Determining the best functional form of reservoir operating policies to maximize the value of integrating seasonal streamflow forecasts into reservoir operations in the Colorado River Basin

Abstract

Droughts are among the most severe natural disasters, but are difficult to prepare for because of their slowonset. In absence of effective drought early warning systems and response plans, water users oftencontinue to pursue short-term economic gains that ultimately come at the expense of longer-termeconomic and ecological sustainability. Fortunately, seasonal forecasts have shown promise for informingadaptive water management policies that can reduce these impacts. One avenue of adaptation is throughreservoir operations, where seasonal streamflow forecasts can enable hedging of releases to favor morefrequent but less impactful water shortages over more devastating impacts down the line. This can beachieved by optimizing reservoir operating policies that define how much water to release from a networkof reservoirs as a function of these seasonal forecasts. However, the best functional form for such policiesis an open research question.The goal of this project is to compare alternative formulations of reservoir operating policies conditionalon seasonal climate forecasts to see which is most effective in reducing economic and ecological droughtimpacts. We investigate this question in the Colorado River Basin (CRB). Commonly termed the“lifeblood of the West,” the Colorado River provides irrigation water for over 5.5 million acres ofagricultural land, drinking water for more than 40 million people, and over 4000 MW of installedhydropower capacity. Yet managing this system is becoming increasingly challenging due to ongoingclimatic and anthropogenic drought conditions that jeopardize water security and endanger the river’secological health. To improve water security in the basin, this project aims to optimize reservoir operatingpolicies in Lake Mead, Lake Powell, and three upstream reservoirs by coupling a RiverWare reservoirmodel of the CRB with Borg, a multi-objective optimization algorithm, to reduce the frequency andseverity of water shortages to the Upper Colorado River Basin (UCRB), Lower Colorado River Basin(LCRB), and Mexico. In this study, we explore different ways of incorporating seasonal forecasts of theinflows to these five reservoirs into their operating policies considering different functional forms of theoperating policies using a model-free, closed loop optimal control method called Direct Policy Search(DPS). Specifically, we compare using logistic and Gaussian Radial Basis Functions (RBFs) for thereservoir rules. Our work illustrates the value of integrating streamflow forecasts into reservoir operatingrules for drought management, while also providing insights into how to formulate the policies tomaximize that value.