Summary
The total dissolved gas forecaster is in its second year of predictions. Because of changes in operations in recent years and data availability at many tailrace sites, the forecaster is currently fit to data from 1995-1998. 1998 was a more moderate flow year with a few newly modified structures and has thus yielded some important information about forebay and tailrace gas distributions. New flow deflectors were added to John Day and Ice Harbor dams since the 97 season. The flow deflectors were added to reduce gas production and hence new gas equations were added for the 98 season. Also new to 1998 was the tailrace water quality monitor added to Wells Dam.A few changes were made mid-season due to reflect this new information and to significantly improve the tdg model in CRiSP.
Powerhouse Flow Entrainment
Because gas levels were significantly lower in 1998 than in the last two years, a larger variety of forebay conditions were available to fit the forebay model. A new feature was added to the CRiSP model that allowed not only the generation of TDG at a tailrace, but also allowed partial gassing of the powerhouse flow due to entrainment by the spillway flow. Previously TDG from the forebay was passed through the powerhouse, and the tailrace flows from the spillway and powerhouse were either instantaneously mixed or mixed at a slower rate as the river moved downstream. There seemed to be noticeable entrainment below Wanapum, Lower Granite and Little Goose Dam resulting in more TDG in the downstream forebay than if only the spillway flow had been gassed. Entrainment coefficients were fit to 1995-1998 forebay data. Entrainment coefficients seem to improve the overall fit, though a detailed analysis still needs to be done. The change in the model is reflected in the forecasts dated May 18,1998 or later.Ice Harbor Dam
Ice Harbor's new tdg production equation was based on water quality and spill data from March 98. This was enough to predict the 98 season. At the end of the year, a new equation will be fit using the whole year's data.John Day Dam
John Day Dam's new tdg production equation was based on early spill data and was provided by Army Corps of Engineers. This equation significantly underestimated the amount of tdg produced at JDA. A new equation was fit to the year-to-date water quality data through June 8, 1998. All forecasts prior to this date had the original estimate for gas production, and all forecasts after this date use the new production equation.The new equation improved the underestimation, however there still seems to be an underestimate of TDG at John Day. This could be the result of CRiSP currently using daily average values for spill. There is significantly more spill at JDA during the night. This is causing an underestimation of gas at night. The day equation is a "flatter" equation, and so the underestimation of the night TDG was enough to throw off the daily average. This could be improved if spill was imported into the model on a hourly or every 6 hour average to capture the day/night differences in spill.
Lower Monumental Dam
Lower Monumental Dam had a poor fit in 1998. Previous years also tended to underestimate TDG. A new equation was fit for both night and day dynamics, as well as a new entrainment coefficient (see above), in order to improve TDG predictions at this dam. Predictions after May 18th reflect the new model coefficients for LMN.
Wells Dam
The new water quality monitor, WELW, added to Wells Tailrace has provided information for the first time on TDG levels below Wells Dam. A new equation was fit to WELW first on May 11, and then on June 11; each resulting in a better fit. CRiSP had previously been significantly overestimating the TDG levels produced by Wells. The old equation was based on forebay data at Rocky Reach from the high gas years 96-97 and proved to be inadequate. This equation will be refit once the full year's data is in.