Water use of four street tree species in Bangkok, Thailand

Water use of four selected tree species commonly used on Bangkok streets under well-watered conditions was investigated in two studies during the mid and late periods of the characteristic monsoonal dry season. Field grown trees of Cassia fistula, Pterocarpus indicus, Lagerstroemia loudonii, and Swietenia macrophylla, were harvested as per local nursery practices, potted into 20 L containers with organic media and allowed to establish for three weeks prior to each study. Tree water use was measured gravimetrically during the cooler mid monsoonal dry period in late January, and again during the hotter late monsoonal period in late March, concurrent with local weather variables used to calculate reference evapotranspiration (ETo). Tree water use was normalized to depth units by leaf area determined at the end of each study, then divided by ETo to calculate a water needs index, Kc. Daily ETo increased from an average 3.8 to 5.1 mm day(-1) early to late monsoonal dry period. P. indicus and S. macrophylla water use was relatively constant over both studies, each ranging between 1-1.4 mm day(-1), with an average Kc of 28% of ETo, but were not well correlated to ETo. By contrast, C fistula and L. loudonii had higher but much more erratic water use than the other two species. C fistula had the highest water use rate in the first study, averaging 3 mm day(-1), but did not exceed 2 mm day(-1) in the second study, an inverse relationship to ETo. L. loudonii had intermediate water use, approximately 2 mm day(-1) in the first study, but ranged from 1.6-3 mm day(-1) in the second, averaging 44% of, but was very weakly related to, ETo. This information, particularly for P. indicus and S. macrophylla, can be used for improved street tree irrigation management during Bangkok's dry monsoonal period.