ORIGINAL PAPER

Shuqin Wan Æ Yaohu Kang

Effect of drip irrigation frequency on radish (Raphanus sativus L.) growthand water use

Received: 16 May 2005 / Accepted: 28 June 2005� Springer-Verlag 2005

Abstract Irrigation frequency is one of the most impor-tant factors in drip irrigation scheduling, and a properirrigation frequency can establish moderate moist andoxygen conditions in the root zone throughout the cropperiod. Field experiments on the effects of irrigationfrequency on radish growth and water use were carriedout in 2001 and 2002. The experiment included six irri-gation frequencies: once every day, once every 2 days,once every 3 days, once every 4 days, once every 6 daysand once every 8 days. There was no significant differ-ence among the six treatments on radish developmentand yield, but significant differences in radish rootsdistribution and market quality were found. Radishesirrigated once every 3 days had well-developed rootsthroughout the crop period, the lowest cracking rate andthe least number of radishes of Grade 3. The observa-tion results of lysimeter in 2002 showed that radishevapotranspiration decreased as irrigation frequencydecreased, and the general changing tendency of 2-dayET of high irrigation frequency was related to that of 2-day evaporation. It is recommended that radish irriga-tion frequency should be once every 3 days and theirrigation amount should be estimated according to theevaporation of 20 cm diameter pan in the North ChinaPlain.

Introduction

Radish (Raphanus sativus L.) is an important root veg-etable and is widely planted in China. It is a short

duration crop with high growth rate and sensitive towater stress. The deficiency and overabundance of soilwater do harm to its yield and quality (Singh and Che-ema 1972; Park and Fritz 1982; Barker et al. 1983).Therefore, a frequent, uniform supply of water is ex-tremely important for its growth, yield and quality.

Drip irrigation, a modern irrigation method, canreadily establish a nearly constant water regime in theroot zone (Beese et al. 1982), and ensure plants growingunder proper soil water for the optimum yield and size.

Drip irrigation management is based on the frequentreplenishment of water loss by evapotranspiration (ET).There is varying agreement over the effect of irrigationfrequency on crop water use. Goldberg et al. (1971)indicated that the high irrigation frequency could reduceevaporation and deep percolation, and establish afavorable soil moisture and oxygen condition in the rootzone throughout the crop period. Smasjtrla et al. (1985)pointed out that to minimize deep percolation and tomaintain nearly constant high soil water potential, highfrequency (multiple applications per day) irrigationsshould be recommended. Whereas some scholars figuredout that practicing irrigation water management withminimal plant water stress by maintaining a high soilmoisture content between irrigations can lead to signif-icant deep percolation losses (Levin et al. 1979b).Meshkat et al. (2000) concluded that an excessively highirrigation frequency could cause the soil surface to re-main wet and the first stage of evaporation to persistmost of the time, and resulted in more water loss.

Findings from Goldberg and Shmueli (1970) indi-cated that yields of cucumber (Cucumis sativus L.) andmelon (Cucumis melo L.) planted in sandy soil would bereduced if irrigation frequencies were beyond one day.Bucks et al. (1974) experimented on clay soil showingthat the yields of cabbage (Brassica oleracea L.) woulddrop by the increase in irrigation frequency. Resultsprovided by Levin et al. (1979a) on the similar clay soilillustrated that different irrigation frequencies (once ev-ery day, twice every week and once every week) had nodistinct effects on apple (Malus pumila Mill.) yield.

Communicated by J. Ayars

S. Wan Æ Y. Kang (&)Key Laboratory of Water Cycle and Related Land SurfaceProcesses, Institute of Geographical Sciences and NaturalResource Research, Chinese Academy of Sciences,11 A, Datun Road, Anwai, Beijing 100101, ChinaE-mail: kangyh@igsnrr.ac.cnTel.: +86-10-64856516Fax: +86-10-64856516

Irrig Sci (2005)DOI 10.1007/s00271-005-0005-9

Radin et al. (1989) indicated that the relationship be-tween water and cotton (Gossypium herbaceum L.) washighly dependent on the frequency of irrigation when thefruit load was heavy, but was generally independent ofirrigation frequency before and after the period. Trialsconducted by Oktem et al. (2003) suggested that among2-, 4-, 6- and 8- irrigation frequencies, a 2-day irrigationfrequency, with 100% ET water application was optimalfor sweet corn (Zea mays L.) grown in semi-arid regions.Potato (Solanum tuberosum L.) was subjected to sixirrigation frequency treatments (1-, 2-, 3-, 4-, 6- and 8-irrigation frequencies) to evaluate the effects of severalirrigation frequencies on potato yield, ET and water useefficiency (WUE) by Kang et al. (2004) and the resultsshowed that potato yield, ET and WUE increased asirrigation frequency increased, and the highest yield, ETand WUE values were achieved with an irrigation fre-quency of once a day.

The objectives of this study are: (1) to measure theeffect of drip irrigation frequency on radish growth,yield, water use and WUE; and (2) to define the basis forirrigation scheduling of drip-irrigated radish and waterresource planning in the North China Plain.

Materials and methods

Experimental site

Field experiments were conducted at Luancheng Agro-ecosystem Station (LAES), Chinese Academy of Sci-ences. LAES is located in Luancheng County, HebeiProvince (Latitude: 37�53¢N; Longitude: 114�41¢E; 50 mabove sea level). Average annual precipitation is about480 mm, mainly concentrated from July to September.The spring and early summer are normally quite dry.The dominant soil is silt loam with an average bulkdensity of 1.53 g/cm3, and organic matter content in thetillage layer is about 1�1.2%. Field soil water capacity(gravity content) is about 22.5%. The soluble mineralcontent of groundwater is less than 0.5 g/l. The watertable is about 28 m deep and groundwater contributionto the root zone is therefore negligible.

Experimental design

The experiments included six irrigation frequencytreatments in 2001 and a reference and six irrigationfrequency treatments in 2002. The six irrigation fre-quency treatments in 2001 and 2002 were as follows: (1)once every day (F1), (2) once every 2 days (F2), (3) onceevery 3 days (F3), (4) once every 4 days (F4), (5) onceevery 6 days (F6), and (6) once every 8 days (F8). In2001, the irrigation quantity for F1 was adjusted daily tomaintain the soil matric potential at 20 cm depthimmediately under drip emitter close to �25 kPa.Though the irrigation quantity for F1 was adjusted dailyin 2001, when the daily ET was great, the daily irrigation

quantity could not completely meet radish water con-sumption. So in 2002, in order to determine irrigationquantity for F1 per day, a reference treatment (R) wasdevised. The R treatment was designed to control thesoil matric potential at 20 cm depth immediately underdrip emitter higher than �25 kPa. All the treatmentswere replicated three times and they followed a completerandomized block design.

Each treatment plot was a submain unit of a dripirrigation system comprising a flow meter, a valve and apressure gauge at the entrance of the unit to control theoperating pressure and measure the irrigation volume.Thin-wall drip tape (Chapin Watermatics) with 0.3 memitter spacing and a flow rate of 1.12 l/h at the oper-ating pressure of 0.042 MPa was placed on the center ofraised beds.

Irrigation management

In both years, irrigation for F1 was applied daily, unlessthe soil matric potentials of F1 were too high after rain.In 2001, the average irrigation quantity for F1 was about2.5 mm per day and adjusted daily to maintain the soilmatric potential at 20 cm depth immediately under dripemitter was close to �25 kPa. In 2002, the irrigationamount per day for F1 was equal to the ET value of theprevious day of R treatment, which was measured by thelysimeter. The irrigation of R treatment was applied onlywhen the soil matric potential at 20 cm depth immedi-ately under drip emitter was close to �25 kPa, and theaverage irrigation amount was about 2.5 mm per time. Inboth years, irrigation amount for F2, F3, F4, F6 and F8was the cumulative values of F1.

Agronomic practices

In 2001 and 2002, about 37.5 m3/ha well-rotted cowmanure was uniformly applied to all plots before fieldwas ploughed. About 3�4 days later, 1 kg carbamideand 1.25 kg compound fertilizer (monoammonia phos-phate) were uniformly applied to each plot when the soilwas bedded. The total applications of N, P2O5 and K2Owere 182, 71 and 52 kg/ha, respectively. The soil wasdisced and bedded 0.8 m apart and 0.15 m high. Everydrip irrigation plot contained seven beds, and the area ofeach plot was 5.6·6 m2. Radishes were double-rowplanted on each bed with row spacing of 0.3 m andinterplant spacing of 0.25 m (Fig. 1).

The two radish varieties are both planted in autumn,and most of their characteristics are alike. Seeds ofradish cv. ‘Dahongpao’ and ‘Mantanghong’ were plan-ted on July 31 and August 17 during 2001 and 2002,respectively. After emergence, seedlings were thinned toleave only one seedling at each location maintaining aplant population of approximately 100,000 plant/ha.Radishes were harvested on October 19 and November 9in 2001 and 2002, respectively.

Observation and equipments

Soil matric potential

In 2001, one set of mercury tensiometers with 30 sensorswas installed in F1 treatment to observe soil matricpotential distribution. In 2002, another six sets of mer-cury tensiometers were installed in F2, F3, F4, F6, F8and R treatments. Each set of mercury tensiometer in-cluded 30 sensors and the sensors placement was thesame for all treatments. There were five series of sensorsin the vertical transect perpendicular to the drip tape atfive horizontal distances (0, 10, 20, 30, and 40 cm) andsix vertical soil depths (10, 20, 30, 50, 70 and 90 cm)(Fig. 1). After radish establishment, the tensiometer at20 cm depth immediately under drip emitter in Rtreatment was observed every 2 h during the daytime inorder to determine the irrigation time.

Radish water use

In 2002, for each treatment, one weighing lysimeter wasinstalled in the center of one of the radish plots tomeasure water use of each treatment. Each lysimeterconsisted of an inner tank for crop cultivation and anouter tank for protection and drainage reservoir. Thevolume of the inner tank was 0.36 m3

(0.8 m·0.5 m·0.9 m), and a filtering layer of coarsesand and gravel, 0.15 m thick, was overlain by arepacked soil profile of 0.7 m. The rooting depth ofradishes is generally less than 0.3 m under drip irrigation

so there was adequate depth for the roots. The topsoil ofthe inner tank was shaped to the same forms as fieldbeds, and at the bottom of the inner tank, a pipe servingas a drainage outlet connected the tank to the outertank. Four radishes were cultivated in each tank with adensity of 100,000 plants/ha equal to the field. The driptapes for the beds were installed across the lysimeters forirrigation. There was a moveable electric weighing sys-tem to weigh the lysimeters one at a time. The lysimeterin R was weighed every day, and others were weighedonce every two days.

Twenty centimeter diameter pan evaporation over canopy(EW20) and weather data

In 2002, a 20-cm diameter evaporation pan was installedover radish canopy in plot R on September 15 (29 daysafter seeding). The starting height of the evaporationpan was 15 cm above the ground and was adjusted withthe growth of radish. On October 17 (61 days afterseeding), the pan reached height of 50 cm above theground, and was kept at this height till the harvest ofradish. Pan evaporation was observed at 8:00 AM daily.The meteorological data were obtained from LEASweather station.

Radish growth

Three representative plants were randomly selected andfixed in one plot of each treatment for leaf area mea-surement at 10-day intervals during radish-growingseason in 2001 and 2002. Three other representativeplants were randomly selected and sampled in anotherplot of each treatment for dry mass (leaf and succulentroot) investigation at 10-day intervals in 2001 and 2002.

In 2001, root dry weight density for each treatmentwas obtained from soil cores extracted between rowswith an auger (55 mm in diameter, 10 cm high with avolume of 237.46 cm3) at the leaf development stage(September 22) and at harvest (October 19). In eachplot, the distances to the center of raised beds for sam-pling were 0, 7.5, 15, and 22.5 cm, and sample depthswere 0�10, 10�20, 20�30, 30�40 cm depth on Sep-tember 22 and 0�10, 10�20, 20�30, 30�40, 40�60 cmdepth on October 19.

In 2002, radishes in the middle three rows of one plotof each treatment were harvested for analysis. Freshfruit weight (g/fruit) for size categories were as follow:>500 g (Grade 2), 250�500 g (Grade 1) and <250 g(Grade 3).

Statistical analysis

The treatments were run as a single-factor analysis ofvariance (ANOVA). The ANOVA was performed ata=0.05 level of significance to determine if significantdifferences existed among treatment means. The multiple

Fig. 1 Placement of tensiometers for each experimental treatment

comparisons were done for significant effects with theLSD test at a=0.05.

Results and discussion

Weather and irrigation

The average weekly temperatures of 2001 were higherthan those in the corresponding period of 2002, and theaverage weekly evaporation was similar during the2 years. Total rainfall was 75.0 mm and 68.9 mm in2001 and 2002, respectively. The rainfall distributionwas 68% and 69% rainfall in the first 4 weeks (radishseedling stage), 22% and 7% in the middle 4 weeks(radish leaf development stage), and 10% and 24% in

the last 4 weeks (radish succulent root formation stage)in 2001 and 2002, respectively (Table 1).

Irrigation treatments were initiated on August 25(25 days after seeding) and on September 24 (38 daysafter seeding) in 2001 and 2002, respectively. Prior tothat date, all the treatments were given uniform irriga-tions to ensure germination. In 2001, the water appliedto F1�F8 treatment was about 81.5 mm. In 2002,according to the previous day ET of R treatment, theaverage amount applied to F1 was about 3.9 mm perday. The irrigation amount for R treatment was56.5 mm and for F1�F8 treatment was about 102.6 mm(Fig. 2). The water applied to F1�F8 treatment in 2002was about 20 mm more than that in 2001 though theweather in 2002 was cooler than that in 2001. This isbecause the general soil moisture status in 2002 wasbetter than that in 2001 due to the water applied to eachtreatment according to the ET of R treatment observedevery day.

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Fig. 2 Water applied for radishes under different irrigationfrequency treatments during radishes growing period in 2001 and2002

Table 1 Weather data during radish different development periods of 2001 and 2002

Week Temperature(�C) Cumulative rain-fall (mm)

Pan evaporation(mm/day)

Maximum Mean Minimum

2001 2002 2001 2002 2001 2002 2001 2002 2001 2002

1 31.7 30.1 26.5 24.7 22.1 19.4 5.2 1.2 4.7 3.82 31.3 31.9 24.5 26.0 17.8 21.1 0.0 9.2 5.6 4.73 29.7 27.8 24.2 21.3 19.7 16.2 42.6 2.9 3.5 4.14 30.2 26.9 24.6 19.5 19.4 14.2 3.5 34.7 5.0 3.65 29.5 25.2 22.5 17.4 15.4 10.7 0.0 3.2 4.6 4.06 28.0 27.3 20.8 18.8 14.6 9.7 0.2 1.5 4.0 4.57 28.2 26.7 21.6 16.9 15.7 7.9 0.0 0.0 3.4 6.58 24.1 22.7 17.8 14.2 12.9 7.3 16.2 0.0 3.6 4.69 25.1 20.7 17.3 13.6 10.3 8.1 1.4 16.0 4.3 2.910 20.6 13.6 14.1 8.0 7.8 3.6 3.8 0.0 3.5 2.811 21.1 11.1 15.6 5.6 10.4 1.0 1.8 0.2 2.7 2.612 20.0 13.1 13.6 5.8 7.3 �0.4 0.3 0.0 3.3 2.5

Soil Matric Potential

Temporal patterns

In 2001, only one set of mercury tensiometers with 30sensors was installed in F1 treatment. Figure 3 illus-trates the soil matric potentials at 10, 20, 50 and 90 cmdepths immediately under drip emitter for F1 treatmentin 2001. The soil matric potentials values at 10 and20 cm depths were kept basically higher than �25 kPaduring the whole growing period. The soil matricpotentials values from 50 to 90 cm depths were above�10 kPa at first, and began to decrease successively withthe growth of radish. In the whole period, the soil matricpotentials from 10 to 90 cm depth were always above�30 kPa, which means that there was no drought stressfor radish.

In 2002, another five sets of mercury tensiometerswere installed in F2, F3, F4, F6 and F8 treatments. Thesoil matric potentials at 20 cm depth immediately underdrip emitter for different drip irrigation frequencytreatments in 2002 are showed in Fig. 4. The soil matricpotentials values for F1, F2, F3 and F4 treatments weregenerally higher than �25 kPa and fluctuated slightlythroughout the whole growing period, whereas those for

F6 and F8 oscillated a little remarkably. So the resultsindicate that the average soil matric potentials increasedas irrigation frequency increased during radish growingperiod.

Spatial variations

Figure 5 illustrates the spatial distribution of soilmatric potentials in a vertical transect perpendicular tothe drip tape for each treatment before and after irri-gation in 2002. On September 24 (38 days after seed-ing), the spatial distributions of soil matric potentialsof all treatments were very similar, and there was atendency for the soil matric potentials values to in-crease gradually from about �35 to �5 kPa as soildepth increased. At September 25, 1 day after irriga-tion, the spatial distributions of soil matric potentialswere obviously different. The average soil matricpotentials for F1 and F2 changed a little after irriga-tion, whereas those for F6 and F8 increased drastically.On October 10 and 11 (54 and 55 days after planting),at radish succulent root formation stage, the averagesoil matric potentials for F1 and F2 were high, andabove �10 kPa in the root zone no matter before or

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Fig. 3 The changes of soilwater potentials in 10, 20, 50and 90 cm depths immediatelyunder emitters for F1treatments in 2001

after irrigation. The average soil matric potentials val-ues for F3 and F4 varied in a range from �20 to�10 kPa before irrigation and were higher than�10 kPa after irrigation. In contrast, the average soilmatric potentials for F6 were much lower (October 6).The soil matric potentials values increased graduallyfrom about �40 to �5 kPa as soil depth increasedfrom 0 to 90 cm, and after irrigation, the average soilmatric potentials increased rapidly to about �15 kPa(October 7). Despite a longer irrigation interval, theaverage soil matric potentials before irrigation for F8were higher than those for F6. This may be due to alarger amount of irrigation water applied for F8

treatment than for F6 treatment at the last irrigation.Similarly, the soil matric potentials for F8 increasednotably as well after irrigation. Both F6 and F8treatments showed increases in soil matric potential atdepths of 70 and 90 cm after irrigation, indicating thatirrigation was more than sufficient to meet ET.

The results indicate that before irrigation, as theirrigation frequency decreased, the average soil matricpotential values through the whole root zone decreased,and the dry domain in the soil expanded. However, afterirrigation, as the irrigation frequency decreased, theaverage soil matric potentials values in the root zoneincreased greatly.

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Fig. 4 The changes of soilwater potentials in 20 cm depthimmediately under emitters fordifferent irrigation frequencytreatments in 2002

Fig. 5 Transect of spatial distribution of soil water potentialsperpendicular to drip tape for irrigation frequency treatmentsbefore and after irrigation at leaf development and succulent rootformation stages in 2002

Radish growth

Leaf development

Changes in leaf area index (LAI) from seedling stage toharvest for different irrigation frequency treatments in2001 and 2002 are presented in Fig. 6. The figures showsigmoid shapes for the LAI versus time relationship inboth years. During seedling period, the LAI values forall treatments were small, and began to increase at leafdevelopment stage. When near the succulent root for-mation stage, the LAI values of different treatmentsreached their maximum successively, and then decreaseda little at the end of experiments. At harvest, LAI of thesix treatments followed a F2>F3�F6�F8>F4�F1

order and F3>F4>F2>F6>F8>F1 order in 2001and 2002, respectively. In both years, the highest irri-gation frequency (F1) resulted in the least LAI. It isbecause a high irrigation frequency (irrigating once ev-ery day) caused a very humid region in the root zone andreduced the oxygen diffusion into the soil, which affectedthe activity of crop enzyme, weakened crop photosyn-thesis (Pezeshki 1994; Liao and Lin 1994; Huang et al.1994), and inhibited the development of leaf area.

Succulent root development

The growth response of radishes to the different irriga-tion frequency can be presented by tracking the devel-opment of circumferences of radish succulent roots. Thecircumference accretion versus time relationship for eachtreatment in 2001 and 2002 (Fig.7) were sigmoid shapes.During the early growing period, radish succulent rootswere small and began to expand rapidly about 25 daysafter planted. At the late period (about 65–70 days afterplanting), root circumference accretion rates began toslow.

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Fig. 6 The changes of LAI during radish growing periods forirrigation frequency treatments in 2001 and 2002

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Fig. 7 The succulent root body expanding during radish growingperiods for irrigation frequency treatments in 2001 and 2002

Dry mass accumulation

Figure 8 indicates the succulent roots dry massesaccretion versus time relationships were double-sigmoidshapes in 2001 and 2002. At first, the dry masses ofsucculent roots for each treatment accumulated slowly,and began to increase rapidly on September 12 andOctober 2 in 2001 and 2002, respectively. About half amonth later, the dry masses accumulated slowly again.The slowly growing stage lasted about 10 days, and thenthe dry masses increased dramatically again beforeharvest. It is obvious that radishes were harvested earlyin the harvest season in 2001.

Root distribution

Figures 9 and 10 illustrate the roots dry weight densityfor the six treatments on September 22 and October19 in 2001, respectively. On September 22, most ofradish roots of each treatment developed in the soil15 cm horizontally to the center of raised beds at thedepth of 0�20 cm. The highest irrigation frequencytreatment (F1) led to the lowest roots dry weightdensity, about 150·10�6 g/cm3, and the lowest irriga-tion frequency treatment (F8) resulted in the highestroots dry weight density, about 1,100·10�6 g/cm3. Theroots dry weight density for F2, F3, F4 and F6treatments were about 420, 840, 710 and 620·10�6 g/cm3, respectively. When harvested (on October 19), theroots dry weight density for most treatments (exceptF1 and F3) was less than that on September 22, butthe spatial distributions of roots were similar. Most ofradish roots developed in the soil 15 cm horizontallyto the center of raised beds at 0�20 cm depth. Theroots dry weight density for F1, F2, F6 and F8treatments were about 450·10�6 g/cm3, and that forF3 and F4 treatment was about 850 and 650·10�6 g/

cm3, respectively. It can be seen that the root dryweight density for F3 treatment kept high throughoutthe whole growing period.

Fresh root yields and market quality

The highest yield was recorded at F2 in 2001 and F3 in2002, and followed a F2>F3>F6>F4>F8>F1 orderin 2001 and F3>F4>F2>F6>F8>F1 order in 2002(Table 2). However, according to the statistical analysis,the differences in average root fresh weight and yieldamong the six irrigation frequency treatments in bothyears were not significant.

Among all the six treatments in 2002, F3 had thelowest cracking rate (1.3%) and the fewest radishes(1.4%) of Grade 3 (W<250 g). These results indicatethat irrigation once every 3 days was favorable forradish growth.

Irrigating too frequently, like F1 treatment, resultsin a very humid region in the root zone, and rootgrowth and function are affected by inadequate oxygendiffusion into the root zone. In addition, when the soilis too wet, the parenchyma cells in the xylem of rootexpand quickly, but the cells in the phloem and peri-derm cannot expand accordingly, and cause rootcracking. On the other hand, too long irrigationintervals, like F8 treatment, results in dramatic fluctu-ations of soil moisture in the root zone, and bringabout cyclic water stress on radish root growth. Whenthe soil moisture is lacking, the development of theroot is restrained and the lignification of periderm isincreasing. Subsequently, when the soil is wet, the rootexpands rapidly, but the cells in periderm cannot ex-pand correspondingly, and then cause the root tocrack. Therefore, both too high and too low irrigationfrequencies are not good for radish normal growth.Irrigation once every 3 days (F3) may establish afavorable soil moisture and oxygen conditions in theroot zone throughout the growth period, and isfavorable for the development of radish.

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Fig. 8 Development of succulent root dry masses during radishgrowing periods for different irrigation frequency treatments in2001 and 2002

Radish evapotranspiration under different irrigationfrequencies

Figure 11 demonstrates 2-day paired cumulative ET forall treatments determined using the weighing lysimetersand the corresponding 2-day evaporation (EW20) fromSeptember 14 to November 6, 2002. During radishmiddle growing period, from the first 2-day to theeighteenth 2-day period (September 14 � October 18),radish ET values were relatively high for all of thetreatments, and the corresponding EW20 values werealso high. During this period, the maximum 2-day ETvalues for F1, F2, F3, F4, F6 and F8 were 14.0, 19.7,13.2, 14.2, 10.7 and 11.9 mm, respectively, and themaximum 2-day EW20 was 13.2 mm. With the declining

of temperature and the senescing of radish, radish ETvalues became relatively low for all of the six treatments.From the 19th 2-day to the 27th two-day (from October18 to the end), the maximum 2-day ET values for F1,F2, F3, F4, F6 and F8 were 8.3, 6.6, 7.4, 7.7, 8.0 and5.7 mm, respectively, and the maximum 2-day EW20

value was 8.1 mm.The regression between 2-day paired cumulative ET

and the corresponding 2-day EW20 from September 14to November 6, 2002 were showed in Fig. 12. Theregression equations for F1 to F4 were similar. The slopefor F1 to F4 treatment was between 0.85 and 0.93, andthe R2 between 0.71 and 0.76. The regression equationsfor F6 and F8 were similar. The slope for F6 and F8treatments were both 0.69, but the R2 were only 0.69 and

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0.0 200.0 400.0 600.0 800.0 1000.0 1200.0

(F4) Root weight density (10-6*g/cm3)

0

7.5

15

22.5

0

10

20

30

40

0.0 200.0 400.0 600.0 800.0 1000.0 1200.0

(F6) Root weight density (10-6*g/cm3)

Dep

th (

cm)

0

10

20

30

40

0.0 200.0 400.0 600.0 800.0 1000.0 1200.0

(F8) Root weight density (10-6*g/cm3)

0

7.5

15

22.5

Fig. 9 The root dry weightdensity for different treatments onSeptember 22, 2001

0.63, respectively. So the longer irrigation interval was,the lesser the ET tracked the corresponding evaporation.The general changing tendency of the 2-day ET of highirrigation frequency treatments (F1–F4) during radishgrowing period was related to that of EW20. This is inagreement with results obtained by Yuan et al. (2001).He used 20 cm pan measurements to determine thewater requirement of tomato planted in unheatedgreenhouse, and found that when the soil matric po-tential was kept higher than �20 kPa at 15 cm depth,the cumulative value of ET was approximately equal tothe cumulative value of water surface evaporation of a20 cm pan installed over tomato canopy.

Figure 13 illustrates the seasonal cumulative EW20

and seasonal cumulative ET for different treatmentsduring radish growth period from September 14 toNovember 6, 2002. The general tendency for all the sixtreatments was similar to that for EW20. The seasonalcumulative ET and seasonal cumulative EW20 followeda F1 (188 mm) > EW20 (185 mm)> F2 (177 mm) � F3(177 mm) > F4 (169 mm) > F8 (165 mm) > F6(149 mm) order. The highest ET (F1) was 23 mm (14%)more than the lowest value (F8). The seasonal cumula-tive ET for F1 was even higher than the cumulative panevaporation. It infers that F1 (irrigating once every day)may cause more water loss.

0

10

20

30

40

50

60

0.0 200.0 400.0 600.0 800.0

(F1) Root weight density (10-6*g/cm3)

Dep

th (

cm)

0

10

20

30

40

50

60

0.0 200.0 400.0 600.0 800.0

(F2) Root weight density (10-6*g/cm3)

0

7.5

15

22.5

0

10

20

30

40

50

60

0.0 200.0 400.0 600.0 800.0

(F3) Root weight density (10-6*g/cm3)

Dep

th (

cm)

0

10

20

30

40

50

60

0.0 200.0 400.0 600.0 800.0

(F4) Root weight density (10-6*g/cm3)

0

7.5

15

22.5

0

10

20

30

40

50

60

0.0 200.0 400.0 600.0 800.0

(F6) Root weight density (10-6*g/cm3)

Dep

th (

cm)

0

10

20

30

40

50

60

0.0 200.0 400.0 600.0 800.0

(F8) Root weight density (10-6*g/cm3)

0

7.5

15

22.5

Fig. 10 The root dry weightdensity for different treatmentson October 19, 2001

The seasonal cumulative value for F6 was so low thatit was out of the common knowledge. By checking theradishes planted in the lysimeter of F6, the result indi-cates that the weights of the four radishes were 833.8,622.1, 368.3 and 482.1 g, respectively. From the data, itis obvious from the differences in individual weights thatthe cumulative curve may not reflect the general patternfor F6 treatment.

Figure 14 shows the general relationship betweenradish seasonal cumulative ET and different irrigationfrequency treatments from September 14 to November6, 2002. The result indicates that radish seasonalcumulative ET decreased as the irrigation frequencydecreased. A significant power relationship between

radish ET (mm) and irrigation frequency (F: days) wasderived as:

ET ¼ 186:9F �0:0622 ðR2=0:94Þ ð1Þ

Water use efficient under different irrigation frequencies

WUE is the relation between yield and ET, and iscomputed based on fresh root yield dividing by thewater consumption. Figure 15 illustrates the generalrelationship between radish WUE and different irriga-tion frequency treatments. Radish WUE increased withthe decreasing of irrigation frequency, and reached the

Table 2 Yield and market quality of radish as affected by irrigation frequency in 2001 and 2002

Average freshroot weight (g)

Yield(Mg/ha)

Rate ofcracking (%)

Fresh root weight distribution

W>500 g Grade 2 250 g<W<500 g Grade 1 W<250 g Grade 3

2001F1 488.1a 48.8aF2 506.3a 50.6aF3 502.8a 50.3aF4 496.9a 49.7aF6 498.5a 49.9aF8 492.5a 49.2a2002F1 449.9a 45.0a 5.6a 34.7c 55.6a 9.7aF2 473.7a 47.4a 5.1a 48.1abc 43.0ab 8.9aF3 481.5a 48.2a 1.3b 52.6ab 46.0ab 1.4bF4 475.0a 47.5a 7.8a 56.3a 35.9b 7.8abF6 469.4a 46.9a 6.7a 38.7bc 57.3a 4.0abF8 459.1a 45.9a 7.1a 38.5bc 52.9a 8.6a

The same letters are not significant difference at 0.05 level, and different letters mean significant difference at 0.05 level

0

2

4

6

8

10

12

14

16

18

20

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Times (two-day)

Tw

o-da

y E

T a

nd tw

o-da

y E

W20

(m

m)

F1 F2

F3 F4

F6 F8

EW20

Fig. 11 Two-day pairedcumulative ET curves fordifferent irrigation frequencytreatments during radishgrowth period from September14 to November 6, 2002 usingweighing lysimeters

maximum value when irrigation frequency was aboutonce every 6 days, and then WUE began to decrease asirrigation frequency decreased. The relation betweenWUE (kg/ha mm) and irrigation frequency was ex-pressed by polynomial as following:

WUE ¼ �2:17F 2 þ 24:4F þ 220 ðR2=0:94Þ ð2Þ

F3 treatment had the lowest cracking rate, the lessradishes of Grade 3, and the moderately high WUE, soirrigating once every 3 days is suggested for radish (cv.

Mantanghong) drip-irrigated scheduling, and the irri-gation frequency is also suitable for radish variety‘‘Dahongpao’’.

Summary and conclusions

Different irrigation frequency treatments did affecttemporal and spatial distribution of soil water in theroot zone. As the irrigation frequency decreased, theaverage soil matric potential values through the wholeroot zone decreased, and the dry domain became larger.Moreover, the variability of soil matric potential valuesat 0�50 cm depths before and after irrigation increasedas irrigation frequency decreased.

0

y1 = 0.92x + 0.50R2 = 0.74

y2 = 0.93x - 0.30R2 = 0.71

y3 = 0.85x + 0.75R2 = 0.72

y4 = 0.90x - 0.17R2 = 0.76

0

2

4

6

8

10

12

14

16

2 4 6 8 10 12 14

Two-day EW20 (mm)

Tw

o-da

y E

T (

mm

)F1

F2F3

F4F1

F2F3

F4

y6 = 0.69x + 0.87R2 = 0.69

y8 = 0.69x + 1.30R2 = 0.63

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12

Two-day EW20 (mm)

F6

F8

F6

F8

14

Fig. 12 The relationships between 2-day paired cumulative ETfrom weighing lysimeters and the corresponding 2-day EW20fromSeptember 14 to November 6, 2002

0

20

40

60

80

100

120

140

160

180

200

28 32 36 40 44 48 52 56 60 64 68 72 76 80 84

Days after planting

Seas

onal

cum

ulat

ive

ET

and

EW

20

(m

m)

F1 F2

F3 F4

F6 F8

EW20

Fig. 13 Seasonal cumulative evaporation and season cumulativeradish ET for different irrigation frequency treatments duringradish growing period from September 14 to November 6, 2002

y = 186.9x-0.0622

R2 = 0.94

100

120

140

160

180

200

1 2 3 4 5 6 7 8

Irrigation frequency (days)

Seas

onal

cum

ulat

ive

ET

(m

m)

Fig. 14 Relationship between seasonal cumulative ET (fromSeptember 14 to November 6) and irrigation frequency

Different irrigation frequencies had no significantimpacts on radish growth and development, but they didaffect radish roots distribution and market quality verymuch. Radishes irrigated once every 3 days had well-developed roots throughout the crop period, the lowestcracking rate and the least number of radishes of Grade 3.

Radish seasonal cumulative ET decreased as irriga-tion frequencies decreased. The highest ET (F1) was23 mm (14%) more than the lowest value (F8). RadishWUE increased with the decrease in of irrigation fre-quency, and reached the maximum value when irrigationfrequency was about once every 6 days, and then WUEbegan to decrease as irrigation frequency decreased.Furthermore, the general changing tendency of 2-daypaired cumulative ET of high irrigation frequency wasrelated to that of corresponding 2-day evaporation of20 cm diameter pan.

In the North China Plain, irrigating once every3 days is suggested for radish drip-irrigated scheduling,and the irrigation amount may be determined based onthe evaporation of 20 cm diameter pan installed overradish canopy. Most conclusions in this study werebased on only 1-year data, and many further researchesshould be made.

Acknowledgements This study is the part works of the Project40125002 supported by National Science Fund for DistinguishedYoung Scholars and the Project 2002AA2Z4061 supported byNational High Technology Research and Development Program

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y = -2.17x2 + 24.4x + 220

R2 = 0.94

200

220

240

260

280

300

1 2 3 4 5 6 7 8Irrigation frequency (days)

Wat

er u

se e

ffic

ienc

y (k

g/ha

/mm

)

Fig. 15 Relationship between radish WUE and irrigation fre-quency