In California, irrigation is essential to provide moisture for seed germination, alleviate soil crusting, and to provide most of the water needed by a crop during the growing season.  Sugarbeets are most commonly irrigated by the furrow method, but sprinklers are used where topography, high water table, or other special conditions make the furrow system difficult to use.  Sprinkler irrigation, though more costly, has advantages in improving seedling emergence and in using less water in the early stages of plant growth.

Preparation for furrow irrigation starts with land leveling to provide the proper grade for careful water control.  Slopes from 0.05 to 0.2 percent down the irrigation furrows will make it possible to do a good job of “subbing” water past the seed line of planting beds and will provide adequate fall for irrigating when plants are large.

Effects of Moisture Stress

When plants lose water from their leaves faster than their roots can absorb it from the soil, internal water deficits develop, growth is slowed, and the plants may wilt.  Sugarbeets wilt easily, and soil moisture depletion can bring about a rapid, severe wilt and subsequent destruction of leaves through contact with the hot, dry soil surface, seriously affecting yield.  Even with plenty of water, sugarbeets may wilt slightly during the afternoon on hot, dry days.  Such wilting does not indicate a need for irrigation.  However, if the plants wilt early in the day, or if recovery is slow in the late afternoon as temperatures and light intensity decline, an irrigation is needed.

The sugar concentration in beet roots may increase about one percentage point when plants are allowed to silt slightly just prior to harvest.  This is a risky practice since prolonged wilt will reduce total sugar production through the slowing of root growth.

Water Use

The crop surface loses water in two ways: 1) water vapor passes from the leaves into the air (transpiration); and 2) water is lost from the soil through evaporation.  The total loss of water through these two processes is known as evapotranspiration (ET).

After the plants have developed a canopy of leaves that completely shades the soil surface, the rate of ET depends on climatic conditions.  High temperatures, intense sunlight, wind, and low humidity increase the ET rate.  In areas such as the Central Valley, the ET during the warm months of June, July and August may be 0.3 acre-inch of water per acre per day.  Hot, dry winds may increase this rate considerably.  It will be necessary to apply from 18 inches of water per acre per season in a cool climate, where the soil is filled by plentiful winter rain, to as much as 48 inches in a hot, dry climate with limited winter rain.

Guides to Irrigation

The frequency and amount of irrigation required by a beet crop also depends on the extent of its root system.  After planting, about 2 ½ months of good growing weather are required for a sugarbeet crop to develop a good root system.  In well-drained, open soils, roots can easily reach 5 to 6 feet but obtain most of their moisture from the top 3 feet.  Root penetration may be limited by a high water table, compacted layers, or other factors.

It is good practice to have the soil filled with water to field capacity at the time of planting or soon after to provide moist soil for the rapid extension of roots.  When beets are planted during a rapidly warming period (e.g. May in the Sacramento Valley) it may be necessary to apply three or four light irrigations during the first 2 ½ months until a deep root system is established.  Heavier irrigations are required during mid season when the crop has a complete canopy of leaves and is transpiring at very high rates.  Each irrigation should be sufficient o replace the water used by the crop as well as to take care of losses within the irrigation system.  Intervals between irrigations may be as short as seven days on soils of low water holding capacity to as longa s 21 days for heavier soils.

The amount of water in a soil that can be used by a crop depends on the physical properties of the soil.  A loamy sand may hold only 1 acre-inch of available water per acre per foot depth of soil while a clay loam holds about 2 acre-inches per foot depth per acre.  An estimation of irrigation frequency is obtained by considering the water storage capacity of the soil (Table 1), the depth of rooting, and the rate of evapotranspiration (Table 2).

Days to use available water = (Inches of available water/foot root depth)/(Evapotranspiration, inches/day)

For example, a three month old sugarbeet crop will draw about 80% of its water from the first 30 inches of soil.  A loam soil stores about 1.75 acre-inches of water per acre per foot of depth (Table 1).  At an ET of 0.3 inches/day (Table 2: 9.1/.31 _ 0.3).  This amount of water will be exhausted in about 1.75(2.5)/0.3 = 15 days.  An irrigation frequency of about two weeks would be required to prevent undue moisture stress.

It is good practice to check for depth of water depletion and water penetration by using a soil tube or auger before and after an irrigation.  By this method, you can see how well water has been applied and make corrections accordingly.  This can also be accomplished by installing soil moisture measuring devices such as tensiometers or gypsum blocks at two or more depths at the beginning, middle and near the end of the irrigation runs.  Tensiometers placed at 18 and 30 inches can be very useful in scheduling irrigations in light textured soils and for checking on adequacy of penetration (see “Questions and Answers About Tensiometers”, AXT-92, Cooperative Extension, University of California).

In areas of a field where soil is lighter, or where water penetration is poor, plants tend to wilt sooner than in the rest of the field.  These areas may be used as indicators of the need for irrigating the entire field.   However, certain fungi and nematodes will cause plants to wilt even when the supply of soil moisture is plentiful.  Roots should be examined for these agents when patches of wilting occur.

Effects of Irrigation Schedules and Moisture Stress on Production

The sugarbeet is sensitive to soil moisture stress, wilts easily, and production can be damaged if wilting is excessive.  The crop, however, is tougher than most of us think and can stand a certain amount of stress after the root system is well established.  Experiments by Doneen and Loomis at Davis, Miller, Hale and Yamada at the West Side Field Station, and Ehlig and Lemert at the USDA Field Station at Brawley illustrate how sugarbeet production can be maintained with fewer irrigations and sometimes considerably less water than is now used by most farmers.

UC Davis - 1961.  (L. D. Doneen and R. S. Loomis) Table 3 shows the effects of irrigation frequency on sugarbeet production on a Yolo loam soil.  The crop was planted May 14, 1961.  The results are averages of two harvest dates on September 14 and October 27.  The wet treatment was irrigated every five to six days after crop canopy was nearly complete.  The dry treatment was irrigated at the first sign of wilting, and the medium frequency was between the wet and dry.  Dry plus stress meant that the plants were allowed to wilt about three days before irrigation.  Note that a limited amount of wilting, the dry treatment, did not decrease sugar yield and considerably reduced the number of irrigations necessary to produce the crop.

West side Field Station - 1963.  (R. J. Miller, V. Q. Hale, and H. Yamada)   This experiment was conducted on a Panoche clay loam soil and involved three levels of nitrogen fertilization.  There were no interactions of irrigation with nitrogen fertilization.  The irrigation treatments 1, 2, 3, and 4 of Table 4 were to irrigate when soil moisture tensions reached 0.6 var at 30-, 24-, 18-, and 12-inch soil depths, respectively.  All plots received approximately the same amount of irrigation water, bu the numbers of irrigations for treatments 1 through 4 were respectively 9, 10, 12, and 13.

In this experiment it was noted that the sugarbeets utilized the available moisture at the 30-inch depth as efficiently as at the 12-inch depth, and that when about 3 acre-feet of water were used the number of irrigations could range from 9 to 13 without significantly affecting crop production.

Brawley field Station - 1973-74.  (C. Ehlig and R. D. Lemert)   At this location a weighing lysimeter was planted to sugarbeets at the same time a field experiment was planted on Holtville silty clay soil.  The lysimeter was used to estimate the daily water use.  Actual water use, however, was determined by soil sampling.  Preirrigation and irrigations for germination on September 28 filled the soil profile with water to at least 5 feet.  Differential irrigation treatments were started on December 20.  The six irrigation treatments applied 10% more, the same as, and 10, 20, 30, or 40% less water than was used by the sugarbeets in the lysimeter.   The researchers accomplished this by applying 4 inches of water to the plots when the cumulative evapotranspiration (ET) of the lysimeter indicated a use of 3.6, 4.0, 4.4, 4.8, 5.2, and 5.8 inches, respectively, since the previous irrigation.  By the last irrigation on May 31 the driest treatment had received three irrigations less than the wettest.  From the December 20 starting date, water applications varied from 23 inches on the driest to 36 inches on the wettest treatment.  Total water use from date of emergence to harvest on June 25 varied from 35.9 inches in the driest treatment to 47.2 inches in the wettest.

Soil sampling between irrigations indicated that the sugarbeets in the driest treatment used some of the water originally stored in the soil to a depth of 4 feet in addition to the water applied at each irrigation.  As the season progressed the driest treatments showed symptoms of water stress by afternoon wilting and used less water.  Beets were harvested on May 13 and June 25.  No significant differences were found in sugar yield among treatments at either date.  At the final harvest the average yield was 49.9 tons of roots per acre at 15.7% sucrose.

The researchers concluded that sugarbeets grown on this type of soil are able to adjust to gradually imposed conditions of water stress and that significant water savings may be made by applying up to 40% less water after the root system is well established without a significant reduction in crop yield.

A key point with regard to the above three experiments is that the soil profile to the depth of sugarbeet rooting was filled with water at the start of the growing season.  As the season progressed some roots utilized soil water from lower depths (3 to 5 feet), and thus could get by with lesser amounts of irrigation water.

Last Irrigation Prior to Harvest.  For a summer or fall harvest sugarbeets can take considerable water stress just prior to harvest without reducing crop yield.  A 1963 Kern County experiment on Hesperia find sandy loam showed that considerable wilting could be tolerated after the last irrigation prior to harvest without seriously reducing sugar yield (Table 5).

Differences in water stress at harvest in this experiment were indicated by the appearance of tops.  Plants not irrigated for 30 days had lost many leaves, those not irrigated for 20 days were visibly wilted with some leaf burning and those irrigated 10 days prior to harvest had begun to wilt.

A similar result was obtained at the Imperial Valley field Station in 1961 (Figure 1).  In this clay loam soil small restrictions in root growth due to water deficits were apparent from the time the plants first wilted, three weeks after the last irrigation, but increases in sucrose concentration largely made up for the lack of gain in root yield.

For sugarbeets to be harvested in the early winter or the following spring a terminal irrigation about September 1 to rewet the soil profile to about 3 feet should be sufficient to carry the crop through.  With a medium-textured soil and the rapidly declining evapotranspiration rates at this time of year, such an irrigation should prevent appreciable crop loss for a winter or early spring harvest.
 
 
 

Table 1. Available water holding capacity for various soil textures

	Available Water
Texture	Range  (inches/foot)	Average  (inches/foot)
Very coarse to coarse textured sand (loamy sands)	0.5 - 1.25	0.9
Moderately coarse textured sandy loams and fine sandy loams	1.25 - 1.75	1.5
Medium texture.  Very fine sandy loams to silty clay loam	1.5 - 2.3	1.9
Fine and very fine texture.  Silty clay to clay	1.6 - 2.5	2.1
Peats and mucks	2.0 - 3.0	2.5