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From the Rocky Mountains in the US to the Sea of Cortez in Mexico, the Colorado River is one of the most managed and monitored rivers in the world. As decreasing river flows from drought and climate change continue to impact the Colorado River Basin, the US federal government, seven US states, and Mexico have signed agreements that would reduce deliveries, including a Drought Contingency Plan (DCP), an agreement among the US states and the Bureau of Reclamation, and Minute 323 (a treaty agreement between the US and Mexico). As part of the DCP, the US Bureau of Reclamation (Reclamation) committed to increasing conservation efforts to augment storage in Lake Mead. This paper presents an analysis of Reclamation's Decree Accounting Reports that record water deliveries and conservation savings from 1964 to 2019. Overall, this analysis illustrates that (1) the agency has implemented significant conservation actions over the last three decades, (2) the agency has responded to increased accounting needs as dictated by national and international management agreements, (3) some actions have resulted in environmental costs, and (4) future actions could be designed to avoid environmental harms while also augmenting Lake Mead levels. As climate change continues to constrain Colorado River water supply, detailed accounting may help reveal areas for potential efficiencies or demonstrate where the greatest levels of savings have been reached while ensuring that environmental and social benefits are preserved. Over the last 100 years, humans have changed the path and personality of the Colorado River by distributing water to cities, communities, and agricultural fields to support over 40 million people. Throughout the twentieth century, the US federal government, seven US, and Mexico signed agreements enabling development of Colorado River water. By the early 21st century, as it became evident that water demands and climate change were reducing available supplies, these same governments signed agreements to reduce pressure on the river. All these agreements have, in part, been tracked by the US federal agency that manages the river, the Bureau of Reclamation (Reclamation), through Decree Accounting Reports. Since 1964, these reports have recorded how water is or is not accounted for that might be important to the future of the river. This paper examines those reports and reveals four main findings: (1) Reclamation has implemented numerous water conservation projects, (2) they have responded to new management agreements, (3) some of their actions have impacted the environment, and (4) there are proactive ways they can reduce environmental impacts and bolster water supplies in the future. [ABSTRACT FROM AUTHOR]

Utilities are increasingly considering the use of temporary water transfers to augment their supplies during periods of drought, an alternative that is often less expensive than expanding safe yields through new infrastructure. Understanding the volume and timing of transfers is important for developing contracts between buyer and seller and can be challenging due to the transient nature of drought, a situation complicated by the uncertainties associated with climate change. While transfer arrangements have received some attention in the literature, the effects of climate change on such agreements remain unexplored. This paper investigates these impacts using an improved method for developing new hydro-climate scenarios. A technique for producing stochastic time series of inflows is described, one which effectively replicates the autocorrelation present in the historic record. Unlike autoregressive (and similar) models that assume complete stationarity, the modified fractional Gaussian noise (mFGN) method preserves the seasonal patterns in the correlation structure, thereby providing some advantages when modifying historical streamflow records to reflect alternative hydro-climate scenarios. Alternative scenarios are developed for the Research Triangle region of North Carolina, an area with several utilities currently seeking to use a system of risk-based transfer agreements as a means to meet demand during droughts. This study simulates hydrologic conditions and transfer activity from 2010 to 2025 under a variety of climate scenarios. Results indicate that increased variability in inflows, with no change in the mean, corresponds to slight increases in transfer activity. However, when increased variability is paired with modest decreases in expected inflows (7%), transfer activity is doubled. [ABSTRACT FROM AUTHOR]

The identification and prediction of drought events depend on the integrity of the dataset employed. Streamflow is a good indicator of surface water availability and has dominated the literature on frequency analysis of hydrological droughts and water management. However, gauged measurements are impaired by climate and land use changes, especially in large, modified watersheds. Hence, their use in drought prediction is limited because they may violate the assumption of stationarity unless a naturalized observation series is obtained. In this paper, a land surface model is used to generate runoff in the Rio Grande/Río Bravo del Norte basin. Land use land cover is kept constant so that changes are a result of climatological variations. The river threads across several climatic zones; therefore the basin is divided into different regions for analysis. Using statistical theory of runs, water deficit duration and severity and drought interarrival time are extracted from 3-month Standardized Runoff Index series for each region and for the whole basin. Copulas are used to develop joint distribution functions of water deficit duration and severity. Nine copulas, two from the extreme value family and seven from the one-parameter families of the Archimedean copulas, are tested. Different copulas are deemed suitable for each region because they are subject to different climatologic conditions, which affect the nature of deficit events, especially at the extremes. Conditional probability models of duration and severity are developed and compared for two regions-one experiencing alpine climate at the headwaters and one in the semiarid portion of the basin. Further, conditional probabilities derived from model runoff are compared with that based on observed precipitation. Finally, univariate and conditional return periods based on duration and severity are computed for the basin and their significance in long-term water resources planning is discussed. [ABSTRACT FROM AUTHOR]

Hydroclimatological records are increasingly examined for evidence of trends and shifts that may assist in prediction of future climate change scenarios. This study investigates the trend and step changes in U.S. streamflow over a 52-year period (1951–2002) using data from 639 unimpaired streamflow stations categorized according to the hydrologic unit codes. This is particularly relevant since the issue of climate change is of interest to many, and studies have indicated an abrupt change in climate around the year 1976/77. Trends were evaluated using three statistical tests: Spearman’s rho, Mann-Kendall, and linear regression, and step changes were evaluated using the rank sum and student’s t test. The temporal resolution used for the study included water year (Oct–Sept), autumn–winter (Oct–Mar), and spring–summer (Apr–Sept) periods. Additionally, April 1 snow-water equivalent (SWE) data for 121 SNOTEL stations for the period 1941 to 2004 were used to test for the trends in the western U.S. The multiple statistical tests provided robust results for regions with significant changes. Results indicated that the Mississippi and Missouri regions have an increasing trend in streamflow quantity. The Pacific Northwest and South Atlantic-Gulf regions have streamflow decreasing due to a step change in climate. Decreasing trends for the SWE were noted for a number of stations in the states of Oregon and Utah. [ABSTRACT FROM AUTHOR]