Global warming and changes in drought

Nature Portfolio (Springer Nature) | December 20th, 2013

Several recently published studies have produced apparently conflicting results of how drought is changing under climate change. The reason is thought to lie in the formu

Several recently published studies have produced apparently conflicting results of how drought is changing under climate change. The reason is thought to lie in the formulation of the Palmer Drought Severity Index (PDSI) and the data sets used to determine the evapotranspiration component. Here, we make an assessment of the issues with the PDSI in which several other sources of discrepancy emerge, not least how precipitation has changed and is analysed. As well as an improvement in the precipitation data available, accurate attribution of the causes of drought requires accounting for natural variability, especially El Niño/Southern Oscillation effects, owing to the predilection for wetter land during La Niña events. Increased heating from global warming may not cause droughts but it is expected that when droughts occur they are likely to set in quicker and be more intense.

Going Beyond Low Flows: Streamflow Drought Deficit and Duration Illuminate Distinct Spatiotemporal Drought Patterns and Trends in the U.S. During the Last Century

American Geophysical Union (AGU) | August 30th, 2022

Streamflow drought is a recurring challenge, and understanding spatiotemporal patterns of past droughts is needed to manage future water resources. We examined regional p

Streamflow drought is a recurring challenge, and understanding spatiotemporal patterns of past droughts is needed to manage future water resources. We examined regional patterns in streamflow drought metrics and compared these metrics to low flow timing and magnitude using long-term daily records for 555 minimally disturbed watersheds. For each streamgage, we calculated streamflow drought duration (number of days) and deficit (flow volume below a specified threshold) for each climate year (April 1–March 31). We identified drought using five thresholds (2%–30%) and two approaches: variable thresholds with unique values for each day of the year, and a fixed threshold based on all period-of-record flows. We then analyzed drought trends using the Mann-Kendall test with persistence adjustment for 1921–2020, 1951–2020, and 1981–2020, and computed correlations between annual streamflow drought metrics and climate metrics using values from a monthly water balance model. Spatial patterns in drought metrics were consistent between variable and fixed approaches, though fixed threshold durations were typically longer and variable threshold deficits larger. High interannual variability in drought duration emerged in the central, interior west, and southwestern U.S., with high deficit variability in the interior west. Drought metrics were weakly correlated with low flow magnitude and timing, providing unique information. Drought duration and deficit increased in the southern and western U.S. for both 1951–2020 and 1981–2020, particularly using fixed thresholds, and paralleled trends in aridity. Projections of continued aridification for the southern and western U.S. may increase drought durations and deficits and intensify water availability impacts.

Governing California Through Climate Change

Little Hoover Commission | July 1st, 2014

State government, which guided California through the instability of the Great Depression, managed the profound disruptions of World War II and steered, during following

State government, which guided California through the instability of the Great Depression, managed the profound disruptions of World War II and steered, during following decades, one of the nation’s great population, development and innovation booms, faces again an historic governing challenge in climate change. A $2 trillion annual economy and the needs of nearly 40 million residents ride on the outcome of the state’s preparations and response. Climate change, which most scientists believe has already begun, promises decades of wilder weather and great uncertainty regarding the scale of annual precipitation, wildfire activity, sea level rise and daily temperatures. These changes have powerful implications for agricultural production, air quality, real estate values, electricity generation, public health and California’s renowned quality of life.

GRACE Groundwater Drought Index: Evaluation of California Central Valley groundwater drought

Remote Sensing of Environment (Elsevier) | July 3rd, 2017

Quantitative approaches to assess the complexity of groundwater drought are hindered by the lack of direct observations of groundwater over space and time. Here,

Quantitative approaches to assess the complexity of groundwater drought are hindered by the lack of direct observations of groundwater over space and time. Here, we present an approach to evaluate groundwater drought occurrence based on observations from NASA's Gravity Recovery and Climate Experiment (GRACE) satellite mission. Normalized GRACE-derived groundwater storage deviations are shown to quantify groundwater storage deficits during the GRACE record, which we define as the GRACE Groundwater Drought Index (GGDI). As a case study, GGDI is applied over the Central Valley of California, a regional aquifer undergoing intensive human activities and subject to significant drought periods during the GRACE record. Relations between GGDI and other hydrological drought indices highlight our ability to capture drought delays unique to groundwater drought. Further, GGDI captures characteristics of groundwater drought that occur as a result of complex human activities and natural changes, thus presenting a framework to assess multi-driver groundwater drought characteristics.

Green Light for Adaptive Policies on the Colorado River

Water (MDPI) | December 21st, 2021

The Colorado River is a critical source of water supply for 40 million people in nine states spanning two nations in western North America. Overallocated in the 20th cent

The Colorado River is a critical source of water supply for 40 million people in nine states spanning two nations in western North America. Overallocated in the 20th century, its problems have been compounded by climate change in the 21st century. We review the basin’s hydrologic and water management history in order to identify opportunities for adaptive governance to respond to the challenge of reduced system flows and distill the ingredients of past successes. While significant advances have been made in the first two decades of the 21st century, these past actions have not been sufficient to halt the declines in the basin’s reservoirs. We find that the mix of federal, state, and local responsibility creates challenges for adaptation but that progress can be made through a combination of detailed policy option development followed by quick action at hydrologically driven moments of opportunity. The role of directives and deadlines from federal authorities in facilitating difficult compromises is noted. The current state of dramatically decreased overall flows has opened a window of opportunity for the adoption of water management actions that move the river system toward sustainability. Specific measures, based on the existing institutional framework and on policy proposals that have circulated within the Colorado River community, are suggested.

Guidelines for the Small Community Drought Relief Program

California Department of Water Resources (DWR) | September 20th, 2022

How unusual is the 2012–2014 California drought?

American Geophysical Union (AGU) | December 30th, 2014

For the past three years (2012–2014), California has experienced the most severe drought conditions in its last century. But how unusual is this event? Here we use two

For the past three years (2012–2014), California has experienced the most severe drought conditions in its last century. But how unusual is this event? Here we use two paleoclimate reconstructions of drought and precipitation for Central and Southern California to place this current event in the context of the last millennium. We demonstrate that while 3 year periods of persistent below-average soil moisture are not uncommon, the current event is the most severe drought in the last 1200 years, with single year (2014) and accumulated moisture deficits worse than any previous continuous span of dry years. Tree ring chronologies extended through the 2014 growing season reveal that precipitation during the drought has been anomalously low but not outside the range of natural variability. The current California drought is exceptionally severe in the context of at least the last millennium and is driven by reduced though not unprecedented precipitation and record high temperatures.

Human-Induced Changes in the Hydrology of the Western United States

American Association for the Advancement of Science (AAAS) | February 22nd, 2008

Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regi

Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regional, multivariable climate change detection and attribution study, using a high-resolution hydrologic model forced by global climate models, focusing on the changes that have already affected this primarily arid region with a large and growing population. The results show that up to 60% of the climate-related trends of river flow, winter air temperature, and snow pack between 1950 and 1999 are human-induced. These results are robust to perturbation of study variates and methods. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States.

Hydroclimate Report Water Year 2015

California Department of Water Resources (DWR) | December 23rd, 2016

Water year 2015 added a fourth year to the ongoing drought in California, with observations indicative of a changing climate, including record warmth. The Water year ende

Water year 2015 added a fourth year to the ongoing drought in California, with observations indicative of a changing climate, including record warmth. The Water year ended with record high temperatures, and preceded a period of historically low precipitation that started in 2012. April snowpack measurements in 2014 tied the historic record low of 1977. Expectations of a developing El Niño event in the eastern tropical Pacific fueled notions that water year 2015 would be better. However, during the first two months of the water year, warm temperatures persisted and precipitation continued to fall short of expectations.  The developing El Niño event stalled as California headed into the heart of its wet season. In 2014, the snowpack level was 25% of its average on April 1. That mark was shattered on April 1, 2015 when snowpack amounted to a meager 5% of average. Satellite-imagery compares the Sierra Nevada snowpack near the end of March 2015 to the average conditions in water year 2011. This extreme low in snowpack exceeds end-of-century climate projections. When accompanied by the record warm temperatures experienced in the 2015 water year, there is conversation of California having shifted to a new climate “normal”.

Hydroclimate Report Water Year 2016

California Department of Water Resources (DWR) | September 5th, 2017

Water year 2016 ended up being different than expected, from a climate standpoint. Water year 2015 marked the 4th year of drought in California, ending with record high t

Water year 2016 ended up being different than expected, from a climate standpoint. Water year 2015 marked the 4th year of drought in California, ending with record high temperatures, record low precipitation, and a record low snowpack. Forecasts were for a strong, wet El Nino during water year 2016, with the potential to rival large flood producing events of 1983 and 1998, higher than average sea levels, increased wave action and potential damage along the coast, warmer than average temperatures, and above-average precipitation producing storms. As a result of the cooling of the tropical Pacific sea surface temperatures, and the decay of El Nino, only some of the anticipated impacts were realized, above-average precipitation and greater than average snowpack did not come to pass. While statewide air temperatures averaged lower than the record setting 2015 water year they were still well above the long-term record, ranking water year 2016 warmest 117 of 121 since 1895. Precipitation was above normal in the Northern Sierra with the majority of precipitation falling in January and March. Precipitation in the Southern Sierra was on average with the long-term trend. Differences in Northern and Southern Sierra precipitation were due to the majority of Atmospheric River landfalls occurring in the north including the two strongest occurring in the first two weeks of March while only two Atmospheric Rivers made landfall south of Monterey Bay. Statewide snowpack was 15 percent below average and impacted the April-July snowmelt with streamflow on the Sacramento and San Joaquin Rivers by 32 percent and 22 percent below average respectively. Water year type was classified as “Below Normal” for the Sacramento River system and “Dry” for the San Joaquin. Overall, water year 2016 was an improvement to the previous four years of drought conditions with enough precipitation to offset some of the large deficits in water storage reservoirs.