2004 Hydrologic-Institutional Model (H-I Model) Supreme Court of the US State of Kansas, Plaintiff v. State of Colorado 125 S.CT 526

In this case Kansas contested the amount of monetary damages that it should receive based on the decisions of prior cases acknowledging Colorado's violations of the Arkansas River Compact 1949. Specifically, Colorado had violated the following provision in the Compact, "that the waters of the Arkansas river, as defined in Article III, shall not be materially depleted in usable quantity or availability for use to the water users in Colorado and Kansas under this compact by such future development or construction" (Colo Rev Stat Article 69). To monitor future extractions and Colorado's future Compact compliance, the states would employ a computer model, Hydrologic-Institutional Model (H-I Model).

Regarding the model, Kansas argued that administrators not employ the recommendation of the special master to measure compliance in 10-year periods but in 1-year periods. The Supreme Court over-ruled this objection. According to the brief, "the Master found that Model results over measurement periods less than 10 years are highly inaccurate, but that the Model functioned with acceptable accuracy over longer periods of time" (p. ). The justices further explained the purpose of the Model as a tool "to determine just what the precise water flows into Kansas would have been had Colorado not allowed increased consumption of ground water after 1949" (p. 533). Although the states had challenged assumptions of the Model, the Supreme Court referenced the Administration, a body established in the Arkansas River Compact 1949 Article VIII, with the authority to settle disputes. If the Administration failed to settle differences, the parties could resort to arbitration.

The brief described the Model's ability to measure "whether Colorado's post-1949 wells deplete the river of usable water that the Compact makes available for Kansas" (p. 536) in the following manner: "It does so by trying to account for almost every Arkansas-River-connected drop of water that arrives in, stays in, or leaves Colorado, whether by way of rain, snow, high mountain streams, well pumping of underground water, evaporation, canal seepage, transmountain imports, reservoir storage, or otherwise . . . With all 'switches' turned on, the Model predicts how much river water will leave Colorado for Kansas during a given month . . . To obtain a figure representing an unlawful depletion (or lawful accretion) under the Compact, the Model subtracts from this figure (the actual flow) a number representing a hypothetical prediction of how much water would have flowed into Kansas had Colorado not dug and operated post-1949 wells. The Model obtains this prediction through a computer rerun with the Model's "post-1949 wells" switch turned off . . . The final figure is then adjusted to reflect depletions to usable, as opposed to total, flow. . . . Not surprisingly, the Model's ability to calculate depletions has proved highly controversial, leading to many Model modifications during this litigation" (p. 536).

The justices continued their discussion by asserting that application of the Model and the Amended Rules and Regulations Governing the Diversion and Use of Tributary Ground Water in the Arkansas Basin (Use Rules) would result, theoretically, in zero depletions, "that is, there would be no difference between actual flow and what the flow would have been under precompact conditions" (p. 538). The Use Rules "protects both Kansas water users and senior Colorado users by insisting that Colorado users with junior rights (and in particular those who obtain water from post-1949 wells) replace the river water that they use. They must either (1) buy replacement water, say, from the Rockies' western slope or (2) buy land irrigated under pre-1949 water rights and remove it from cultivation . . . In practice, junior users belong to one of three associations that conduct these transaction, reporting the details monthly to the Colorado State Engineer's Office, and receiving replacement "credits," which they divide among their members" (pp. 537-538).