Irrigation Management Basics

Small fruit, vegetable and nursery crops generally require irrigation to produce the quality the market demands and the yield the producer needs. Irrigation technology has changed dramatically over the last thirty years. Drip or trickle irrigation has been widely adopted by large and small-scale farmers.  But many growers prefer the ease and flexibility of sprinkler irrigation.  Independent of which system or mix of systems a grower uses, irrigation management can be challenging.

An irrigation program must match the changing demands of the crop with the water supplied.

For direct seeded vegetables, the planting bed is often irrigated prior to seeding to create a soil moisture reserve. This may not be necessary in the first spring plantings but is crucial in establishing later plantings. 

After seeding, the beds are kept moist to encourage rapid germination.  As seedlings emerge and become established, time between irrigations increases and the amount applied each time also increases as the root and the leaf systems develop. Much the same process is followed with transplants except that the initial requirement for a constantly moist top inch of soil is not as crucial.

As the canopy develops, water demand tracks both the increase in leaf area of the crop and the demand put on that crop by weather.  Water moves out of the plant “capture zone’ in several ways.

  • First, a significant percentage of soil moisture is lost by downward movement through the soil.
  • Second, there is a modest loss of soil moisture through upward movement of water to the soil surface. Once at the surface, it evaporates.
  • Finally, plants move a tremendous amount of water through their leaves. This is called transpiration. Water is taken up through the root system and exits the plants through small leaf openings called stomata, found mostly on the undersides of leaves.  As the leaf area of the plant increases, so does the moisture lost through the crop canopy. Hot and/or windy days accelerate moisture loss from leaves.

The transpiration flow cycles mineral nutrients from the soil into the plant, cools the leaf surface as it evaporates, and serves as the medium for most biochemical processes in the plant.

Water also acts as a key structural element for the herbaceous parts of plants. When plants are under extreme water stress, they wilt. Confronted with milder stress, the stomata shut down to conserve moisture. When that happens, no carbon dioxide enters the leaves and photosynthesis grinds to a halt. As a result, fewer sugars are produced and root and shoot growth slows. This can become a vicious cycle when moisture in the existing root zone becomes limiting. As crops move into their critical productive stages (e.g. onions bulbing, squash fruit forming, broccoli heading and the like) the impact of moisture shortfalls is profound and economically costly.

So with the knowledge that adequate moisture is the key to good crop production, how do you define the irrigation intervals for your crops and allocate the water among competing crops or plantings? What are some of the factors that come into play?

Root development

Crops have characteristic effective rooting patterns and depths. But these characteristics can be modified by soil type, irrigation water distribution (especially with drip systems), organic matter, compaction, and plow pans.  For vegetables, 70% of the soil moisture will come from the upper 50% of the effective rooting depth. This is where the largest fraction of active roots are found.  Most vegetables have an effective rooting depth of 12-20 inches.

Soil infiltration rate and water holding capacity

Dust off your soil survey because it contains some great information on water movement and retention specific to your property.  Tables describe how fast water moves though your various soil types in inches (or fractions of inches) per hour at different depths. It will then give the available water capacity (AWC) as inches of water per inch of soil depth. The available water capacity is the difference between the total amounts of water the soil can hold just short of saturation down to the lower limit of the permanent wilting point (PWP).

When the permanent wilting point is reached, the plant can no longer access the residual moisture as it becomes tightly bound to soil particles. Sandy soils show high infiltration rates but generally not much storage capacity. So you can irrigate the profile quickly but it doesn’t last.

Maximum allowable depletion (MAD)

This is the amount of AWC that can be depleted without hurting crop yield or quality.  It is monitored in the effective rooting zone. For most crops, MAD is between 40-60% of the available water capacity. If your soil dries below that point for any period of time, the crop is at risk.

There are sophisticated soil-moisture measuring devices like tensiometers that are used by large commercial operations.

The simplest method to assess water availability in the root zone is to use a soil probe to remove cores of soil in the crop row.

The soil probe is also useful for checking the distribution of water delivered by your irrigation system. Drip systems show moisture spread and infiltration differences depending on soil characteristics, flow rate, and time the system runs. The soil probe also allows you to check the accuracy of the next technique for water management: water or “checkbook” budgeting.

Water budgeting starts with an estimate of the available water capacity in the crop’s root zone (AWC/in x effective root zone). Then irrigate to have a full “checkbook” if the AWC isn’t already fully loaded. You manage your checkbook by following the crop and weather removal of moisture paired with additions either through rain or irrigation. It isn’t a difficult technique once you get used to it.

Weather stations located throughout the Pacific Northwest provide data from which the evapotranspiration  (ET) demand can be calculated on a daily basis for crops grown in each area. The ET calculation combines the water lost from surface evaporation (largely temperature and wind driven) and water lost through the crop canopy (temperature, wind and leaf area driven) less any rainfall. It is often referred to as the crop water use and is measured in inches per day.  The Oregon data can be found on the AgriMet web site hosted by the Bureau of Reclamation. 

Look at the list of crop abbreviations on the AgriMet home page to make sense of the data. There are several assumptions embedded in these calculations.

  • First, there is an assumed normal planting time and crop development cycle.
  • Second, many crops (for example tomatoes and peppers) are not listed at all. However, it is often possible to pick listed “surrogate” crops to base your decisions on.
  • Finally, the water loss projections assume good weed control. If that is not the case, you would need to add a fudge factor to take into account the water weeds remove. Typical mid-summer moisture losses once a crop canopy is near full are between .25 and .35 inches per day. Extremely hot weather can push this to.40+ in./day.

Crop profiles and supporting information are found in the outstanding Western Oregon Irrigation Guides EM 8713, which should be required reading for crop managers. See bibliography at the end for details.

Most crops are irrigated when available water capacity (AWC) reaches 50%. Onions can only lose 30% of the AWC. Beyond that, they don’t size normally. The following example shows two soils on different parts of the same farm:

Depth (in)

Soil Depth (in) Infiltration rate (in/hr) AWC (in water/in of soil depth)
Sauvie Silt Loam 0-15 0.2-0.6 .19-.21
15-39 0.2-0.6 .19-.21
39-60 2.0-6.0 .15-.17
Burlington Fine Silt Loam 0-12 2.0-6.0 .15-.15
12-60 6.0-20 0.9-.10

The Burlington fine silt loam has an available water capacity 0f 2.22 inches in the 18 inch effective rooting zone for onions. The Sauvie silt loam, by contrast, has a 3.60 inch AWC. Since onions fail to size properly if the AWC goes lower than 70% (down 30%), irrigation needs to be applied for the Burlington soil when moisture removed equals .67 inches while the Sauvie soil has a little larger buffer of 1.08 inches.


Soil Burlington FSL Sauvie SL
MAD 30% of AWC 30%
Rooting depth 18 inches 18 inches
AWC (0-12")
  • 1-12" = .14in/in = 1.68 inches
  • 12-18" = .09in/in = .54 inches
.2in/in = 18 x .20 = 3.20 inches
AWC x MAD 2.22 x .30 = .67 inches 3.60 x .3 = 1.08 inches

AgriMet evapotranspiration data (what moisture the crop/weather is removing in inches):

ET Aurora Forest Grove
7/14 .26 .24
7/15 .28 .30
7/16 .30 .30
7/17 .29 .31
7/18 .33 .38
7/19 .32 .34
7/20 .34 .37
7/21 .31 .30
7/22 .30 .31
7/23 .22 .26

Looking at the Burlington soil with a full AWC of 2.22 inches, how often would you have to irrigate? Comparing the MAD of .67 inches with the average rate of depletion in the root zone of .30 inches/day (this was a hot period), you would have to water about every 2.2 days. For the Sauvie soil with a MAD of 1.08 inches, the timing would be about every 3.5 days.

Is it possible to assume greater effective rooting depth for the onions? If so, you could store more water in the soil below 18 inches and you would have a larger AWC and thus might be able to spread out the irrigation a little longer. In addition, building organic matter in a field can improve the water holding and can also stretch out the intervals between irrigations. Other crops effectively reach below 18 inches thus leading to longer watering intervals.

The final piece of this puzzle is sorting out how long to irrigate. Each irrigation system has its own performance characteristics that need to be understood. The key criteria are:

  • How much water is delivered per minute?
  • Over what area?
  • How uniformly?

Drip/trickle irrigation systems generally deliver a more uniform pattern since they are relatively unaffected by wind. However, soil characteristics can profoundly affect the width and depth of the delivered water pattern in drip systems, so routine checking with a soil probe will help manage the timing of the irrigation.

A sprinkler irrigation system with 7/64 nozzles with a placement of 40’ x 40’ and set to deliver 2.68 gallons/minute (50 psi)  will deliver .16 inches/hour in the covered area. Taking into account a 15-30% loss due to sprinkler inefficiencies (evaporation, wind drift, etc.)  and the need to replace .67 inches from the Burlington soil leads to an irrigation set of 5 hours. The infiltration rate of this soil and the output of the systems are not limiting.

The same exercise repeated for the Sauvie soil requires 7-8 hours to get to the 1.08 MAD but would not have to be repeated as often.  The infiltration rate of .2-.6 inches/hour for this soil is also not limiting with this irrigation set-up.

Distribution inefficiencies are less on non-windy days and also vary by the type of system. Solid set systems are the most inefficient in that their position doesn’t move so the distribution issues there at the beginning remain through the growing season. Offset hand move systems are the best sprinkler set-up.

Drip systems can be calibrated in the same manner using the delivery constants at a given water pressure (psi). Pay special attention to the drip pattern in your soil and crop row placement.

Mixed vegetable farms have the most complex irrigation challenges. The diversity of crop production, market, and harvest cycles can drive the irrigation manager crazy.

  • Can a given crop be overhead watered at night without provoking disease?
  • What is the cost of pumping? 
  • How do you sort between competing crops that need water (crop stage, ultimate value, possible deeper root system in one, etc.)?
  • If irrigation systems overlap crops, are you shorting one crop and over-saturating a new seed or transplant bed?
  • What do you do if a crop needs water but a new order has come in that requires the crop to be partially harvested the next day and that isn’t possible if it has just been irrigated (on some soils).
  • Is it possible to design a cropping system/cycle that is impossible to water adequately in a very hot event? Yes, it is.  How do you handle those choices?

Planning around crop irrigation requirements over the growing year, knowledge of your various soils, a handy soil probe, and thoughtful investments in irrigation infrastructure will support all your other efforts and lead to great crops and a productive year.

To get more information on irrigation budgeting and management, see following publications:

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