A Detailed GIS Modeling Study for Two Different Environments: Simulating Runoff and Sediment Yields for a Variety of Smaller Experimental Watersheds Near Tombstone (USA) to Plan Measurements Near Nanning (China)
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Accurately predicting runoff and sediment yields measured at the outlet of a large watershed can be challenging. The assessments, however, have the potential to be helpful for local land management, policy implementations and rehabilitation. Process-based models represent physical processes and have shown their advantage of dealing with the hydrological responses at a smaller spatial and temporal scale. The WEPP model is a well-developed process-based model founded on the fundamentals of hydrology, erosion mechanics, plant growth, and open channel hydraulics. GeoWEPP is a GIS interface that connect WEPP with GIS technology, which overcomes limitations of WEPP in multiple aspects, including data processing and spatial visualization. The Walnut Gulch Experimental Watershed (WGEW) is a 150km2-watershed located in Tombstone in Southern Arizona. Continuous long-term core data collection network is available in this area, including precipitation, runoff and sediment yields. The study area concentrates on six nested subwatersheds (watershed 101 to 106) at Lucky Hills, a small subwatershed within the WGEW that has very detailed observations. The primary objective of this study is to simulate 55 years of runoff and sediment yields for these six subwatersheds using GeoWEPP. In contrast, the Nala watershed is a 3km2-watershed located in Nanning, China, which is a recently established watershed and detailed datasets are not yet available. Therefore, the second objective is to use GeoWEPP with estimated parameter settings to assess the expected range of likely observations to assist in designing the location and dimensions of measurement equipment for certain locations within the newly established watershed. Input parameters are adjusted based on watershed 101 for runoff (research-grade 1m-Digital Elevation Model (DEM), but with no sediment yield measurements) and watershed 102 for sediment yields (publicly available DEM, but with sediment yield measurements) in order to obtain accurate simulation results, and validated based on other watersheds. The simulation performance for the runoff generated by input data in three resolutions shows that the input data in 5m-resolution leads to a more accurate result comparing to the input data in 1m or 3m-resolution. This might be caused by the over-sensitivity to micro-topographic variations of input DEM in higher resolutions, which is not taken into consideration in the model. 22-year short-term analysis (for all watersheds) and 53-year long-term analysis (for five watersheds) of the simulation performance for the runoff in the study area was performed and compared. The results indicate that uniform input parameters generated based on one subwatershed can be used and accurately predict the runoff in other watersheds in the study area. However, the results have a tendency to over-estimate runoff, and become more significant with the distance from the watershed that was used for parameter estimation. This result points towards challenges of accurately representing channel parameters in the modeling (e.g. potentially accounting for transmission losses of infiltrating water in channels). For sediment yields, the results indicate that the short-term soil properties parameterized for one watershed may lead to significantly different results for neighboring watersheds. This experience for the detailed analysis of the small, nested watersheds in US enabled us to use the GeoWEPP and recommend design requirements for a planned research watershed near Nanning, China.