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Aquarius is a state-of-the-art computer model devoted to the temporal and spatial allocation of water among competing uses in a river basin. The model is driven by an economic efficiency operational criterion requiring the reallocation of stream flows until the net marginal return in all water uses is equal. This occurs by systematically examining, using a nonlinear optimization technique, the feasibility of reallocating unused or marginally valuable water storage and releases in favor of alternative uses. Because water-system components can be interpreted as objects of a flow network, the model considers each component as an equivalent node or structure in the programming environment as well.

Overview and Applicability

Mountain stream below Dream Lake, Rocky Mountain National Park (photo compliments of National Park Service).
Mountain stream below Dream Lake, Rocky Mountain National Park (photo compliments of National Park Service).

Aquarius is an analysis framework rather than a single dedicated model for water allocation. The model was implemented using an object-oriented programming (OOP) language (C++). Water systems are ideal candidates for modeling under an OOP framework, where each system component (e.g., reservoir, demand area, diversion point, river reach) is an object in the programming environment. Aquarius supports the following water uses (system components): 

  • Storage reservoir

  • Hydropower plants

  • Agricultural water use

  • Municipal and industrial water use

  • Instream recreation water use

  • Reservoir recreation water use

  • Fish habitat protection

  • Flood control area

Actual Versus Efficient Water Allocation

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Under the dominant water allocation doctrine in the Western United States, the Prior Appropriation Doctrine, the water available for a new application is reduced by the sum of all prior established rights. A priority-based allocation in a heavily appropriated river can become inefficient as values change if institutional barriers or market failures impede voluntary transfers of rights from lower-valued to higher-valued uses. Because institutional barriers and market failures commonly affect Western water, actual water allocations may be different from the economically efficient allocations achieved using Aquarius. 

It may be helpful to compare the "actual allocation" with an "efficient allocation". Such a comparison may indicate promising opportunities for private water trades or, where such trades are hampered or precluded by institutional barriers, may indicate areas where institutional reforms can allow for a more efficient water allocation. Where water developments are publicly financed, the comparison may indicate directions that the public entity should consider to increase the efficiency of the project. Aquarius facilitates such comparisons by characterizing an efficient allocation, subject to the analyst's ability to specify demand functions for the key water uses. 

Model Implementation

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Although the present version of the model implements only a monthly time step, Aquarius was conceived to simulate the allocation of water using any time interval, including days, weeks, months, and time intervals of nonuniform lengths. Future versions of the model will support these other time steps. 

Aquarius can be used in a fully deterministic optimization mode, for general planning purposes, or in a quasi-simulation mode, with restricted foresight capabilities. The model distinguishes between the period of analysis, used to specify the length of the whole segment of time for which the model will simulate system operation, and the optimization horizon, used to specify how far into the future the model should look to build the optimal operational policies. Setting the optimization horizon equal to the period of analysis produces a full-period optimization. 

Formulating a water allocation problem entirely within the domain of the objective function allows the user to redirect the water allocation process in any direction in real time, directly from the screen, as the optimization progresses. This unique feature provides an expeditious and innovative mode of exploring "what if" scenarios.

How to Interact with the Model

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The user interacts with the model through the network worksheet screen (NWS), which represents the water system of interest using the inherent capability of the object-oriented paradigm for graphical representation. In the NWS, each water system component corresponds to an object, a graphical node or link, of the flow network. These components are represented by icons, which are pictorial representations of the objects. By dragging and dropping these icons from the menu, the model creates instances of the objects on the screen.

Components can be repositioned anywhere in the NWS or can be removed. Once nodes (e.g., reservoirs, demand areas) are placed, they are linked by river reaches and conveyance structures. This operation occurs by left-clicking on the outgoing terminal of a node and then on the incoming terminal of the receiving node. This procedure facilitates the assembly or alteration of water systems by connecting their system components in the NWS. The creation and alteration of flow networks is further facilitated by copying and inserting an object or whole portions of an existing network onto the same or a new NWS. Copy/paste creates new instances of the object(s) and duplicates their data structure, creating clones of the original objects. 

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Download Aquarius verion 10 here (.exe file).


Diaz, Gustavo E., Thomas C. Brown, and Oli Sveinsson. 2000. Aquarius: A Modeling System for River Basin Water Alloction. General Technical Report RM-GTR-299-revised, Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station, 172 p.


Aquarius runs on a personal computer under most Microsoft Windows operating systems (95/98/NT/2000).


Diaz, Gustavo E.; Brown, Thomas C.; Sveinsson, Oli. Aquarius: A Modeling System for River Basin Water Allocation. Available online at

National Strategic Program Areas: 
Wildland Fire and Fuels
National Priority Research Areas: 
Watershed Management and Restoration
RMRS Science Program Areas: 
Human Dimensions
RMRS Strategic Priorities: 
Human-Landscape Interactions; Water & Watersheds