A global transition to flash droughts under climate change

American Association for the Advancement of Science (AAAS) | April 13th, 2023

Flash droughts have occurred frequently worldwide, with a rapid onset that challenges drought monitoring and forecasting capabilities. However, there is no consensus on w

Flash droughts have occurred frequently worldwide, with a rapid onset that challenges drought monitoring and forecasting capabilities. However, there is no consensus on whether flash droughts have become the new normal because slow droughts may also increase. In this study, we show that drought intensification rates have sped up over subseasonal time scales and that there has been a transition toward more flash droughts over 74% of the global regions identified by the Intergovernmental Panel on Climate Change Special Report on Extreme Events during the past 64 years. The transition is associated with amplified anomalies of evapotranspiration and precipitation deficit caused by anthropogenic climate change. In the future, the transition is projected to expand to most land areas, with larger increases under higher-emission scenarios. These findings underscore the urgency for adapting to faster-onset droughts in a warmer future.

A greener grass: Can the environmental impact of cannabis cultivation be reduced?

Nature Portfolio (Springer Nature) | August 28th, 2019

A Guide for Using the Transient Ground-Water Flow Model of the Death Valley Regional Ground-Water Flow System, Nevada and California

U.S. Geological Survey (USGS) | November 4th, 2006

This report is a guide for executing numerical simulations with the transient ground-water flow model of the Death Valley regional ground-water flow system, Nevada and Ca

This report is a guide for executing numerical simulations with the transient ground-water flow model of the Death Valley regional ground-water flow system, Nevada and California using the U.S. Geological Survey modular finite-difference ground-water flow model, MODFLOW-2000. Model inputs, including observations of hydraulic head, discharge, and boundary flows, are summarized. Modification of the DVRFS transient ground-water model is discussed for two common uses of the Death Valley regional ground-water flow system model: predictive pumping scenarios that extend beyond the end of the model simulation period (1998), and model simulations with only steady-state conditions.

A Guide to California Water Right Appropriations

California State Water Resources Control Board (SWRCB) | January 1st, 2000

A guide to California's water right permit process written by the State Water Resources Control Board, dated January 2000. TABLE OF CONTENTS California Water Righ

A guide to California's water right permit process written by the State Water Resources Control Board, dated January 2000. TABLE OF CONTENTS California Water Rights Permit Issuance - A look at the process Things to consider before filing your application Filing your applicationj Appendices

A Guide to Ordinary High Water Mark (OHWM) Delineation for Non-Perennial Streams in the Western Mountains, Valleys, and Coast Region of the United States

U.S. Army Corps of Engineers (USACE) | August 14th, 2014

Federal regulations define the ordinary high water mark (OHWM) as “that line on the shore established by the fluctuations of water and indicated by physical cha

Federal regulations define the ordinary high water mark (OHWM) as “that line on the shore established by the fluctuations of water and indicated by physical characteristics such as a clear, natural line impressed on the bank, shelving, changes in the character of soil, destruction of terrestrial vegetation, the presence of litter and debris, or other appropriate means that consider the characteristics of the surrounding areas” (U.S. Congress 1986). Under Section 404 of the Clean Water Act (CWA), the OHWM defines the lateral extent of federal jurisdiction in non-tidal waters of the United States (WoUS) in the absence of adjacent wetlands (U.S. Congress 1977). Thus, accurate and consistent OHWM delineation practices are essential for proper implementation of the CWA. The dynamic nature of stream systems and fluvial processes presents challenges for OHWM delineation. Natural sources of variability in river and stream systems (e.g., climate, sediment supply, landscape position, etc.) are compounded by direct and indirect anthropogenic sources of variability (e.g., watershed alteration, dam emplacement and removal, climate change, etc.). Thus, it is challenging to impose a consistent measure of “ordinary” high flow conditions across systems in which the hydrology and geomorphology can vary greatly in both space and time. OHWM delineation in non-perennial (i.e., intermittent and ephemeral) streams can be especially challenging. The U.S. Army Corps of Engineers (USACE) defines intermittent streams as having “flowing water during certain times of the year, when groundwater provides water for stream flow. During dry periods, intermittent streams may not have flowing water. Runoff from rainfall is a supplemental source of water for stream flow” (USACE 2012). Ephemeral streams have “flowing water only during, and for a short duration after, precipitation events in a typical year. Ephemeral stream beds are located above the water table year-round. Groundwater is not a source of water for the stream,” and “[r]unoff from rainfall is the primary source of water for stream flow” (USACE 2012). In contrast to both intermittent and ephemeral streams, perennial streams have “flowing water year-round during a typical year. The water table is located above the stream bed for most of the year,” and “[g]roundwater is the primary source of water for stream flow” (USACE 2012). Given the less persistent streamflow regimes characteristic of non- perennial streams, particularly ephemeral systems, the characterization of ordinary high water flows is perhaps more challenging than in perennially flowing systems. Moreover, depending on climate, vegetation, and other related factors, the appearance of some OHWM indicators may vary greatly between wet and dry seasons or between relatively infrequent flow events, more so than in many perennial streams. Mountainous terrain can present additional challenges to OHWM delineation. For instance, the relatively steep and confined valleys in which mountain streams commonly flow can restrict the development of some alluvial features (e.g., flood-plains, bankfull benches, etc.) that are typical of low-gradient systems and that may help to identify the OHWM. Thus, in non-perennial mountain streams, it is often difficult to determine what constitutes ordinary high water and to interpret the physical and biological indicators established and maintained by ordinary high water flows. Challenges and inconsistencies pertaining to OHWM delineation practices are becoming increasingly relevant in mountainous parts of the western U.S. in light of expanding development. This increased pressure on fluvial systems highlights the need for accurate, consistent, and repeatable OHWM delineation practices in this region. These factors, combined with the particular challenges of OHWM delineation in non-perennial mountain streams, provided the impetus for developing this delineation guide. This guide presents the concepts, field indicators, and methods for assessing, delineating, and documenting the OHWM in non-perennial streams in the Western Mountains, Valleys, and Coast (WMVC) Region of the United States (Figure 1). The information presented here is based on the findings of Mersel et al. (2014) (discussed in Section 1.5) and on years of field observations and data gathering in the WMVC Region and in other regions of the U.S. by the authors and other contributing experts. The remainder of Section 1 provides background information regarding the concept of the OHWM and pertaining to stream hydrology and geomorphology in general. Section 2 discusses and provides examples of the specific field indicators used to identify the OHWM in non-perennial streams in the WMVC Region. Section 3 discusses field methods for delineating the OHWM and addresses additional techniques and lines of evidence that may help in problematic delineation scenarios. The information presented here is technical guidance and does not define, amend, or replace any existing regulations, laws, or legal guidance related to the OHWM or to the regulation of WoUS. Furthermore, determining whether any stream is a jurisdictional WoUS is beyond the scope of this document and involves further assessment in accordance with regulations, case law, and clarifying guidance. This guide pertains to non-perennial streams in the WMVC Region of the U.S., and while the information presented here may have a wider applicability to other regions or to perennial rivers within the WMVC Region, this has not been tested or validated. This manual serves as a companion to A Field Guide to the Identification of the Ordinary High Water Mark (OHWM) in the Arid West Region of the Western United States (Lichvar and McColley 2008) as these two regions—the WMVC and the Arid West—are interspersed with one another. Best professional judgment is required to determine which manual is most appropriate for any given location within these two regions. The technical guidance presented here aims to provide an informed and consistent approach to OHWM delineation within the WMVC Region; however, OHWM delineation is not a precise practice. The OHWM can take on a variety of appearances and characteristics and may change over time due to natural or anthropogenic causes. Best professional judgment and consideration of the unique characteristics of each project site are always required.

A Guide to Water Quality Requirements under the Sustainable Groundwater Management Act

Stanford University | April 17th, 2019

California’s Sustainable Groundwater Management Act (SGMA) requires Groundwater Sustainability Agencies (GSAs) develop Groundwater Sustainability Plans (GSPs) that ach

California’s Sustainable Groundwater Management Act (SGMA) requires Groundwater Sustainability Agencies (GSAs) develop Groundwater Sustainability Plans (GSPs) that achieve sustainability within twenty years of Plan adoption. Meeting this goal requires GSAs to avoid causing any of the six undesirable results that are central to SGMA, including undesirable result number 4 (UR No. 4) – “significant and unreasonable degraded water quality” throughout the basin. Unlike most other undesirable results, UR No. 4 is the subject of robust federal, state and local regulatory regimes carried out by a number of different entities. While the State and Regional Water Resources Control Boards have the broadest responsibilities to protect groundwater quality, many other agencies also have relevant responsibilities. The other distinctive feature of UR No. 4 is that groundwater management that prevents chronic lowering of groundwater levels may be insufficient to prevent significant and unreasonable degradation of groundwater quality. For example, significant changes in groundwater pumping can potentially mobilize naturally occurring constituents or cause migration or expansion of contaminant plumes even when the extracted amount is within the basin’s sustainable yield. This Guide is intended to assist GSAs along with other agencies and stakeholders in understanding the meaning of UR No. 4, and to provide insights into how GSAs can successfully fulfill their legal responsibilities regarding groundwater quality. To avoid UR No. 4 under SGMA, this Guide recommends that GSAs do the following: 1. Understand the existing regulatory regime for groundwater quality and, as necessary, confer with entities that have regulatory authority over groundwater quality in their basins. (See Section 3 of this Guide). 2. Consider existing federal, state and local groundwater quality standards when developing groundwater quality minimum thresholds in their basins. Federal, state or local regulatory requirements were established after careful scientific study, legal review and procedural steps mandated by law. They are designed to protect public health and welfare, and they provide the clearest indication of the point beyond which there is a real risk of an undesirable result. (See Section 3.1). 3. Work closely with their Regional Water Quality Control Boards to address all groundwater quality issues. GSAs working in the Central Valley are especially encouraged to work with the Central Valley Regional Board to address salt and nutrient groundwater contamination. Programs in this region are continuing to evolve and will require that GSAs remain aware of changing regulatory requirements. Additionally, there are many groundwater quality monitoring efforts in this region that are generating a lot of groundwater quality data. These efforts should be aligned with any groundwater monitoring efforts being undertaken by GSAs. (See Sections 3.4.2 and 4.3.2). 4. Use existing groundwater quality data and information to (1) assess and prioritize groundwater quality issues in their basins, (2) meet requirements that GSPs include a description of current and historical groundwater conditions within the basin and (3) serve as either the basis for or a complement to local groundwater quality monitoring programs developed by GSAs. 5. Consider the following approach (discussed in Section 3.2) if they identify groundwater quality problems that either arose or were exacerbated after January 1, 2015: • Where there is a significant groundwater quality problem that is clearly under the purview of another agency, confer with that agency and seek to confirm a reasonable plan to address the groundwater quality problem. If such a plan exists, the water quality problem and the plan should be referenced in the GSP. Section 3 and Table 1 in the Guide provide information on existing regulatory programs and the agencies responsible for their enforcement. • Where a significant groundwater quality problem is not clearly under the purview of another agency, or the responsible agency is unable to confirm a reasonable plan to address the problem, confer with Regional or State Board staff, and perhaps affected parties, to identify a reasonable plan to address the problem. If no reasonable plan is identified, and remediating the problem is impractical, excessively wasteful of resources or otherwise infeasible, the GSA should include in the Plan an explanation of the problem and the reasons why remediation is impractical or infeasible. 6. Consider the following approach (discussed in Section 5) if they are planning projects or actions that may affect groundwater quality: • Assess the potential impacts that the projects or actions are likely to have on groundwater quality over time. If the GSA concludes that the proposed project or action could have unacceptable adverse impacts on water quality, GSAs should consider options such as revising the project to avoid or mitigate the project’s impacts or developing alternative projects that avoid water quality problems. • Ensure regulatory compliance for any GSP management actions (e.g., recharge projects, water banking) that have the potential to negatively impact water quality in their basins or adjacent basins. (Sections 3 and 5 provide information on regulatory requirements and considerations associated with groundwater recharge and active basin management, respectively.) 7. Develop supplemental groundwater monitoring networks where necessary. Many basins being managed under SGMA have significant water quality data gaps. Thus, GSAs in many basins will likely need to develop or expand local groundwater quality monitoring networks. Decisions to do so should be informed by (1) the extent and quality of existing groundwater quality in the region, (2) the level of groundwater development, (3) the severity of existing groundwater quality issues and their proximity to domestic or irrigation wells, public water supply sources or other vulnerable users, (4) current or proposed groundwater management actions or projects, (5) local resources, and (6) the hydrogeologic complexity of the basin. GSAs are not responsible for enforcing existing water quality standards or collecting data to support existing water quality programs. However, they are responsible for avoiding “significant and unreasonable” degradation of water quality in their basins. Working with federal, state and local water quality monitoring and enforcement entities will help GSAs understand groundwater quality issues in their basins, enabling them to develop GSPs that avoid UR No. 4.

A Historical Perspective of the Interagency Ecological Program: Bridging Multi-Agency Studies into Ecological Understanding of the Sacramento-San Joaquin Delta and Estuary for 40 Years

Interagency Ecological Program (IEP) | February 8th, 2013

The Interagency Ecological Program (IEP) has been in existence in some form for almost 50 years. As the San Francisco Estuary evolved due to natural and man-caused pertur

The Interagency Ecological Program (IEP) has been in existence in some form for almost 50 years. As the San Francisco Estuary evolved due to natural and man-caused perturbations, the IEP has responded by creating and re-creating itself to ensure the best understanding of the Estuary as it was influenced by water project implementation. The goal of the IEP has always been to provide this understanding to managers responsible for water project implementation in the most protective and efficient manner given social, political, and environmental constraints. On the whole, IEP has done an outstanding job of providing consistently useful data and information in a highly complex environment. As new programs and constraints are developed to cope with the ever changing natural environment and demands of society, an understanding of the history of IEP is valuable, and to learn from past experience is wise. This descriptive history is meant to document in some detail the development, strengths, and weaknesses of this one-of-a-kind multi-agency consortium that has attempted to bridge the gap between water management and estuarine science in the most significant Estuary on the west coast of North America. Sources of information for this account are personal interviews with program participants, annual reports, IEP Newsletters, coordinator meeting summaries, director meeting summaries, management team meeting notes, and personal experiences of the author. This history may err on the side of excessive detail, but that has been intentional; the author believes it is important to have detailed information available in a single reference so that interested parties can readily obtain knowledge about IEP. Some individuals may not want to read the entire report, but can easily be guided to topics of interest by consulting Appendix A, which lists significant events that influenced the IEP by page number in the report.

A holistic stochastic model for precipitation events

Nature Portfolio (Springer Nature) | February 7th, 2025

In the western United States, much of the annual precipitation falls during relatively few storm events. When precipitation is measured as daily (or hourly, etc.) accumul

In the western United States, much of the annual precipitation falls during relatively few storm events. When precipitation is measured as daily (or hourly, etc.) accumulations, these events appear as sequences of various durations. We describe a trivariate probability distribution and assert that it provides a natural description for the durations, magnitudes, and maximum intensities of such events. We show that this distribution, in its most straightforward application, indeed describes precipitation events composed of daily observations at most long-term weather stations across the western United States, allowing for a flexible assessment of specific event probabilities and return periods with respect to their three defining components. This characterization opens the door to further understanding of risk associated with out-of-sample meteorological events.

A hybrid machine learning model to predict and visualize nitrate concentration throughout the Central Valley aquifer, California, USA

Science of the Total Environment (Elsevier) | June 9th, 2017

Intense demand for water in the Central Valley of California and related increases in groundwater nitrate concentration threaten the sustainability of the groundwater res

Intense demand for water in the Central Valley of California and related increases in groundwater nitrate concentration threaten the sustainability of the groundwater resource. To assess contamination risk in the region, we developed a hybrid, non-linear, machine learning model within a statistical learning framework to predict nitrate contamination of groundwater to depths of approximately 500m below ground surface. A database of 145 predictor variables representing well characteristics, historical and current field and landscape-scale nitrogen mass balances, historical and current land use, oxidation/reduction conditions, groundwater flow, climate, soil characteristics, depth to groundwater, and groundwater age were assigned to over 6000 private supply and public supply wells measured previously for nitrate and located throughout the study area. The boosted regression tree (BRT) method was used to screen and rank variables to predict nitrate concentration at the depths of domestic and public well supplies. The novel approach included as predictor variables outputs from existing physically based models of the Central Valley. The top five most important predictor variables included two oxidation/reduction variables (probability of manganese concentration to exceed 50 ppb and probability of dissolved oxygen concentration to be below 0.5 ppm), field-scale adjusted unsaturated zone nitrogen input for the 1975 time period, average difference between precipitation and evapotranspiration during the years 1971–2000, and 1992 total landscape nitrogen input. Twenty-five variables were selected for the final model for log-transformed nitrate. In general, increasing probability of anoxic conditions and increasing precipitation relative to potential evapotranspiration had a corresponding decrease in nitrate concentration predictions. Conversely, increasing 1975 unsaturated zone nitrogen leaching flux and 1992 total landscape nitrogen input had an increasing relative impact on nitrate predictions. Three-dimensional visualization indicates that nitrate predictions depend on the probability of anoxic conditions and other factors, and that nitrate predictions generally decreased with increasing groundwater age.

A hydrological simulation dataset of the Upper Colorado River Basin from 1983 to 2019

Nature Portfolio (Springer Nature) | January 20th, 2022

The Upper Colorado River Basin (UCRB) is one of the most over-allocated basins in the world. It provides water for 40 million people in Colorado and downstream states of

The Upper Colorado River Basin (UCRB) is one of the most over-allocated basins in the world. It provides water for 40 million people in Colorado and downstream states of Arizona, California, Nevada, and Utah. However, the UCRB water capacity is decreasing as a combined result of climate change and anthropogenic activities. Understanding the dynamic of the UCRB water cycle is crucial for long-term and short-term water resources management.