ABSTRACT
In the upper part of the Central Plain of Thailand which covers about 38,000 km2, due to the government price-subsidized policy, farmers tend to grow rice more often now, resulting in a high demand for irrigation water, with the latter being drafted increasingly from groundwater resources. This has not only put pressure on the regional aquifers but, owing to river-groundwater interactions, also on the surface waters in the region. As part of a major national effort, conjunctive water use patterns are to be explored to optimise the water resources in the upper Central Plain for the various stake-holders involved. In the present study, conjunctive use pattern of surface- as well as groundwater are investigated by field surveys and groundwater flow modelling, using the MODFLOW model. The groundwater model of the upper Central Plain is developed to simulate the groundwater movement over the last 10 years, before an investigation of the groundwater use was conducted. The latter can be categorized into three main types, namely, industrial, domestic and agricultural. Groundwater use patterns were considered for different seasons (wet and dry) and different water availability situations. A pilot area in the study area was selected to investigate the actual water use patterns, farmers’ irrigation behaviour and constraints; i.e. harvest terms, groundwater pumping hours, pumping ability, etc. The conjunctive patterns vary significantly in response to the water availability situation, such that the portion of groundwater covering the total water-demand in years of wet, normal, dry and drought con-ditions is 13%, 17%, 13%, and 19%, respectively. The groundwater ratio in the dry and wet season is 6% and 38%, respectively, of the demand. Finally, the future groundwater demand in the groundwater basin has been predicted and it is found that the conjunctive use pattern is a key factor for the estima-tion of groundwater consumption and for the assistance in the proper conjunctive planning in order to mitigate future water shortages and to sustain the groundwater resources in the area.
