Studies on the Growth Phase and Effect of Fertilizer Application to Rice Plants in Malaya : III. Seasonal variations of evapo-transpiration and evapo-transpiration ratio of indica rice plants with special reference to leaf area

Abstract

The seasonal changes in irrigation requirement were measured 1967 to 1969 in the Northwest coast of West Malaysia using three varieties, Ria (IR-8), Bahagia (sister line of IR-5) and Radin Ebos 33 (local long term variety) under flooded field condition. The evaporation ratio (E/Em, E: evaporation from water surface in paddy field, Em: pan evaporation in meteorological observation field), transpiration ratio (T/Em, T: transpiration from plant surface) and evapo-transpiration ratio (ET/Em) were mainly used for discussion in order to eliminate the weather effects. The results obtained are summarized as follows: 1. The E/Em gradually decreased due to the mutual shading of paddy plants according to the increase of leaf area index (LAI) and dry matter of plants (table 1, fig.1-3). On the contrary against for the above the T/Em and ET/Em were smaller during the early stage of growth and later on increased gradually. Two peaks were noticed the maximum tillering stage as a rule which fell roughly on 45 days after transplanting and heading stage (fig.1-2). The fornler peak was lower than the later, followed by a depression between two peaks. 2. The changes in the T/Em were largely due to the size of growth particularly LAI. However this was not always parallel with LAI and the exponential function was obtained between the T/Em per unit leaf area and LAI (fig.3). As such the T/Em per unit leaf area was constant and scarcely affected by growth when LAI became larger than 3.5. The fact and a depression of transpiration mentioned above might be largely due to the mutual shading of paddy plants. 3. The mean value of T/Em was about 0.6 and was almost equal to that of E/Em (table 1). The ET/Em showed the mean value of l.2 though it varied 1.0-1.2 during the early stage, 1.2∼1.3 during the maximum tillering stage and 1.4 during heading stage (fig.1∼2). Daily mean value of ET were 6.4 mm in the Off season (dry period) and 5.9 mm in the Main esason (wet period). It was also found that the longer in growing period, the greater in total ET. 4. The seasonal changes of ET was the dominant factor which caused the changes in irrigation requirement. Under the continuous flooded condition, the irrigation requirement including rainfall for Bahagia 130-140 days variety, was estimated approximately to be l, 340 mm (9.5 mm/day) during the 0ff season and 980 mm (7.4 mm/day) during the Main season (table 2). The 38% difference in the irrigation requirement could be attributed to the greater Em and lO days increase of growing period of the former.