Highway construction often causes substantial adverse environmental effects, both during and after the construction phases. To assess the impact of highway construction on surrounding environment and mitigate its adverse influences, I-99 Environmental Research is being conducted. This study is a component of I-99 Environmental Research and mainly focuses on hydrological modeling of the highway construction watersheds.Recent research in basic hydrological theories and related fields were reviewed. Several Geographic Information System (GIS) techniques in hydrological modeling were discussed. Recent GIS-based hydrological applications are mostly applied to large, natural watersheds. However, the watersheds in this research are very small and their topographic characteristics are severely changed by construction. There are a few difficulties in applying the GIS-based watershed models in the project. Several improvements were made to apply GIS-based watershed model to highway watersheds using Watershed Modeling System (WMS). The WMS model employs Soil Conservation Service Unit Hydrograph (SCS UH) to generate hydrograph and has the shortcoming of predicting earlier peak time and higher peak discharge.To overcome the WMS weakness, a new model - Highway Watershed Model (HWM) was developed. The HWM model uses a new type of unit hydrograph, the Linear Exponential Unit Hydrograph (LEUH) in generating runoff from rainfall. Dimensionless Unit Hydrograph (DUH) in LEUH consists of linear rising part and exponential recession part. HWM has the ability to describe different watersheds using different LEUHs, which reflect the watershed unique hydrologic response characteristic. In both WMS and HWM models, an attempt was also made to find out the relationship between antecedent moisture condition (AMC) and curve number (CN). The diagram fitting of AMC-CN from HWM is better than that from WMS. It is recommended that this issue may be studied further using additional instrumentation to measure the time variation of soil moisture conditions. Although the WMS is widely used, HWM produces more reliable and better results than WMS in the studied watershed. The peak discharge and peak time are difficult to simultaneously model perfectly.
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