CROP COOLING WITH SPRINKLERS

INTRODUCTION

The use of sprinkler irrigation equipment to cool the aerial environment ofcrops is a recent innovation in agriculturalproduction. The benefits to be gainedinclude increased yield and quality of thecrop and the extension through climaticmodification of the normal geographiclimits of specific crop production.

Plant water potential is influencedmore by evaporative demand of the atmosphere than by soil water availability,particularly when soil water is maintainedwithin the tensiometer range (1, 4, 9).The significance of changes in plant waterpotential is not well defined (1) but thereis evidence (7) that the changes can becontrolled by mist irrigation with resultant beneficial yield response.

Plant growth is restricted by waterdeficits caused by excessive transpirationduring the heat of midday (10). Simultaneous plant and air temperatures maydiffer widely and plants may respond toair temperature changes of only a fewdegrees Celsius (13). A change of a fewdegrees in leaf temperature can make amajor difference in the biological functions of plants (6).

Beans, peas, potatoes, tomatoes, strawberries, and tree fruits are among thecrops reported to have critical maximumtemperatures from which they can beprofitably protected (5, 7, 9, 11, 15).Little has been recorded about critical

temperatures but 90°F (32.2°C) has arbitrarily been assumed to be the upperthreshold temperature for most temperate zone crops (2, 7).

Reductions in air and soil temperatureproduced by sprinkling with conventionaland low-rate irrigation equipment havebeen documented (2, 9, 11, 12, 14, 15).Improved crop growth has often beenassociated with these environmental mod

ifications, but in many instances thedegree of modification has not been

RECEIVED FOR PUBLICATION APR. 20,1972

E.H. Hobbs

Member CSAE

Research Branch, Agriculture CanadaResearch Station

Lethbridge, Alberta Tl J 4B1

interpreted on a climatic basis. None ofthe studies reported has been conductedin a climate similar to that of southernAlberta, which features hot dry summersbut which has few extreme maximum

temperatures (8). The sprinkler methodof irrigation has been increasing in popularity to the extent that it predominatesin some localities and many systemscurrently in use have a crop-cooling capability. This paper describes crop-coolingexperimental techniques used in southernAlberta and relates the results to prevailing meteorological conditions.

EQUIPMENT AND EXPERIMENTALPROCEDURE

Most crop cooling described in theliterature has been done when air temperature exceeded 90°F (32.2°C). At Lethbridge, on the average, air temperaturehas exceeded 90°F (32.2°C) on only fiveoccasions for a total of 16 h annually. Itwas considered impractical to assess cooling on the basis of so few extremetemperatures. But, on the average, airtemperatures exceed 80 F (26.7 C) 6,16, 14, and 5 d in June, July, August, andSeptember, respectively, or a total ofnearly 150 h. Consequently, this lowertemperature was used as a base.

The experimental design consisted of12 plots, each 40 ft (12.2 m) square,arranged in three replications. Each four-plot replication contained two irrigationtreatments on which cooling was superimposed and two on which cooling wasnot superimposed. Irrigation and coolingtreatments were randomized within each

replication. The replicated design wasprovided for the physiological assessmentof the crops being studied under thecooling regime. Equipment availabilitylimited the instrumentation for coolingassessment to one complete replication.

Sprinklers for both irrigation and cooling were of full-circle, single-nozzle, conventional design and were located at thecorners of the plots. Sprinkler nozzles oftwo sizes, 13/64 inch (5.2 mm) and 7/64inch (2.8 mm), were used for the twoirrigation treatments but for cooling

4/64-inch (1.6-mm) nozzles were used.

For cooling, a temperature controllerlocated near the pump actuated an electric pumping unit as long as the ambienttemperature exceeded 80°F (26.7 C).Cooling was arbitrarily cycled 16 min onand 14 min off by a time clock. Thepurpose of the on-off cycling and the useof small-sized nozzles was to reduce the

amount of water applied to the plots. Thecooling treatment was intended only tocool the crop environment and not tosupply water for crop use.

Catch cans were placed on a 10-ft(3.0-m) grid over the plot area to determine the actual amount of water appliedto the soil and plant surfaces duringirrigation and cooling and to provide datafor calculating distribution uniformitycoefficients by the Christiansen (3)method. Coefficients for 25 consecutive

irrigations averaged .75 and ranged from.70 to .83.

The cooling sprinklers, operating at 55psi pressure, applied water at the rate of0.050 inch (1.27 mm) per h on the 40-ft2(12.2-m2) plot area. Operational cyclingproduced a net application rate of 0.026inch (0.66 mm) per h, a rate similar tothat of evaporation from a free watersurface during midsummer. Distributionuniformity coefficients for cooling applications were similar in magnitude tothose for irrigation. During highly evaporative conditons, however, many of thecatch cans were dry immediately after acooling application.

Water for cooling and irrigation waspumped from a small reservoir suppliedfrom the Irrigation District canal. Watertemperature was as high as 80°F (16.7°C)during the hottest part of the day. Thepump intake was equipped with a 4/64-inch (1.6-mm) screen. This screen, thesettling reservoir, and a thorough flushingof the cooling system after installationeffectively prevented nozzle-pluggingproblems.

During 1969 and 1970, data wererecorded for potatoes, cult Netted Gem,and in 1971 for green bush beans, cult

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 1, JUNE 1973

Executive. Tensiometers and neutron-

scattering techniques were used to schedule irrigations and monitor soil moisture.The effects of cooling and irrigation wereseparated by insuring that irrigations wereapplied only on those days or duringperiods of the day that cooling was notrequired.

Hourly maximum air temperature andwind data were obtained from the CanadaDepartment of Environment meteorological station located about 4 miles (6.4 km)south of the test plots. Relative humiditywas determined daily with a psychrom-eter at about the time that ambienttemperature was greatest. Additionalhumidity measurements occasionallywere made in the immediate plot area.Air temperature within the plant canopyin all four plots of one replication wasrecorded on thermographs. The sensingelements were located in the crop row 4inches (10.2 cm) above the soil surfaceand were shielded from the sun and fromdirect contact with the sprinkler spray.Soil temperature at a depth of 4 inches(10.2 cm) under the crop row also wasrecorded. The maximum daily temperature reduction due to cooling was determined from the thermograph charts foreach of 88 cooling days during the 3 yr ofthe tests. These data were related bysimple and multiple correlation to relativehumidity, maximum ambient air temperature, and wind velocity at the time ofmaximum temperature reduction. Regression equations relating these indices totemperature reduction were also calculated.

RESULTS AND DISCUSSION

Sprinkling reduced the canopy air temperature by a maximum of 16 F (8.9 C),but the average reduction, under a widerange of weather conditions and over anextended period .of time, was 6 F(3.3°C). As a rule the cooling effect warsgreatest in midafternoon (Figure 1) anddecreased as the day progressed. Therewas usually some residual effect in theevening that occasionally was still apparent the next morning. Air temperature inthe plot canopy was not always the sameas that at the controller. Consequently,towards evening the cooling equipmentdid not necessarily shut off preciselywhen the canopy temperature of thenoncooled plots fell to 80°F (26.7°C).On the cooled plots the canopy temperature at the time the cooling sprinklerswere shut off was below ambient temperature by the amount that the treatmentwas influencing temperature at that time.In the highly vegetative potato crop,cooling always maintained the plant canopy air temperature below 80 F

Figure 1. Typical effects of cooling on canopyair temperature.

Figure 2. Temperature changes resulting fromcycling of the cooling system underhighly evaporative conditions.

Figure 3. Temperature changesresulting from anormal field-type irrigation.

(26.7 C), but with beans the canopytemperature occasionally was higher thanthis.

The temperature recovery during the14-min off period varied, depending uponthe prevailing weather, from practicallynothing to about 4°F (2.2°C). Figure 2depicts the well-defined cycling patternthat occurred under high evaporative conditions. Chesness and Braud (2), in asimilar study in Louisiana, applied waterat rates of 0.08 inch (2.03 mm) per h onstrawberries and reported a recovery of40% during a 15-min cycling period.Generally, though, as in Figure 1, the

CANADIAN AGRICULTURAL ENGINEERING, VOL. IS, NO. 1, JUNE 1973

cycling pattern was much less sharplydefined and the temperature recovery wasin the order of 10-20% of the maximum

temperature reduction.

Air temperature in the plant canopywas also influenced during sprinkling forirrigation (Figure 3). The residual temperature reduction was more pronouncedafter irrigation than after low-rate coolingbecause of the greater wetting of the soilsurface. The effect was quite apparent thefollowing day but was dissipated completely within 48 h.

Soil temperature was also influencedby sprinkler cooling but to a much lesserextent than air temperature, the reduction usually being between 1 and 2 F(0.6 and 1.1°C).

Although the rate of application ofcooling water approximated the evaporation rate, the soil became slightly moisterin the cooled plots. The occasional residual effect of cooling evidenced on themorning after cooling also indicated somemoisture carryover. This increase in soilmoisture can be attributed directly to acontribution from the cooling water andindirectly to a reduction in transpirationduring cooling. Reduced transpirationdue to lowered temperature and increasedhumidity is indicated. The magnitude ofthe reduction is the basis for continuingstudies. On a seasonal basis, however,both water application to and evapotrans-piration from the cooled plots were greater than from the noncooled plots. Thusthe reduction in transpiration was outweighed by the amount of cooling waterapplied, and the cooling treatment actually decreased the overall efficiency ofwater use. It does not seem practical tofurther reduce the size of the coolingnozzles but the duration of the coolingtreatment might be reduced.

During the 88 d on which cooling tookplace, the maximum air temperature recorded was 97°F (36.1°C), but on 67 ofthe days the temperature did not exceed90°F (32.2°C). Relative humidity rangedbetween 10 and 50% but two-thirds ofthe time was between 20 and 35%. Windvelocity varied from 2 to 25 mph (3.2 to40.2 km/h) but exceeded 15 mph (24.1km/h) only 25% of the time. Thus therange of weather conditions prevailingduring the study was not extreme.

When the three meteorological indices— humidity, maximum air temperature,and wind — were simply correlated totemperature depression by cooling, humidity had the greatest influence, the rela-tionshop being negative (r = .794). Maximum air temperature (r = .602) and wind(r = .513) were less well related.

TABLE I REGRESSION EQUATIONS, MULTIPLE CORRELATION COEFFICIENTS, ANDSTANDARD DEVIATIONS FOR TEMPERATURE REDUCTION (A t) VS. HUMIDITY (x ), MAXIMUM AIR TEMPERATURE (* ), AND WIND (x )

Tempreduction

(°F) Constant

Relative

humidity

(%)

Max air

temp

(°F)

Wind

velocity(mph)

Multiplecorrel.

coef. SD

At =

At =

At =

15.62

13.48

-.29

-.261xl-.236^-.192jct

+.111jc2+.H4jc2 +.145x3

.794

.819

.840

1.92

1.83

1.74

The low correlation with wind is not

surprising. Under the weather that prevailed during the cooling process (airtemperature above 80°F (26.7°C)) windvelocities were usually neither extremenor widely variable. Consequently theywould not have contributed large orfluctuating amounts of convective heat.But there was usually sufficient wind toremove the moist air from the plantcanopy and thus effect the cooling process. The correlation between wind and

air temperature reduction was slightlyhigher for the potato crop in 1969 and1970 than for the bean crop in 1971(.552 vs. .385), probably because of theeffect of the differences in foliage densityof the two crops.

The three indices were related in mul

tiple regression to temperature depression(Table I). As indices were successivelyintroduced into the equation, the multiple correlation coefficient increased andthe standard deviation decreased, but thechanges were not large. The introductionof wind caused a greater reduction inresidual variance than did the introduc

tion of temperature. Thus wind assumeda higher level of importance when considered in multiple correlation than whenconsidered in simple correlation. Theequations indicate the relative contribution of each of the indices used andprovide a means of estimating broadly,within the limits of the meteorologicalvariations encountered, the effect on thecrop environment of sprinkler cooling.

SUMMARY

Irrigated plots of potatoes and bushbeans were sprinkled intermittently withwater whenever the ambient air temperature exceeded 80 degrees Fahrenheit(26.7 degrees Celsius). Conventional butlow-volume irrigation equipment wasused to apply the cooling water at a nettheoretical application rate of 0.026inches (0.66 millimeters) per hour. Over a3-year period and 88 applications thissprinkling rate effectively reduced plantcanopy air temperature. The average temperature reduction was 6 Fahrenheit degrees (3.3 Celsius degrees) but reductionsas large as 16 Fahrenheit degrees (8.9Celsius degrees) occurred. The amount ofcooling achieved was dependent upon theweather prevailing during cooling. Humidity exerted the greatest single influence,maximum air temperature was next inimportance, and wind was less well related. When all three meteorological indiceswere considered together, their combinedinfluence was greater than that of anysingle index. The multiple regressionequations indicate, within the range ofweather conditions prevailing during thestudy, the amount of temperature reduction likely to be achieved with field-typesprinkler irrigation equipment.

REFERENCES

1. Cary, J.W. and J.L. Wright. 1971. Response of plant water potential to theirrigated environment of southern Idaho.Agron. J. 63: 691-695.

2. Chesness, J.L. and H.J. Braud. 1970.Sprinkling to reduce heat stressing ofstrawberry plants. Agr. Eng. 51: 140-141.

3. Christiansen, J.E. 1942. Irrigation bysprinkling. Univ. Calif. Bull. 670. 124 pp.

4. Denmead, O.T. and R.H. Shaw. 1962.Availability of soil water to plants asaffected by soil moisture content andmeteorological conditions. Agron. J. 54:385-390.

5. Dolan, M.W.systems. R.M.Oregon. 8 pp.

6. Gates, D.M. 1964. Leaf temperature andtranspiration. Agron. J. 56: 273-277.

7. Gilbert, D.E., J.L. Meyer and J.J. Kissler.1971. Evaporation cooling of vineyards.Trans. Amer. Soc. Agr. Eng. 14: 841-843,859.

8. Hobbs, E.H. 1970. The agricultural climate of the Lethbridge area, 1902-1969.Agrometeorol. Publ. 1. Res. Sta., Can.Dept. Agr., Lethbridge, Alberta. 13 pp.

9. Howell, T.A., E.A. Hiler and C.M.H. vanBavel. 1971. Crop response to mist irrigation. Trans. Amer. Soc. Agr. Eng. 14:906-910.

10. Kramer, P.J. 1963. Water stress and plantgrowth. Agron. J. 55: 31-35.

11. Peterson, L.E. and J.L. Weigle. 1970.Varietal response of potatoes to air conditioning irrigation. Amer. Potato J. 47:94-98.

12. Robinson, F.E. 1970. Modifying an aridmicroclimate with sprinklers. Agr. Eng.51:465.

13. Tanner, C.B. 1963. Plant temperatures.Agron. J. 55: 210-211.

14. Van den Brink, C and R.L. Carolus. 1965.Removal of atmospheric stresses fromplants by overhead sprinkler irrigation.Mich. Agr. Exp. Sta. Quart. Bull. 47:358-363.

15. Wheaton, R.Z. and E.H. Kidder. 1966. Tocontrol heat stress in plants. Agr. Eng. 47:325.

1963. Solid-set

Wade and Co.,sprinklerPortland,

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 1, JUNE 1973