Mid-term Probabilistic Operations Model (MTOM)

Overview

The Mid-term Probabilistic Operations Model (MTOM) is one of three river system models developed and maintained by Reclamation to support operational decision-making and planning for the Colorado River Basin. All three models are implemented in the RiverWare modeling platform and share some key features. RiverWare is an object-oriented modeling platform in which the objects may be inflow points, river reaches, diversions, reservoirs, canals, pipelines, and other water-resource features.

MTOM was developed directly from the 24-Month Study Model (24MS) so the two models have very similar structures and representations of operations. Compared to 24MS, MTOM extends the timescale of projection out to 5 years (60-68 months, depending on the starting month), vs. 2 years, and MTOM also allows for the input of a large ensemble (typically 35) of inflow traces to facilitate probabilistic risk analysis, as well as more robust testing of the inputs themselves, vs. the deterministic single-trace runs of 24MS.

MTOM is used for mid-term (2-5 years) system projections for risk-based operational planning and analysis. Operationally, MTOM is paired with the Colorado River Simulation System (CRSS) to create a set of "hybrid" mid-term (5-year) projections that leverage the input and output capabilities of both models. These MTOM/CRSS runs are performed twice per year, in January and August. MTOM is also run on its own without CRSS to create simpler probabilistic analyses; these projections are not readily available but can be requested from Reclamation.

Relevance

The envelope of the MTOM (and MTOM/CRSS) projections of Lake Powell and Lake Mead levels over the 5-year mid-term period provides guidance to Reclamation and basin stakeholders about the probability of undesirable system outcomes over that period (e.g., Lower Basin delivery reductions, reservoir levels falling below the power pool). This information can help water managers and users prepare for those outcomes in accordance with their own risk tolerances.

Model Structure and Operation

Figure 1. The Mid-term Probabilistic Operations Model (MTOM) in object view, including the 12 reservoirs in the bullet list at left, which are shown with the purple triangles. (Source: Reclamation)

MTOM takes a set of projected inflow sequence and runs them through prescribed operational rules at 12 Reclamation reservoirs to project the monthly conditions at those reservoirs over the next 60-68 months, depending on the starting month of the run. A total of 19 inflow points (vs. 16 in 24MS) are used to represent both inflows to those reservoirs and intervening flows between reservoirs. The three additional inflow points allow the automation in MTOM of a process that is done manually in 24MS.

The 5-year inflow sequences are provided by the NOAA Colorado Basin River Forecast Center (CBRFC). The 35 sequences are generated by CBRFC's modeling system for seasonal streamflow forecasts; each sequence begins with the basin moisture conditions as of the MTOM run date and then is driven by the historical temperatures and precipitation for one of 35 overlapping 5-year periods sampled from the 1981-2015 observational record. This method is effectively the same as that used by CBRFC for their ensemble streamflow prediction (ESP) forecasts for seasonal streamflows, just over a longer time period.

Water demands are handled in MTOM the same way as in 24MS. Upper Basin water demands are not explicitly modeled (with three exceptions); instead, the inflow sequences from CBRFC have the expected Upper Basin diversions and consumptive use already subtracted from the flows that would have occurred under natural conditions. The three exceptions are the Gunnison Tunnel, Azotea Tunnel, and Navajo Indian Irrigation Project, which are represented as separate diversions in MTOM. Lower Basin water demands are modeled based on monthly schedules provided by the water users.

The output from MTOM consists of ~5 years of monthly inflows, projected elevations and storage, releases downstream, and hydropower generation (if applicable) for the same 12 reservoirs as in 24MS. But as indicated above, this "raw" output is seldom distributed in its entirety. Instead, the main operational use of MTOM output is to initialize CRSS for the MTOM/CRSS mid-term risk analyses. For this purpose, only the end-of-year 1 reservoir elevations projected by MTOM are retained; the 35 sets of elevations are passed on to CRSS to be each used as the starting point for multiple CRSS runs that project the system outcomes for years 2-5.

In the most recent (January 2021) MTOM/CRSS analyses, two sets of inflow sequences were used for years 2-5 in CRSS: 113 sequences sampled from the 1906-2018 natural flow hydrology (Full hydrology), and 31 sequences sampled from the 1988-2018 hydrology (Stress Test hydrology). Because all of the CRSS sequences branch out from the end of each of the 35 MTOM sequences for year 1, the MTOM/CRSS coupling produces a total of 3,955 traces (35 x 113) for years 2-5 under the Full hydrology, and 1,085 traces (35 x 31) for years 2-5 under the Stress Test hydrology.

Reclamation then computes and reports the percent of these traces in which particular event or system condition occurs, as shown in Figure X. These percentages represent the actual future risk only if all of the modeling assumptions are correct, so it is more reasonable to treat the numbers as conditional risk estimates, where the conditions are the key modeling assumptions.

Data and Tools

Colorado River System 5-Year Projected Future Conditions

The MTOM/CRSS projections of future conditions of the Colorado River system are updated by Reclamation at least twice annually (January and August). The linked page shows the results of the most recent projections in tabular form, along with a description of the modeling approach and assumptions.

Colorado River System Projected Future Conditions - Alternative Future Hydrology Scenarios

This Reclamation page shows the same results of the latest MTOM/CRSS projections as the Colorado River System 5-Year Projected Future Conditions page, but in time-series form, and with an explanation of the development of the alternative future hydrology scenarios, i.e., the Stress Test hydrology.

Additional Resources

State of the Science Report

Chapter 3 of the State of the Science report, Primary Planning Tools, describes the three primary Reclamation system models in detail.

MTOM/CRSS General Modeling Information

This Reclamation page summarizes the key components of the MTOM/CRSS runs: the models' setups and assumptions, the inflow scenarios, and the reservoir operations rules.

MTOM Technical User Guide

This Reclamation document provides step-by-step guidance for running MTOM and analyzing the results produced by the model. The intended audience for this guide is technical modeling personnel and researchers.

New and Notable Research (2020-present)

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Summary

Contact

MTOM is maintained and updated by Reclamation Upper Colorado and Lower Colorado regional staff who are based at the University of Colorado Boulder's Center for Advanced Decision Support for Water and Environmental Systems (CADSWES), in consultation with Reclamation staff in other offices. The RiverWare modeling platform was also developed at CADSWES.

For additional information or questions, contact them at ColoradoRiverModeling@usbr.gov.