Study on Integration of Monitoring and Modeling for Analyzing the Effects of Land Use Changes on Water Quality and Stream Temperature in Chichiawan Creek
Date Issued
2010
Date
2010
Author(s)
Lee, Tsung-Yu
Abstract
Formosan Landlocked Salmon (Oncorhynchus masou formosanus), an endangered species that can be only found in Chichiwan Creek in Taiwan, is not only the natural asset for Taiwan but also for the world. The decreasing habitat due to human activities motivates us to understand the temporal and spatial distribution of water quality and stream temperature in this watershed. The investigation and understanding on the mechanism of water quality and stream temperature changes within the watershed will help the managers to efficiently resotre the habitat.
Through the intensive investigation on water samples in temporal and spatial aspects, it was found that the expropriated vegetable farms (inactive vegetation) still contributed lots of NO3-N and PO4-P to the stream. Even if the expropriated farms has stopped growing vegetable for 2~3 years, the annual NO3-N and PO4-P yield from the inactive vegetation were still as high as 356.57 kg-N/ha/yr and 4.21 kg-P/ha/yr, respectively. Although the yields were much lower than those from the current vegetable farms (2698.1 kg-N/ha/yr and 5.38 kg-P /ha/yr), they were still higher than the yields from the current orchard (124.16 kg-N/ha/yr and 1.19 kg-P/ha/yr) indicating the consequences of fertilization in the past. If all the rest vegetable farms were expropriated, the reduction of annual NO3-N and PO4-P export among all the subwatersheds would reach 58.7%?80.9% and 2.6%?15.1%, respectively. Most of the NO3-N was leached out via the light and moderate rainfall events. It is not like PO4-P that often attached to the surface of eroded sediment was flushed out in the very short time span, i.e. typhoon events. According to the results of flow pathway, it was found that when typhoon invaded, surface runoff contributed more than 50% streamflow and then subsurface runoff and groundwater. Except storm events, groundwater discharge was the primary contributor then subsurface runoff. However, we still need more field observations to validate our conclusion on flow pathway.
For study on stream temperature, it was found that the orientation of river and riparian canopy would affect stream temperature. Hence, several planting strategies along Chichiawan Creek have been evaluated. Upstream planting having the best performance on reducing daily maximum stream temperature was strongly recommended. If the riparian canopy in the upstream could create shades on the entire stream surface at any time, more than 1 km suitable habitat would be created for Formosan Landlocked Salmon. To improve the stream temperature model, cloud effect and surface/subsurface runoff input has been incorporated into the model making it capable of continuously simulating stream temperature when rain or shine. The well-validated model can be applied to evaluate the impacts of short-term weather variabilities on stream temperature. The finding, the effect of cloud is more siginificant than surface/subsurface runoff input, support the idea of riparian planting which can diminish solar radiation.
To keep monitoring water quality in Chichiawan Creek is essential to understand the consequent effects of farm land expropriation. The Chichiawan watershed can be an ideal example demonstrating the effects of land use changes on water quality. Stream temperature is one of the most critical environmental factors for Formosan Landlocked Salmon. To keep monitoring and looking for alternatives to reduce stream temperature is the most critical issue, particularly important because of stress from global warming. The results of this research can be a reference for the associated authorities to manage the watersheds to mitigate the impacts on aquatic habitat resulting from climate changes and land use alterations.
Subjects
Non-point Source Pollution
Pollutant Transport
Stream Temperature
Riparian Planting Strategy
Formosan Landlocked Salmon
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