Empirical Models of Shallow Groundwater and Multi-Hazard Flood Forecasts as Sea-Levels Rise

American Geophysical Union (AGU) | February 8th, 2025

Knowledge of coastal hydrogeology and hazards as groundwater responds to sea-level rise (SLR) can be improved through installation of shallow groundwater monitoring piezo

Knowledge of coastal hydrogeology and hazards as groundwater responds to sea-level rise (SLR) can be improved through installation of shallow groundwater monitoring piezometers and continuous observations. Interpolation of site data enables mapping of the present-day state of groundwater elevation, depth to groundwater (DTW), their temporal statistical variation, and differing spatial responses to tides and rainfall. Future DTW and its variability can be projected under increments of SLR, with assumptions and caveats, to show where and when episodic and/or permanent inundation can be expected. This methodology is outlined in a case study of Dunedin, New Zealand, which enabled comparison of rising groundwater's contribution to pluvial flooding and groundwater emergence with coastal inundation. Changes in relative land exposure with SLR shows evolution in flood hazard from current pluvial-dominated events, into “flooding from below” and groundwater emergence, in advance of any overland coastal inundation. Dunedin exemplifies how groundwater transfers effects of SLR surprisingly far inland, but the lowest-lying or shoreline-proximal suburbs are not necessarily the most vulnerable. Unlike coastal inundation, rising groundwater is unconstrained by protective topography and presents as a creeping hazard, or contributor to hazards such as pluvial flooding, which can be widespread, occurring already and difficult to defend against. The empirical models contain assumptions and uncertainties important to the veracity of results and application. While conservative (“risk averse”) and a compromise from computationally expensive numerical solutions, their value is in providing the spatial and temporal precision needed for multi-source hazard assessment and holistic adaptive planning.

Empirical Return Periods of the Most Intense Vapor Transports during Historical Atmospheric River Landfalls on the U.S. West Coast

American Meteorological Society (AMS) | August 21st, 2018

Atmospheric rivers (ARs) come in all intensities, and clear communication of risks posed by individual storms in observations and forecasts can be a challenge. Modest AR

Atmospheric rivers (ARs) come in all intensities, and clear communication of risks posed by individual storms in observations and forecasts can be a challenge. Modest ARs can be characterized by the percentile rank of their integrated water vapor transport (IVT) rates compared to past ARs. Stronger ARs can be categorized more clearly in terms of return periods or, equivalently, historical probabilities that at least one AR will exceed a given IVT threshold in any given year. Based on a 1980–2016 chronology ofAR landfalls on the U.S. West Coast from NASA’s Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), datasets, the largest instantaneous IVTs—greater than 1700 kgm21 s21—have occurred in ARs making landfall between 418 and 468Nwith return periods longer than 20 years. IVT values with similar return periods are smaller to the north and, especially, to the south (declining to ;750 kgm21 s21). The largest storm-sequence IVT totals have been centered near 42.58N, with scatter among the top few events, and these large storm-sequence totals depend more on sequence duration than on the instantaneous IVT that went into them. Maximum instantaneous IVTs are largest in the Pacific Northwest in autumn, with largest IVT values arriving farther south as winter and spring unfold, until maximum IVTs reach Northern California in spring.

Escalating Hydrological Extremes and Whiplashes in the Western U.S.: Challenges for Water Management and Frontline Communities

American Geophysical Union (AGU) | May 14th, 2025

Estimates of Natural and Unimpaired Flows for the Central Valley of California: Water Years 1922-2014 (draft)

California Department of Water Resources (DWR) | March 1st, 2016

Estimating regional water supplies that would have occurred absent human activities is a common practice in water resources planning. In this report, such theor

Estimating regional water supplies that would have occurred absent human activities is a common practice in water resources planning. In this report, such theoretical water supply estimates are referred to as “unimpaired” flow. Since 1980, the Department of Water Resources (Department) has periodically published estimates of Central Valley unimpaired flows. In spite of the Department’s previous attempts to distinguish between natural conditions and its calcula tion of theoretical unimpaired flows, unimpaired flow estimates have frequently been used as a surrogate measure of natural conditions, presumably because natural flow estimates were unavailable. A major objective of this report is to clarify the conceptua l differences between natural and unimpaired flows. In this report, the term “unimpaired” flow is used to describe a theoretically available water supply assuming existing river channel conditions in the absence of (1) storage regulation for water supply a nd hydropower purposes and (2) stream diversions for agricultural and municipal uses. Unimpaired flow estimates are theoretical in that such conditions have not occurred historically. In pristine watersheds which have undergone little land use change, unim paired flow estimates provide a fixed frame of reference to develop relationships between precipitation, runoff, and water supply based on long - term hydrologic records. For many years these relationships were based on the assumption of stationarity, i.e. that the past is a good indicator of the future. However, global warming now requires hydrologists and water resources managers to analyze non-stationary processes, requiring more sophisticated tools and techniques to quantify future water supplies. This report updates and extends the Department’s previous published estimates of unimpaired flows for 24 Central Valley sub-basins and the Delta. Monthly unimpaired flows are presented for water years 1922 - 2014.

Estimates of present and future flood risk in the conterminous United States

Environmental Research Letters (IOP) | February 28th, 2018

Past attempts to estimate rainfall-driven flood risk across the US either have incomplete coverage, coarse resolution or use overly simplified models of the flood

Past attempts to estimate rainfall-driven flood risk across the US either have incomplete coverage, coarse resolution or use overly simplified models of the flooding process. In this paper, we use a new 30 m resolution model of the entire conterminous US with a 2D representation of flood physics to produce estimates of flood hazard, which match to within 90% accuracy the skill of local models built with detailed data. These flood depths are combined with exposure datasets of commensurate resolution to calculate current and future flood risk. Our data show that the total US population exposed to serious flooding is 2.6–3.1 times higher than previous estimates, and that nearly 41 million Americans live within the 1% annual exceedance probability floodplain (compared to only 13 million when calculated using FEMA flood maps). We find that population and GDP growth alone are expected to lead to significant future increases in exposure, and this change may be exacerbated in the future by climate change.

Estimating evapotranspiration change due to forest treatment and fire at the basin scale in the Sierra Nevada, California: Forest Disturbance and Evapotranspiration Change

Ecohydrology (Wiley) | April 6th, 2018

We investigated the potential magnitude and duration of forest evapotranspiration (ET) decreases resulting from forest‐thinning treatments and wildfire in west‐slope

We investigated the potential magnitude and duration of forest evapotranspiration (ET) decreases resulting from forest‐thinning treatments and wildfire in west‐slope watersheds of the Sierra Nevada range in California, USA, using a robust empirical relation between Landsat‐derived mean‐annual normalized difference vegetation index (NDVI) and ET measured at flux towers. Among forest treatments, the minimum observed NDVI change required to produce a significant departure from control plots with NDVI of about 0.70 was −0.09 units, corresponding to a basal‐area reduction of 29.1 m2/ha (45% reduction) and equivalent to an estimated ET reduction of 153 mm/year (21% change; approximate mean annual precipitation = 1,000 mm). Intensive thinning in highly productive forests that approached prefire‐exclusion densities reduced basal area by 40–50%, generating estimated ET reductions of 153–218 mm/year (21–27% change) over 5 years following treatment. Low‐intensity underburn treatments resulted in no significant change in ET. Examining the cumulative impact of wildfires on ET between 1990 and 2008, we found that the lower and wetter American River basin (5,310 km2) generated more than twice the ET reduction per unit area than those in the higher and drier Kings River basin (4,790 km2), corresponding to greater water and energy limitations in the latter and greater fire severity in the former. A rough extrapolation of these results to the entire American River watershed suggests that ET reductions due to forest thinning by wildfire could approach 10% of full natural flows for dry years and 5% over all years.

Estimating Historical California Precipitation Phase Trends Using Gridded Precipitation, Precipitation Phase, and Elevation Data

California Department of Water Resources (DWR) | July 18th, 2014

Climate change projections for California indicate reduction in the percentage of precipitation that falls as snow, and increase in the percentage that falls as rain, due

Climate change projections for California indicate reduction in the percentage of precipitation that falls as snow, and increase in the percentage that falls as rain, due to warmer temperatures in the future. Previous work has shown overall decreases in rainfall / snowfall ratios for the western US over the last 60 years. Of interest in this study is the quantification of snow/rain ratios for smaller regions, specifically in California. Estimating cumulative precipitation phase ratios for specific regions is difficult due to large differences in local precipitation. In California, the high relief of the surface topography makes such estimates particularly difficult. The low spatial resolution of suitable precipitation/snow water equivalent monitoring stations contributes to the difficulty in quantifying the trends for sub-state sized regions of interest in this study. The present exploratory study develops and describes a methodology that uses readily available research data sets to produce gridded estimates of historical rainfall as a fraction of total precipitation for areas comprising the major water-supply watersheds of California. Using this methodology, statistically significant increases in the ratio of annual liquid to total precipitation are seen for large areas in the northern part of the State and northern Sierra over the water year. No significant annual trends are seen for regions in the central and southern portions of the Sierra. Future work extending the analysis to distinct elevation ranges and seasonality would provide more refined conclusions.

Estimating the potential economic impacts of climate change on Southern California beaches

Climatic Change (Springer) | November 24th, 2011

Climate change could substantially alter the width of beaches in Southern California. Climate-driven sea level rise will have at least two important impacts on beaches: (

Climate change could substantially alter the width of beaches in Southern California. Climate-driven sea level rise will have at least two important impacts on beaches: (1) higher sea level will cause all beaches to become more narrow, all things being held constant, and (2) sea level rise may affect patterns of beach erosion and accretion when severe storms combine with higher high tides. To understand the potential economic impacts of these two outcomes, this study examined the physical and economic effects of permanent beach loss caused by inundation due to sea level rise of one meter and of erosion and accretion caused by a single, extremely stormy year (using a model of beach change based on the wave climate conditions of the El Niño year of 1982/1983.) We use a random utility model of beach attendance in Southern California that estimates the impacts of changes on beach width for different types of beach user visiting public beaches in Los Angeles and Orange Counties. The model allows beachgoers to have different preferences for beach width change depending on beach size. We find that the effect of climate-driven beach change differs for users that participate in bike path activities, sand-based activities, and water-based activities. We simulate the effects of climate-related beach loss on attendance patterns at 51 public beaches, beach-related expenditures at those beaches, and the non-market (consumer surplus) value of beach going to those beaches. We estimate that increasing sea level will cause an overall reduction of economic value in beach going, with some beaches experiencing increasing attendance and beach-related earnings while attendance and earnings at other beaches would be lower. We also estimate that the potential annual economic impacts from a single stormy year may be as large as those caused by permanent inundation that would result from a rise in sea level of one meter. The economic impacts of both permanent inundation and storm-related erosion are distributed unevenly across the region. To put the economic impacts of these changes in beach width in perspective, the paper provides simple estimates of the cost of mitigating beach loss by nourishing beaches with sand.

Evaluating GFS and ECMWF Ensemble Forecasts of Integrated Water Vapor Transport along the U.S. West Coast

American Meteorological Society (AMS) | October 28th, 2022

Atmospheric rivers (ARs) are long and narrow regions in the atmosphere of enhanced integrated water vapor transport (IVT) and can produce extreme precipitation and high s

Atmospheric rivers (ARs) are long and narrow regions in the atmosphere of enhanced integrated water vapor transport (IVT) and can produce extreme precipitation and high societal impacts. Reliable and skillful forecasts of landfalling ARs in the western United States are critical to hazard preparation and aid in decision support activities, such as Forecast-Informed Reservoir Operations (FIRO). The purpose of this study is to compare the cool-season water year skill of the NCEP Global Ensemble Forecast System (GEFS) and ECMWF Ensemble Prediction System (EPS) forecasts of IVT along the U.S. West Coast for 2017–20. The skill is analyzed using probability-over-threshold forecasts of IVT magnitudes ≥ 250 kg m−1 s−1 (P250) using contingency table skill metrics in coastal Northern California and along the west coast of North America. Analysis of P250 with lead time (dProg/dt) found that the EPS provided ∼1 day of additional lead time for situational awareness over the GEFS at lead times of 6–10 days. Forecast skill analysis highlights that the EPS leads over the GEFS with success ratios 0.10–0.15 higher at lead times > 6 days for P250 thresholds of ≥25% and ≥50%, while event-based skill analysis using the probability of detection (POD) found that both models were largely similar with minor latitudinal variations favoring higher POD for each model in different locations along the coast. The relative skill of the EPS over the GEFS is largely attributed to overforecasting by the GEFS at longer lead times and an increase in the false alarm ratio.

Evaluating historical changes in a mussel bed community in northern California

Nature Portfolio (Springer Nature) | January 14th, 2025