No abstract

The increasing application of pesticides in the uplands of northern Thailand has increased the transfer of pesticides to surface water. To assess the risk of pesticide use for stream water quality, we monitored the concentrations of seven pesticides (atrazine, dichlorvos, chlorpyrifos, dimethoate, chlorothalonil, (α-, β-) endosulfan, cypermethrin) frequently used in the Mae Sa watershed (77 km(2)) in water and sediment samples over a period of one and a half years (2007-2008). All investigated pesticides were recorded in the river. Chlorpyrifos was detected most often in water samples (75% at the headwater station), while cypermethrin was most often found in riverbed (86%) and in all suspended sediment samples. The highest concentrations of the pesticides were detected during the rainy season. About 0.002 to 4.1% by mass of the applied pesticides was lost to surface water. The risk assessment was based on the risk characterization ratio (RCR). The RCRs of dichlorvos in water, (α-, β-) endosulfan, and cypermethrin in water and sediments were higher than unity indicating that they are likely to pose a threat to aquatic ecosystem. Finally, we discuss the role of sampling design on ecotoxicological risk assessment. Our study shows that pesticide contamination of surface waters is an environmental issue in the Mae Sa watershed and that measures need to be undertaken to reduce the loss of pesticides from soil to surface waters.

No abstract

Highland agriculture is intensifying rapidly in South‐East Asia, leading to alarmingly high applications of agrochemicals. Understanding the fate of these contaminants requires carefully planned monitoring programmes and, in most cases, accurate simulation of hydrological pathways into and through water bodies. We simulate run‐off in a steep mountainous catchment in tropical South‐East Asia. To overcome calibration difficulties related to the mountainous topography, we introduce a new calibration method, named A Nash–Sutcliffe Efficiency Likelihood Match (ANSELM), that allows the assignment of optimal parameters to different hydrological response units in simulations of stream discharge with the Soil and Water Assessment Tool (SWAT) hydrological model. ANSELM performed better than the Parasol calibration tool built into SWAT in terms of model efficiency and computation time. In our simulation, the most sensitive model parameters were those related to base flow generation, surface run‐off generation, flow routing and soil moisture change. The coupling of SWAT with ANSELM yielded reasonable simulations of both wet‐season and dry‐season storm hydrographs. Nash–Sutcliffe model efficiencies for daily stream flow during two validation years were 0.77 and 0.87. These values are in the upper range or even higher than those reported for other SWAT model applications in temperate or tropical regions. The different flow components were realistically simulated by SWAT, and showed a similar behaviour in all the study years, despite inter‐annual climatic differences. The realistic partitioning of total stream flow into its contributing components will be an important factor for using this hydrological model to simulate solute transport in the future. Copyright © 2014 John Wiley & Sons, Ltd.

No abstract