(1) Background: Federal land managers in the US are charged with risk-based decision-making which requires them to know the risk and to direct resources accordingly. Without understanding the specific factors that produce risk, it is difficult to identify strategies to reduce it. (2) Methods: Risk characterized by U.S. land managers during wildfires was evaluated from 2010–2017 to identify factors driving risk perceptions. Annotation from 282 wildfires in two regions with distinctive risk profiles, the Northwest and Southwest Geographic Areas, were qualitatively coded using the risk assessment framework of hazards, values, and probability from the Relative Risk Assessment in the Wildland Fire Decision Support System (WFDSS). (3) Results: The effects of climate on seasonal severity, fuel condition, and fire behavior emerged as the most influential factors driving risk perceptions and characterizations of risk in both regions. Monsoonal precipitation extended the longevity of landscape barriers, especially in the Southwest. The results suggest that a scarcity of values at risk and a mild fire environment produce low risk fires regardless of location, while high risk fires reflect specific local values and geography, under the umbrella of dry climate. (4) Conclusions: the climatic contrasts between the two regions highlight how influential climate change will be on future characterizations of wildfire risk.

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Factors Influencing Risk during Wildfires: Contrasting Divergent Regions in the US


The Las Vegas metropolitan area in Nevada has experienced extensive urban growth since 1950 coincident with regional and local climate change. This study explores the nonstationary flood history of the Las Vegas Wash (LVW) watershed by deconstructing it into its constituent physical drivers. Observations and reanalysis products are used to examine the hydroclimatology, hydrometeorology, and hydrology of flash flooding in the watershed. Annual peak flows have increased nonlinearly over the past seven decades, with an abrupt changepoint detected in the mid-1990s, which is attributed to the implementation of flood conveyance systems rather than changes in land use. The LVW watershed exhibits two pronounced flood seasons, associated with distinct synoptic atmospheric circulations: winter floods linked to inland-penetrating atmospheric rivers and summer floods linked to the North American monsoon. El Niño–Southern Oscillation also plays a role in modulating extreme rainfall and the resultant floods because annual maximum daily rainfall totals positively correlate with El Niño, with Spearman’s correlation coefficient of 0.36 (p value < 0.05). Winter maximum daily rainfall totals have increased since 1950, whereas summer daily rainfall maxima have decreased. The trends in hydrometeorological drivers interact with urbanization to shift flood seasonality toward more frequent winter floods in the LVW watershed. A process-based understanding of the flood hydrology of the watershed also provides insights into flood frequency analysis and flood forecasting.

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The Nonstationary Flood Hydrology of an Urbanizing Arid Watershed

The dry and hot American Southwest under the present and future climates

Changes in the North American Monsoon (NAM), a circulation system that transports moisture into western Mexico and the southwest U.S., can have substantial impacts on water resources and agriculture. Here, we utilize future projections from Phase 6 and 5 of the Coupled Model Intercomparison Project (CMIP) to assess monthly changes in land precipitation and the leading mechanisms resulting from anthropogenic climate change. Historical CMIP6 simulations of seasonal precipitation demonstrate skill in reproducing NAM rainfall, but mimic precipitation biases observed in previous CMIP generations. Future climate projections from the SSP5-8.5 pathway produce reductions in precipitation that persist throughout the monsoon season (June–August) but are balanced by precipitation increases during the late monsoon season (September–October) but are not shown in CMIP5 projections. Atmospheric moisture budget analysis reveals that early monsoon rainfall deficits are associated with a combination of greater evaporative demand, a negative dynamic response of vertical moisture advection, and anomalous subsidence. Increases in late monsoon season rainfall are attributed to a positive change in the dynamical term of vertical moisture advection and increases in upward motion. Although minimal changes in total land NAM rainfall are observed, seasonal shifts and the persistence of drier conditions can have significant ecological and societal consequences. 

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Papers

Future Land Precipitation Changes Over the North American Monsoon Region Using CMIP5 and CMIP6 Simulations