Simulation of potential impacts of land use/cover changes on surface water fluxes in the Chaophraya river basin, Thailand

[1] The Chaophraya river basin in Thailand is one of the most significant land use/cover change (LUCC) regions where clearing of natural vegetation to cropland or secondary forest has occurred over several decades. The LUCC affects surface water fluxes (SWF), including transpiration, interception loss, evaporation, and runoff, because the SWF components are controlled by aerodynamic characteristics, soil moisture content, and potential photosynthetic activities according to vegetation type. Therefore, to understand the potential impacts of LUCC on SWF in the Chaophraya river basin, the simple biosphere model 2 (SiB2) with paddy scheme was run in uncoupled mode with forcing in keeping with the International Satellite Land Surface Climatology Project Initiative I data set. The simulation results revealed the following: (1) The absorbed radiation energy of forest was larger than other vegetation types although the Bowen ratios are similar. (2) The characteristics of SWF for each vegetation type were significantly controlled by soil water content, photosynthetic activity, and leaf area index. (3) The SWF index (W) and the relative SWF index (RW) were suggested as scaling parameters to estimate the seasonality of SWF as follows (definitions are given in section 3.4): Wi = i) and RWi = . (4) The WS were divided into two groups: the low seasonality SWF type (WS ≤ 0.5) consisting of forest or paddy field and the high seasonality SWF type (WS > 0.5) consisting of grassland or crop. (5) Runoff of the Chaophraya river basin decreased because of expanding crop area and paddy fields, an increase in the evaporation during the rainy season caused by changes in various vegetation types to paddy fields, and an increase in transpiration during the dry season caused by an expanding crop area.

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