Hydrological impacts of forest conversion to agriculture in a large river basin in northeast Thailand

Small‐scale experiments have demonstrated that forest clearance leads to an increase in water yield, but it is unclear if this result holds for larger river basins (>1000 km2). No widespread changes in rainfall totals and patterns were found in the 12 100 km2 Nam Pong catchment in northeast Thailand between 1957 and 1995, despite a reduction in the area classified as forest from 80% to 27% in the last three decades. Neither were any detectable changes found in any other water balance terms nor in the dynamics of the recession at the end of the rainy season. When a hydrological model calibrated against data from the period before the deforestation was applied for the last years of the study period (1987–1995), runoff generation was however underestimated by approximately 15%, indicating increased runoff generation after the deforestation. However, this was mainly due to the hydrological response during one single year in the first period, when the Q/P ratio was very low. When excluding this year, neither analysis based on the hydrological model could reveal any significant change of the water balance due to the deforestation. More detailed land‐use analysis revealed that shade trees were left on agricultural plots as well as a number of abandoned areas where secondary growth can be expected, which is believed to account for the results.