Assessment of groundwater dynamics in Quaternary aquifers of the Phrae Basin,northern Thailand,using isotope techniques

Hydrogeology Journal - The Quaternary sequences in the Phrae Basin, northern Thailand, can be classified into three aquifers based on depth. The groundwater dynamics and recharge of these aquifers...     

Variations of characteristics of consecutive rainfall days over northern Thailand

The Chao Phraya basin, Thailand, is frequently inundated by flooding during the southwest monsoon period. Most floods coincide with consecutive rainfall days. This study investigated consecutive rainfall days during the southwest monsoon period at 11 stations over northern Thailand, the upstream area of this basin. The Markov chain probability model was used to study the consecutiveness of days with at least 0.1, 10.1, and 35.1 mm of rainfall. The consecutive length of rainfall days from the model showed good agreement with the observed value. A chi-square test of independence was applied to assess the significance of the consecutiveness, and it was found that days with at least 10.1 mm of rainfall tend to be consecutive over the entire area. Moreover, days with at least 35.1 mm of rainfall were found to be consecutive over the joint area where the mountainous region meets the plain area. However, the consecutiveness of days with less than 10.1 mm of rainfall was not obvious. The rainfall amount on days with at least 10.1 mm of rainfall was also calculated and it showed lower values over the mountainous region than over the plain. Hence, this study established the characteristics of consecutive rainfall days over the plain, mountainous region, and joint area.     

Response of the flood peak to the spatial distribution of rainfall in the Yom River basin,Thailand

The response of the flood peak to the spatial distribution of rainfall has been reported in basins with nonuniform characteristics. However, prioritization of the influences of these characteristics is still poorly understood. This study evaluated the variability in the flood peak with the spatial distribution of rainfall at Sukhothai (city) in the Yom River basin, Thailand, and investigated the influence of the basin characteristics on the flood peak. For each of the 2-, 5- and 10-y rainfalls with durations of 24, 48 and 72 h, 1000 simulated rainfall events with various spatial distributions were generated according to the observed data by using a Monte Carlo analysis and Cholesky randomization. The floods from these rainfalls were then simulated, and the peak discharges were evaluated. The flood peaks from 24-h rainfalls were usually small but highly variable and could be extremely large when the rainfalls were concentrated over the mountainous region. The flood peaks from 48 to 72-h rainfalls were consistently large and correlated with the rainfalls over the joint area between the mountainous region and plain area. The basin characteristics that influenced the response of the flood peak to the spatial distribution of the rainfall appeared to depend on the rainfall duration and magnitude. For short-duration rainfalls, the response was mainly influenced by the surface storage when the rainfall was small and by the terrain steepness when the rainfall was large. For long-duration rainfalls, the response was mainly influenced by the soil percolation rate.     

Determination of rainfall data for direct runoff prediction in monsoon region: a case study in the Upper Yom basin,Thailand

Natural Hazards - In a monsoon region, flooding is usually caused by heavy rainfall events in conjunction with saturated soil. A hydrograph in the wet season consists of multiple flood waves from...     

Impacts of land-use changes on watershed discharge and water quality in a large intensive agricultural area in Thailand

The objectives of this research were to evaluate the effect of land-use change on streamflow, sediment and water quality data along the Lower Yom River, Thailand, covering an intensively agricultural area of 14 613.6 km², and to assess the relative impact of point and non-point sources of pollution from multiple-land-use watersheds. Long-term calibration and validation of the SWAT (Soil and Water Assessment Tool) model was performed on data for 2000–2013. Land-use change led to a 13–49% increase in runoff in the basin and resulted in 37–427% increased sediment yield. The amount of NO₃-N load doubled in the upper and middle parts of the study area, while the increase in PO₄^3– ranged from 37 to 377%, reflecting the increase in agricultural lands and urban areas. It is concluded that the changed land use is closely associated with the quantity of runoff, sediment yield and the NO₃-N and PO₄^3– concentrations.     

Flood mitigation due to extreme rainfall events in the inner Bangkok,Thailand

In the past decades, the influence of climate change has caused changes in the amount of rainfall in many areas which may affect the flood assessment and mitigation. This research aims to determine amount of rainfall which impacts on changes of the water levels in canals and evaluate the appropriate mitigation measures for floods in the inner Bangkok area, Bangkok Noi and Bangkok Yai districts of Bangkok. The maximum 1-day rainfall during 1997–2010 was determined under different return periods of 2, 5, 10, 25, 50 and 100 year. The MIKE 11 model was then applied to assess changes of the water levels in canals caused by design rainfall events for those return periods. The flood mitigation was also proposed by applying various pumping capacities and initial water levels, incorporating with building dykes and a floodgate. This study has found that the highest flood-risk areas are along Chak Phra and Bangkhunnon canals and the eastern part of Jakthong Canal while the lowest flood-risk area is Bangkok Yai district. Flood caused from the 10-year rainfall can be mitigated by building dykes with the height of 0.75 m [mean sea level (MSL)] and maintaining the initial water level of 0.70 m (MSL). Furthermore, it has also been found that flood caused from the 25-year rainfall can be mitigated by building the floodgate to prevent the flowing back water at Wat Yangsuttharam Canal. However, 50- and 100-year rainfalls seem to cause floods which are too large to mitigate.