The Role of Hydrology in the Resolution of Water Disputes

ydrology is irrelevant to adjudication. This is a statement any western water lawyer might make. Most western states, on paper, rely on the doctrine of prior appropriation to allocate this resource with its variable supply (Hutchins 1971). Thus in times of drought, rather than re-allocate the supply to share shortage, use is eliminated in order of reverse priority. Those first to the stream take a full supply; those last go without. Adjudication need only sort out the various priorities; the ditch rider will do the rest.But consider a layperson’s translation of the statement “hydrology is irrelevant to adjudication” – i.e. understanding the water resource is irrelevant to its allocation. To anyone but the most reverent adherent to prior appropriation as the panacea, this is absurd. Historically we have built dams rather than face the harsh consequences of shutting off junior water uses. In modern times, we buy senior water rights to serve more economically or politically successful junior needs. In rare instances we rely on allocation through prior appropriation. In all three cases, understanding the water resource is essential.Adjudication does not exist in a vacuum. It is not an end in itself, but rather a means of defining the legal basis on which future water allocation decisions will be made. Management and enforcement of water rights in a complex system can only occur against the backdrop of a useable database defining and relating the many rights within it. A water transfer cannot occur without a complete definition of the right being transferred. This means not only a definition of priority, use and quantity – elements commonly defined in adjudication — but because transfer can only occur in prior appropriation states with no injury to other water uses (e.g. Idaho Code §42-222(1)), an understanding of the effects of altering diversion and return flow is required. Design of physical solutions to mitigate the harsh impacts of prior appropriation requires an analysis of the impacts of the proposed solution on the many water rights. Even strict enforcement of prior appropriation may require sophisticated knowledge of the water resource in complex systems involving rights from surface and ground water sources that are hydrologically connected. Because these decisions can only be made with an understanding of the water resource, adjudication should not proceed without laying the groundwork for that understanding.The advent of high speed computing has given us access to means to handle mathematically described relations between the complex variables that comprise and affect a water resource, and thus provides a tool for understanding the interplay between the resource and water use. This paper will discuss the importance of development and use of this tool – the hydrologic model – in adjudication and subsequent water management. Because hydrologic modeling can be misused due to the black-box nature of the tool from the viewpoint of the non-scientist, this paper will begin with background on development of models and stress the steps in which water managers, lawyers and policymakers must play a role to assure that the model developed will serve the purpose they intend. The paper will then describe the compilation of data and development of a hydrologic surface water model for use in adjudication, settlement and water management in the Milk River, Montana, followed by discussion of development of a hydrologic ground water model for enforcement and conjunctive management of