VU GIA THU BON INFORMATION CENTRE (VIETNAM)
Being located in the southern direction of the Bach Ma hill range and in the eastern direction of the West Truong Son mountain range, the climate in the VGTB river basin shows similar characteristics like the South Central Vietnamese climate with relatively warm winters, dry summers affected by dry westerly winds and a strong monsoon impacted rainy season with typhoons lasting from September to December.
Rainfall during the wet season accounts for 65-80 % of the total annual rainfall, with 40-50 % of the annual rainfall in October and November causing regularly severe floods (IMHEN, 2011). The average annual precipitation of four selected stations from 2000-2013 (CHM Quang Nam) can be seen in the follwing figure:
The dry season lasts from January to August and is regularly accompanied by droughts which usually occur from February to April and account for 3-5 % of the total annual rainfall. In May and June there is a second rainfall peak, which is pronounced towards the north-western part of the area.
Rainfall in the basin is unevenly distributed, both in space and time and has a strong orographic component ranging from about 2000 mm in the flood plain to more than 4000 mm in the upper parts of Song Vu Gia and with the highest rainfall measured in Tra My. However, there are only twelve rainfall stations in the river basin from which none is above 400 m of elevation.
The annual average temperature varies between 24.5°C and 25.5°C in the mountains and between 25.5°C to 26.0°C at the coast and tends to increase gradually from North to South. Temperature data is only available for the climate stations in Da Nang and Tra My for the period 1977 to 2010. Maximum temperature occurs in June and July with maximum temperatures of above 40°C on days of dry westerly winds, and minimum temperatures in December and January. The lowest recorded daily temperatures at both stations have been as low as 10°C, but the monthly mean temperatures are above 20°C for all months. According to IMHEN (2010) the following climate values were observed and calculated:
- The relative humidity is around 77% in the dry season (April to September) and can reach up to 93% in wet season (October to March).
- The total annual radiation is in average about 140-150 kcal/cm2. The Annual Radiation balance ranges in average 75-100 kcal/cm2.
- The average annual sunshine hours vary from 1800 hours in the high mountains to more than 2000 hours in the coastal plain.
- The average annual wind speed is 0.7-1.3 m/s in the mountains, whereas it is 1.3-1.6 m/s in the coastal plain. The maximum observed wind speed is 34m/s in summer, and 25 m/s in the rainy season at the Tra My station. In the coastal plain area, the wind speed reaches 40 m/s.
Hydro-meteorological Data sources: Data are collected by the Centers for Hydrology and Meteorology in each Province. In this case, the precipitation and temperature data from 1977-2010 were obtained from IMHEN.
References and further reading:
Alexandersson, H. (1986) A homogeneity test applied to precipitation data, International Journal of Climatology, 6(6): 661-675.
Souvignet, M., Laux, P., Freer, J., Cloke, H., Thinh, D. Q., Thuc, T., Cullmann, J., Nauditt, A., Flügel, W.-A., Kunstmann, H., Ribbe, L. (2013) Recent climatic trends and linkages to river discharge in Central Vietnam, Hydrol. Process, doi: 10.1002/hyp.9693.
Laux, P., Phan, V.T., Lorenz, Ch., Thuc, T., Ribbe, L., Kunstmann, H. (2012) Setting up Regional Climate Simulations for Southeast Asia, High Performance Computing in Science and Engineering, 12, Conference Proceedings, Wolfgang E. Nagel, Dietmar B. Kröner, Michael M. Resch (Eds.), Springer (in press).
Yatagai, A., Kamiguchi, K., Arakawa, O., Hamada, A., Yasutomi, N., Kitoh, A. (2012) APHRODITE: Constructing a Long-term Daily Gridded Precipitation Dataset for Asia Based on a Dense Network of Rain Gauges, Bulletin of the American Meteorological Society, 10.1175/bams-d-11-00122.1.