A line-source irrigation trial was carried out during two years to quantify water use, biomass accumulation and sugar yields of an October-planted sugarbeet crop in the San Joaquin Valley of California, at a site with low winter rainfall. Irrigation water was applied at rates varying from approximately 110 to less than 25% of estimated crop ET. Measurements with a neutron access probe were used to schedule irrigations and estimate water recovery from the soil to a depth of 2.75 m. Harvests were made in May, June and July each year. In the first year, winter rainfall was twice the long-term average, and sugarbeet root yields failed to respond to irrigation water gradients until the final harvest in July. In the second year, rainfall was below average and root yields responded to the irrigation gradient at all three harvests, with response increasing with successive harvests. Sucrose concentration was increased by deficit irrigation. In deficit treatments, sugarbeets recovered water to a depth of greater than 2.75 m. Water use, yield, sucrose concentration, DM partitioning and water recovery are discussed in this paper. Over-wintered sugarbeets can be produced with low amounts of irrigation water.

Keywords: Sugarbeet, irrigation, linesource.

Introduction

Conflicting demands for surface water supplies and concerns about ground water overdraft have reduced the amount of water available to farmers for irrigation in California's San Joaquin Valley. Growers in portions of the valley face severe restrictions on the amount of saline drainage water they can generate. Sugarbeet can be planted in early spring and harvested in the fall, or planted in the fall and harvested in the summer. Fall planting offers several potential advantages compared to spring planting, including: improved irrigation water use efficiency, reduced loss of NO₃-N to groundwater during the winter, increased tolerance to salinity and rhizomania and reduced pest management requirements compared to spring planting. Sugarbeet may also be able to recover water from deep in the soil profile or from shallow water tables, reducing the amount of saline drainage water.

Previous reports on water use by sugarbeet in California (Hills et al.,1990; Howell et al., 1987; Ghariani, 1981) and elsewhere (Hang and Miller, 1986a&b; Winters, 1980; Miller and Aarstad,1976) have focused primarily on water needs and soil-water relations of spring planted crops, which typically are exposed to high levels of solar radiation and high temperatures during the growing season. Evapotranspiration requirements of 900 to 1200 mm have commonly been reported (Dunham, 1993; Hills et al., 1992), requiring proportionally higher irrigation applications depending on the location and year. In contrast, fall established crops in the San Joaquin Valley develop during the cooler, rainy period of the year, with the potential of significantly lowering irrigation water requirements. However, water use, patterns of water recovery and effects of varying irrigation amounts on crop biomass, sugar yield and root quality have not been studied during this period in the San Joaquin Valley.

The objectives of this project were to:

Sugarbeets were planted in 0.76 m rows in early October in 1992 and again in 1993 at the University of California's Westside Research and Extension Center (WSREC) near Five Points, in western Fresno County, using a single line-source irrigation systems (Grimes et al. ,1992; Hanks et al., 1974). Rows were parallel to the single sprinkler line in the plot's center and there were 20 on each side of the line source. Rows and the line source were oriented slightly northwest to southeast to parallel the prevailing wind direction. Table 1 summarizes relevant management information for both year's experiments. The soil is a Panoche clay loam (thermic, Typic Torriorthent), is deep and has a substantial available water holding capacity (275 mm in a 2.5 m deep profile). This soil's properties have been well-characterized previously at that site (Nielsen et al., 1964) and are summarized in Table 2. All rows were sprinkler irrigated uniformly two or more times to establish stands and the soil profile was saturated at the start of the experiment each year. All subsequent irrigations were applied with the line source system. Neutron access tubes were installed ( in even numbered rows from 2 to 16 ) after plants were established in November or early December to a depth of 2.75 m. Frequent measurements of water content using a sealed source neutron probe were made and used to estimate the quantity of water extracted and the depth and time of extraction. Irrigations were applied when the amount of water in the surface 15 cm was depleted to 40 % of available water in row 4 (100% ET treatment). Collection cups were placed in the same rows as access tubes to measure water applied. The amount of water applied by each harvest date in 1993-94 is presented in figure 1. Three harvests were carried out each year in May, June and July, collecting parallel 9.1 m long sections from each row. Fresh and dry weights for roots and leaves plus crowns were determined. Data reported are rolling averages of three adjacent rows. Weather data was collected at a nearby CIMIS weather station located at the Center and is presented in Table 3. Crop evapotranspiration was estimated using the equation:

Water was recovered by sugarbeets in stressed treatments at depths greater than 2.75 m. To estimate an upper limit for the amount of water recovered that was not measured, a relationship between ET_(crop) and total biomass was developed by regressing total DM yields versus ET from treatments receiving water in amounts within 2% of the reference ET treatment. This relationship (Eq. 2), taken to be constant (Hanks, 1983), was used to estimate actual water use.
 

TOTAL BIOMASS / ET_(crop) = Q_et (Eq. 2)
 

Results and Discussion
 

In 1992-93, rainfall for the season was greater than twice the long-term average (370 mm) at the WSREC site. Combined with the high available water holding capacity of the Panoche clay loam soil (275 mm per 2.5 m depth), between 600 and 700 mm of water were available to the beet crop without supplemental irrigation. Consequently, there was little response to irrigation in 1992-93 until the last harvest in early July (figure 2). In contrast 1993-94 was a dry year (143 mm rainfall) and response to irrigation treatments was more pronounced. Most of the data reported is from that year. Total plant biomass, root and sucrose yield for 1993-94 are reported in figure 3. The regression equations for the lines drawn are in Table 4. For all three harvest dates, total biomass increased linearly with irrigation amounts spanning the range of 85% to 5% ET_(crop). Root DM and sucrose yields did not respond linearly. Instead, there was a broad range of irrigation inputs with essentially similar root and sucrose yields (45 to 75% of ET_(crop)) in July, indicating the potential for savings of irrigation water at this site compared to full irrigation treatments. With each successive harvest, irrigation levels above approximately 90% of ET_(crop) reduced sucrose yields. Hang and Miller (1986a) reduced water application rates to 35 to 50 % of estimated ET_(crop) without reducing sugar yields on a loam soil.

Comparisons of rates of DM accumulation for sugarbeets observed in this experiment, with those reported from other analyses in California and elsewhere for sugarbeet are presented in Table 5. Erie and French (1965) and Ehlig and LeMert (1979) reported higher consumptive water use (900 to 1200 mm) by fall established sugarbeets in the arid desert climates of the Imperial and Salt River Valleys, respectively. Tanner and Sinclair (1983) suggested that substantial improvements in water use efficiency (Y/ET) could be achieved by matching crop production to locations and times when vapor pressure deficits were lower, compared to places and times when they are higher. Fall establishment and winter production in the San Joaquin Valley compared favorably to summer production in the same areas, and to winter production in more arid regions with respect to WUE.

 
Sucrose Concentration and DM Partitioning

In May, sucrose concentration (FW basis) was unaffected by irrigation treatment (figure 4). By June, however, sucrose concentration was observed to decline with increasing irrigation amounts. By July, sucrose concentrations in stressed treatments also had declined and were highest in moderately stressed treatments. Hillls et al., (1990) observed similar phenomena in a summer trial in the Sacramento Valley of California. DM partitioning between roots and leaves plus crowns was affected by irrigation levels. More DM was present in leaves and crowns early in the season than later, even in well watered treatments (figure 5). For all harvests, above approximately 90% of estimated ET_(crop), top to root ratios tended to increase substantially, suggesting that applying water at 100% of ET is wasteful.
 

Water Recovery

Data on volumetric soil water content as a function of depth at two harvests, comparing the reference and high stress irrigation treatments is presented in figure 6. In late May, volumetric soil moisture at the 2.75 m depth was identical between the stressed and nonstressed treatments. In late July, the stressed treatment had depleted available soil water to the depth measured with the neutron access tube and appeared to be recovering additional water at still greater depths. Winters (1980) estimated that a sugarbeet crop recovered water from a depth of 3 m in a Texas study.

The relationship between crop transpiration and total DM production is considered to be constant for a given set of conditions and approximately similar among sites for a given species and among different species, distinguishing between C3 and C4 plants (Tanner and Sinclair, 1983). Hanks (1983) analyzed data from a number of studies carried out under semi-arid conditions correlating yield and ET_(crop) for maize, wheat and alfalfa. He found a consistently linear relationship between total biomass and ET_(crop) over a wide range of climates and years, though cultivars, and site-year interactions influenced the value of the slope within a limited range. Root yields from stressed treatments in July equalled approximately 75% of the highest yielding irrigation treatments, but crop ET was estimated at only 40% of the reference (100 %) ET treatment. Assuming the same constant relationship holds for all irrigation treatments, the amount of water recovered by stressed treatments that was not measured using neutron probe data and equation 1 could be calculated. These calculations are presented in figure 7 and estimate an upper limit for water recovery from soil at depths greater than 2.75 m. WUE may also have been higher for plants in stressed treatments. Assuming uniform WUE among treatments, severely stressed treatments may have recovered greater than 50%of their water at depths greater than 2.75 m. Wallender et al. (1979), reported a similarly high amount of recovery by cotton on a Panoche clay loam soil in western Fresno County. When Hills (Hills et al., 1990) withheld water from a spring planted crop at Davis, CA, compensatory soil water recovery was observed to a depth of slightly greater than 2 m, and ET_(crop) differences between the adequately watered and non-irrigated crops were equal to 269 mm by the end of the season. Yields of the stressed crop were 75 to 80 % of the fully irrigated ones, while ET_(crop) was approximately 65%. Differences in response between the two experiments reflect differences in the two soils, but may also reflect the superior capacity of fall planted crops to withstand stress.
 

Conclusions

1. Highest sucrose yields in 1993-94 were produced at irrigation levels over a range 45 to 75% of the estimated ET_(crop).

2. Sugarbeet root and sucrose yields in the western San Joaquin Valley of California often are high compared to other regions of the world. High yields and low vapor pressure deficits in winter compared to other arid and semi-arid regions in summer result in high Q_et and S_et values.

3. Increasing stress resulted in increasing sucrose concentrations as stress increased, but as water application rates declined below 45% of ET_(crop), sucrose concentrations declined again. Top to root DM ratios increased greatly at irrigation levels above 90% of ET_(crop), resulting in wasteful use of water.

4. Significant water recovery occurred in stressed treatments form soils deeper than 2.75 m. The amount recovered may have e been greater than 50% of actual ET_(crop). The capacity of sugarbeet in this and similar soils to recover large amounts of water at depth results in high irrigation WUE and improves the water economy of the entire cropping system by recovering residual irrigation water unavailable to shallow rooted crops. At the same time, such recovery reduces the amount of saline drainage water that must be disposed.
 

References

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