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Basics of irrigation scheduling

Soil is plant’s water reservoir. This reservoir has upper and lower limits of water that it can store for crop water use. Understanding different water thresholds in the soil-water reservoir are important for irrigation scheduling. 

Soil water reservoir terms

Figures 1 and 2 indicate the soil water reservoir and plant growth at various thresholds:

soil water graphic
Figure 1. Soil water reservoir components.

Saturation

It is the soil water content where all soil pores are filled with water and water readily percolates or drains out from the root zone by gravitational force.

Field capacity (FC)

It is the amount of water that remains in the soil after all the excess water at saturation has been drained out. Usually, when sandy soils are allowed to drain for approximately 24 hours after saturation, field capacity is reached. In heavier textured soils that have more silt and clay, it take up to 2-3 days after saturation to reach FC.

Permanent wilting point (PWP) 

When plants take up all the available water for a given soil, soil dries to a point that it cannot supply any water to keep plants from dying.

Available water holding capacity (AWC)

This is the maximum amount of water that soil can store to be extracted by the plants. It is the water held between field capacity and permanent wilting point. The total available water in the soil root zone for a specific crop is equal to the crop’s rooting depth multiplied by the available water-holding capacity per unit depth of the soil.

soil water and plant relationships
Figure 2. Soil water and plant growth.

Soil type differences

soil moisture graph
Figure 3. Soil moisture conditions for various soil textures. Source: The COMET program. University Corporation of atmospheric research.

Different soil types have different AWCs. For example, coarse soils, such as sands and sandy loam, have relatively large pores when compared to a finer textured soil such as clay (Figure 3). Fine soils, like clays or clay loams, have small mineral particles and very small pores.

Having a larger number of small pores means that a fine textured soil can hold more water than a coarse textured soil. Tables 1, 2 and 3 show AWC by different soil textures and examples of the available water-holding capacity of two typical irrigated soils, respectively. Get detailed look at your field’s soil using Web Soil Survey.

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Maximum allowable depletion or deficit (MAD)

Management allowable depletion specifies the maximum amount of soil water the irrigation manager chooses to allow the crop to extract from the active rooting zone between irrigations. Only a portion of the available water holding capacity is easily used by the crop before crop water stress develops. As illustrated in Figure 1, soil’s MAD is less than its total AWC.

Historically, irrigations have been planned to prevent the soil water deficit from exceeding 50 percent of the total available water capacity in the rooting zone. But research states that the depletion limit can be varied to optimize the field's production depending on the crop, stage of growth, soil water capacity, and the irrigation system's pumping capacity.

Management allowable depletion is usually expressed as a percentage of the total available water capacity in the rooting zone. It needs to be converted to inches of soil water for a specific crop and soil situation. To convert depletion percentage to inches of water, multiply the given depletion percentage by the total available water in the root zone.

For example, if a 30 percent depletion limit is desired for a soil holding 3.50 inches of water, the depletion level in inches of soil water would be 1.05 inches:

.30 x 3.50 inches = 1.05 inches

Table 4 lists recommended management allowable soil water depletion limits and management strategies for several irrigated crops grown in Minnesota. These recommendations result from several research projects in the North Central states and published guidelines from other states.

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Management guidelines for developing a management plan and setting management allowable depletion (MAD) limits can be found in Irrigation management strategies

Soil water deficit

This is the amount of water removed by the crop from the active rooting depth. Likewise, it is the amount of water required to refill the root zone to bring the current soil moisture conditions to field capacity. Soil water decreases as the crop uses water (evapotranspiration) and increases as precipitation (rainfall or irrigation) is added. Expressed in soil water deficit, evapotranspiration increases the deficit and precipitation decreases it. It is usually expressed in inches of water and can be estimated or measured by several methods described later sections.

For irrigation scheduling, whenever the soil water deficit is equal to or higher than MAD, irrigation should be triggered. Irrigation amounts should be refilling the rooting zone to field capacity while also leaving some room for possible precipitation.

Crop rooting depth

Crop rooting depth determines how much soil water can be accessed by the crops. A shallow-rooted crop has less access to soil water as compared to a deep-rooted crop. Table 5 lists suggested rooting depths for irrigation water management of commonly irrigated crops in Minnesota. Each crop can potentially develop a greater rooting depth.

Table 5. Crop rooting depth for irrigation water management

Crop Depth (inches)
Alfalfa (established) 48
Corn, sugarbeet 36
Potato, small grain 24
Soybean, field bean 24
Tomato, muskmelon 12-24
Broccoli, cauliflower 12-18
Blueberry, strawberry 12-18

However, because most of a plant’s roots are located in the upper portion of the root zone, irrigation water applications are generally managed to a shallower depth than the crop’s full rooting depth. For annual crops, this rooting depth isn’t usually achieved until 30 to 50 days after planting. Note that local soil and climatic conditions may reduce these values. Figure 4 shows the typical water extraction pattern in a uniform soil profile.

soil water extraction pattern graphic
Figure 4. Typical water extraction pattern in uniform soil profile. Source: Chapter 3 “Crops”, Irrigation Guide, Natural Resources Conservation Service- National Engineering Handbook .

Acknowledgements

The author wishes to thank former University of Minnesota colleagues Joshua Stamper and Jerry Wright for their previous development efforts in earlier iterations of this content.

Vasudha Sharma, Extension irrigation specialist

Reviewed in 2019

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