Climate Model Output Downscaling 

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One of the seven climate models downscaled for this study predicted a decrease in precipitation for the Occoquan watershed through the year 2100. The other six climate models projected increased precipitation, suggesting flow volume increases are likely for the modeled runoff. The low precipitation climate model output was combined with two others to create the ensemble used for analysis. Three scenarios of future change were modeled to represent the effects of climate change (S1), land use/demand change (S2), and joint climate and land use/demand change (S3).
In meeting the research objectives described in Chapter 1, care was taken to ensure both data and methods used were drawn from established entities and techniques. Local input was gathered from local entities where possible and global data were gathered from entities with inter-governmental support and recorded histories of work, which helped increase certainty when using statistical methods to relate data at different scales. Using these data the objectives were successfully attained in the following ways:

  • Objective 1: As shown in Chapter 2.2, very good agreement was attained for the calibrated hydrologic model using established correlation techniques. This agreement ensures that the calibrated model is a good representation of the physical watershed, and provided meaningful output for analysis.
  • Objective 2: Chapters 1 and 2.3, along with Appendix B, describe how the downscaling of GCM output relied heavily on observed local and regional data, allowing this study to produce useful data for water supply managers. The downscaling process produced a weather forecast similar to the historic time series but influenced by the GCM used, allowing for a direct comparison for changes between the historic and future hydrologic outputs.
  • Objective 3: The land use model, as shown in Chapter 2.4, showed a strong agreement when validated using an independently collected land use data set. This validated model was then adjusted to correspond to the land use profile used for calibrating the hydrologic model creating a continuous time series of land use change suitable for direct input into the hydrologic model.
  • Objective 4: The information provided in this chapter and Chapter 2.6 demonstrate the effectiveness of the forecast streamflows generated using the methods in this study, by showing the clear distinctions between the future impacts of climate change, of land use/demand change, and the joint effects of both climate and land use/demand change. The ensemble created by averaging the highest precipitation GCM, the lowest precipitation GCM, and the GCM that best matched the historic streamflow, from the remaining, for the years 1980-2000, can be defined as the forecasted output of greatest likelihood, and provides the analysis of highest certainty.
  • Objective 5: The low flow analysis techniques, as shown in Chapters 2.5 and 2.6, provided a broad assessment of low flow variations for both runoff and reservoir storage, thereby transferring the modeling results into readily available and useful data. Using multiple techniques for both runoff and reservoir storage provided increased certainty in the analysis results.

Conclusions and Discussion

Figures 3.1-1 and 3.1-2 show increases in annual flow rate into the Occoquan reservoir between the ensemble historic hydrograph and scenarios S1 and S2 respectively. The increase in the hydrograph from climate change is shown in S1, and supports the projections made by the Intergovernmental Panel on Climate Change (IPCC) that stipulate change will occur with more variation and intensity (higher peaks and lower minimum flows) rather than a direct increase or decrease in overall precipitation (IPCC 2007). The increase from land use/demand change is shown in the S2 hydrographs that demonstrate a large step increase from the historic to 2040, with a smaller increase between 2040 and 2070, and practically no increase between 2070 and 2100. These differences can be seen quantitatively by the average annual flow rate, averaged over the century, listed in Table 3.1-1. The lack of increase in the latter part of the century is a consequence of the saturation of urban area within the watershed.
The preceding figures highlight the likelihood of increased runoff in the Occoquan watershed from both climate and land use change. While climate change is expected to alter the intensity and timing of runoff, these impacts will be relatively minor compared to the increased runoff from land use change for the greater part of this century (through year 2070). Towards the end of this century, as urbanized area reaches its maximum, the incremental impacts of climate change become larger than the incremental impacts from land use change. As stated in the IPCC AR4 (IPCC 2007) the most intense effects of climate change will be realized in the latter part of this century. This timing sets up the possibility for the Occoquan watershed to reach maximum development capacity at the same time as changes to infrastructure may be required to deal with increasingly severe impacts brought about by climate change.
The current management practices for the Occoquan watershed include the use of reclaimed water supply, multiple source water supplies, and regional inter-agency drought coordination. The modeling in this study shows that with these practices in place the Occoquan reservoir will support the most extreme demands projected to be placed on it by both climate and land use/demand change.

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Future Research

This study focused on low flows (drought), the water supply of primary concern to water managers, and used techniques to incorporate future projections of climate variability and land use change, for a full assessment of impacts to the watershed. While ensuring supply during times of drought is the first part of a thorough watershed management plan, additional topics should be considered for future research in order to maintain a clean and reliable source of drinking water. These watershed management topics include but are not limited to:

  • The water quality impacts of changes in rainfall intensity from climate change along with increased urban area runoff. The increase in urban area will change the composition of nutrient loading in the runoff from the watershed. Also, increases in total runoff are likely to increase the total nutrient load from both natural and urbanized land use areas.
  • Reduction in groundwater supply from changes in rainfall patterns and expanded urbanization and demand. As impervious area increases with expanded urbanization more water is swept from the surface as opposed to percolating into groundwater aquifers. This process can be compounded by changes in the timing and intensity of rainfall patterns.
  • The impacts from peak flow and flood variations like changes in sediment transport, along with stream bed erosion and damage to bridge and dam foundations. Increased peak flow is likely correlated to increases stream bed erosion. This erosion will increase the amount of sediment and debris transported through the watershed. Increased sediment and debris is likely to intensify erosion to civil structures, along with possibly increasing the siltation of reservoirs, within the waterway.
  • The use of different statistical techniques for downscaling and data treatment to better define the uncertainty of analysis. The use of more climate model outputs may increase the statistical certainty of the final simulations. Also, changes to the statistical downscaling techniques can increase the GCM imparted variations in the projected local weather signal. Increase in the use of downscaling methods, and using multiple downscaling techniques, can develop consensus results amongst independent studies that convey greater certainty of projected outcomes.m

1 Introduction and Overview 
1.1 Research Objectives
2 Low Flow Variations in Source Water Supply for the Occoquan Reservoir System Based on a 100-Year Climate Forecast
2.1 Introduction
2.2 HSPF Model Calibration for the Occoquan Watershed
2.3 Climate Model Output Downscaling
2.4 Land Use Model
2.5 Water Supply Model Experimentation
2.6 Results and Analysis
3 Conclusions 
3.1 Conclusions and Discussion
4 References 
5 Appendices
Low Flow Variations in Source Water Supply for the Occoquan Reservoir System Based on a 100-Year Climate Forecast

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