Sediment transport, streamflow, and climate change: long-term resilience of the Bay-Delta

Percent change in peak precipitation days (>95th percentile) (top panel), April 1st snow water equivalent (mm) by end‐of‐century (2070–2099) (bottom panel) compared to historical baseline (1980–2009) for 10 models for representative concentration pathways (RCP) 4.5 and 8.5, and ensemble averages.

To assess potential future trends in streamflow and sediment supply, the calibrated Hydrological Simulation Program - FORTRAN (HSPF) model (Stern et al., 2016) was run using 20 future climate projections (scenarios) with varying air temperature and precipitation. These 20 scenarios consist of 10 global climate models; each run using two representative (greenhouse gas) concentration pathways (RCPs), RCP 4.5 and RCP 8.5. RCP 8.5 represents a future with very high global population, slow economic growth and a technological change, and a high dependence on fossil fuels (Riahi et al., 2011), and RCP 4.5 represents a future in which greenhouse gas emissions are partially mitigated through technological advances and policy changes (Thomson et al., 2011). Comparisons between each scenario's historical period (WY1980–2009) and the end-of-century 30-year period (WY2070–2099) were analyzed to quantify projected changes in climate. 

The global climate model results for the Sacramento River Basin indicate that 60% of scenarios projected increases in average precipitation, with a general model consensus of increased peak precipitation, and an all model consensus of increased temperatures, with varying magnitudes that broadly depend on RCP. In California, snow is the primary source of surface water storage, and the April 1st snowpack measurement is a vital metric of the potential water resources available through the dry summer months. In our model domain, which is below the major reservoirs, modeled April 1st snow water equivalent (SWE) in the Sacramento River basin decreased dramatically across all future scenarios by end‐of‐century and is consistent with other studies assessing changes in snowpack in the Sierra Nevada (Gergel et al., 2017; Rhoades et al., 2018; Thorne et al., 2015). 

Projected precipitation changes were the main driving mechanism for changes in the magnitude of modeled streamflow. Current climate projections indicate increases in precipitation frequency and intensity for the Sacramento River Basin, which leads to increased peak streamflow. To a lesser extent, air temperature influenced streamflow by increasing evapotranspiration decreasing SWE, and changing the timing of snowmelt. Seasonal changes in streamflow by end-of-century from the historical baseline are evident across all scenarios, which affects the timing of water resources, water quality, and water temperature through the end of the warm season.  

The changes in precipitation and air temperature had varying effects on flow, sediment, and suspended sediment concentration (SSC), and mean sediment loads were more sensitive to changes in climate than mean streamflow and SSC. 

Total annual sediment load (million metric tons, Mt) for the Sacramento River at Freeport location, for representative concentration pathway (RCP) 4.5 scenarios (gray lines) with 30‐year running average for RCP 4.5 ensemble in blue (top panel) and RCP 8.5 scenarios (gray lines) and 30‐year running average for RCP 8.5 ensemble in red (bottom panel).

Significant consequences arise from either too much or too little sediment. Although we found statistically significant increases in five of the 20 scenarios and the RCP 4.5 and 8.5 ensembles, the nonsignificant trends of a leveling off or decline of sediment are also plausible outcomes. A leveling off or continued decline of sediment could significantly deteriorate the health and resiliency of the Bay‐Delta as the climate continues to change. Increases in the most intense and frequent storms and atmospheric rivers projected in most scenarios would generate an increased amount of runoff and sediment transport. Temperature increases lead to earlier snow melt and therefore create larger magnitude floods earlier in the wet season and thus higher sediment loads than previously experienced in the watershed. The risk of increased intense flooding imposes additional failure risk to the Bay‐Delta levees, dams, and other infrastructure components that are already in need of reinforcing and repair. Water quality may become an increased concern with higher sediment loads since sediment can transport contaminants such as pesticides, herbicides, nutrients, and mercury. 

Even with the many caveats and assumptions of the combined modeling approach described here, results of the scenarios can be useful to managers who need to plan for the middle- and long-term time scales. Projected increases in peak flows and sediment suggest a number of possible management actions, including floodplain restoration to capture sediment during peak events, increasing groundwater recharge to manage peak flows and prepare for droughts, and bolstering flood control structures. 

References 

Gergel, D. R., Nijssen, B., Abatzoglou, J. T., Lettenmaier, D. P., & Stumbaugh, M. R. (2017). Effects of climate change on snowpack and fire potential in the western USA. Climatic Change, 141(2), 287– 299. https://doi.org/10.1007/s10584-017-1899-y 

Rhoades, A. M., Jones, A. D., & Ullrich, P. A. (2018). The changing character of the California Sierra Nevada as a natural reservoir. Geophysical Research Letters, 45, 13,008– 13,019. https://doi.org/10.1029/2018GL080308 

Riahi, K., Rao, S., Krey, V., Cho, C., Chirkov, V., Fischer, G., Kindermann, G., Nakicenovic, N., & Rafaj, P. (2011). RCP 8.5—A scenario of comparatively high greenhouse gas emissions. Climatic change, 109(1–2), 33. https://doi.org/10.1007/s10584-011-0149-y 

Stern, M. A., Flint, L. E., Minear, J., Flint, A. L., & Wright, S. A. (2016). Characterizing changes in streamflow and sediment supply in the Sacramento River basin, California, using hydrological simulation program—FORTRAN (HSPF). Water, 8(10) 432 p, 432.  https://doi.org/10.3390/w8100432 

Thomson, A. M., Calvin, K. V., Smith, S. J., Kyle, G. P., Volke, A., Patel, P., Delgado‐Arias, S., Bond‐Lamberty, B., Wise, M. A., Clarke, L. E., & Edmonds, J. A. (2011). RCP4. 5: A pathway for stabilization of radiative forcing by 2100. Climatic change, 109(1‐2), 77. https://doi.org/10.1007/s10584-011-0151-4 

Thorne, J. H., Boynton, R. M., Flint, L. E., & Flint, A. L. (2015). The magnitude and spatial patterns of historical and future hydrologic change in California's watersheds. Ecosphere, 6(2), 1– 30.