Drought Attribution Studies and Water Resources Management

American Meteorological Society (AMS) | February 16th, 2023

Drought impact on cropland use monitored with AVIRIS imagery in Central Valley, California

Science of the Total Environment (Elsevier) | November 19th, 2022

During 2012–2016 California experienced the longest and most severe drought in the last centuries. This water scarcity led to an increase in non-cultivated croplands du

During 2012–2016 California experienced the longest and most severe drought in the last centuries. This water scarcity led to an increase in non-cultivated croplands during this period. The objective of this study was to quantify agricultural trends in the Central Valley (California) at peak growth from 2013 to 2016 during the drought and in 2017–2018 post-drought. For this purpose, we analysed yearly official harvested area reported at county level for the main crops and compared them to visible-shortwave infrared (VSWIR) spectra acquired by the Airborne Visible/Infrared Imaging-Spectrometer (AVIRIS-classic) over 2334 km2 of the Central Valley each year. Multiple Endmember Spectral Mixture Analysis (MESMA) was applied to AVIRIS data to estimate green vegetation (GV), non-photosynthetic vegetation (NPV) and soil fractions in crop fields each year. MESMA and crop reports (R2 = 0.9) showed that soil (i.e.; non-cultivated areas) increased during the summers of the drought; with the smallest GV area in 2015, the second year classified with exceptional drought in this period. According to MESMA, 34 % of the cropland was covered by GV in 2015, and 69.5·104 ha according to the crop reports. MESMA also registered the highest value of soil area in 2015 (48 %). The year with most cultivated area was 2017, the wettest year in the studied period, with 54 % of the croplands covered by GV and 75.2·104 ha. This study verified that the non-cultivated areas increased in the Central Valley during the exceptional drought period and validated the use of AVIRIS imagery to monitor broad-scale cropland use changes in future climatic extreme events.

Drought impacts to water footprints and virtual water transfers of the Central Valley of California

American Geophysical Union (AGU) | June 23rd, 2017

Drought water right curtailment analysis for California’s Eel river

American Society of Civil Engineers (ASCE) | February 1st, 2018

Water users in California’s hybrid water rights system have different legal priorities to available surface water in times of water scarcity. A set of two linear progra

Water users in California’s hybrid water rights system have different legal priorities to available surface water in times of water scarcity. A set of two linear programming models was developed to determine curtailments of water use under drought conditions according to riparian and appropriative water right doctrines with spatially varying water availability and water rights within a basin. The models were implemented in spreadsheets and extended to estimate water right reliability and factors of safety in water rights administration. Alternate methods for calculating water use curtailments are discussed. Curtailments from the models are compared with actual water shortage notices issued by the state for the Eel River, California for June 30, 2014. Analyzing water use curtailments with an algorithm in spreadsheet software offers a mechanistic, transparent, accessible, and precise approach derived from legal doctrines to support water rights administration during drought.

Drought, agricultural adaptation, and sociopolitical collapse in the Maya Lowlands

National Academy of Sciences (NAS) | May 5th, 2015

Paleoclimate records indicate a series of severe droughts was associated with societal collapse of the Classic Maya during the Terminal Classic period (∼800–9

Paleoclimate records indicate a series of severe droughts was associated with societal collapse of the Classic Maya during the Terminal Classic period (∼800–950 C.E.). Evidence for drought largely derives from the drier, less populated northern Maya Lowlands but does not explain more pronounced and earlier societal disruption in the relatively humid southern Maya Lowlands. Here we apply hydrogen and carbon isotope compositions of plant wax lipids in two lake sediment cores to assess changes in water availability and land use in both the northern and southern Maya lowlands. We show that relatively more intense drying occurred in the southern lowlands than in the northern lowlands during the Terminal Classic period, consistent with earlier and more persistent societal decline in the south. Our results also indicate a period of substantial drying in the southern Maya Lowlands from ∼200 C.E. to 500 C.E., during the Terminal Preclassic and Early Classic periods. Plant wax carbon isotope records indicate a decline in C4 plants in both lake catchments during the Early Classic period, interpreted to reflect a shift from extensive agriculture to intensive, water- conservative maize cultivation that was motivated by a drying climate. Our results imply that agricultural adaptations developed in response to earlier droughts were initially successful, but failed under the more severe droughts of the Terminal Classic period.

Droughts impede water balance recovery from fires in the Western United States

Nature Portfolio (Springer Nature) | January 2nd, 2024

A steady rise in fires in the Western United States, coincident with intensifying droughts, imparts substantial modifications to the underlying vegetation, hydrology and

A steady rise in fires in the Western United States, coincident with intensifying droughts, imparts substantial modifications to the underlying vegetation, hydrology and overall ecosystem. Drought can compound the ecosystem disturbance caused by fire, although how these compound effects on hydrologic and ecosystem recovery vary among ecosystems is poorly understood. Here we use remote sensing-derived high-resolution evapotranspiration (ET) estimates from before and after 1,514 fires to show that ecoregions dominated by grasslands and shrublands are more susceptible to drought, which amplifies fire-induced ET decline and, subsequently, shifts water flux partitioning. In contrast, severely burned forests recover from fire slowly or incompletely, but are less sensitive to dry extremes. We conclude that moisture limitation caused by droughts influences the dynamics of water balance recovery in post-fire years. This finding explains why moderate to extreme droughts aggravate impacts on the water balance in non-forested vegetation, while moisture accessed by deeper roots in forests helps meet evaporative demands unless severe burns disrupt internal tree structure and deplete fuel load availability. Our results highlight the dominant control of drought on altering the resilience of vegetation to fires, with critical implications for terrestrial ecosystem stability in the face of anthropogenic climate change in the West.

Droughts in California

Public Policy Institute of California (PPIC) | April 1st, 2021

Dry Me a River: Ecological Effects of Drought in the Upper San Francisco Estuary

University of California, Davis (UC Davis) | May 24th, 2024

Dry Year Program (1978)

California State Water Resources Control Board (SWRCB) | January 1st, 1978

The importance of water was emphasized to all Californians during the 1975-76 water year. A high pressure system persisted off the West Coast for months, preventing Pacif

The importance of water was emphasized to all Californians during the 1975-76 water year. A high pressure system persisted off the West Coast for months, preventing Pacific storms from depositing moisture on major areas of the State. As a consequence, the 1975-76 period turned out to be the third driest water year in recorded State history, being surpassed only by the droughts of 1923-24 and 1930-31. With most of the State•s surface reservoirs substantial l y depleted and meager prospects for normal runoff, Governor Brown - on the last day of 1976 - announced an eight-point drought emergency program. In effect, the emergency program marshaled the resources and personnel of all state agencies in an effort to ease, as much as possible, the social and economic impact on Californians if the drought should continue for a second year . As it developed, 1976-77 became the driest year of record, amply justifying the Governor's foresight. The State Water Resources Control Board (State Board) role in the emergency program was to develop a program of water conservation in cooperation with the University of California; to determine the quantity of water which could be expected in critically dry areas; and to protect the priority of uses of that limited supply in accordance with the State's water rights law. This was particularly important in the case of agriculture, the State's No. 1 industry. It appeared likely that the available supply of water would not meet the needs of farms and orchards for the full growing season or meet the full demand of municipal and industrial users and instream needs. The State Board was concerned that water users would inadvertently interfere with the water rights of others unless forewarned of the extent of availability of water to them. In addition, the warnings would be conducive to early implementation of conservation measures. The State Board established the Dry Year Program as a function within its Division of Water Rights ( Division) . This program, designed to protect and enforce the priorities of users of surface water had four objectives : 1. To identify critical drought-impacted areas in Northern and Central California. 2. To provide information on water conservation and shortage to diverters in these areas. 3. To assure that the limited water supply available would be used in accordance with established rights. 4. To take enforcement action against violations of permit and license conditions, illegal diversions and waste or unreasonable use of water.

Dynamical and Thermodynamical Causes of Large-Scale Changes in the Hydrological Cycle over North America in Response to Global Warming

American Meteorological Society (AMS) | October 15th, 2014

The mechanisms of model-projected atmospheric moisture budget change across North America are exam- ined in simulations conducted with 22 models from phase 5 of t

The mechanisms of model-projected atmospheric moisture budget change across North America are exam- ined in simulations conducted with 22 models from phase 5 of the Coupled Model Intercomparison Project. Modern-day model budgets are validated against the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis. In the winter half year transient eddies converge moisture across the continent while the mean flow wets the west from central California northward and dries the southwest. In the summer half year there is widespread mean flow moisture divergence across the west and convergence over the Great Plains that is offset by transient eddy divergence. In the winter half year the models project drying for the southwest and wetting to the north. Changes in the mean flow moisture convergence are largely responsible across the west but intensified transient eddy moisture convergence wets the northeast. In the summer half year widespread declines in precipitation minus evaporation (P 2 E) are supported by mean flow moisture divergence across the west and transient eddy divergence in the Great Plains. The changes in mean flow convergence are related to increases in specific humidity but also depend on changes in the mean flow including increased low-level divergence in the U.S. Southwest and a zonally varying wave that wets the North American west and east coasts in winter and dries the U.S. Southwest. Increased transient eddy fluxes occur even as low-level eddy activity weakens and arise from strengthened humidity gradients. A full explanation of North American hydroclimate changes will require ex- planation of mean and transient circulation changes and the coupling between the moisture and circulation fields.