The Kok River basin situated in the Golden Triangle delta in northern Thailand is becoming to serve as an important regional trade hub for the Yunan Province in China; Myanmar, Laos P.D.R and Thailand, as laid out in the quadrangle economic cooperation policy. As a consequence of the latter, the Kok river basin is expected to experience major economic growth in the near future which, despite its positive social impacts, should exert some environmental stress on the natural resources of the region, namely, water resources. Imminent climate change across the region as a whole may further exacerbate these adverse effects.
Although groundwater is still at present time abundant in the Kok River basin and is supposed to support the water needed to sustain the envisioned future economic growth, no thorough investigation with regard to the quantity (yield) and the future sustainability of the groundwater resources in the basin exists up-to-date. Here we investigate this issue by means of a 3D numerical groundwater flow model (MODFLOW) for the Kok river basin composed of 4 aquifer basins, namely, Upper Chiang Rai, Wiang Papao, Mae Saruay and Mae Sai aquifers, whereby the focus of the study is on the estimation of the future sustainable groundwater yield under external stresses on this aquifer due to groundwater pumping.
Based on first observational results of a recent exploratory hydrogeological investigation of the Thai Department of Groundwater Resources (DGR), the aquifer system is modeled with a top unconfined aquifer layer and three underlying confined layers, each of them separated by an aquitard. The groundwater flow model is calibrated in steadystate and transient mode using observed piezometric heads for the various aquifer layers for the year 2009. The sustainable yield of the 4 aquifers system is then computed based on a definition of the DGR, whereby sustainable yield is “the maximum total pumping rate that ensures that the average
piezometric head in each layer does not fall below a vertical distance of 20 meter from the land
surface in the next 20 years”.
Employing these constraints for the future head variation into the model, the transient MODFLOW computations result in a total sustainable yield of 1.3, 12.36, 2.71 and 28.41 m3/Rai (1600 m2)/day for the Upper Chiang Rai, Wiang Papao, Mae Saruay and the Mae Sai aquifer, respectively. These yield differences reflect the different hydraulic conditions and the hydrogeology of the 4 aquifers. Thus, the Mae Sai aquifer is the most productive one, with a hydraulic conductivity going up to 190 m/day and a total thickness of 200 meters which results in rather high values for the transmissivity. Although the Upper Chiang Rai aquifer has horizontal hydraulic conductivities of up to 494 m/day, it is the least productive one, as its total thickness is just 44 meters. Using a zone budget module within the groundwater model, the sustainable yields have then been calculated also subdistrict- wise.
Although still preliminary, the present modeling study should give policy makers a first tool at hand for future sustainable groundwater resources management in the Kok river basin. The next step of the study will be the incorporation of the effects of regional climate change on the sustainable yield estimation, as changing future seasonal rainfall pattern and temperatures will most likely affect groundwater recharge to the regional aquifers in an adverse way, putting extra stress on the groundwater system.