Evapotranspiration of pineapple field in okinawa

Abstract

Field experiments have been conducted in order to estimate the evapotranspiration from the pineapple field in Okinawa for more than two years covering from planting through harvest time of the crop. Water balance method has been applied to 10 plants of the Smooth Cayenne variety planted in a lysimeter with a standard planting density, 4.000 plants per 10 ares, in Okinawa. Two experiments were carried out separately; one in a commercial pineapple field and the other in an expermental field on our campus where buildings stand in the vicinity of the patch of grass surrounding the lysimeter. Irrigation interval was once in about every 10 days, and 50 mm of water was supplied at one irrigation time. However it can not be denied that more than 50 mm of water was often supplied in one irrigation interval due to additional supply by rainfall. Results obtained are as follows: 1. Evapotranspiration for the first 12 months after planting was 721 mm in the commercial pineapple field and 684 mm in the experimental field on our campus. These values are 59% and 62%, respectively, of the value in the first 24 months after planting. Maximum value of 4.4 mm/day in 10-day average was obtained right after planting. 2. It was observed that evapotranspiration in vegetative growth stage decreases as leaf area index of the crop increases. This relationship can be expressed by the following empirical formula: [formula ommited] where, Etis evapotranspiration, I is leaf area index, and A and α are empirical constants. Values for A and α obtained in the experiment on the campus are 3.56 and 0.286, respectively, where measured values of evapotranspiration have been adjusted to those when air temperature is 25°C; and short-wave radiation on horizontal surface 350 ly/day. The adjustment was done using a linear regression between evapotranspiration and air temperature and that between evapotranspiration and short-wave radiation on horizontal surface all in middle and late growing stages of the crop. 3. Partial correlation coefficient between evapotranspiration and air temperature and that between evapotranspiration and short-wave radiation on horizontal surface were calculated using three correlation coefficients, which were obtained from each two of evapotranspiration, air temperature, and short-wave radiation on horizontal surface all in monthly average in middle and late growing stages of the crop. Partial correlation coefficient between evapotranspiration and air temperature was +0.80, showing from a statistical test that fairly high correlation may exist between the two parameters. Partial correlation coefficient between evapotranspiration and short-wave radiation on horizontal surface was +0.55, showing that high correlation may not exist between the two parameters in the pineapple field where the crop is in middle and late growing Stages. 4. There is not much difference in evapotranspiration between one of the data obtained by P. C. Ekern at Wahiawa, Hawaii and that by the author in Okinawa, when the maximum and the minimum in monthly average and total values in the first 12 months after planting were compared. Evapotranspiration in winter months in Okinawa has been measured less than that in one of Ekern's experiments. This may be mainly due to the difference in climate between the two places. © 1974, The Society of Agricultural Meteorology of Japan. All rights reserved.