Canopy Resistance as Affected by Soil and Meteorological Factors in Potato

E (Rn G) cp{es[T(z)] e(z)}/ra (ra rc)/ra [1] Precision irrigation requires a method of quantifying the crop water status or root zone depletion of water to determine when and how where E is the latent heat of vaporization (J m 2 s 1), much water to apply to the soil. Changes in canopy resistance (rc) the change of saturation vapor pressure with temperaand canopy temperatures have the potential of being used as a crop ture, the psychrometric constant (kPa K 1), Rn the water status indicator for irrigation management. A study was connet radiation (J m 2 s 1), G the soil heat flux (J m 2 ducted on potato (Solanum tuberosum L.) grown in northern Egypt s 1), the density of dry air (kg m 3), cp the specific at Shibin El-Kom on an alluvial loamy soil for winter (20 Sept. 2001 heat of air (J kg 1 K 1), es the saturation vapor pressure through 20 Jan. 2002) and spring (1 Feb. 2002 through 20 May 2002) (kPa), T(z) the air temperature at position z (K), e(z) seasons to determine if rc derived from energy balance and plant the vapor pressure of water in atmosphere at position z, parameters could be used to determine the onset of water stress and ra the aerodynamic resistance (s m 1), and rc the canopy the amount of water required to refill the soil profile. Diurnal rc was resistance (s m 1). determined for well-watered conditions and achieved minimum values of 20 and 10 s m 1 at noontime during winter and spring periods, The dependence of canopy resistance on environmenrespectively. A power relationship of 0.86 for well-watered condital factors offers the potential for feedback control of tions was developed between rc and net radiation (Rn) at various plant irrigation. Irrigation management requires that the intergrowth stages. In deficit soil water conditions, rc increased linearly relationships among plant, soil, and weather factors be with decreasing available soil water (ASW), with a change in potato quantified to determine when to irrigate and how much rc of 0.75 and 0.39 s m 1 per percentage ASW for 1 and 2 MJ m 2 water to apply. Canopy resistance determined from leaf h 1 of Rn at midgrowth, respectively. A ratio of actual/potential canopy or canopy temperatures represents a plant parameter resistance (rc/rcp) was derived to normalize the meteorological differaffected by plant characteristics, e.g., leaf area index ences between growing seasons. This ratio was 2.5 when 50% of ASW (LAI), height, and maturity. Soil factors (ASW content was removed and can be used as a parameter to determine the need and soil solution salinity) and weather factors (Rn and for irrigations using weather factors and canopy temperature. Canopy wind speed) also affect canopy resistance. The companresistance increased linearly with increasing soil solution salinity, elecion term in Eq. [1], ra, can be determined using plant trical conductivity, when the soil solution was above the threshold height and wind speed. These terms represent a combisoil salinity value. A ratio of rc/rcp was found to normalize the effects nation of both plant and weather factors directly affectof different environments across saline and water deficit conditions. ing crop water use. Monteith (1965) showed that transpiration rate physically depends on relative changes of surface temperaE of plant water status provides a basis ture and ra. He concluded that ra depends on Reynolds for more efficient irrigation management. There number of air and can be determined from wind speed, are several different methods of estimating plant water characteristic length of plant surface, and the kinematic status, e.g., leaf water potential, leaf or canopy temperaviscosity of air. For Reynolds numbers between 103 and ture, or rc (Stewart, 1984). Of these methods, one of 3 103, ra ranges between 20 to 2 s m 1. From field the least explored terms has been canopy resistance. studies, he found barley (Hordeum vulgare L.) rc inCanopy resistance represents a bulk resistance to water creased from 30 s m 1 in mid-June to 70 s m 1 at the end of July. During this period, total LAI decreased vapor or mass transfer from the collection of leaves. from 10 to 6. The increase of rc was caused by a decrease Canopy resistance was proposed by Monteith (1965) as in total leaf area, by an increase in the resistance of an expansion of the energy balance equation to more individual leaves due to senescence, or by a combination closely link the biological factors with meteorological of both effects. He also revealed that an increase of conditions. An examination of the Penman–Monteith Sudan grass (Sorghum hordense L.) rc was related to an equation shows the direct relationship between evapoincrease of stomatal resistance in leaves that had transpiration (ET) and canopy resistance as depicted in reached maturity. An increase of stomatal resistance Eq. [1]: for grass at O’Neill, NE, was correlated with decreasing soil moisture content. On 13 August, when soil moisture was 6% of wet weight, the surface resistance was 200 s K.H. Amer, Agric. Eng. Dep., Menoufiya Univ., Menoufiya, Egypt; m 1, but after 18 d without rain, soil moisture decreased and J.L. Hatfield, USDA-ARS Natl. Soil Tilth Lab., 2150 Pammel Drive, Ames, IA 50011. Received 14 Mar. 2003. *Corresponding author ([email protected]). Abbreviations: ASW, available soil water; DOY, day of year; EC, electrical conductivity; ET, evapotranspiration; LAI, leaf area index; Published in Agron. J. 96:978–985 (2004).  American Society of Agronomy ra, aerodynamic resistance; rc, canopy resistance; rcp, potential canopy resistance; Tl – Ta, leaf–air temperature differences. 677 S. Segoe Rd., Madison, WI 53711 USA