Citrus Water Management

Julian W. Sauls
Professor and Extension Horticulturist
Texas AgriLife Extension

January, 2008

Citrus, a perennial evergreen tree, requires available soil moisture throughout the year--generally a minimum of 45 to 50 inches of water annually. Because average annual rainfall in the Lower Rio Grande Valley ranges from 17 to 27 inches across the citrus-production area and rainfall distribution through the year is irregular, supplemental irrigation is essential for sustained economic citrus production.

Moreover, the irregular seasonal distribution patterns of rainfall frequently result in the need to apply more total irrigation water than merely the difference between natural rainfall and total annual water requirement. This occurs when rainfall amounts at a given time exceed what is needed to replenish soil moisture to field capacity.

The amount and timing of irrigation applications are dependent upon tree age, soil type, weed management, climate and irrigation system. Mature trees require more water than young trees but young trees require more frequent irrigation because of limited root systems. Sandy soils have a lower water holding capacity than heavier soils, thus, requiring more frequent irrigation, preferably with less water per irrigation. Orchards which are tilled or disked for weed control require more water than those under chemical weed control, because of the use of water by weeds and the drying of surface soil from cultivation.

Applied water, whether by irrigation or rainfall, is used first to replenish the soil reservoir. Water in excess of that necessary to replenish available soil moisture percolates through the soil profile to the ground water table, carrying with it salts which have accumulated in the soil profile. A 3-inch rain may provide leaching of a fine sandy loam soil, but may not completely replenish the soil moisture of a clay loam soil.

Available soil moisture is depleted in two ways-loss by evaporation from the soil surface and use by plants for growth and transpiration. Climate directly and indirectly influences both evaporation and transpiration, primarily through temperature, but also through wind and relative humidity. The combined use and loss of available soil moisture, called evapotranspiration, determines the citrus orchard water requirement at any given time during the year.

Citrus Water Requirement

Citrus in Texas has long been perceived as a major user of irrigation water, possibly because of its perennial nature, but more likely because of an average of the five to seven irrigations per year which were somewhat common when water supplies were more normal. Based upon the erroneous assumption that each irrigation used 6 inches of water, five to seven irrigations would amount to 30 to 42 inches on an annual basis. With an annual water requirement of 45 to 50 inches of water and about half that supplied by rainfall, it is not difficult to understand the perception.

Mature citrus under flood irrigation.

In a demonstration of surge flow irrigation in citrus in Cameron County in the early 1990's, a 10-acre block of grapefruit under regular disking and irrigated with gated pipe and temporary borders, required about 21 inches of water for the non-surge flow half of the orchard and only about 18 inches for the surge flow half. That calculates to 4.2 and 3.6 inches per acre, respectively.

Because of drought conditions, a number of growers and irrigation districts installed flow meters in an effort to stretch their available allocations. To the surprise of practically everyone, citrus growers were consistently applying less than 4.0 to 4.5 inches of water per acre. During 2000 and 2001, Rio Farms at Monte Alto used 26 and 25 inches, respectively, in five irrigations each year on 329 acres of its citrus.

Orchards under microsprayers and drip irrigation are consistently using less than 15 inches of irrigation water per year--even during recent years of below-normal rainfall. Clearly, citrus is not a major water-using crop at all. Indeed, it is becoming increasingly apparent that citrus growers have historically been using water amounts well within their 30-inch irrigation rights.

Water Sources and Quality

The primary source of irrigation water for the Valley is the Rio Grande River, impounded in the reservoirs of Falcon Lake near Zapata and Lake Amistad near Del Rio. Its impoundment, accounting and apportionment to the U.S. and Mexico is under the jurisdiction of the International Boundary and Water Commission under terms of the 1944 Water Treaty and some 308 Minute Orders that have attached to it. The IBWC provides a monthly status report of ownership of the waters in storage, after which the Rio Grande Watermaster of the Texas Natural Resources Conservation Commission has responsibility for accounting, allocation and release of U.S. waters to municipalities, irrigation districts and other water rights holders. An in-depth discussion of those operations and key provisions of the Treaty and some of its Minute Orders is in preparation for later publication.

Probably all citrus acreage in the Valley has Class "A" water rights, which is 30 inches of water per acre annually. Rights, however, are not synonymous with allocations, as the latter are wholly dependent upon the amount of water which is available in the reservoirs. In times of drought and low supply such as have occurred over the last several years, allocations will be significantly less than rights. Indeed, numerous irrigation districts ran out of water during the summer of 1998, an event which is being repeated in the summer of 2002.

To obtain water for irrigation, the citrus grower must order and pay for the water, then arrange for the time of delivery to the orchard. At that point, distribution of the water within the orchard is through whatever irrigation system the grower has in place. Gravity flow systems are most common, but pressurized systems are also in operation.

River water is considered moderately saline, usually containing 700 to 1,200 ppm total salts. However, salinity can increase during drought periods to potentially harmful levels that require more careful water management to avoid yield reductions, particularly at delivery points farthest from the River.

In some areas, ground water exists in sufficient volume and of sufficient quality to be used in citrus irrigation. Well water should contain less than 1 ppm boron and have a sodium absorption ratio below 8 for use as irrigation water. Total salinity should be less than 1,200 ppm. Yield reductions of about 10 percent occur with prolonged use of 1,500 ppm water, perhaps as much as 25 percent at salinity levels of 2,000 ppm.

Irrigation Systems-Gravity Flow

Conventional flood irrigation is the primary system used in Texas citrus production. Acceptable irrigation efficiency with minimal labor requirement is possible if the system is properly designed and installed prior to orchard planting. Proper design includes land leveling to the appropriate grade, with underground supply lines, permanent valves and permanent borders within the orchard to deliver and control the water. Because soil permeability affects the length and width of irrigation runs, as well as the necessary grade to assure even distribution of water across the orchard, growers should consult with the Farm Service Agency of the USDA for assistance in land leveling and irrigation system design.

Permanent Valves and Borders
Permanent orchard valve.

The most efficient of the gravity-flow irrigation options is the use of underground pipes that lead to orchard valves at the upper end (head) of the properly graded orchard. Typically, there will be one valve for each adjacent pair of rows, with each pair of rows separated from adjacent pairs by means of permanent borders. Generally, permanent borders with a settled height of 1 foot are sufficient to confine the water within a two-row pan.

Because of supply limitations in recent years, some growers increased the height and width of permanent borders dramatically, thereby decreasing the effective surface area that was irrigated, reducing irrigation time as well as the amount of water applied. Unfortunately, the creation of such large borders in existing, mature orchards severed most of the root system at or just inside the tree canopy on the bordered side of the row. Aside from the obvious effect of such extensive root damage on growth and production in that and subsequent seasons, there were a number of cases of increased tree damage and even loss from infections by soil-borne Ganoderma fungus. Such borders would be advantageous in young orchards, but the negative aspects may be too great to create them in mature orchards.

Surface Pipe and Borders

Temporary border for irrigation.

Both poly pipe and gated pipe are used in conjunction with either permanent or temporary borders to distribute water within the orchard. Irrigation efficiency is about equal to that of underground lines and valves, but labor requirements are substantially higher. Efficiency drops a little with temporary borders, and labor requirements are higher, as the temporary borders have to be put up and knocked down for each irrigation.

Open Ditch and Borders

Some growers still use an open ditch to distribute water from the source valve to the orchard, in combination with temporary borders within the orchard. This is the least efficient of the gravity-flow systems, as seepage and deep percolation of water from the ditch can be significant, especially on lighter soils. Labor requirements are the highest of the three, as the ditches and borders must be put up and knocked down for each irrigation. Too, the ditch bank is normally cut by hand to divert water into each pan, which also requires the use of tarps (lunas) to control the water in the ditch.

Strip Watering

Strip borders for young orchards.

Strip flooding is a modification of conventional flood irrigation used to conserve water by the use of strip borders along each side of each tree row, thereby limiting irrigation to half or less of the orchard floor. Strip irrigation is practiced mainly during orchard establishment to deliver water more quickly to young trees. Under permanent valve and pipeline systems and chemical weed control, strip irrigation borders are a little more difficult to install, maintain and use, but it can be done. In other flood systems with tillage for weed control, the strip borders may be built and removed several times each season.

Floor Management Considerations

Permanent irrigation border.

The choice of gravity flow system also involves consideration of weed control methods. Part of the efficiency of permanent valves and permanent borders is due to trunk-to-trunk herbicide use to preclude any unwanted vegetation. Not only is no water lost to weed growth, neither is water flow impeded by weeds. Too, a crusted, packed soil surface expedites water movement down the row, thereby shortening the run time, with less water lost to deep percolation.

Orchards with temporary borders are necessarily subjected to some means of tillage for weed control in the tree row. Tilled soil impedes water flow, thereby increasing run time and deep percolation. Too, weeds typically germinate and grow, using available soil moisture, before tillage implements can be employed for their elimination. Tillage also increases soil moisture loss by exposing moist soil to the air where it quickly dries out.

Irrigation Systems-Pressurized

Pressurized irrigation systems utilize pumps to deliver water under low pressure, usually about 20 psi, through a series of mains, submains and lateral lines directly to the individual trees where the water is distributed through one or more emitters at each tree. The pipeline system is underground, but the lateral lines are typically aboveground or only very shallowly covered. Because irrigation intervals are a matter of a few days to a week, near constant access to water is a must. If access to a permanently-charged irrigation canal in not available, an on-site reservoir or a source of quality groundwater is essential. Aside from access to water, such systems require a reliable power source for the pump, the best filtration that can be afforded, an automatic backflush system and a water meter. These systems are readily adaptable for the application of fertilizers and some herbicides in the irrigation water. Land leveling is not required for these systems, although leveling assures better utilization of rainfall.

These systems can cost between $750 and $1500 per acre to install, depending on the system and the necessary specifications of equipment for a particular orchard. Growers should proceed only with the assistance of personnel who have the necessary qualifications and expertise to design, install and operate these systems of irrigation.

Drip Irrigation

Drip irrigation systems involve lateral lines into which the individual water emitters are built, with one or more emitters for each tree. The water output per emitter may be selected from 0.5 to 4.0 gallons per hour. The number and spacing of emitters at each tree determines the total water output for each tree over time and the total surface area wetted. It is generally accepted that a minimum of 50 percent of the surface area covered by the tree canopy should be wetted.

Different line diameters, emitter spacings and emitter outputs are available. For orchard use, the lines are usually semi-rigid polyethylene tubing, half an inch in diameter, with emitters spaced every 3 feet and having an output of approximately 1 gallon per hour. In most Valley citrus soils, the wetted pattern at each emitter would be about 3 feet in diameter. Thus, a single line per row should provide adequate water for an orchard for the first 3 or 4 years, after which an additional line or lines would be needed to meet the increased water requirement of older trees.

Drip systems require the highest degree of filtration to prevent clogging of emitters and reduction in flow. Chemical treatment may be required periodically to clean and flush the lines and emitters. Soil salinity at the edge of the wetted pattern does increase, but rarely becomes problematic except during an extended absence of leaching rainfall. The application of fertilizers and some herbicides through the system improves efficiency and reduces overall costs.

With proper design and operation, drip systems provide adequate soil moisture for optimal growth and production while using only about 12 to 15 inches of water per acre per year. Such savings result from better water use efficiency, negligible evaporation and deep percolation losses and from the fact that water is applied to only a limited soil surface area, under which the absorptive root system of the tree is concentrated.

One drawback to drip irrigation in Texas citrus is that in years of adequate rainfall, the roots grow far beyond the drip-irrigated zone of soil. However, such roots necessarily die during subsequent extended dry spells such as have occurred several times over the last few years. The other drawback is that drip systems provide no cold protection for the orchard. Some growers operate the system too long, thereby waterlogging the wetted zone where the roots are concentrated and pushing water too deeply into the soil profile. In the main, the wetted pattern below a drip emitter is more or less pear shaped, as lateral movement is limited.

Microsprayer Irrigation

Microsprayer/microsprinkler irrigation systems utilize only one emitter per tree, usually situated on a riser or stake to hold the emitter upright in place. Emitters are available with outputs of 4 to 30 gallons or more per hour. Most will wet an area up to 7 to 8 feet from the emitter, thereby producing a wetted surface area equal to or slightly greater than the canopy area of a mature tree.

Microsprayer irrigation of citrus.

Microsprayers differ from microsprinklers in that the former usually have no moving parts, spraying water outward in a pre-designed configuration or pattern. Microsprinklers, however, have a revolving head that directs one or two streams of water outward as it turns. Because of wind-blown soil particles, microsprinklers sometimes quit revolving, so that the water is deposited in only one or two spots.

Several types of microsprayers are available, including a double emitter style with one mounted at the appropriate height for irrigation and the other secured at the appropriate height in the tree scaffolding for cold protection. The selection of mode is controlled by a small, inline valve located at the lower sprayer--it must be set manually at each tree. Probably the best design for a microsprayer is one with a "floating" deflector head that drops into the orifice after irrigation, thereby sealing it against insect entry, especially of ants and snails which like to oviposit in the orifices.

Good filtration, regular maintenance and periodic flushing and chemical treatment will greatly reduce the occasional clogging problems experienced. Salinity buildup in the soil has not been a problem. Long-term usage in Texas citrus has averaged 11 to 15 inches of water per acre per year. Moreover, microsprayer systems can provide some measure of cold protection, particularly those delivering 16 gph or higher. Uniform water distribution and the elimination of water stress through frequent applications result in superior orchard performance.

Irrigation Scheduling

Basically, citrus trees do not exhibit visible symptoms of water stress until most of the available soil moisture has been depleted. However, fruit set can be adversely affected at soil moisture depletion levels of only 40 to 50 percent, particularly from pre-bloom through June. Consequently, extensive research in Florida and other citrus production areas has led to the recommendation that water be applied at one-third depletion during January through June and at two-thirds depletion over the remainder of the year.

The objective of irrigation is to maintain available soil moisture as near optimum as can be accomplished to avoid moisture stress. This objective is not practically attainable with flood irrigation. During the first several days post irrigation, the soil is waterlogged to the extent that root function ceases for lack of oxygen. This is followed by a week, at best, of good balance between soil moisture and aeration, so roots function normally. As the available soil moisture is depleted, that remaining becomes increasingly more difficult to extract, requiring considerably more energy to be expended on the absorption of water at the expense of growth and development.

Available soil moisture capacity can be roughly calculated for the upper 3 to 5 feet of an orchard soil by using the soil maps and charts in county soil surveys available at local Farm Service Agency offices. Locate the orchard on the maps, note the soil designation (letters or numbers) and look it up in the appropriate table (Table 5 for Cameron, Table 17 for Hidalgo and Table 14 for Willacy). Multiply available water capacity by the inches of rooting depth to obtain both the high and low values, then average the two to obtain a practical value.

Depletion of available soil moisture can be measured indirectly as a mathematical computation which correlates Class "A" pan evaporation data with evapotranspiration. Given that citrus requires about 45 to 50 inches of water annually, evapotranspiration models that are based on 60 percent of pan evaporation (the crop coefficient, as it is called) indicate close correlation with that total amount of water. However, classic work in Arizona, which compared predicted citrus water use with actual use as measured by neutron probe, revealed that the models were fairly accurate in the spring and the fall months, but overestimated the amounts needed in winter and underestimated the amounts needed in the summer. When these findings were applied to conditions in Texas, it was determined that the citrus coefficient should be adjusted for the winter and the summer periods.

As a general rule, the crop coefficient for Texas citrus is 50-55 percent of pan evaporation during December through February, 60-65 percent during March through May and September through November, and 70-75 percent during June, July and August. Pan evaporation data for a number of Valley locations are reported daily on KURV-710 radio and on the Texas ET Network.

The grower should total the pan evaporation values since the last irrigation or significant rainfall and multiply that by the appropriate coefficient percentage for the time of year to determine how much water has been used in the interim. That value can then be compared to the calculated soil moisture capacity to determine the depletion level, as well as to determine how much water is needed to bring soil moisture levels back to field capacity.

Tensiometers and gypsum blocks are relatively inexpensive instruments which growers can use to directly measure available soil moisture and thereby schedule irrigations. Both require servicing and reading (recording) at periodic intervals to be effective. For whatever reasons, few growers use these devices.

In the absence of actual measurements or calculated estimates of soil moisture for more precise irrigation scheduling, some general rules can be combined with grower experience to determine the need to irrigate. Under flood systems, irrigation should begin in late January or early February and continue at 20 to 35-day intervals until a final irrigation in mid to late November. The irrigation interval should be adjusted for rainfall and time of year.

Drip irrigation systems are designed for near-daily use during the season except during periods of rainfall. The length of time the system is on will vary from just a few hours during the cooler months to a maximum of 16 to 18 hours during mid-summer. It is considered detrimental to run the system for more than 18 hours continuously, as waterlogging develops and root function in the wetted zone slows or ceases altogether. A 10-hour drying-out period should be employed prior to resuming irrigation.

Microsprayer systems normally are operated at weekly intervals during the season except during prolonged rainy periods. The length of time the system is on can be varied from just a few hours during the cooler months to 12 to 14 hours during mid-summer. During periods of extreme drought and heat of mid-summer, irrigation frequency can be increased, if necessary, to provide the needed water. With the exception of operation for freeze protection, microsprayer systems should not be operated continuously for more than 16 to 18 hours, to preclude waterlogging in the root zone. Regardless of the type of irrigation system and other production inputs, final productivity, fruit size and fruit quality can only be as good as the management of supplemental irrigation in the citrus orchard.

Drought Management

Irrigation water supplies have been critically low over most of the last four years, and several irrigation districts which ran out of water in the summer of 1998 may again in the summer of 2002. The experience of 1998 proved that citrus trees can go without additional water much longer than was believed possible, as some orchards went 90 or more days between irrigation in late May or early June and significant rainfall in early September. Neither fruit size nor production appeared to have been affected when harvest was completed. What did happen, however, is that tree growth and development were dramatically reduced (only a light mid-summer growth flush occurred) and many orchards began to exhibit alternate bearing the following season, which trend continues to date.

Drought is a double-edged sword with regard to water conservation. On the sone side, drought imposes water conservation simply because there isn't enough water to go around. On the other side, irrigators who fully expect to run out of water before the season ends tend to water more heavily when they do irrigate. It is unlikely that such overwatering is beneficial, as the excess water typically percolates well below the effective root zone of most crops.

Given the prospects of a short supply, citrus growers should concentrate their limited water resources during the critical fruit set period from just pre-bloom through near the end of May, as moisture stress during those 3 months can dramatically reduce production. Worry about sizing the crop when that time comes, as shown by the experience of 1998.

Bear in mind that the potential size of citrus fruit is already established by June first, as the total number of cells in the fruit has been determined by that time. Fruit size increases thereafter are solely due to the enlargement of those existing cells. Obviously, adequate soil moisture during the summer would accelerate the sizing process, but size can still be achieved if adequate rainfall (or irrigation) occurs in late August into the fall.

If groundwater of higher than desired salinity must be used, greater amounts than normal should be applied at each use to leach at least some existing salts out of the root zone. Hopefully, leaching rains will occur before significant salinity problems occur from prolonged use of over-saline water.

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This page last revised January 7, 2008

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