Irrigation Fundamentals

Part 1, The Basics

By Ed Butts, PE

For the most part I have been fortunate throughout my professional water industry career. I have had the privilege of working on many diverse and complex engineering projects and worked alongside some truly top-notch engineers and designers.

One was Jim Grimm, my boss and one of my earliest engineering mentors while I was employed at Stettler Supply Co. The many things he taught me—not the least his ethical, personal, and business philosophies—have stayed with me throughout my career.

One of the many skills he passed on to me was in irrigation system design, primarily sprinkler irrigation, including hand and solid set, hard and soft hose travelers, pivots, and linears.

Even though I learned enough to subsequently become certified as an Agricultural Sprinkler Designer by the Irrigation Association, I no longer practice irrigation system design much these days. However, I maintain my certification and still try to keep up with the newest technology and refresh my working knowledge whenever I can.

The topic of irrigation is such a comprehensive one it would require a dozen or more Engineering Your Business columns to adequately cover it all. Thus, this month, as the first installment of a six-part series on irrigation system design fundamentals, techniques, and methods, we will outline and discuss the basic concepts of planning an irrigation system. This will include basic terms and definitions, fundamental concepts of irrigation, and considerations to help decide if irrigation is even needed.

Even with six columns we will certainly not be able to cover all the nuances and fine points of planning or designing an irrigation system. I believe, however, we will provide enough basic knowledge to enable you to explore this important element of water use in greater detail.

Definition of Irrigation Terms

Like many other water system definitions, irrigation has many terms seemingly unique to the industry. Following are some of the most common terms that primarily apply to irrigation.

Note many of the terms with different names have substantially the same definition as others. The most important and frequently used terms related to irrigation are indicated by an asterisk (*) before the term.

*Application Efficiency (%): The ratio of the average depth of irrigation water infiltrated or stored in the root zone (net) available for plant growth as opposed to the average total (gross) depth of irrigation water applied.

*Application Rate (in./hr, mm/hr): The gross rate in which water is applied to a given area, usually expressed in units of water depth per time.

*Available Water (%, in./ft, mm/mm): The amount of water released between in-situ field capacity and permanent wilting point. Also referred to as available water-holding capacity.

Capillary Water (%, inches, mm): Water held within the capillary or small pore regions of the soil, usually with soil water pressure (tension) greater than 4.78 psi (1/3 bar). Capillary water can move in any direction.

*Carryover Soil Moisture (%, inches, mm): Moisture stored in a soil’s root zone during the winter or early spring while the crop is dormant or before it is planted or starts to sprout. This moisture is available to help meet the initial consumptive water needs of the crop.

Climates:

  • Arid Climate: Climate characterized by low rainfall and high evaporation potential. A region is usually considered as arid when precipitation averages less than 10 inches per year.
  • Humid Climate: Climate characterized by high rainfall and low evaporation potential. A region generally is considered as humid when precipitation averages more than 40 inches per year.
  • Semiarid Climate: A climate characterized as neither entirely arid nor humid, but between the two conditions. A region is usually considered to be semiarid when precipitation averages between 10 and 20 inches per year.
  • Sub-Humid Climate: A climate characterized by moderate rainfall and moderate to high evaporation potential. A region is usually considered to be sub-humid when precipitation averages more than 20 but less than 40 inches per year.

*Consumptive Irrigation Requirement (inches, mm, feet): The depth of applied irrigation water, exclusive of precipitation, stored soil moisture, or groundwater required consumptively for full crop production.

*Consumptive Use (Evapotranspiration): The unit amount of water used on and by a given plant or area from transpiration, building of plant tissue, or evaporated from adjacent soil, snow, or intercepted precipitation in any specified time. Consumptive use may be expressed in several types of units. They include volume per unit area such as acre-inches or acre-feet of water per acre, in rates per area including gallons per minute (gpm) or cubic meters per hour (m3/hr) per acre, or in depth such as inches, feet, or millimeters per acre.

*Consumptive Water Requirement (inches, mm): The amount of water potentially or actually required to meet the evapotranspiration needs of vegetative areas so that plant production is not limited or stunted from a lack of water.

Crop Growth Stages: Periods of like-plant functions during a growing season. Usually four or more progressive periods are identified: starting with planting of the seed, into development, through maturity, and finishing with harvest.

  1. Initial: Between planting or when growth begins to approximately 10% of ground cover
  2. Crop Development: Between approximately 10% of ground cover to 70% to 80% of ground cover
  3. Mid-Season: From 70% or 80% of ground cover to the beginning stage of maturity
  4. Late: From the beginning stage of maturity up to the final stage of harvest.

*Distribution Uniformity (%, decimal): The ratio of uniformity of the volume or rate of irrigation water applied over a specific area versus the total volume or rate of water applied over the same area, per time unit.

Effective Precipitation (inches, mm): Precipitation in the form of rainfall or snow falling during a growing period or season available to meet the consumptive water requirements or replace the soil moisture used for a crop. Does not include deep percolation loss below the root zone nor surface runoff or wind drift losses.

*Evapotranspiration (in./day, in./week, mm/week, mm/ day): Combination of the water transpired or consumed by a plant due to vegetative growth processes and evaporated from the soil and exposed plant surfaces.

  • Crop: Crop evapotranspiration is the quantitative amount of evapotranspiration within the area of a field only associated with the growing of a crop. This is the same as plant-water requirement, is often called the plant’s uptake value.
  • Potential: Rate at which water, if available, would be removed from the soil and plant surfaces; expressed as the latent heat transfer per unit area or its equivalent depth of water per unit area.

Hydraulic Conductivity (in./hr, ft/day, mm/hr):

  • Coefficient describing the ease at which a soil’s porosity permits vertical and lateral water movement.
  • Combined soil-water characteristic describing the ability of water to easily flow through a particular soil.

*Infiltration or Intake Rate (in./hr, mm/hr): The downward (vertical) rate of flow of water into and through the soil occurring at the air-soil interface (upper horizon) or volume of water infiltrating through a horizontal unit area of soil surface by gravity force at a specific time interval. The instantaneous intake rate reflects the higher rate dry or de-watered soil will readily accept water over the first one to two initial hours of application, which is thereafter followed by the lower basic or average intake rate reflective of the soil’s intake rate after the initial one to two hours as it becomes saturated and the rate of water percolation without starting surface runoff drops.

*Field Capacity (%): The moisture percentage on a dry-weight basis of a soil after rapid drainage has taken place following an application of water, provided there is no static water table within capillary reach of the root zone. This moisture percentage usually is reached within two to four days after an ordinary irrigation. The precise time-interval depends on the specific type of soil classification.

*Growing Season (days, months): The elapsed period of time, often occurring during the frost-free period, in which the climate, hours of daily sunlight, and soil conditions are conducive to soil preparation and plowing, followed by planting, development, maturity, and harvest of a specific crop or plant. This is usually indicated in days or months.

*Irrigation Depth or Demand:

  • Gross (inches, mm): The total depth of irrigation water applied to a specific area of land, which may or may not meet the total water requirement of the crop (i.e., needed to account for waste, runoff, or percolation losses by leaving an extra measure of water storage in
    the soil for anticipated rainfall or pending harvest).
  • Net (inches, mm): The actual amount (depth) of applied irrigation water stored and available in the soil for direct plant use and consumption or moved through the soil for leaching out excess salts. This also includes any water applied for maintaining crop quality and/or temperature modification (i.e., frost control, cooling of plant foliage, and plant/fruit protection). By strict definition, the net irrigation depth is always less than the gross irrigation depth. Application or soil losses such as evaporation, runoff, wind drift, and/or deep percolation are not included.

*Irrigation Efficiency (%): The percentage of irrigation water stored in the soil and available for consumptive use by the crops after allowance for application losses. When measured at or in the field, it is designated as the field or net irrigation efficiency. When measured at the point of diversion or source, it may be termed as the project or gross efficiency.

*Irrigation Interval, Cycle, or Frequency (minutes, hours, days):

  • Interval: Average time interval between the commencement
    of successive irrigation over a given field or area.
  • Cycle: For turf or landscape irrigation: a preprogrammed time period for a series of automatic sprinklers to activate to irrigate a specific zone or area followed by a progression to the next sequenced zone in the order of programming. It is usually set up to irrigate the same zone each day for a time period of 10-30 minutes.
  • Frequency: Elapsed time in days between consecutive irrigation events. Usually considered the maximum allowable passage of time between irrigation applications during the peak evapotranspiration period of the crop.

*Irrigation Period or Cycle Time (minutes, hours): Time period required to apply one irrigation application to a given design area during the peak consumptive use period of the crop being irrigated. For turf and landscape irrigation, the cycle time usually occurs over a series of minutes; for agricultural irrigation, it is generally timed in hours.

*Irrigation Gross Water Requirement (inches, mm): The crop’s consumptive use (net) irrigation water requirement in inches/mm divided by the irrigation efficiency (%). The calculated amount of water needed to replace soil water used by the crop (soil water deficit), for leaching undesirable elements and salts through and below the plant root zone, plus satisfy all losses including wind drift, percolation, runoff, and uneven distribution. The irrigation water requirement is generally calculated only after proper considerations are made for effective precipitation.

Moisture Percentage: The percentage of moisture in the soil based on the weight of the oven-dried material.

*Precipitation Rate (in./hr, mm/hr): Rate at which an individual sprinkler or system applies water to a given area; not to be confused with the soil intake rate which is the rate the soil accepts water. This is synonymous and interchangeable with the previously defined application rate.

  • Instantaneous Precipitation (Application) Rate (in./ hr, mm/hr): The maximum initial application rate, usually localized, that a sprinkler or other type of application device applies water onto the soil.
  • Net Precipitation Rate (in./hr, mm/hr): Amount of water actually reaching the ground. It is the gross precipitation rate minus the losses that occur between the sprinkler and the surface.
  • Sprinkler Precipitation Rate (in./hr, mm/hr): Precipitation rate of a group of heads used together and all having the same arc, spacing, and flow generally applied for landscape or turf irrigation.
  • System or Basic Precipitation Rate (in./hr, mm/hr): Precipitation rate for a system as the average precipitation rate of all sprinklers factored over a given area, regardless of the arc, spacing, or flow rate of each separate head.

*Root Zone (inches, feet, mm): Total depth of soil plant roots readily penetrate and in which the predominant root activity and water consumption occurs. Area of the soil from which the crop roots extract water and nutrients but may also be used as a portion of the root zone in equations where soil characteristics change within the root zone.

Transpiration: The net quantity of water absorbed through the crop roots and transpired plus that which is used directly in the building of plant tissue. It does not include evaporation from the soil or intercepted precipitation. It is expressed in terms of volume per unit area or as depth in feet or inches.

*Uniformity Coefficient (fraction, %): A fractional or percentile measurement of the uniformity of irrigation water application. It equals the average depth of irrigation water infiltrated minus the average absolute deviation from this depth, all divided by the average depth of water infiltrated into the soil.

Water Uses (generally denotes a legal or statutory term that varies with states):

  • Consumptive: Total amount of water consumed by vegetation for transpiration or building of plant tissue, plus the unavoidable evaporation of soil moisture, snow, and intercepted precipitation associated with vegetal growth.
  • Non-Consumptive: Water that leaves the selected region and not considered consumptive. Examples include runoff, deep percolation, and canal spills.
  • Beneficial: Beneficial use of water supports the production of crops: food, landscape, turf, ornamentals, or forage. Many regulatory agencies use “beneficial use” as the legal definition of proper water application and use.
  • Non-Beneficial: Water utilized in plant growth that cannot be attributed to beneficial uses.

*Water Storage Efficiency: The ratio of the amount of water stored in the root zone during irrigation to the amount of water needed to fill the root zone to field capacity.

*Water Use Efficiency: The ratio of the yield per unit area to the applied irrigation water per unit area.

*Wilting Point: The moisture percentage of the soil below which little or no plant growth occurs.

Zone (irrigation or sprinkler): Section or portion of an irrigation system served by a single control valve. Zones are usually comprised of similar sprinkler and plant material types with similar water requirements and pressure.

(Source: Glossary of Irrigation Terms, Version 7/1/2017, Irrigation Association.)

Fundamental Concepts of Irrigation Practices

For me, the fundamental tenet of a good irrigation system design is to provide just enough of the needed water and nutrients to the crop or plant at the required uptake point and rate without applying excessive or wasteful amounts of either.

It may seem a little confusing to have the term “nutrients” included in the definition above. However, many irrigation systems are now equipped to help bolster the needed nutrients that may be lacking or unavailable in sufficient quantities to support plant growth in the form of industrial-prepared chemical solutions.

For example, many of these important nutrients—such as nitrogen, calcium, potassium, and phosphorus—can be applied as a single or combined fertilizer formulation to the irrigation water as it is pumped and delivered from the source to the field and then crop in the form of an “NPK” fertilizer.

This is generally conducted as a combined pumping system by using a positive displacement pump interlocked to operate with the source pump and proportionally pump the fertilizer into the flow stream, often called fertigation.

Another similar process, referred to as herbigation, can inject herbicides into the flow stream to control weeds and other noxious plants that may retard or stop the proper growth of the crop.

In both cases, when applying fertilizers or herbicides to an irrigation system, appropriate check valves, backflow prevention, electrical safeguards, and pump lockouts must be employed to protect the crop from overapplication of the chemical—and most importantly, to protect the well from siphonage back to the source. This is especially the case for the application of nitrogen. Siphonage can rapidly lead to nitrate
contamination of the well and aquifer if allowed to operate in this manner for an extended time.

It is always important to remember no one system is best for every application. Once your customer decides to consider an irrigation system, as the designer you must assist them by examining several important factors. To summarize:

  • The crop’s daily and seasonal water needs, root depth, crop rotation or cover crop impacts
  • Soil: type, structure, consistency, horizon depths, water-holding capacity, permeability
  • Environmental issues: prevailing wind direction and speed, local climate, sun exposure, shading
  • Physical characteristics of irrigable areas: acreage, slopes, obstructions, irregularities, legal and geographic obstructions, property limits, boundaries, setbacks, shape of irrigated fields
  • Electrical or fuel system, unit cost, local availability
  • Type of water source: shallow or deep well, river, pond, lake, other
  • Water source: water quality, flow rate, volume, water rights, cost (if water is purchased)
  • Pumping system specifics: type of pump for applicable source, capacity and head, source protection
  • Irrigation system specifics: type, initial cost, water application gross and net rates, depth and management, ease of setup or reset, system efficiency, adaptability to possible relocation to other fields (if applicable)
  • Ongoing system operation and maintenance: equipment and labor requirements, costs
  • Adequate source and crop protection when using fertilizers or herbicides.

There may be additional considerations in some situations, such as whether or not the customer owns or is renting or leasing the land they intend to farm. The customer may also own some existing equipment such as a well, pumping plant, or irrigation equipment they wish to adapt the system to work with.

Ultimately, there are two basic economic factors to be considered when planning any irrigation system: the initial capital investment and the annualized costs.

The capital investment provides an idea as to the initial and upfront expenditures that must be made to purchase and construct the system. Annualized costs provide a prorated operating cost basis per year or growing season.

Is Irrigation Even Appropriate?

Before setting up any agricultural or turf irrigation system, the following four essential elements must be individually and collectively considered to know if irrigation is even right for the intended land and application, to select the right system, and to plan its use accordingly for good crop growth management and greatest water use efficiency.

1. Understanding crop watering needs

The first factor any designer should consider before planning to use an irrigation system is the water requirements of the type and variety of plants being grown.

Different plants must receive water in different ways and at different stages of growth. It is essential to understand the specific water  requirements at each of the various stages by accounting for evapotranspiration, runoff, and wind drift to determine how watering should be scheduled and when total water amounts are adequate.

By using this information, along with examining the systems available from local irrigation system providers, you can find the best one to suit the water requirements.

2. Examining all potential environmental and water availability factors

This may seem like a “no-brainer” but is often ignored, stepped over, or around.

Environmental factors are often those generally impacting the specific region that may not apply to an area just a few miles away. They include those elements of nature in which humans hold little reasonable control.

Among these are the necessary photosynthesis gained from sunlight but without the impact from too many shaded areas that may block the sun. Avoiding the harmful effects from excessive heat, humidity, and radiation. Impacts from lateral, swirling (tornadic), or excessive winds that can dislodge germinating crops or severely disrupt sprinkler irrigation.

The second element is often water availability. This alone often controls the type and size of irrigation system that can be used, especially with mechanized systems.

For example, drought or water-limited/starved regions may not possess ready access to enough well or surface water needed to apply up to 10 gpm per acre onto a crop with sprinkler irrigation techniques in the permitted time frame, necessitating consideration of more expensive drip irrigation.

In other areas with adequate water quantity but a well with severely sandy conditions, the water quality may be so inferior that applying it to a crop will damage or even kill the plant from excessive thirst as a consequence of sealing the soil surface with sand. This can also cause surface runoff of applied water, destroying the pumping equipment, and likely the well itself.

In other cases, the salinity or boron hazard may be high enough that additional water must be applied to flush or leach the salts from the soil or the water cannot be used at all. Finally, water rights must always be considered. Are water rights present or available for the intended use and region and in the required flow rate and volume? Can existing owned rights be transferred or other water rights purchased and moved to the proposed field for the intended use from another location or site for the proposed use?

3. Understanding the field’s terrain, dimensional, and geographical limitations

Figure 1. Soil textural triangle.

It is often not enough to know you need to apply irrigation to a crop if you cannot get the water to it. Terrains and slopes (topography) are case-by-case and locational issues that can potentially impact one farm while not affecting a neighbor whatsoever.

This factor is most predominant and of greater concern for sprinkler and flood irrigation and can result in situations where crops on an upper slope are metaphorically dying of thirst while those in the lower region of the same field are literally drowning.

It can also impact the method of irrigation used, since many mechanized irrigation systems such as center pivots, lateral moves, and hard hose reel systems cannot operate on excessive or undulating slopes.

Unlike most of the other conditions, improper management of a terrain issue can also have lasting consequences on the farm and future growing practices due to possible soil erosion and topsoil relocation or loss. The potential problems associated with terrain and soil erosion must be carefully weighed during initial planning and design and thereafter managed during operation to prevent crop damage in future years.

The next important factor involves the dimensional and geographic limitations and physical/legal boundaries that may impact the method of irrigation. Many irrigation systems such as center pivots, lateral moves, and hard hose Big-Gun systems are only cost-effective irrigation methods when they can be applied over adequate acreage in the allotted time to recover their initial and operating costs. Dimensional restrictions, such as irregular or triangular shaped fields, can also greatly affect the type of irrigation system that can be economically or practically used.

Finally, geographic or legal restrictions—particularly physical or legal impediments such as property boundaries, rights of way, easements, setbacks, roads, ditches, creeks, streams, lakes, gullies, valleys, or similar obstructions—can drastically interfere with the travel or permitted application of most mechanized methods of sprinkler irrigation. This can result in a shorter wheel-line, and center pivot or lateral move length, raising the system’s cost per acre and labor costs.

Another example may necessitate relatively short travel runs for hard hose systems designed to operate at twice the distance, resulting in lower system efficiency and higher labor costs to move and reset the machine more often than desirable.

Besides potentially preventing continued movement across a busy road or property line, for example, the mere presence of the road or property line next to operating sprinklers can present liability issues. This can be due to sprinkler overspray onto vehicle surfaces or adjacent residences and structures, resulting in possible damage or slick road surfaces.

4. Understanding the soil type

Beyond understanding the moisture requirements of the specific plant species or crop, you must also know the type of soil and and how it can affect the plants and irrigation scheduling.

Soil testing can provide you with essential information about the soil’s structure, horizons (layers), infiltration rate, and water-holding capacity. These details are critical when planning for efficient irrigation that allows plants to absorb the most water while losing the least to runoff or evaporation. The soil texture refers to the weight proportion of the separates (sand, silt, and clay) in a given soil for particles less than 2 millimeters.

The soil textural classes are defined as follows. They are from the Soil Survey Manual, which is published by the U.S. Department of Agriculture:

  • Clay: 40% or more clay, 45% or less sand, and less than 40% silt.
  • Clay Loam: 27% to 40% clay and more than 20% to 46% sand.
  • Loam: 7% to 27% clay, 28% to 50% silt, and 52% or less sand.
  • Loamy Sand: Between 70% and 91% sand and the percentage of silt plus 1.5 times the percentage of clay is 15% or more; and the percentage of silt plus twice the percentage of clay is less than 30%.
  • Sand: More than 85% sand, the percentage of silt plus 1.5 times the percentage of clay is less than 15%.
  • Sandy Clay: 35% or more clay and 45% or more sand.
  • Sandy Clay Loam: 20% to 35% clay, less than 28% silt, and more than 45% sand.
  • Sandy Loam: 7% to 20% clay, more than 52% sand, and the percentage of silt plus twice the percentage of clay is 30% or more; or
    less than 7% clay, less than 50% silt, and more than 43% sand.
  • Silt: 80% or more silt and less than 12% clay.
  • Silt Loam: 50% or more silt and 12% to 27% clay, or 50% to 80% silt and less than 12% clay.
  • Silty Clay: 40% or more clay and 40% or more silt.
  • Silty Clay Loam: 27% to 40% clay and 20% or less sand.

The relationship of these various groups of soil and how to determine the precise type of soil based on the percentage of each soil group is shown graphically in Figure 1 as the classic soil textural triangle.

Irrigation Is Appropriate; Now What’s the Best Method?

Based on considerations of the plant species; watering requirements; terrain, land, and geographic limitations; and the soil type and structure, the designer can compare the various types of irrigation systems in order to recommend the most suitable one for the customer’s
needs.

In many instances, irrigation systems require additional components such as soil moisture tensiometers and weather monitors, computerized programming, satellite communication, and other advanced features allowing the system to run and monitor itself once
programmed. Modern irrigation systems run the gamut from flood to drip irrigation, in high- and low-pressure and low-head water application methods.The system can consist of either a pressurized or gravity-fed arrangement depending on the water source, topography,
and the best way to deliver water to the plants.

Recent technological advances in irrigation technology and how they can benefit your customer’s operation can also be integrated with the selected system.

This concludes Part 1 of our series. We will continue next month with an expanded overview on soil-plant-water relationships and the consumptive use of plants.

Until then, work safe and smart.


Ed Butts, PE, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.