The water rights and accounting-driven, multi-reservoir, integrated operations of Colorado's Arkansas basin are remarkably complex and present many modeling challenges that have put RiverWare's capabilities and flexibility to the test. Rather than being dictated from the top down by a controlling agency (which is oh-so-much easier to simulate), the basin's overall operations and conditions are instead driven in a bottom-up manner as the aggregation of the decisions made by dozens of entities operating to their own independent and interdependent objectives throughout the system.

Utilizing the Water Rights Solver, the ABRW model simulates the allocation of native flow on a daily timestep to over 300 unique water rights. Many of these water rights, especially changed ones, have unique and detailed limitations, stipulations, and rules about how much, when, and how the yields can be used, stored, and/or exchanged or otherwise transferred around the basin. While these intricacies can seem trivial, they often determine the owner and fate of significant amounts of native flow, and thus can be critical for model performance. RiverWare's extremely flexible rule-writing framework has proved to be crucial in allowing for appropriate simulation of these important complexities.

Due to these complex water rights, as well as the requirements of several significant transbasin import projects, the accounting and sub-accounting system throughout the Arkansas basin is incredibly complex and infeasible to simulate using only RiverWare's traditional accounting framework. Again, leveraging RiverWare's flexibility, a framework was developed to allow traditionally accounts to be subdivided and tracked individually (and that subaccount into sub-subaccounts, and so on), in a manner analogous to how a reservoir is initially divided into its primary storage accounts. To allow for continued refinement and adaptation to meet various needs, this framework is designed in a way such that accounts of different levels (i.e., one that is lumped and one that is broken down to multiple levels) can still interact to simulate various trades, leases, or joint operations.

Simulation of detailed water rights and complex accounting were both crucial when the ABRW model was used to support Pueblo Water's water rights change and exchange cases of its Bessemer Ditch agricultural water rights. In order to legally change the rights to allow for their municipal and storage uses, under Colorado Water Law's relatively new anti-speculation conditions, Pueblo Water was required to justify the need for these water rights to meet their demands within their 50-year planning horizon. Twenty planning scenarios, 100-year return interval, 6-year long design drought, were developed to envelope various potential levels of demand growth, climate change impacts, and curtailment of imports. The ABRW model was then used to provide an analysis of Pueblo Water' system conditions and the yields from their various water sources throughout the Arkansas basin under each scenario. Since the model responds dynamically to alternative hydrologic inputs, it was able to simulate the system under the extreme drought conditions, even though a drought of that magnitude doesn't exist in the available period of historical hydrology. Furthermore, the model was able to approximate the additional impacts that projected decreases in runoff due to climate change would have on the yields of their various water sources. The model results showed than under nearly all the planning scenarios, Pueblo Water would have a need for the yields from the Bessemer Ditch water rights to be able meet their delivery needs and operational objectives through the design drought period.