Summary
Irrigation water management is the process of applying the right amount of water at the right time to support plant growth while minimizing water, energy, and nutrient losses, and preserving soil quality. Effective irrigation keeps soil moisture within a range that avoids both water stress (too dry) and waterlogging (too wet).
To do this well, irrigators need to understand:
- How water moves into, through and out of the soil
- How climate influences soil moisture
- How much water different crops require for optimal growth at different crop stages
Knowing how to identify and find information about your soil and crop water needs is the first step in developing your irrigation water management plan.
Soils
Soil is a porous medium and is made up of four main components: minerals, organic matter, water and air. The typical composition is approximately 45% minerals, 25% water, 25% air and 5% organic matter, although this varies with climate, soil type and management activities such as tillage. Microorganisms make up only a small fraction by weight, but they play a critical role in nutrient cycling and soil structure.
Each component has a specific function for plant growth.
- The mineral component of the soil consists of particles of sand, silt and clay. The relative percentages of these soil particles give rise to the soil’s texture.
- Air within the soil matrix provides oxygen for root and microbial respiration.
- Organic matter consists of decomposed and decomposing plants and animals. Organic matter has a high capacity to hold and supply the nutrients and water needed to support plant growth.
- Microorganisms are the primary decomposers of raw organic matter and are essential for nutrient cycling and overall soil health.
|
Particle |
Diameter (mm) |
Specific surface area (cm2/g) |
|---|---|---|
|
Very coarse sand |
2.0–1.0 | 11 |
|
Coarse sand |
1.0–0.5 | 23 |
|
Medium sand |
0.50–0.25 | 45 |
|
Fine sand |
0.25–0.10 | 91 |
|
Very fine sand |
0.10–0.05 | 277 |
|
Silt |
0.05–0.002 | 454 |
|
Clay |
<0.002 | 8,000,000 |
From King et al., 2003
Soil texture
Soil texture affects many properties of the soil, such as infiltration (how quickly water enters the soil), permeability (soil’s ability to permit water to flow through its pores), water-holding capacity and storage, aeration and nutrient retention.
- Sandy soils have large pore spaces, allowing water to enter and drain quickly.
- Clay soils have very small pores, causing water to move slowly but increasing water-holding capacity.
- Silt and loam soils fall in between, often providing a good balance of storage and drainage.
To determine the texture of your soil, conduct a simple field test using the texture-by-feel method along with the soil texture triangle (Figure 2). This simple field technique helps you estimate your soil’s textural class based on the proportions of sand, silt, and clay.
Using the soil texture triangle
The U.S. Department of Agriculture soil texture triangle identifies 12 textural classes based on the percentage of sand, silt, and clay. Each side of the triangle represents one of these three particle types, with values ranging from 0% to 100%. Lines extending from each side show how the percentages intersect to define a soil’s texture
- Sand percentages increase from right to left across the base of the triangle.
- Clay percentages increase upward along the left side.
- Silt percentages increase downward from the top right.
To find your soil’s textural class, locate the percentage of each particle on the appropriate side of the triangle and trace the lines until they meet. The point where the three lines intersect is your soil’s textural class.
Example: Using the textural triangle, a soil with 68% sand, 24% clay and 12% silt, where the three lines intersect corresponds to a sandy clay loam.
You can also determine the textural class by using the Natural Resource Conservation Service Soil Texture Calculator.
Pore space, infiltration and permeability
Soil texture influences how water moves through and is stored within the soil profile (Figure 3A–C). Differences in the size and arrangement of soil particles create variations in pore space, infiltration rate, and water holding capacity — all of which determine how irrigation water should be applied.
Soil particle size
Soil particle size strongly influences how water moves through the soil profile, how water adsorbs (holds onto) the particle, and how much water can be stored for plant use.
Soil particles vary widely in size:
- Sand (0.05–2.0 mm) — largest particles
- Silt (0.002–0.05 mm) — intermediate
- Clay (less than 0.002 mm) — smallest particles (Table 1; Figure 3A)
Particles larger than sand, such as gravel (greater than 2mm in size), are considered coarse fragments and can strongly influence water movement by creating large pore spaces.
To visualize the size differences, consider the following animal-size analogy:
- Clay particles are like ants — extremely small and tightly packed.
- Silt particles are like butterflies — small but loosely arranged.
- Sand particles are like dogs — much larger and more separated.
- Gravel particles (if present) are like elephants — very large with wide gaps between them.
Coarse particles (sand, gravel) create larger spaces between particles, while fine particles (clay) pack closely together (Figure 3A).
Pore space
Pore space is the open area between soil particles (Figure 3C). These pores allow air and water to move through the soil.
- Sandy soils have the largest individual pore space, which allow rapid drainage and aeration
- Clay soils have the smallest pore space, which slows water movement but increases water-holding capacity.
Infiltration and permeability
Infiltration rates describe how quickly water enters and passes through the soil (Figure 3C).
Because pore spaces in sandy soils are large, water infiltrates and permeates the soil rapidly.
In clay soils, water enters and moves through the soil profile more slowly (Figure 3C).
Porosity
Porosity refers to the total volume of pores compared to the total volume of the soil.
While individual pore sizes are smaller in clay soils compared to sandy soils, the overall volume of pores is greater in clay soils than in sandy soils, allowing clay soils to retain more water and nutrients (Figure 3D).
Important: These textural differences have direct implications for irrigation management. Coarse-textured (sandy) soils require more frequent irrigations with smaller applications, while fine-textured (clay) soils can be irrigated less often but with larger amounts per event to replenish the root zone.
Available water capacity
When rainfall or irrigation water enters the soil, it fills the pore spaces and is held in different ways depending on soil texture and particle-water interactions (Figure 4). Soil water exists in three primary forms relevant to irrigation, according to the USDA National Engineering Handbook:
1. Gravitational water
This is the water that fills all large soil pores immediately after irrigation or heavy rainfall when the soil is fully saturated.
- It drains downward through the soil profile under the force of gravity.
- It is not held by the soil matrix, so plants cannot use it.
- Drainage continues until only capillary-held water remains — this marks field capacity, or FC.
2. Capillary water
Capillary water is the plant-available water held in the soil after gravitational water drains.
- Capillary water is held in small and medium pores by capillary forces and surface tension.
- It moves in response to plant uptake, soil drying, and gradients in water tension (including upward movement during evaporation).
- Constitutes most of the water contributing to plant growth.
3. Hygroscopic water
Hygroscopic water forms a thin water film tightly adsorbed onto the surface of soil particles.
- It is held at very high soil-water tension.
- It cannot be extracted by plants, even under severe stress.
- Hygroscopic water persists even when the soil appears dry.
Soil-water tension (also called matric potential) describes how tightly water is held by soil particles — the drier the soil becomes, the higher the tension and the harder it is for plants to extract water.
Field capacity and permanent wilting point
After gravitational water drains, the soil reaches field capacity — the point at which the remaining water is held by capillary forces and can be easily extracted by plants. As plants continue to use water, soil-water tension increases.
Soil texture strongly influences how fast this tension increases. Although sandy and clayey soils may contain similar total amounts of water at a given time, clay soils retain a larger portion of that water in small pores and on particle surfaces, holding it at much higher tension. This makes it more difficult for plant roots to extract. In contrast, sandy soils hold water at lower tension, allowing plants to extract water more easily. But sands also store far less plant-available water because they drain quickly.
As soil continues to dry, plant-available water decreases and soil water tension (matric potential) increases. Eventually, the remaining water is held so tightly to soil particles that roots can no longer extract it, even though moisture is present. This point is known as permanent wilting point, where plants begin to wilt irreversibly due to insufficient available water.
Available water capacity
Available water capacity, or AWC, is sometimes called soil water-holding capacity. AWC is the water held in the soil between field capacity and permanent wilting point.
- This is the plant-available portion of soil water.
- AWC varies by soil texture:
- Sands: low AWC (large pores, low retention).
- Loams: moderate AWC.
- Clays: high total water but high tension — only a portion is available.
Available water storage
Available water storage, or AWS, is the total amount of plant-available water a soil can hold within a specific depth.
Example
If a soil’s AWC is 0.14 in/in, then a 24-inch rooting depth can store:
Maximum allowable depletion
To maintain optimal plant growth, irrigators should know how much water can be safely depleted from the soil before the crop experiences stress, impacting growth and yield. Maximum allowable depletion, or MAD, expresses the fraction of total available water that can be depleted before a crop begins to experience yield-reducing stress.
MAD percentages are available for many crop types and growth stages. Once you know the AWC, AWS and the crops’ MAD, you can determine how much water is needed to refill the soil profile back to field capacity. Typical MAD values can be found in Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements.
Know the soils on your property
The Web Soil Survey is a valuable tool for identifying and understanding the soils on your property. By entering your farm address or site coordinates, you can generate a soil map that identifies your soil map units and provides general descriptions such as soil depth, texture, permeability and available water capacity. Additional information including soil moisture content at field capacity and at permanent wilting point is available under the “Soil Properties” tab.
For more information and guidance in using the web soil survey, contact your local Natural Resource Conservation Service office.
Resources
Plants
Understanding the soil is only part of irrigation water management — you must also understand how your crop grows and how it responds to different levels of soil water availability, including when too much or not enough water is applied. Crops have different soil water requirements. Having enough water in the soil depends on several factors:
- Weather conditions (solar radiation, humidity, temperature, wind)
- Soil type and texture
- Available water capacity
- Infiltration rate
- Permeability
- Crop characteristics such as rooting depth, growth stage and drought tolerance
Knowing the effective root depth of your crop provides information on how much water to manage in the soil profile for optimum plant growth. The effective root zone is typically the upper half of the maximum rooting depth, where roughly 70% of root water uptake occurs (Figure 5). A deep-rooted crop will have a greater effective root depth than a crop with a shallow root system.
Why this matters: Understanding how plants access and use soil water helps irrigators avoid both water stress and overirrigation by matching irrigation frequency to actual crop demand.
Together, these factors determine how quickly the soil profile dries and how often irrigation will be required.
Evapotranspiration
Evapotranspiration, or ET — sometimes called crop water use — is the combined loss of water from the soil and plant surfaces.
Evapotranspiration (ET) = Evaporation + Transpiration (Figure 6)
Evaporation
Evaporation occurs when water at the soil surface or on plant surfaces vaporizes and returns to the atmosphere. This water does not contribute to plant growth because it never enters the plant root system.
Transpiration
Transpiration is the process by which water absorbed by plant roots moves through the plant and evaporates from leaf surfaces.
Plants transpire water primarily through stomata, small openings on the underside of leaves (Figure 7).
When the stomata are open:
- Water vapor (H2O) exits the leaf into the atmosphere.
- Carbon dioxide (CO2) enters the leaf for photosynthesis.
- Oxygen (O2) is released as a byproduct.
Because ET determines how fast soil water is depleted — through soil evaporation and plant transpiration — it helps irrigators understand how quickly the soil reservoir (AWC/AWS) is being drawn down between irrigations. This helps irrigators determine both when the next irrigation should occur and how frequently irrigations will be needed during different crop growth stages.
Weather networks: AgriMet
One way to estimate ET is through on-farm weather stations or nearby AgriMet stations. AgriMet is an automated agricultural weather network with stations across the Northwest operated by the U.S. Bureau of Reclamation with support from USDA Agricultural Research Service, the USDA National Resource Conservation Service, land grant universities, the Extension Service, electric utilities and other partners.
AgriMet weather stations measure:
- Air temperature
- Relative humidity
- Precipitation
- Wind speed and direction
- Solar radiation
- Soil temperature
- ET
- Other weather parameters
AgriMet provides daily and monthly crop water use estimates for the current and past seasons. AgriMet stations provide valuable data for forecasting and scheduling irrigation.
|
Textural class |
Available water capacity (inches per foot of depth) |
|---|---|
| Coarse sand | 0.25–0.75 |
| Fine sand | 0.75–1.00 |
| Loamy sand | 1.00–1.20 |
| Sandy loam | 1.20–1.40 |
| Fine sandy loam | 1.40–2.00 |
| Silt loam | 2.00–2.50 |
| Silty clay loam | 1.80–2.00 |
| Silty clay | 1.50–1.70 |
| Clay | 1.20–1.50 |
From: Noble Research Institute, 2013.
|
Crop (established) |
Effective rooting depth (feet) |
Allowable depletion (%) |
|---|---|---|
| Alfalfa | 4.0 | 60 |
| Beans | 2.0 | 50 |
| Corn | 3.0 | 50 |
| Grapes | 3.0 | 65 |
| Orchard | 3.0 | 50–65 |
| Potato | 2.0 | 30–40 |
| Grass pasture/hay | 2.0 | 60 |
| Small grains | 3.0 | 50 |
How soil, plant and water relationships relate to irrigation water management
Soil acts as a reservoir that stores water and nutrients for plant growth. To manage when and how much water to apply, it is critical to know:
- How much water your soil can hold (AWC).
- How deep the crop roots can access that water (effective root depth).
- Your crop water needs.
- How much water has been depleted since the last irrigation or rainfall event (ET or soil moisture data).
Determining these elements will help you better manage and apply irrigation water.
Example: Determining irrigation depth for grass hay on sandy loam soil
1. Determine soil texture and AWC
Identify the dominant soil type in your field and look up the AWC for that soil texture. You can find these values in Table 2 or by using the Web Soil Survey. For this example, we assume the soil is sandy loam, which has an AWC of 1.4 inches per foot.
2. Find the crop’s effective rooting depth
Use published values or Table 3 to determine the effective rooting depth for the crop. For pasture/grass hay, the typical effective rooting depth is 2 feet.
3. Calculate total available water storage
AWS represents the total amount of plant available water stored within the effective rooting depth at field capacity.
4. Apply maximum allowable depletion (MAD)
The crop’s MAD determines the percentage of the available water the crop can use before experiencing stress.
For grass hay, MAD = 60% (Table 3)
MAD = AWC*MAD
MAD = 2.8in*0.60
MAD = 1.7 inches
The crop can use 1.7 inches of water before irrigation is required.
This 1.7 inches represents the maximum amount of water that can be depleted from the soil before the next irrigation should be applied to avoid stress and yield reduction.
References and resources
- Allen, R.G., L.S. Pereira, D. Raes and M. Smith. 1998. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. Food and Agriculture Organization of the United Nations.
- Bubl, Chip. 2010. Irrigation management basics. OSU Extension.
- Hortau. 2021. Soil tension’s relationship to soil type and salts.
- Peters, R.T., and J. Davenport. 2012. Managing Irrigation Water for Different Soil Types in the Same Field, Washington State University Publication FS086E.
- Noble Research Institute, Soil and Water Relationships.
- Shareeja, D. Soil Water: Importance, Concepts and Classification, in Soil Management.
- USDA Natural Resource Conservation Service. 1991. National Engineering Handbook, Section 15, Irrigation, Chapter 1 Soil-Plant-Water Relationships, 2nd Edition.
- USDA Natural Resources Conservation Service. 1997. National Engineering Handbook, part 652, Irrigation Guide.
- Washington State University Extension, Oregon State University Extension and University of Idaho Extension, Irrigation in the Pacific Northwest, Tutorials on the Basics of Plant-Soil-Water Relations.
Acknowledgments
This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2021-38640-34695 through the Western Sustainable Agriculture Research and Education program under project No. WPDP22-020. USDA is an equal opportunity employer and service provider. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the view of the U.S. Department of Agriculture.
Reviewers included Matthew Alongi, Natural Resources Conservation Service State Irrigation Engineer; Mylen Bohle, Associate Professor, Emeritus; Troy Peters, Professor and Extension Irrigation Engineer, Irrigated Agriculture Research and Extension Center, Washington State University; Rex Barber of Big Falls Ranch, Terrebonne, Oregon; Mike Macy of Macy Farms, Culver, Oregon; Patrice Spyrka of Tumalo Alpen Ranch, Tumalo, Oregon; Greg Mohnen of Triple S Ranch; Darrell Abby of Rock Island Ranch
Contributors included Kurt Moffit, Natural Resources Conservation Service; and Gordon Jones, Associate Professor of Practice, Oregon State University.
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