Air Temperature and Vapor Pressure Changes Caused by Sprinkler Irrigation¹

Abstract

The downwind effect of evaporation from sprinkler spray was studied in the field to determine if air temperature and vapor pressure were changed enough to influence plant growth and water use. Wet-bulb and dry-bulb temperature profiles were measured upwind and at three distances downwind from a sprinkler lateral before and during sprinkling. Wind-speed and direction were also measured. Air temperature generally was reduced less than 1 C, and vapor pressure in the air was increased less than 0.8 mb. This amount of change in the air temperature and humidity is not likely to be sufficient to cause any significant change in plant growth or evaporative loss of water. Additional index words:Evaporation loss, Evaporative cooling, Spray evaporation. S PRINKLER irrigation exposes the applied water to the atmosphere in a manner which enhances evaporation. The evaporation process cools the droplets, enabling heat to be drawn from the air through which the droplets pass, and adds water vapor to the atmosphere. The increase in atmospheric vapor pressure and decrease in air temperature caused by sprinkler irrigation are of interest in crop production because significant changes in these microclimatic variables could either benefit or retard plant growth depending on existing conditions and plant requirements. Cool-season plants growing under warm weather conditions might experience improved growth, while a warm-season crop might be retarded.In this paper, changes in vapor pressure and air temperature just above the crop surface are compared under field conditions with and without the operating sprinklers. Changes in these two parameters are directly dependent on the amount of water evaporated, a quantity often termed "evaporation loss." Generally, evaporation loss includes the evaporation from the spray plus that from the wetted foliage. Wettedfoliage evaporation under sprinkles' irrigation has been studied with full-cover crops of alfalfa (Medieago saliva L.), oats (Avena sativa L.), sudangrass [Sorghum sudanense (Piper) Stapf I , and ryegrass (Lolium multiflorum Lam.) under arid conditions with the findings that evapotranspiration from the wetted foliage was approximately equal to that from nonwetted, actively growing foliage with adequate soil moisture (1, 6, 10). Therefore, we concluded that the influence of sprinklers on air temperature and vapor pressure over a well-watered crop is mainly from spray evaporation. THEORYEvaporation of spray is difficult to measure because errors are additive in catch methods and salt concentration or electrical conductivity methods and exaggerate the estimate of evaporation. In catch methods the procedure is to measure the volume of water reaching the soil surface, subtract it from the volume applied, and call the difference "evaporation loss." However, the difference not only includes the evaporation loss from the spray, but also that from the catch device (catch cans or plastic covered area) and the mist carried beyond the measurement site, and other possible incomplete catch e...

Evaporation Research: Review and Interpretation

“…For droplet diameters of 0.3 to 1.5 mm, Kincaid and Longely (1989) validated the sprinkler evaporation model pre sented in their paper against measurements using the micro needle syringe method presented in Kincaid (1989). Comparisons as related to droplet size and wind speed for simulated impact sprin klers operated at 414 kPa with 4.76 mm nozzles (Thompson 1993b Mclean et al (1994) as follows: Kohl and Wright (1974) and Dadiao and Wallender (1985) showed that sprinkler droplet size was propor tional to nozzle diameter. Hills and Gu (1989), Dadiao and Wallender (1985), and Edling (1985) found that the droplet size at any distance from the sprinkler is partially a function of the nozzle size.…”

Sprinkler irrigation scheduling of winter wheat in the North China Plain using a 20 cm standard pan

“…The reason for these results may be that the pan at meteorological station is exposed to higher wind speed because it is placed 70 cm above ground surface. Other reasons may be the lower air temperature and vapor pressure deficit in the sprinkler irrigated fields as compared with those at meteorological stations due to the cooling evaporation of sprinkler water and transpiration of leaves (Kohl and Wright 1974;Kang et al 2002).…”

“…The reason for these results may be due to the fact that pan evaporation measured above top canopy used in methods A and B could represent the real field potential evaporation. The ET 0 computed by standard meteorological data used in method C may be not appropriately represent the field microclimatic condition in the sprinkler irrigated field because the temperature and humidity were significantly affected during sprinkler irrigation day and the following few days and in turn influence the actual ET (Kohl and Wright 1974;Kang et al 2002).…”

Sprinkler Irrigation Changes Maize Canopy Microclimate and Crop Water Status, Transpiration, and Temperature

“…In addition, Steiner et al (1983b) in one of the two studied years, and Liu and Kang (2006) reported that these microclimatic changes lasted for several days after the irrigation event. The microclimatic changes can also affect areas downwind from the irrigated area (Kraus, 1966; Kohl and Wright, 1974; Longley et al, 1983).…”