes to irrigate each field is 28, 25, 21, and 19 hours, respectively. Thus, the cycle time is 93 hours or nearly 4 days. According to Table 2, a trigger of 76 centibars may be appropriate.

Figure 2 is an example of Watermark data early in the irrigation season. Notice that more sensors are included in the weighted average as the centibars in the deeper sensors begin to increase.

Figure 2.
FIGURE 2. Number of Watermark sensors included in weighted average.

During the first quarter of the graph, only the 6-inch and 12-inch centibars are increasing, so only two sensors are included in the weighted average for example date A. During the middle half, the 24-inch centibars are increasing, so three sensors are included for example date B. During the final quarter of the graph, the 36-inch centibars are increasing, so all four sensors are included for example date C.

Table 3 shows the weighted average calculations on the three example dates. Based on the chosen trigger of 76 centibars, irrigation would be suggested for example date B but not for example dates A and C.

TABLE 3. Weighted average calculations for three example dates.

Sensor depth Date A (two sensors) Date B (three sensors) Date C (four sensors)
6″ 0.5 x 62 cb = 31 cb 0.25 x 104 cb = 26 cb 0.17 x 72 cb = 12 cb
12″ 0.5 x 52 cb = 26 cb 0.25 x 108 cb = 27 cb 0.17 x 60 cb = 10 cb
24″ 0.50 x 54 cb = 27 cb 0.33 x 51 cb = 17 cb
36″ 0.33 x 30 cb = 10 cb
Weighted average 31 cb + 26 cb
57 cb
26 cb + 27 cb +
27 cb = 80 cb
12 cb + 10 cb +
17 cb + 10 cb =
49 cb

For more information, please contact Extension Irrigation Specialist Drew Gholson at or (662) 390-8505.

This publication is a contribution of the National Center for Alluvial Aquifer Research (NCAAR), the Mississippi State University Extension Service, and the Row-Crop Irrigation Science Extension and Research (RISER) initiative. NCAAR is supported by the Agricultural Research Service, United States Department of Agriculture, under Cooperative Agreement number 58-6001-7- 001. RISER is supported jointly by the Mississippi Soybean Promotion Board, Mississippi Corn Promotion Board, Mississippi Rice Promotion Board, Cotton Incorporated, and Mississippi Peanut Promotion Board.

The information given here is for educational purposes only. References to commercial products, trade names, or suppliers are made with the understanding that no endorsement is implied and that no discrimination against other products or suppliers is intended.

Publication 3541 (10-20)

By Jacob Rix, Extension/Research Associate; Himmy Lo, PhD, Assistant Extension/Research Professor; Drew Gholson, PhD, Assistant Professor; and Mark Henry, Extension Associate, Delta Research and Extension Center.

Copyright 2020 by Mississippi State University. All rights reserved. This publication may be copied and distributed without alteration for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.

Produced by Agricultural Communications.

Mississippi State University is an equal opportunity institution. Discrimination in university employment, programs, or activities based on race, color, ethnicity, sex, pregnancy, religion, national origin, disability, age, sexual orientation, genetic information, status as a U.S. veteran, or any other status protected by applicable law is prohibited. Questions about equal opportunity programs or compliance should be directed to the Office of Compliance and Integrity, 56 Morgan Avenue, P.O. 6044, Mississippi State, MS 39762, (662) 325-5839.

Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. GARY B. JACKSON, Director


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Drew Gholson, Irrigation Specialist
By Drew Gholson, Irrigation Specialist and Dan Roach, Ext. Associate June 25, 2021 19:51
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