Canopy resistance as affected by soil and meteorological factors in potato

Precision irrigation requires a method of quantifying the crop water status or root zone depletion of water to determine when and how much water to apply, to the soil. Changes in canopy resistance (r(c)) and canopy temperatures have the potential of being used as a crop water status indicator for irrigation management. A study was conducted on potato (Solanum tuberosum L.) grown in northern Egypt at Shibin El-Kom on an alluvial loamy soil for winter (20 Sept. 2001 through 20 Jan. 2002) and spring (1 Feb. 2002 through 20 May 2002) seasons to determine if r(c) derived from energy balance and plant parameters could be used to determine the onset of water stress and the amount of water required to refill the soil profile. Diurnal r(c) was determined for well-watered conditions and achieved minimum values of 20 and 10 s m(-1) at noontime during winter and spring periods, respectively. A power relationship of -0.86 for well-watered conditions was developed between r(c) and net radiation (R-n) at various plant growth stages. In deficit soil water conditions, r(c) increased linearly with decreasing available soil water (ASW), with a change in potato r(c) of 0.75 and 0.39 s m(-1) per percentage ASW for 1 and 2 MJ m(-2) h(-1) of R-n at midgrowth, respectively. A ratio of actual/potential canopy resistance (r(c)/r(p)) was derived to normalize the meteorological differences between growing seasons. This ratio was 2.5 when 50% of ASW was removed and can be used as a parameter to determine the need for irrigations using weather factors and canopy temperature. Canopy resistance increased linearly with increasing soil solution salinity, electrical conductivity, when the soil solution was above the threshold soil salinity value. A ratio of r(c)/r(cp) was found to normalize the effects of different environments across saline and water deficit conditions.