Dennis Lettenmaier

University of California, Los Angeles | UCLA · geog

Ongoing runoff declines in the Colorado River Basin have been shown to be predominately driven by decreasing albedo from warming‐driven snow‐cover loss, especially in late‐spring (hereafter snowmelt‐radiation feedback). Here, we explore the feedback's impact on annual runoff sensitivity to warming across the western U.S. (WUS) using hydrologic mode...

Streamflow forecasts are critical for water and environmental management, especially in the water-short Western U.S.. Land Surface Models (LSMs), such as the Variable Infiltration Capacity (VIC) model and the Noah-Multiparameterization (Noah-MP) play an essential role in providing comprehensive runoff forecasts across the region. Virtually all LSMs...

Unlabelled: The frequency and severity of wildfires in the Western United States have increased over recent decades, motivating hypotheses that wildfires contribute to the incidence of coccidioidomycosis, an emerging fungal disease in the Western United States with sharp increases in incidence observed since 2000. While coccidioidomycosis outbreak...

Plain Language Summary Summer streamflow is a critical water resource in the generally dry summers of the western U.S. (WUS), and is routinely forecasted using spring snowpack and/or winter total precipitation as primary predictors. However, climate warming leads to reduced snowpacks, exacerbates summer low flows, and reduces the accuracy of snow‐b...

Atmospheric rivers often cause extreme precipitation at landfall, leading to floods. However, the underlying physical mechanism of atmospheric rivers and their linkage with flooding in India remain unrecognized. Here we use reanalysis and observations to diagnose the drivers of atmospheric rivers and unravel their crucial role in extreme precipitat...

Predictions of drought onset and termination at subseasonal (from two weeks to one month) lead times could provide a foundation for more effective and proactive drought management. We used reforecasts archived in NOAA’s Subseasonal Experiment (SubX) to force the Noah Multi-parameterization (Noah-MP), which produced forecasts of soil moisture from w...

Accurate estimates of snow water equivalent (SWE) based on remote sensing have been elusive, particularly in mountain areas. However, there now appears to be some potential for direct satellite-based SWE observations along ground tracks that only cover a portion of a spatial domain (e.g., watershed). Fortunately, spatiotemporally continuous meteoro...

Mountain snowpacks are transitioning to experience less snowfall and more rainfall as the climate warms, creating more persistent low- to no-snow conditions. This precipitation shift also invites more high-impact rain-on-snow (ROS) events, which have historically yielded many of the largest and most damaging floods in the Western United States. One...

We analyze uncertainty in model-based estimates of Probable Maximum Precipitation (PMP) as used in dam spillway design. Our focus is on model-based PMP derived from WRF (Weather Research and Forecasting) model reconstructions of severe historical storms, amplified by the addition of moisture in the boundary conditions (so-called Relative Humidity M...

Background Drought is an understudied driver of infectious disease dynamics. Amidst the ongoing southwestern North American megadrought, California (USA) is having the driest multi-decadal period since 800 CE, exacerbated by anthropogenic warming. In this study, we aimed to examine the influence of drought on coccidioidomycosis, an emerging infecti...

Accurate remote sensing-based snow water equivalent (SWE) estimates have been elusive, particularly in mountain areas, however, there now appears to be some potential for direct satellite-based SWE observations along ground tracks that only cover a portion of a spatial domain (e.g., watershed). Fortunately, spatiotemporally continuous meteorologica...

Significance How will increasing wildfire activity affect water resources in the water-limited western United States (WUS)? Among basins where >20% of forest burned, postfire streamflow is significantly enhanced by an average of approximately 30% for 6 y. Over 2015 to 2020, several large WUS basins experienced >10% of forest burned. Climate project...

Future climate warming over the Western U.S. (WUS) is projected to be greater in summer than winter. Previous model‐based studies of large river basins in the WUS showed much different annual streamflow responses to warming in warm versus cool seasons. However, it remains unclear how the relative responses annual streamflow relative responses to se...

Plain Language Summary Floods are among the most common and costliest of natural disasters. Extreme rainfall is a major cause of flooding and has become more common as global temperatures have increased. However, many studies show that the number and size of flood events remain mostly unchanged. It is possible that climate change is affecting diffe...

Background Coccidioidomycosis is an emerging infection in the southwestern United States. We examined the effects of precipitation and temperature on the incidence of coccidioidomycosis in California during 2000-2020, and estimated incident cases attributable to the California droughts of 2007-09 and 2012-15. Methods We analyzed monthly California...

Groundwater depletion is a major threat to agricultural and municipal water supply in California's Central Valley. Recent droughts during 2007–2009 and 2012–2016 exacerbated chronic groundwater depletion. However, it is unclear how much groundwater storage recovered from drought‐related overdrafts during post‐drought years, and how climatic conditi...

A new book presents recent advances in the modeling and remote sensing of droughts and floods of use to emergency response organizations and policy makers on a global scale.

We examine the characteristics of extremely severe and extensive floods across the Western U.S., which we term superfloods. We develop a system to score each day from 1950 to 2010, across the conterminous U.S. west of the Continental Divide, according to the severity of flooding and the extent of the area affected on that day. In order to augment t...

Plain Language Summary Atmospheric rivers (AR), commonly defined as corridors of concentrated water vapor in the atmosphere, can yield large amounts of snow accumulation when they make landfall during the cold season. As over half of the naturalized streamflow in the Colorado River originates from water released by snow melt, it is of great signifi...

Hydrologic models predict the spatial and temporal distribution of water and energy at the land surface. Currently, parameter availability limits global-scale hydrologic modelling to very coarse resolution, hindering researchers from resolving fine-scale variability. With the aim of addressing this problem, we present a set of globally consistent s...

This book provides an overview of historical and current research activities, products, application, and evaluation of drought and flood monitoring and forecast. It also presents how relevant products are being used for risk analysis and disaster response. Overall, there have been successful models, products, systems, and case studies at both globa...

Extreme daily precipitation contributes to flooding that can cause significant economic damages, and so is important to properly capture in gridded meteorological data sets. This work examines precipitation extremes, the mean precipitation on wet days, and fraction of wet days in two widely used gridded data sets over the conterminous U.S. (CONUS)....

Atmospheric rivers (ARs) are responsible for up to 90% of major flood events along the U.S. West Coast. The timescale of subseasonal forecasting (two weeks to one month) is a critical lead time for proactive mitigation of flood disasters. The NOAA/Climate Testbed Subseasonal Experiment (SubX) is a research-to-operations project with almost immediat...

Seasonal snow water equivalent (SWE) accumulation in California’s Sierra Nevada is primarily governed by a few orographically enhanced snowstorms. However, as air temperatures gradually rise, resulting in a shift from snow to rain, the governing processes determining SWE accumulation versus ablation become ambiguous. Using a network of 28 snow pill...

Millions of people across the globe are affected by droughts every year, and recent droughts have highlighted the considerable agricultural impacts and economic costs of these events. Monitoring the state of droughts depends on integrating multiple indicators that each capture particular aspects of hydrologic impact and various types and phases of...

We examine the drought variability over the Conterminous United States (CONUS) for 1915-2018 using the Noah-MP land-surface model. We examine different model options on drought reconstruction including optional representation of groundwater and dynamic vegetation phenology. Over our 104-year reconstruction period, we identify 12 great droughts that...

In response to the outbreak of the COVID-19 pandemic, many governments instituted “stay-at-home” orders to prevent the spread of the coronavirus. The resulting changes in work and life routines had the potential to substantially perturb typical patterns of urban water use. We present here an analysis of how these pandemic responses affected Califor...

The California state government put restrictions on outdoor residential water use, including landscape irrigation, during the 2012-2016 drought. The public health implications of these actions are largely unknown, particularly with respect to mosquito-borne disease transmission. While residential irrigation facilitates persistence of mosquitoes by...

The response of annual runoff volume to sub‐annual climate warming is highly uncertain, and the governing mechanisms remain poorly understood, challenging adaptive water management. A typical exemplar is the Western United States, where climate models project substantially stronger warming in the warm season (April to September) than in the cool se...

Compared to differences in snow accumulation predicted by widely-used hydrological models, there is a much greater divergence among otherwise “good” models in their simulation of snow ablation process. Here, we explore differences in the performance of VIC, Noah-MP, Catchment and SSiB3 in their ability to reproduce observed snow water equivalent (S...

Recent research has reported the great influence of springtime land surface temperature (LST) and subsurface temperature (SUBT) over the Tibetan Plateau (TP) on downstream region summer precipitation, indicating the potential application of LST/SUBT on subseasonal to seasonal (S2S) prediction. In this study, we employed both observational data and...

Mountain regions are experiencing more pronounced climate change than other global land areas. How have vegetation dynamics responded to these changes and what are the implications for hydrology? To answer these questions, we examine the impacts of changes in mean air temperature ( T _mean ), precipitation ( P ) and winter snow cover extent ( SCE )...

Many studies have examined how runoff (Q) responds to long-term changes in precipitation (P) and temperature (T), but the effects of potential evapotranspiration (PET) have received less attention. We examine observational data sets for P, T, and Q, along with PET estimated from observations, to determine the extent to which derived P and PET runof...

Temperature is widely known to influence the spatio-temporal dynamics of vector-borne disease transmission, particularly as temperatures vary across critical thermal thresholds. When temperature conditions exhibit such 'transcritical variation', abrupt spatial or temporal discontinuities may result, generating sharp geographical or seasonal boundar...

Abstract Observational networks enhance real‐time situational awareness for emergency and water resource management during extreme weather events. We present examples of how a diverse, multitiered observational network in California provided insights into hydrometeorological processes and impacts during a 3‐day atmospheric river storm centered on 1...

We examine reforecasts of flash droughts over the United States for the late spring (April–May), midsummer (June–July), and late summer/early autumn (August–September) with lead times up to 3 pentads based on the NOAA second-generation Global Ensemble Forecast System reforecasts version 2 (GEFSv2). We consider forecasts of both heat wave and precip...

Groundwater plays a critical role in sustaining agriculture in California's Central Valley (CV). However, groundwater storage in the CV has been declining by around 3 km³/year over the last several decades, with much larger declines during the 2007–2009 and 2012–2015 droughts. Managed Aquifer Recharge (MAR) can potentially mitigate existing overdra...

Precipitation extremes are projected to become more frequent along the U.S. West Coast due to increased atmospheric river (AR) activity, but the frequency of less intense precipitation events may decrease. Antecedent soil moisture (ASM) conditions can have a large impact on flood responses especially if pre-storm precipitation decreases. Taken toge...

The Surface Water and Ocean Topography (SWOT) satellite will provide global observations of surface water storage and river discharge at an unprecedented high resolution and accuracy after its scheduled launch in 2021, but a limitation of SWOT observations is their spatial and temporal discontinuity associated with the SWOT orbital characteristics....

The recent availability of freely and openly available satellite remote sensing products has enabled the implementation of global surface water monitoring at a level not previously possible. Here we present a global set of satellite-derived time series of surface water storage variations for lakes and reservoirs for a period that covers the satelli...

Abstract. The recent availability of freely and openly available satellite remote sensing products has enabled the implementation of global surface water monitoring to a level not previously possible. Here we present a global set of satellite-derived time series of surface water storage variations for lakes and reservoirs for a period that covers t...

Groundwater plays a critical supporting role in agricultural production in the California Central Valley (CV). Recent prolonged droughts (notably 2007–2009 and 2012–2016) caused dramatic depletion of groundwater, indicating the susceptibility of the CV’s water supply to climate change. To assess the impact of climate change on groundwater storage i...

Based on a process‐level modeling of the rain‐on‐snow (ROS) events in the period of 1950 to 2013 and in a warmer climate, we quantify the historical and future runoff contribution from ROS to extreme floods and the source of runoff and snowmelt in large ROS events within the conterminous United States (CONUS). We find that the regions impacted most...

Wind is a common ground transportation hazard. In arid regions, wind-blown dust is an added risk. Here, we analyzed the relationship between accidents and wind speed, dust events to study how they may have contributed to vehicular accidents in California. The California Highway Patrol (CHP) reports information about weather conditions that potentia...

Abstract Recent fire seasons have fueled intense speculation regarding the effect of anthropogenic climate change on wildfire in western North America and especially in California. During 1972–2018, California experienced a fivefold increase in annual burned area, mainly due to more than an eightfold increase in summer forest‐fire extent. Increased...

A body of work over the last decade or so has demonstrated that most major floods along the U.S. West Coast are attributable to Atmospheric Rivers (ARs). Recent studies suggest that observed changes in extreme precipitation associated with a general warming of the Western U.S. have not necessarily led to corresponding changes in floods, and changes...

River discharge estimation requires knowledge of bathymetry. However, aside from a few locations where surveys have been conducted, bathymetric data are unavailable, even for major rivers. It has been suggested that water surface elevation and flow width measurements from the upcoming Surface Water and Ocean Topography (SWOT) satellite mission (pla...

Plain Language Summary The mass of water stored in seasonal snowpacks (snow water equivalent) is a key parameter for water supply management. Yet, despite its importance and decades of effort, snow water equivalent has been an elusive variable to characterize over the mountainous areas of the globe, with no existing satellite capable of providing r...

Previous studies have shown limited success in improving streamflow forecasting for snow-dominated watersheds using physically based models, primarily due to the lack of reliable snow water equivalent (SWE) information. Here we use a hindcasting approach to evaluate the potential benefit that a high-resolution, spatiotemporally continuous, and accu...

We examine the relative contributions of root zone soil moisture (SM) and groundwater (GW) storage to trends and interannual variability in total water storage (TWS) from essentially the entire Gravity Recovery and Climate Experiment (GRACE) record (2003–2016). Our study region is the Eastern United States where GW generally is shallow, and we use...

The Third Pole (TP) is experiencing rapid warming and is currently in its warmest period in the past 2,000 years. This paper reviews the latest development in multidisciplinary TP research associated with this warming. The rapid warming facilitates intense and broad glacier melt over most of the TP, although some glaciers in the northwest are advan...

In the summer of 2015, Eastern Washington State (east of 122°W) experienced over 2000 fires that burned a record amount of area, saw the deaths of three firefighters, and caused hazardous air quality throughout much of the region. The area burned during the 2015 fire season (~471 000 hectares) was over three times that of the next largest year in t...

Atmospheric rivers (ARs) are narrow, elongated corridors of high water vapor content transported from tropical and/or extratropical cyclones. We characterize precipitation and snow water equivalent associated with ARs intersecting the western U.S. coast during the cold season (November- March) of water years 1949- 2015. For each AR landfalling date...

Millions of people died due to famines in India in the 19th and 20th centuries; however, the relationship of historical famines with drought is complicated and not well understood. Using station‐based observations and simulations, we reconstruct soil moisture (agricultural) drought in India for the period 1870‐2016. We show that over this century a...

Plain Language Summary While it is known that elongated moisture‐rich atmospheric features, known as atmospheric rivers (ARs), play an important role in the water resources of the mountainous western United States, less is known about how the atmospheric data sets used to diagnose the presence of ARs influence the amount of AR‐attributed snowfall e...

Dramatic declines in snowpack in the western US Mountain snowpack stores huge amounts of water in the western US, supplying much of the water used to grow crops. A team of researchers from Oregon State University and UCLA found that spring snowpack declined almost everywhere, especially in the coastal states and other locations with mild winter cli...

Does elevation-dependent warming hold true above 5000 m elevation? A combination of model- and observation-based evidence over the Tibetan Plateau suggests an absence of elevation-dependent warming (EDW) at high elevations. EDW, which is closely linked to snow-albedo feedback, has important implications for the hydrology of the headwaters of most o...

Despite evidence of increasing precipitation extremes, corresponding evidence for increases in flooding remains elusive. If anything, flood magnitudes are decreasing despite widespread claims by the climate community that if precipitation extremes increase, floods must also. In this commentary we suggest reasons why increases in extreme rainfall ar...