RGDSS_Task3_ModelingEffortReview

Home Browse Search

amount as set forth in the Rio Grande Compact. As modeled, however, this target delivery is adjustable each year by the model user so that the model can strive for either a credit or debit in a given year. For example, the model user may wish to erase an accrued credit so the target delivery would strive for a annual debit equal to the accrued credit. In addition, the user may supply a maximum and a minimum curtailment percentage for each time step. The user, therefore, may override the built-in logic to force any desired amount of curtailment in any time step. Once the actual curtailment percentage is calculated, whether by the model or set by the user through the override logic, the model installs a senior demand for water equal to the curtailment percentage times the sum of the inflows for the next season. This senior demand ensures that the compact delivery is made; the overall effect is the same as calling out the most junior ditches that otherwise would divert in the absence of the compact delivery. It should be noted that the calculations to determine a curtailment are inexact because of the uncertainty introduced by the forecasts; the general idea is to introduce some uncertainty into the curtailment determination because this uncertainty exists in the Division Engineer's calculations, too. If desired, the model can operate with perfect foresight by using actual inflow data in lieu of forecasts. The use of perfect forecasts, i.e. actual inflow data, will reduce the season to season volatility of the curtailment calculations. In addition to the compact standard logic, the model contains a flag to represent a spill situation at Elephant Butte Reservoir. If this flag is positively set in any time step, the model will not enforce any curtailments from the current time step until the end of the calendar year. Water rights, including the Platoro 1973 storage right, are able to store in priority without any Compact curtailment when this flag is set. An accrued debit is wiped out by an Elephant Butte spill. Accrued credits, however, are only reduced by the amount of a user specified value which indicates the amount of credit water spilled. This differential treatment of accrued debits and credits in the event of an Elephant Butte spill is in accordance with the Rio Grande Compact. Two other compact related limits are present in the model, a maximum accrued credit and a maximum accrued debit. Should the model reach the maximum credit, curtailments are shut down until the accrued credit falls below the maximum amount. Conversely, the model enforces 100% curtailment if the maximum debit is reached. Although 100% curtailment would not occur in reality, it is equally unlikely that the Compact debit would reach the maximum amount. The maximum credit is normally set in the CRPM to 75,000 acre-feet, half of the 150,000 acre-foot in the Compact, and the maximum debit is set to 40,000 acre-feet, the amount agreed to in the 1991 Memorandum of Understanding between the State of Colorado, the Conejos District, and the Rio Grande Water Users Association. The maximum and minimum accrued credit are modifiable by the user. The Closed Basin and Norton Drain inflows controlled by the District are not counted as part of the Conejos input index flows. They are, however, counted towards the required deliveries. These flows, therefore, serve to reduce the curtailment that would otherwise have to be obtained from the Conejos River. Another possible use of Closed Basin (and potentially Norton Drain) flows is a non- real-time exchange into Platoro. Under anon-real-time exchange, Platoro is allowed to exchange water into the general pool ahead, or possibly behind, the actual appearance of the Closed Basin flows. Such an exchange would allow Platoro maximum flexibility to store because it overcomes the mismatch of rare yet usually high-volume exchange opportunities at Platoro and the constant yet relatively low instantaneous pumping rates of the Closed Basin. C:\cdss\Task3Mem.doc Review Previous Modeling Efforts Apri16, 1999 -Page 6 of 11