Studies on the characteristic features of water consumption in various crops. IV

Abstract

In order to make clear the characteristics of water consumption of taro plants under upland field conditions, the authors measured both leaf transpiration and evaporation from the soil surface using the chamber method (the vopor-trnsfer method).The measurements were carried out at Taketoyo, Aichi in summer of 1963, 1964 and 1966.1. The amounts of transpiration of taro plants during the growth periods (from late May to early October) were 370mm and 408mm in 1963 and 1964, respectively. For the sameperiod in 1966, the evapotranspiration amount of these plants was 582mm.2. Transpiration rate of taro plants in the early growth stage was little owing to small leaf area, on the other hand the rate of evaporation from the soil surface was much on account of slight covering the soil surface with the foliage of taro plants.The rate of transpiration increased gradually with the increase of leaf area and the rise of air temperature. The maximum transpiration observed was 6.8mm/day on September 10, 1964.About 60% of total amounts of transpiration was transpired for 50 days from July 24 to September 10, 1964. The water requirements were relatively larger (500-800g/g) in the early and late growth stages than the middle growth stage (200-300g/g).3. The relative transpiration increased gradually with the progress of growth, nemely, the increase of leaf area index. This relationship is approximately expressed with the rectangular hyperbolic equation:RT=bL/(1+aL)where RT=relative transpiration, i.e. the value of the rate of transpiration (mm/day) divided by the pan-evaporation (mm/day); L=leaf area index; a and b=constants. The value of inflection point of the hyperbolic curve was about 2.5 of leaf area index, which corresponded to the stage that the soil surface was covered almost thoroughly with the foliage of taro plants.4. Unit relative transpiration, namely, the rate of transpiration (g dm-2 12 hrs-1) divided by the pan-evaporation (mm 12 hrs-1) or daylight hours mean saturation deficit (mm Hg), decreased exponentially with increasing leaf area index. It was approximetely applicable to the above relation that the exponential eqution was given byTp=Tp0e-KLwhere Tp=transpiration power, i.e. unit relative transpiration; Tp0=hypothetical transpiration power when leaf area index was zero; e=base of natural logarithm; L=leaf area index; K=constant.5. The rate of evaporation from the soil surface varied by the covering degrees of the soil surface with the foliage of taro plants.The ratio of the evaporation from the siol surface to the evapotranspiration was about 30% in the stage of largest leaf area index (4.0).6. The largest evapotranspiration rate of taro plants was 9.8mm/day in summer. The daily mean evapotranspiration rate of taro plants was within the range of 5-7mm/day in summer, at Taketoyo, which was estimated by relative evpotranspiration was 1.12-1.14.7. As regards correlations among transpiration, evapotranspiration and various environmental factors, in general transpiration had a positive correlation with leaf area and evapotranspiration had positive correlations with pan-evaporation or solar radiation.