Although floods and droughts are a part of life in Monsoon Asia, there is a growing public perception that they are becoming more severe (i.e. increasing in magnitude and frequency) due to a multitude of factors that include climate change, deforestation, population growth and urbanization. An understanding of factors that modify streamflow, either natural or anthropogenic, is vital to efficient water resources management and reducing vulnerability to both floods and droughts in Thailand. The project featured here examines the rainfall-runoff patterns of catchments in Northern Thailand to provide a better understanding of hydrological processes and their interactions with natural climatic variability and anthropogenic influences over a range of different timescales. It is part of a collaborative initiative between researchers from the UK and Thai government officials from the Irrigation and Forestry departments.
Keywords: rainfall, runoff, monsoon, floods, drought, Northern Thailand
Are floods and droughts increasing/becoming worse?
Rainfall and runoff data for the past 100 years were analysed for the Ping River that flows through Chiang Mai city and into the Chao Phraya River, the most important river system in Thailand. The results showed the following:
- The perception that floods are increasing in magnitude is not true. There is no trend in annual peak flow for the Ping River for the past 100 years despite landuse change.
- Annual peak flow is driven predominantly by climatic variables such as anomalies in monsoon rainfall and tropical storm activity. The relationship between annual peak flow and El Ninõ Southern Oscillation events is unclear.
- Flooding in Chiang Mai city tends to occur late in September when heavy rainfall (due to cyclones or typhoons) coincides with wet catchment soils due to monsoon rainfall from earlier months. This pattern occurs for both small (less than 10 km2) and large catchments (10,000 km2).
- Cyclones and typhoons are the main triggers for floods especially if they occur later in the monsoon season (mid-September onwards).
- Low flows in the Ping River have decreased significantly in the past 100 years. This is attributed to landuse changes from forest to agriculture and urban landcovers.
- Anthropogenic impacts (changes in landcover) have a greater impact on low flows than peak flows. Modelling experiments show that rainfall seasonality plays an important role in determining the appropriate choice of calibration periods for hydrological modelling.
How does the monsoon season affect the hydrological behavior of catchments in Northern Thailand?
The monsoon season in Northern Thailand is caused by winds coming over the Bay of Bengal in the Southwest direction. The season lasts from May to late October/November. The occurrence of heavy rainfall after a distinct dry period results in very changes in catchments that result in very dynamic hydrological behavior.
The results of our study showed the following:
- Changes in catchment wetness due to changing rainfall over the monsoon season affects the hydrological behavour of catchments with very similar physical characteristics (e.g. vegetation, geology, topography).
- Drier and wetter conditions experienced by the catchments over the course of the monsoon season affect the performance of hydrological models. Parameter values and model performance change accordingly with changes in rainfall and catchment conditions during the monsoon. This has implications for flood modeling in this area.
Lim, H.S., Vongtanaboon, S., Richards, K. & Koichiro, K. (2009) Investigating the rainfall-runoff response of a small forested mountain catchment in the seasonally humid tropics using a modelling approach, in D. Marks (ed.) Hydrology in Mountain Regions: Observations, Processes and Dynamics, IAHS Publication 326, 157-166.
Lim, H.S. & Boochabun, K. (2012) Flood generation during the Southwest Monsoon season in North Thailand, in Terry, J.P. & Goff, J. (eds.) Natural Hazards in the Asia-Pacific Region: Recent Advances and Emerging Concepts, Geological Society London, Special Publications, 361, 7-20.
Ziegler, AD, HS Lim, C. Tantasarin, N Jachowski, RJ Wasson. Floods, False Hope, and the Future (2012). Hydrological Processes, DOI: 10.1002/hyp.9260.
Lim, H.S., Boochabun, K. & Ziegler, A.D. Modifiers and amplifiers of anomalous flows on the Ping River in Northern Thailand (1921-2009): implications for flood and drought management. (Submitted for review)
Lim, H.S., Richards, K.S., Ziegler, A.D. & Koichiro, K. Comparing the hydrological behaviour of a pair of adjacent forested catchments in Northern Thailand (submitted for review)