Examining the Effects of Climate Change on Managed Lowland Rivers in the California Bay Delta Watershed

Water that is released from reservoirs can affect the downstream thermal regimes of rivers. During the summer months, these flow releases can lower the river temperature downstream of dams in an extension that mainly depends on the volume and temperature of these releases and the energy exchange with the overlying atmosphere. The benefit of this cooling effect has been suggested as an approach to mitigate the effects of climate change in downstream-regulated rivers. However, anticipated climate change conditions may weaken these cooling benefits, especially in managed lowland rivers (MLRs), as they are subjected to large withdrawals, are shallow, and convey clear water. Here, we show that MLRs in the California Bay Delta Watershed are vulnerable to water temperature increases, especially during future summer months subjected to a future high-emission greenhouse scenario. Low-flow conditions exacerbate this vulnerability, especially at locations downstream of high-flow diversions. By using a physical energy balance model (FLUVIAL-EB) paired with a downscaled climate regional model (CRCM5-RCP8.5), we found that for summer months between 2030 and 2100, longwave and latent heat fluxes will contribute to water temperature increases, while absorbed solar radiation will likely decrease under future climate scenarios. Despite the warming effects of climate change on MLRs in the California Bay Delta Watershed, our findings suggest that increasing the release of hypolimnetic water from reservoirs during summer months can be a viable solution to mitigate the river temperature increase.

水库下泄水流会对河流下游的水温情势（thermal regime）产生影响。夏季时段内，这类下泄过程可降低坝下游河段的水温，降温效果主要取决于下泄水量、下泄水温以及与上覆大气的能量交换。该降温效应被认为可用于缓解受调控下游河流受气候变化的影响。然而，预期的气候变化情境可能会削弱这类降温效益，尤其是在受调控低地河流（managed lowland rivers, MLRs）中——这类河流面临大量取水需求、水深较浅且水体透明度较高。本研究表明，加利福尼亚湾三角洲流域的受调控低地河流易受水温升高影响，在未来高排放温室气体情景下的夏季尤为显著。枯水情势会加剧这一脆弱性，在大流量引水工程下游河段尤为明显。本研究借助物理能量平衡模型（FLUVIAL-EB）与降尺度区域气候模型（CRCM5-RCP8.5）开展分析，结果显示：2030年至2100年的夏季时段内，长波辐射通量与潜热通量将推动水温升高，而未来气候情景下太阳辐射吸收量则大概率呈下降趋势。尽管气候变化会加剧加利福尼亚湾三角洲流域受调控低地河流的水温升高问题，但本研究结果表明，在夏季增加水库底层水的下泄量，可作为缓解河流水温上升的可行方案。