Irrigation

Soil water management is critical for the production of high quality vegetables. Even short periods of moisture stress can affect a crop’s performance. Irrigation is essential in the Intermountain West due to high temperatures and high rates of evapotranspiration. Moisture deficiencies can occur early in the crop production cycle before local irrigation is available, which may delay or reduce emergence or slow early growth. Shortages later in the season often decrease fruit set, size, or quality. Over-irrigating is as detrimental to the crop as water shortage. Too much water can delay harvest, reduce quality, and shorten postharvest life. Table 2.4 lists the critical periods when water is critical for high quality vegetable production.

A crop’s water requirement, termed evapotranspiration (ET), is equal to the quantity of water evaporated (E) from the soil surface and the quantity lost from the plant (transpiration=T). Many factors must be considered when estimating ET. Most weather services provide an estimate of ET based on solar radiation, air temperature, wind speed, and humidity level. Therefore, using ET can improve irrigation management, and taking time to better understand crop water needs can greatly improve yield and quality.

There are many things that affect irrigation requirements. These include crop species and variety, canopy size, plant population, rooting depth, and stage of growth. These all influence transpiration, light absorption, and the rate that water evaporates from the soil. Mature plants use more water than crops which do not have a complete canopy (immature plants, recently transplanted crops). Rooting depths vary with crop species and determines the volume of soil from which the crop can draw water.

Effective Root Depth of Selected Vegetables

Shallow (6-12 inches)	Moderate (18-24 inches)	Deep (30+ inches)
Radish	Cabbage	Asparagus
Broccoli/Kale/Kohlrabi	Cantaloupe/Cucumber/Summer	Squash Pumpkin/Squash
Salad Crops (Lettuce/Spinach/Chard)	Beet/Carrot/Turnip	Watermelon
Garlic/Onion	Eggplant/Potato/Tomato
Pepper	Bean/Pea
	Sweet Corn

Note: Direct seed crops tend to root deeper than transplanted crops. Effective root depth is where the bulk of the root system is located. Some roots do go deeper.

Plant growth stage influences susceptibility to moisture stress. Irrigation is beneficial for newly seeded or transplanted crops as their root systems are not well established. Irrigation after transplanting significantly increases plant survival, especially when soils are dry and ET is high. Irrigation can also increase seed emergence and uniformity and final plant stand. For seeded crops, crusting can be an issue, so the water application rate and volume needs to be carefully regulated. If crusting is common, apply low rates and volumes of irrigation to soften the crust until seedlings emerge. 

Critical Periods during Vegetable Plant Growth when Adequate Water is Required for a Healthy Crop

Crop	Critical Period-Growth Stage
Allium Crops (Garlic/Leeks/Onion)	Bulb sizing
Asparagus Summer	fern growth
Brassica Crops (Broccoli/Cabbage/etc.)	Head formation or sizing
Cucurbits (Cucumber/Melons/Squash/etc.)	Flowering, fruit sizing, and ripening
Legumes (Beans/Peas)	Flowering, fruit set, pod sizing or filling
Potato	Tuber set and enlargement
Root Crops (Beets/Carrots/Radish/Turnips)	Root elongation and enlargement
Salad Crops (Chard/Lettuce/Spinach/etc.)	Leaf enlargement or heading
Solanaceae Vegetables (Eggplant/Pepper/Tomato)	Early flowering, fruit set, and sizing
Sweet Corn	Silking/Tasseling, ear development

Note: Water availability is critical for stand establishment for direct seeded or transplanted crops.

Cultural practices also influence ET and irrigation requirements. Cultivation, mulching, weed growth, and method of irrigation are factors to consider. Cultivation generally increases soil evaporation. Shallow cultivation helps eliminate soil crusts and may improve water infiltration, but if crop roots are damaged by the cultivator, water uptake may be reduced. Plastic or organic mulches generally reduce water use because they reduce evaporation. Weeds compete with the crop for water. Sprinkler irrigation systems which wet the whole field have greater evaporation loss than drip systems that wet only the area around the plant.

Soil type and texture has a big influence on waterholding capacity. Soils with more silt, clay, and organic matter hold more water than sandy or compacted soils. It is the amount of available water (amount of water a plant is able to withdraw from the soil) that is most important. Soils with high available water-holding capacity require less frequent irrigation than soils with low available water-holding capacity. When applying irrigation, consider the soil infiltration rate. Water should not be applied at a rate greater than the rate at which soils can absorb water. If the rate applied is excessive, erosion and runoff can occur.

Water-holding Capacity and Infiltration Rates Based on Soil Texture

Soil Texture	Water Holding Capacity (inch/foot of soil)	Infiltration Rate (inch/hour)
Sand	0.25-0.75	2.0
Loamy Sand	0.75-1.40	1.8
Sandy Loam	1.30-1.80	1.5
Loam	1.70-2.20	1.0
Clay/Silt	Loam 1.60-2.50	0.5
Clay	1.50-2.20	0.2

To accurately schedule irrigations, you need to consider all the above factors. While published ET values are helpful, keep in mind the following points when deciding when and how much to irrigate.

1. Soils vary greatly in water-holding capacity and infiltration rate. Know your soil type and learn how rapidly water infiltrates to minimize runoff.
2. Water loss from soils (evaporation) and plants (transpiration) is greater on clear, hot, windy days than on cool, overcast, humid days. When the weather is hot and dry, ET rates may reach 0.35 inch/day or more.
3. Plastic mulches reduce evaporation from the soil but most rainwater flows off and away from the crop. Organic mulches like straw will absorb rain and sprinkler irrigation water.
4. Most plants do better if soil moisture levels stay just below field capacity (75-90% soil moisture). Small frequent irrigations are better than letting the soil moisture get too dry (40-50% soil moisture) and then applying a heavy irrigation.
5. Assess the rooting depth of the crop and then apply water to recharge the area to field capacity. This will ensure that water reaches active areas of the root zone.
6. If irrigation water or soil has a high salt content, apply enough water to keep salts from accumulating in the soil.

Surface or Sprinkler Irrigation

Drip (also called trickle) irrigation is a method of applying small amounts of water directly to a plant’s root zone. Water is often applied frequently (daily or several times per day) to maintain optimal soil moisture conditions. The advantages of drip systems are:

• Less water is used
• Pesticides, fertilizers, and other materials can be applied uniformly
• Can be used on a wide range of crops
• Especially effective when used with plastic mulches
• Can be automated
• Disease and insect damage may be reduced because leaves remain dry
• Less weed growth between rows because these areas remain dry
• Field operations (spraying, etc.) can continue even during irrigation

Drip systems do have some potential limitations including:

1. Requires a higher level of management
2. Moisture distribution in the soil is limited
3. Smaller soil water reserves are available to plants
4. Equipment can be damaged by insects, rodents, and laborers
5. Requires a higher initial investment cost
6. Must have a constant water supply as irrigation may be needed on a daily basis
7. Sophisticated filtration equipment is needed to clean dirty water sources
8. Offer little in the way of frost protection

To use a drip irrigation system effectively, you need to design the system for the specific crop of interest, maintain a constant pressure throughout the system, and manage the system in accordance with crop growth stages and water needs. Since soils vary greatly in their water-holding capacity and infiltration rates, drip system designs need to take this into account. Also, as plants grow, their water needs increase, so the drip system has to have the capacity to meet this increasing water demand.

Pressure maintenance is important so that the whole field gets the same amount of water. Growers using drip systems need to be vigilant. If there are leaks, clogged lines or damaged tape, this will affect water distribution and may negatively impact the crop.

Finally, irrigation scheduling is needed to determine how often to irrigate (duration) and how much water to apply. Soil moisture monitoring tools are needed to determine irrigation frequency. These tools include soil moisture blocks, tensiometers, and other sensors that measure water available in the crop root zone. These are commonly placed at various soil depths throughout the field to determine whether or not the irrigation has reached a certain depth and to help determine the depth from which plants draw the most water.

More Information

• Drip Irrigation for Commercial Vegetable and Fruit Production (Utah State University Extension)
• Small Acreage Irrigation System Selection (Utah State University Extension)