Seasonal forecasts offer economic benefit for hydrological decision making in semi-arid regions

AbstractIncreasing frequencies of droughts require proactive preparedness, particularly in semi-arid regions. As forecasting of such hydrometeorological extremes several months ahead allows for necessary climate proofing, we assess the potential economic value of the seasonal forecasting system SEAS5 for decision making in water management. For seven drought-prone regions analyzed in America, Africa, and Asia, the relative frequency of drought months significantly increased from 10 to 30% between 1981 and 2018. We demonstrate that seasonal forecast-based action for droughts achieves potential economic savings up to 70% of those from optimal early action. For very warm months and droughts, savings of at least 20% occur even for forecast horizons of several months. Our in-depth analysis for the Upper-Atbara dam in Sudan reveals avoidable losses of 16 Mio US$ in one example year for early-action based drought reservoir operation. These findings stress the advantage and necessity of considering seasonal forecasts in hydrological decision making.</jats:p

Acoustic Doppler Current Profiler Measurements in the Tailrace at John Day Dam

Acoustic Doppler current profilers (ADCPs) were used to measure water velocities in the tailrace at John Day Dam over a two-week period in February 2005. Data were collected by the Pacific Northwest National Laboratory for the Hydraulic Design Section, Portland District, U.S. Army Corps of Engineers (USACE). The objective of this project was therefore to collect field measurements of water velocities in the near-field draft tube exit zone as well as the far-field tailrace to be used for improving these models. Field data were collected during the project using five separate ADCPs. Mobile ADCP data were collected using two ADCPs mounted on two separate boats. Data were collected by either holding the boat on-station at pre-defined locations for approximately 10 minutes or in moving transect mode when the boat would move over large distances during the data collection. Results from the mobile ADCP survey indicated a complex hydrodynamic flow field in the tailrace downstream of John Day Dam. A large gyre was noted between the skeleton section of the powerhouse and non-spilling portion of the spillway. Downstream of the spillway, the spillway flow is constrained against the navigation lock guide wall, and large velocities were noted in this region. Downstream of the guide wall, velocities decreased as the spillway jet dispersed. Near the tailrace island, the flow split was measured to be approximately equal on Day 2 (25.4 kcfs spillway/123 kcfs total). However, approximately 60% of the flow passed along the south shore of the island on Day 1 (15.0 kcfs spillway/150 kcfs total). At a distance of 9000 ft downstream of the dam, flows had equalized laterally and were generally uniform over the cross section. The collection of water velocities near the draft tube exit of an operating turbine unit is not routine, and equipment capable of measuring 3D water velocities in these zones are at the forefront of hydraulic measurement technology. Although the feasibility of measuring 1D water velocity magnitudes has been previously demonstrated by the authors, the feasibility of resolving 3D water velocity vectors given the heterogeneity of the flow field was unknown before this study’s data were collected. Both the 1D and 3D data were collected by deploying three ADCPs on dual-axis rotators directly above the draft tube exit of Turbine Unit 16. These instruments collected 1D data during both the mobile reconnaissance campaign and a later one-week period with zero spillway discharge. During the zero spillway discharge period, Turbine Unit 16 was operated over a range of discharges. Approximately 12 hours of 1D velocity data were collected at low (12 kcfs), middle (16.2 kcfs), and high (19.2 kcfs) turbine discharges. The 1D dataset indicates large differences in flow patterns and RMS velocity fluctuations among the various discharge levels. Results from this project show that it is technically feasible to measure 3D water velocities directly downstream of an operating turbine unit using a narrow beam swath (i.e., 6-degree) ADCP. Data products from these 3D velocity data include a measurement of the draft tube barrel flow distribution (a.k.a., the flow split), directional changes and the general decay of velocity as flow exits the draft tube and enters the tailrace, and a relative measure of the homogeneity of the flow field

Hydraulic Characteristics of the Lower Snake River During Periods of Juvenile Fall Chinook Migration

This report documents a four-year study to assess hydraulic conditions in the lower Snake River. The work was conducted for the Bonneville Power Administration, U.S. Department of Energy, by the Pacific Northwest National Laboratory. Cold water released from the Dworshak Reservoir hypolimnion during mid- to late-summer months cools the Clearwater River far below equilibrium temperature. The volume of released cold water augments the Clearwater River, and the combined total discharge is on the order of the Snake River discharge when the two rivers meet at their confluence near the upstream edge of Lower Granite Reservoir. With typical temperature differences between the Clearwater and Snake rivers of 10°C or more during July and August, the density difference between the two rivers during summer flow augmentation periods is sufficient to stratify Lower Granite Reservoir as well as the other three reservoirs downstream. Because cooling of the river is desirable for migrating juvenile fall Chinook salmon (Oncorhynchus tshawytscha) during this same time period, the amount of mixing and cold water entrained into Lower Granite Reservoir’s epilimnion at the Clearwater/Snake River confluence is of key biological importance to juvenile fall Chinook salmon. Data collected during this project indicates the three reservoirs downstream of Lower Granite also stratify as direct result of flow augmentation from Dworshak Reservoir. These four lower Snake reservoirs are also heavily influenced by wind forcing at the water’s surface, and during periods of low river discharge, often behave like a two-layer lake. During these periods of stratification, lower river discharge, and wind forcing, the water in the upper layer of the reservoir is held in place or moves slightly upstream. This upper layer is also exposed to surface heating and may warm up to temperatures close to equilibrium temperature. The depth of this upper warm layer and its direction of travel may also be of key biological importance to juvenile fall Chinook salmon. This report describes field data collection, modeling, and analysis of hydrodynamic and temperature conditions in the Lower Granite Reservoir during the summer flow augmentation periods of 2002, 2003, and 2004 plus a brief one-week period in 2005 of Lower Monumental, Little Goose, and Lower Granite Reservoirs. Circulation patterns in all four lower Snake River reservoirs were numerically simulated for periods of 2002, 2003, 2004, and 2005 using CE-QUAL-W2. Simulation results show that these models are sufficiently capable of matching diurnal and long term temperature and velocity changes in the reservoirs. In addition, the confluence zone of the Clearwater and Snake rivers was modeled using the 3-D model Flow3-D. This model was used to better understand mixing processing and entrainment. Once calibrated and validated, the reservoir models were used to investigate downstream impacts of alternative reservoir operation schemes, such as increasing or decreasing the ratio of Clearwater to Snake discharge. Simulation results were also linked with the particle tracking model FINS to better understand alterations of integrated metrics due to alternative operation schemes. These findings indicate that significant alterations in water temperature throughout the lower Snake River are possible by altering hypolimnetic discharges from Dworshak Reservoir and may have a significant impact on the behavior of migrating juvenile fall Chinook salmon during periods of flow augmentation