Scheduling Irrigation

There is no simple method to accurately schedule irrigation events since all the above factors interact to determine the actual rate of evapotranspiration and crop water needs. If you wait until crops are wilted to irrigate, the crop will already be damaged. The following items should be kept in mind when deciding when and how much to irrigate, even if you are using the tools and techniques discussed immediately following. 

1. Soils vary greatly in water-holding capacity and infiltration rate. Silt and clay soils and soils high in organic matter can hold much more water than sandy soils low in organic matter (Tables 13 and 14). Soils with a high available water holding capacity need the same amount of water as those with a lower-water holding capacity, but less frequent irrigation. However, when irrigated less frequently, a greater amount of water should be applied per application since fully wetting the root zone is important for root development. 
2. Water loss from plants and the soil surface is much greater on clear, hot, windy days than on cool, overcast, humid days. During periods of hot, dry weather, when the crop is at full canopy, the rate of evapotranspiration or plant water use may reach 0.3 inch/day or about 2” per week. 
3. Research shows that irrigating to maintain soil moisture levels in a narrow range, just below field capacity (60%–80% available soil moisture), results in better crop performance than if the range is broader. Soil moisture monitoring is therefore a more accurate way to determine irrigation needs.
4. Plastic mulches reduce evaporation from the soil but also reduce the amount of water that can reach the root zone from rain. Thus, a large percentage of the natural precipitation should be discounted when scheduling irrigations for crops grown using plastic mulch.
5. On moderate moisture-holding capacity soils, apply 0.25–0.75 inches of water per irrigation event. This will ensure that water reaches active areas of the root zone. The exception is during early crop growth and establishment when lower rates may be appropriate. With sandy soils, daily irrigation applying only what can be used that day is best. Splitting or pulsing the daily application with a 2+ hour break between applications has shown benefits, particularly in very coarse soil.
6. If irrigation water has a high salt content (for example, wells in coastal aquifers or tidal streams), excess water should be applied during each irrigation event to leach any salts before they are concentrated by evaporation. It is necessary to regularly measure the salinity of tidal surface water to prevent crop damage.
7. Total weekly water needs for vegetable crops will increase to full canopy and decrease thereafter. Irrigation rates should be adjusted accordingly. Critical crop stages such as fruiting, or tuber bulking should also be considered in determining weekly irrigation rates (see Table 13). 

Scheduling Irrigation based on Soil Moisture Levels 

Hand-Feel Method: This is the easiest and cheapest method to determine soil moisture levels for irrigation scheduling. Soil samples are collected using a soil probe or shovel, and the moisture level is estimated by “feeling” the soil and comparing it to known conditions. This method can allow for multiple depths of samples and is not susceptible to equipment failures; however, it does require an experienced operator to get consistent results. Charts are available to describe how different soils should look and feel at different moisture contents. See USDA- NRCS guide for Estimating Soil Moisture. 

Water Budget Method: The water budget method is an accounting process of daily water use and rainfall inputs. Plant water use is estimated daily based on crop development and climate conditions and is compared to the soil’s water-holding capacity. Often, pan evaporation is used to keep track of water losses. An evaporation pan is literally a pan with a gauge in it to determine how much water has evaporated, which is correlated with plant evapotranspiration.

Assume that the rooting depth of a crop is 8 inches on a sandy loam, high-organic-matter soil. After a soaking rain or after irrigation, the soil should hold about 1.2 inches of water (8 x 0.15 inches) (calculated using data from Table 14). Irrigation should begin when available water is depleted by 50%, or 0.6 inches in this example. If evapotranspiration removes about 0.22 inches of water per day from the soil, then after 3 days 50% of the available water is depleted, and water is required. With the water budget method, moisture loss is monitored and continuously calculated, and water is replaced based on the calculations. 

Tensiometers: Tensiometers are inexpensive tools for determining irrigation frequency because they indirectly measure water available in the crop root zone. Tensiometers are glass or plastic tubes filled with water, hermetically sealed with a porous ceramic tip at one end that gets submerged in the soil and a vacuum gauge at the other end. As the soil dries, its capillary action will try to suck the tensiometer water through the ceramic tip creating a vacuum. This vacuum is a measure of soil tension, “soil suction” or “matric potential.” Soil tension is a measure of how tightly water is held in the soil and is measured in pressure units of centibars (cb) or kilopascals (kPa). These are different units of measurement of the same condition: soil vacuum. The soil tension measured with tensiometers is an indirect indication of soil moisture content and can be used as an indicator of irrigation requirements.

Table 16 contains guidelines for using soil tension data to schedule irrigation events. Field capacity, the moisture content at which a soil is holding the maximum amount of water it can against the force of gravity, corresponds to soil tension levels ranging from 5 to 10 cb in coarse-textured soils and as high as 40 cb in fine-textured soils. The soil tension range corresponding to the time when irrigation should begin is also influenced by soil texture. In coarse-textured soils, irrigation should begin at soil tensions of 20 to 40 cb. In extremely coarse-textured soils, irrigation may be necessary at even lower tensions. Conversely, medium- and fine-textured soils do not need to be irrigated until soil tensions reach higher values. For all soil types, irrigate when a maximum of 50% of available water has been depleted. Lower depletion allowances may be used depending upon specific crop and management needs.

Table 16: Irrigation Guidelines for Tensiometers 

The utility of tensiometers in fine-textured soils is limited due to the range of detection. When soil dries beyond the 80 cb tension level, the column of water in the tensiometer "breaks," allowing air to enter the device. After breaking tension, the device ceases to operate correctly until it is serviced. Thus, tensiometers are the most practical in sandy or coarse-textured soils where normal soil tension levels are well below the point of breaking tension. In sandy soils it is often desirable to use ½-bar gauges that read in the 0–50 cb range rather than the standard 0–100 cb.

When locating tensiometers in the field, be certain you are aware of soil variability within a given field. Three to four tensiometers per field may be needed to adequately account for this variability and, in addition, two depths (commonly 6" and 12") may be necessary to adequately reflect the most critically stressed areas. The 6" unit indicates when soil moisture near the surface is being depleted (begin irrigating) and the 12" one shows when the moisture has moved to the bottom of the root zone (stop irrigating). In a drip irrigation system, a rule of thumb is to place the tensiometer about 6" from the tape at a depth of one-third the entire root zone. During irrigation, the tensiometer indicates when field capacity has been attained at the depth of the porous tube.

Resistance Meters - Granular Matrix Sensor: Another type of soil moisture sensor gaining popularity in the Northeast is the granular matrix sensor, which is categorized as a resistance meter. This category of sensors provides a reading based on the electrical resistance between two electrodes embedded in the granular matrix within the sensor. The more soil moisture available in the soil, the lower the resistance and the number on the reader. This resistance reading is reported in kilopascals (kPa) or centibars (cb). This measure of resistance can be used to better understand the force a plant root must overcome to extract water from a given soil. 

Resistance sensors are calibrated for each soil type. As noted previously, this is because different soil types have varying levels of plant available water at various soil moisture readings. The irrigation chart (Table 15) can be used as a guide for when to water. In most soils, other than heavy clay, the decision to irrigate would generally happen in the range of 30–60 kPa. Prepare and install sensors according to manufacturer’s recommendations. Locate resistance sensors and meters in a similar manner to tensiometers to give accurate information on soil water depletion.

Volumetric Soil Moisture Sensors: Volumetric soil water sensors such as TDR (Time Domain Reflectometry) and FDR (Frequency Domain Reflectometry) sensors can also measure soil water accurately. They require power sources to operate (battery, solar, wired) and are typically much more expensive than tensiometers and resistance blocks. For irrigation scheduling, sensors at various depths and locations in the field are installed and monitored. Soil moisture is recorded as volume of water per volume of soil. This then can be related to the available soil water percent based on a specific soil type by calibration to produce a soil water curve.

Water Supply Considerations 

While irrigation is extremely important to successful production, the cost of irrigation is largely dependent on an adequate and dependable supply of good-quality water. The water source should be as close as possible to the area to be irrigated to minimize the pumping and supply line cost. Ponds, lakes, streams, springs, groundwater, and municipal water are all potential water sources.

The legal right to withdraw water for irrigation must be verified. Water rights in the eastern United States are called riparian, or landowner, rights because anyone is entitled to use any water associated with land ownership. However, a riparian owner’s rights are not absolute and are subject to reasonable use interpretations. Some states have water use registration and reporting requirements when a certain number of gallons of water are used, and a permit and/or an annual pumping report may be required. Contact your state environmental services department for information on water use regulations, especially if you plan to pull a significant quantity and/or the source is shared with neighbors or other farms.

Water sources must be able to provide water as often as it is needed, though this is becoming increasingly difficult for many farms, as many previously reliable water sources are becoming more precarious. In extreme cases of drought, some commercial growers have even needed to buy in or transport water from other locations. Streams and springs may not provide enough water during the summer when irrigation is needed most.

Sometimes municipal water systems allow connections for irrigation purposes. This option may be expensive if water-use prices are high. If the municipality enforces water rationing, it may not permit irrigation during droughts.

Seasonal Water Demand

Seasonal water demand is the total amount required by the planting for the entire growing season. Natural rainfall can contribute to supplying the seasonal water demand, but because it is unpredictable, supplementation from artificial sources is necessary. In the Northeast, field vegetable production on average requires 1–2 inches of water per week, or 25–30 inches per season. The warmer the annual temperatures, the higher the demand. Calculating a water budget, or the required amount of water needed for your specific crop production mix, allows you to determine whether your water supply can support your crop mix and helps you to make production decisions relating to crop mix, placement and potential to ration water, in a low rainfall year. 

Water Quality

Water quality should be assessed when choosing a water source. Water quality includes its physical, chemical, and biological constituents. 

Physical constituents refer to sand, silt, or other suspended materials in water. While the physical constituents are usually not damaging to vegetable crops, they can damage or clog the irrigation system. A high sand content, for example, damages pumps and sprinklers, and suspended materials can clog trickle systems. Surface water sources, such as ponds and rivers, usually contain more particles and should be evaluated carefully, especially if drip irrigation is being considered. 

Chemical constituents of water refer to pH, dissolved material, proportions of dissolved ions, and any organic compounds such as oil. Although generally not a problem, organic solvents or lubricants in the water can damage plants. If water pH is high (7.0 or above), consider lowering it to prevent nutritional problems in the field. Furthermore, certain pesticides can be deactivated by high-pH water, so it is important to be aware of the water pH. See Fertigation Section below. 

Biological constituents such as bacteria and algae are often present in surface water and in some wells. These bacteria are usually not harmful to crops, but they can affect irrigation system performance. See the section on Drip Irrigation Maintenance and Challenges. When planning water usage for vegetable production, it is also important to consider that human pathogens (e.g., Escherichia coli (E. coli), Salmonella, Campylobacter, and Listeria) can be spread through irrigation water, causing foodborne illness. See Agriculture Water in the Produce Safety section.