|Water most closely approximates a universal solvent: it dissolves minerals present in the soil and serves as the pathway for solutes to enter the vine and through the tissues. Water is a raw material for photosynthesis and is essential for maintenance of cell turgidity, without which cells cannot function properly. It has a very high specific heat which tends to stabilize protoplasm temperature that is subjected to heat or cold. Water in the soil is continuous with that in the plant, and the entire mass of water is in constant upward movement since the shoot constantly loses water is in constant upward movement since the shoot constantly loses water to the atmosphere. Nearly all of the water (over 99 percent) moving upward is lost by transpiration, and only 0.1 to 0.3 percent is converted to chemical compounds. It serves as the cooling system for the plant, keeping the leaves nearer to air temperature through transpiration. This protects the leaves from heat injury and keeps them nearer to optimum temperature for photosynthesis.
Properties of Water
Water has a high latent heat of evaporation, which refers to the energy required to change water from a liquid to a gaseous state. When water reaches the boiling point, the temperature will not rise further until it changes from the liquid to the gaseous state. Before this change occurs, the water will absorb 539.3 cal per gram. The surface temperatures of the earth tend to be stabilized because of the high latent heat of evaporation of water.
The latent heat of fusion refers to the energy required to melt ice. When the temperature of ice has risen to 320F (00C), it will absorb 79. 9 cal per gram before melting; however, 79.7 cal of heat are released when 1 gram of water changes to ice.
Water contracts s it is cooled, but it expands as the temperatures are lowered from 390-320F (40C to 00C). It also has a very high surface tension. Water is a fairly inert medium in which chemical reactions can occur without involving the water itself.
The amount of invisible water vapor in the air is usually expressed as relative humidity, defined as the percentage of the maximum quantity that the air can hold at the prevailing temperature. Warm air can hold more water than cold air. Dew can form at night when atmospheric moisture condenses on cold surfaces, if the sky is clear enough so the soil and plants can lose enough heat to cause their temperature to fall below the dewpoint. Dew is usually an unimportant source of water for grape growing.
Clouds and fog are visible vapors that consist of water droplets or small ice crystals which result from cooling of the air to temperature below its dewpoint. When cooling is due to upward movement of air from the land surface into colder levels of the atmosphere, clouds are formed; when air cooling occurs at or near the land surface, fog is formed. Prolonged presence of water vapors can reduce solar radiation enough to reduce food manufacture by the leaves.
Water can be classified as unavailable, available, and gravitational when based on availability to the grapevine. Gravitational water usually drains rapidly from the root zone, and unavailable water is held so tightly by capillary forces that it is not available to plant roots. Available water consists of the difference between unavailable and gravitational water.
Water drains from soil because of constant gravitational pull. A much longer time is required for clay soils to drain than for sandy soils, so that a sandy soil might lose its gravitational or free water in 1 day, while a clay might require 4 more days for drainage rate is fastest immediately after irrigation.
|After all the gravitational water has drained from the soil, the soil moisture content is said to be at field capacity. This point is usually reached from 1 to 3 days after the soil has been thoroughly wetted by irrigation or rain. A simple way to determine field capacity is to pour water on the soil surface, allow it to drain for 1 to 3 days depending on soil type, then measure the soil water content.
The permanent wilting point refers to the soil moisture content when plants wilt permanently, and is at the lower end of the available moisture range. Plants wilt when they are unable to extract enough water from the soil to supply their needs. Plants may wilt temporarily on a hot windy day and then recover in the cooler part of the day. Field estimates of the wilting point can be obtained by measuring the soil water content in soils with permanently wilted plants, although it may be difficult to find plants in a condition of permanent wilt. A rough estimate by a factor varying from 2.0 to 2.4 . Since the factor depends on the amount of silt, 2.4 should be used for soils with a high silt content.
Available moisture refers to the difference in moisture content between the field capacity and the permanent wilting point. This is the moisture available for plant use. Generally, finer textured soils have a wider range of water between permanent wilting point and field capacity than do coarse textured soils. In any soil, an increased rooting depth in the soil profile as a whole can compensate for a narrow range of available water in other horizons. Grapes often have very deep roots. Within the range of available water, the degree of availability tends to decrease as soil water content declines. Frequently the range of water for survival is greater than that available for good growth.
|Measuring Soil Moisture
Farmers in irrigated areas can determine how deep the soil moisture is by the resistance of the soil to penetration. One can insert a shovel or push a half inch (12.7 mm) pointed steel rod into the ground. The depth that the rod can be pushed into the soil will indicate the depth of wetting, if seriously compacted soil layers exist to resist the rod?s penetration.
Samples of soil can be obtained throughout the root zone of the soil with a soil auger or a soil tube. Soil moisture content can be estimated by examining and feeling the soil. There are also several more accurate methods. In the gravimetric method, soil samples are usually placed in cans with tight lids, weighed and oven dried to a constant weight, and reweighed to determine the loss of water. The percent of water on a dry or wet weight basis can then be calculated.
Another method to express soil moisture is to determine the tension with which it is bound to the soil. Tension is expressed in atmospheres of pressure (bars), and a tensiometer measures the tension by which soil water is held in the soil. It consists of a porous ceramic cup filled with water that can be buried to a desired depth in soil in the vicinity of roots. The cup is connected by a water filled tube to a manometer or vacuum gauge. As the soil dries, it sucks water out through the porous wall of the cup, creating a partial vacuum inside the tensiometer that can be read on a manometer or a vacuum gauge (Fig. 9-2). This power of the soil (soil suction) to with draw water from the tensiometer increases as the soil dries and the gauge reading rises. Tensiometers operate satisfactorily only up to tensions 0.6 bar (-60 centibars), and these are most widely used in medium (loam) to light (sandy) textured soils.
Tensiometers also indicate extremely wet or saturated soil conditions. A reading of 0 to 5 centibars indicates a wet soil in which vine roots will suffer for lack of oxygen; readings above 10 centibars assure the grower that the soil is drained of excess moisture.
Water may be applied to soil in several ways. In surface irrigation water is distributed over the surface of the soil in furrows or basins, or by flooding. Application of water under pressure as simulated rain is termed sprinkler irrigation. There are both portable and permanent sprinkler systems. In sub irrigation water is distributed to the soil below the surface and reaches the vine by upward capillary movement. Drip irrigation refers to the addition of water soil by droplets emitted from small holes in plastic or other types of tubes.
In California water is often distributed by furrow irrigation. Usually there are two or three to a middle between rows usually spaced 10 to 12 ft (3.05-3.67 m) apart. Sometimes crosschecking is used to slow the flow of water and improve penetration. The wetted perimeter is increased, resulting in a more even distribution of water by using two wide bottom furrows. In table grape vineyards semi permanent furrows are often made, and are renewed each year at the beginning of the irrigation season. In summer the soil is not cultivated and weed growth is mowed frequently. Water penetration is improved by reducing compaction from equipment, and the vineyard is less dusty.
Furrow irrigation on steep terrains is difficult and expensive; contouring is necessary, and care must be taken prevent run off and destruction of furrows.
Flood irrigation can be used where large heads of water are available and there is a salinity problem. Flood irrigating in a wide flattened area between the raised berms along the vine rows is increasing in popularity. It spreads the water more widely for better penetration, eliminates the need to establish furrows, and provides a convenient surface for mowing under grass culture. Cross basin irrigation is seldomly practiced currently due to high costs.
Sprinkler irrigation is especially useful on hilly soils, on shallow soils that cannot be levelled, and where land was not levelled for irrigation before planting. It is also used where a sprinkling system has been installed for frost protection.
Water conversation can be enhanced by sprinkler systems. A more uniform distribution of water and elimination of runoff losses result from a well designed sprinkler system. A sprinkler system is 85-95 percent efficient, a border check system 60-90 percent, and a furrow irrigation system 50-90 percent efficient. Irrigation efficiency is a measure of the uniformity of water application over the entire field and the prevention of water losses. Sprinklers achieve a more uniform distribution of water than does furrow irrigation, since the latter often leaves dry areas within the vine row.
Good control of the amount of water applied to a block of grapes can be obtained with sprinklers, making it possible to avoid both drought and a too wet condition. Irrigation time periods (12, 18, 14, or 36 hr) can be regulated according to the depth of soil wetting desired. Sprinklers with proper design and controls can also be operated at any time and location when the vines require water.
Sprinklers located above the vines are called overhead sprinklers and are the preferred type. This system requires tall riser pipes to raise the sprinklers above the vines. In windy locations, care must be taken to select the proper type of sprinkler and proper spacing. The water rate applied to the soil (precipitation rate) must be less than the rate that the soil can absorb the water (infiltration rate), otherwise water will run off the surface and result in nonuniform distribution, water logging of low areas, and perhaps soil erosion. Precipitation rates can vary from 0.1 in. (2.54 mm) per hr or less to 0.25 in. (6.35 mm) per hr, depending on soil texture. Many growers use a rate of 0.10-0.15 in. (2.54-3.81 mm) per hr, as this rate produces no run off on coarse textured soils and is also satisfactory for many fine textured soils, which tend to compact, surface seal, or crust, and require low application rates to prevent runoff.
Although the risers are usually about 18 in. (45.7 cm) above the vines so that foliage does not stop the spray stream, they must be low enough so that mechanical harvesting equipment can clear them (Meyer and Marsh, 1973). Risers should not be over 70 to 72 in. (1.78-1.83 m) high, and are usually secured to the grape stakes or to separate stakes between vines. A sprinkler spacing of 32 X 48 ft (9.75 X 14.63 m) is recommended for good uniformity of water application with vines spaced at 8 X 12 feet (2.44 X 3.66 m). Vines planted at 7 X 12 ft (2.13 X 3.66 m) require a 35 X 48 ft (10.7 X 14.6 m) sprinkler spacing.
Portable sprinklers are difficult to move over trellises and undulating terrain. When sprinkling is done with saline water, it should be carried out at night to minimize salt damage.
Most emitters have a fixed rate of output with a specified water pressure, and some have adjustable rates. A rate ranging from ½ to 2 gal (1.89-7.57 liters) per hr is most common. Most emitters lie on the soil surface, but some are buried at a shallow depth; the tubes of others are strung above the soil surface.
The emitters are part of, or connected to, a small (3/8-3/4 in. (9.65 X 19.05 mm) plastic lateral line. These connect to a plastic main line that receives water from a head, which is the control system where water is measured, filtered, treated with fertilizer in solution, and regulated for pressure and timing of application.
A grower can often irrigate his vines with less than half the usual amount of water and also sometimes obtain greater yields. Labor costs are also lower with the drip irrigation than the other systems, and it is possible to inject fertilizers into the irrigation water and thus avoid the cost of ground application. Frequent irrigations maintain a soil moisture content that does not vary between wet and dry extremes, and most of the soil is kept well aerated.
Adequate filtering with screens or sand is essential to keep the water clean. Irrigations must be frequent, light, and not too long, so that local excessive wetness in soils is avoided. This also minimizes algae growth in laterals. Lateral lines must be designed to carry the maximum expected flow rate with little or no loss of pressure. One should not apply phosphate fertilizers through a drip irrigation system, as phosphate reacts with calcium to form a precipitate that can clog emitters. Iron has also been a problem in drip irrigation. A high concentration of calcium carbonate in the water also tends to concentrate in the emitters. Algae are also a nuisance when drippers are used and their removal by screens and chemical treatments is recommended. Silt and clay should not be introduced to the drippers.
In the San Joaquin Valley of California there are large areas of soil that exhibit slow penetration of water. A few of these soils have an infiltration rate of only 03 in. (0.76 mm) per hr. These soils, whether clay or sand, tend to develop a thin crust of silt or clay on the surface that impedes the rate of water infiltration. Such practices as cultivating the soil before irrigation to break up the crust, sod culture, deep ripping, and use of gypsum which are used to improve water penetration, all have disadvantages (Aljibury, 1973). In the San Joaquin Valley the potential of drip irrigation to increase water penetration appears most important among the various advantages of the system.
In the San Joaquin Valley many soils have a poor and unstable structure that deteriorates upon contact with water. Water may cause deteriorate of soil aggregates by hydration, which disperses the soil structure through swelling, and by physical breakdown of aggregates due to the erosive action of the moving water (Aljibury and Christensen, 1972). The dispersed particles are carried into the upper soil pores and precipitate on the surface as a crust. Rate of water penetration can be increased by using high electrolyte (dissolved salts) irrigation water and addition of gypsum to the soil. The rate of water penetration is in proportion to the degree of soil crust prevention: a cover crop and organic matter added to the soils improves the rate of water penetrations as long as a crust does not dominate. Penetration can also be improved by disturbing the soil crust by mechanical means.
|Vine Symtomps Resulting From a Water Deficit
Shoots grow rapidly in the spring and early summer, and the rate of growth during this period is a sensitive indicator of soil water availability (Vaadia and Kasimatis,1961). As soil water approaches near the shoot tip become shorter. The normal yellowish green color changes to a dark green. Often as the stress increases, tendrils become flaccid and droop. Although leaves do not usually wilt, the older ones turn yellow, leaf margins become desiccated and curled, and the leaves gradually die and absciss. Under severe or prolonged stress in the hotter areas, the growing points of the shoots may dry. When irrigation is resumed during the growth season, however, the vine will begin new growth.
A sudden reduction of water from the vine can cause wilting of leaves and succulent shoot followed by abscission of basal leaves. This may occur on vines growing in shallow soil because of a sudden rise in temperature. Overall vine growth is reduced under drought conditions.
Young clusters at prebloom or bloom stage dry out easily, but more mature berries persist longer under drought conditions. Under stress, berries become wilted or shrivelled. Berries that have colored contain much sugar and are much more resistant to drying than less mature ones, since the fruit can probably obtain water from leaves and other portions of the plant.
Stomatal closure occurs at a leaf water potential of 13 bars, but rate of shoot growth is inhibited at lower negative tensions (Smart, 1974). A reduction in trunk diameter also results when leaf water potential reaches about 7 bars. With accelerated water stress, the angle made by the petiole and lamina midrib increases from about 50 to 800. The drooping of the leaves results from a change in turgor at the junction of the petiole and the lamina, and is a good indicator of water stress (Smart, 1974). Recently matured leaves should be used for measurement of leaf angles (Fig. 9-3).
During the period following fruit set, when berry growth is rapid, withholding water results in smaller berries. This procedure is sometimes used to reduced berry size and cluster compactness and thus to decrease the prevalence of rot resulting from berry splitting at maturation. Berry size reduction as well as reduced shoot growth occurred in Thompson Seedless vineyards in the San Joaquin Valley when preharvest irrigation was cut off in early July (Christensen, 1975). Yields and / or soluble solids of fruit are usually reduced in grapes growing under drought conditions.
|When vines are girdled at fruit set to increase the size of berries of seedless varieties, such as Thompson Seedless, an irrigation should be made prior to or immediately after the operation to protect the vine in case of water stress brought on by sudden hot weather. Tables grapes sometimes crack during maturation. This can often be avoided by irrigating very lightly or using only short periods of sprinkling during maturation. Such berry splitting may be aggravated by restricted transpiration that occurs in humid regions.
After harvest, vines grown under desert conditions require irrigation to maintain the established leaf surface. It is best that new shoot growth does not occur late in the season. Often vines of vigorous varieties will continue to grow or start new growth after harvest and the wood will fail to ripen. Such shoots may be subject to low temperature injury in the winter. Also, late growth utilizes food materials (carbohydrates) that should be saved for the active growth of the subsequent season. Irrigations after midsummer can be reduced or omitted to harden the growth of inherently vigorous varieties. In this case a late irrigation after temperatures have become too low to permit initiation of new growth is needed to furnish the vine?s water requirement, even though it is low at this period.
|Special Irrigation Problems
Vines growing in light soils infested with parasitic nematodes that attack the roots should be irrigated more frequently than soils without nematodes, since the additional water helps compensate for the injured root system that is unable to absorb sufficient water. Vine yard growing areas with a high water table must be furnished with tile the drains to remove the water. Where the soil is high in salinity that can result from use of irrigation water with high salt content, leaching must be used to drain the salt. The land must be levelled to a flat grade and irrigated frequently by flooding to cover the soil surface. Irrigations should also be made in the autumn to reduce salt concentrations, even though there may be no need to supply soil water. Grapevines damaged by salinity slowly recover after the salts are leached.
Insect build up and outbreaks of powdery mildew (Uncinula necator) are usually not affected by surface applications or by sprinkling, However, sulfur should be reapplied as soon as possible after sprinkler irrigation to re-establish normal protection against powdery mildew infection.
Water use reported in Table 9-1 includes rainfall stored in the soil of the root zone and the added irrigation water, but it does not account for surface runoff or deep percolation. Water used by growers is greater than needs shown by the consumptive data. Doubtless there is a desire to be safe and use plenty of water if it is available to avoid deleterious effects of sudden hot spells.
The water holding capacity of various soils varies greatly (Table 9-2). Soils with a high water holding capacity need less frequent irrigation.
A saline soil is caused by the presence of too much soluble salt. Salts are originally formed by weathering and breaking down of rocks, and are often transported by moving surface water to low lying or irrigated areas where they accumulate. Salts may accumulate because there is too little rainfall for sufficient leaching of the soil or because soil drainage is inadequate. Runoff water may accumulate in low areas, evaporate, or be lost by transpiration by plants, and leave their salts in or on the soil. This continuous addition of salts to the soil will eventually result in a salt problem unless adequate drainage is done to leach the salt in the root zone. Leaching in this case is best accomplished during the dormant season, when surface evaporation and vine use of water is low and there is ample time for leaching. Some areas require installation of under drains.
|Factor that Accentuate Salt Accumulation. Several factors that can accentuate salt accumulation in soils and thus reduce yields (Neja, et al., 1974) include the following:
1.Irrigation water of poor quality (high salinity).
2.Poor water management that allows too much salt to accumulate in the root area.
3.Original salt content of soil was high.
4.Poor soil drainage characteristics. Shallow soils, claypans or compact layers that restrict downward water movement and root penetration. Salty perched or high water tables.
5.Semiarid climate with low rainfall and high temperatures, resulting in a high demand for water. Low rainfall causes an insufficient winter leaching, and large amounts of water applied may then add more salts.
6.Heavy applications of fertilizers.
7.Infrequent, heavy water applications during the growing season to soils with shallow claypans. This may cause water to remain above the claypan and can cause roots to rot, thus decreasing water available to the vine.
8.If soil is dried out excessively after infrequent irrigations, salts can concentrate in the soil solution.
9.Uneven topography and variable depths of soil over impervious soil layers can collect drainage water and form water tables.
Effect of Saline Soils on Vines. Vines growing on saline or salty soils may be unable to absorb water rapidly enough for their requirements. This can result in decreased vine growth, yield of fruit, and quality. Previous crop history is of limited use in predicting salinity damage, or sodic or toxic soil conditions because various crops respond differently to such soils. The best way to appraise the salinity status of vineyard soils is to obtain a soil and water analysis.
Sodic Soil Condition (Sodium, Alkali, or Black Alkali Soils). When there is a high amount of sodium in proportion to the amount of calcium and magnesium, a sodic soil condition usually results. Such soils have poor soil structure and water permeability that reduce water percolation, root development, and may impair drainage. When there is an excessive uptake of sodium, leaf burn may occur. When calcium and magnesium are high and sodium is low, there usually is good soil structure and permeability.
Boron and Chloride Toxicity. Excessive absorption of chloride, boron, or sodium can cause leaf burn. A marginal leaf burning proceeds inward, and excessively high levels can result in defoliation and even death of the vine. Although boron is essential for plant growth, it is toxic in higher than normal concentrations; more than 2 to 3 ppm makes it dangerous for irrigation purposes.
Sprinkle irrigation over the vine with water containing an excess of these salts can result in damaging salt absorption. Windblown sprays or mists are especially hazardous because salts concentrate in the spray, fall on the leaves, and are absorbed and not washed off by runoff. Intermitten wetting and drying of leaves by a slowly rotating sprinkler head can also cause a problem. The adverse effects can usually be reduced by sprinkling only at night, when humidity is relatively high and the drying rate is reduced.
Electrical conductivity is measured in millimhos per centimetre, which is , for example, about 640 ppm of table salt (sodium chloride). Water with ECe values less than 1.8 mmho/cm (1150 ppm) should not be injurious to grapes.
Saline soils contain sufficient soluble salts to interfere with crop productivity. Usually they are only slightly alkaline in reaction (pH 7.0-8.5), contain little exchangeable sodium, and have good physical properties. The excess soluble slats can usually be leached from these soils with large amounts of ordinary irrigation water.
Hard water contains appreciable quantities of calcium and magnesium. Such water is called ?hard? because it is difficult to make work up a lather with soap. Calcium and magnesium react with soap to form a curdlike material that leaves greasy rings in washbasins. After water is softened by removal of calcium and magnesium salts, sodium salts remain.
The SAR and salt content of the water have the main impact on permeability of soils. A high soil ESP with a high SAR ratio causes poor physical soil properties such as stickiness. Irrigation practices. An SAR value of 6 to 9 may be hazardous to vines, and a value above 9 should be avoided. High carbonate (CO3) and bicarbonate (HCO3) can increase permeability problems.
Analysis of irrigation waters is generally a satisfactory guide to their suitability for agriculture use. Of greater importance, however, is a through knowledge of drainage, leaching, salt tolerance of grapes, irrigation methods, and soil management practices. Plants do not grow in irrigation water but in a complex plant soil water system.