Abstract. Mountain pine beetle (MPB) outbreaks in the western United States result inwidespread tree mortality, transforming forest structure within watersheds.While there is evidence that these changes can alter the timing and quantity of streamflow, there is substantial variation in both the magnitude and direction of hydrologic responses, and the climatic and environmental mechanisms driving this variation are not well understood. Herein, we coupled an eco-hydrologic model (RHESSys) with a beetle effects model and applied it to a semiarid watershed, Trail Creek, in the Bigwood River basin in central Idaho, USA, to examine how varying degrees of beetle-caused tree mortality influence water yield. Simulation results show that water yield during the first 15 years after beetle outbreak is controlled by interactions between interannual climate variability, the extent of vegetation mortality, and long-term aridity. During wet years, water yield after a beetle outbreak increased with greater tree mortality; this was driven by mortality-caused decreases in evapotranspiration. During dry years, water yield decreased at low-to-medium mortality but increased at high mortality. The mortality threshold for the direction of change was location specific. The change in water yield also varied spatially along aridity gradients during dry years. In wetter areas of the Trail Creek basin, post-outbreak water yield decreased at low mortality (driven by an increase in ground evaporation) and increased when vegetation mortality was greater than 40 % (driven by a decrease in canopy evaporation and transpiration). In contrast, in more water-limited areas, water yield typically decreased after beetle outbreaks, regardless of mortality level (although the driving mechanisms varied). Our findings highlight the complexity and variability of hydrologic responses and suggest that long-term (i.e., multi-decadal mean) aridity can be a useful indicator for the direction of water yield changes after a disturbance.
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Forest Disturbance Feedbacks From Bedrock to Atmosphere Using Coupled Hydrometeorological Simulations Over the Rocky Mountain Headwaters
The mountain pine beetle (MPB) has dramatically influenced high‐elevation pine forests of western North America, with recent infestations causing millions of acres of forest mortality and basal area loss. While ecohydrologic implications of infestation have been studied extensively in recent years, few have explored atmospheric feedbacks of widespread canopy transpiration loss or the potential role of groundwater to amplify or mitigate changes to land energy. This work presents bedrock‐to‐atmosphere simulations of coupled meteorological and hydrologic states over the Colorado headwaters. Analyses compare configurations with (1) default land surface parameters and (2) disturbance simulations with adjusted transpiration parameters in infested cells. An analysis of variance was conducted to identify regions of significant response to mountain pine beetle. Changes to increased soil moisture and Bowen ratios were found to be statistically significant in MPB‐infested areas and in nonlocal valleys, while planetary boundary layer (PBL) response was significant only in high elevations of the headwaters watershed. Temperature‐humidity covariance was evaluated using mixing diagrams; the results suggest that increased surface Bowen ratios from MPB could affect entrainment of dry air from the troposphere. The PBL is hotter, drier, and higher under infested forest conditions, which could have implications to atmosphere‐vegetation feedbacks and forest drought stress. Finally, land‐atmosphere coupling was sensitive to antecedent subsurface moisture. Regions with shallow water tables exhibit greater magnitude response to MPB at the surface and in the PBL, a finding that has repercussions for ecosystem resilience and hydrologic representation in meteorological modeling.
more » « less- NSF-PAR ID:
- 10459951
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 123
- Issue:
- 17
- ISSN:
- 2169-897X
- Page Range / eLocation ID:
- p. 9026-9046
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Biotic disturbances that overlap in space and time may result in important shifts in forest structure and composition, with potential effects on many ecosystem services. Starting in the late 1990s, outbreaks of multiple bark beetle species caused widespread mortality of three co‐occurring conifer species in the ca. 40,000‐km2subalpine zone of the southern Rocky Mountains (SRM), USA. To better understand the implications of such outbreaks, our goal was to determine if overlapping outbreaks of multiple bark beetle species caused greater tree mortality than single‐species outbreaks in stands with multiple susceptible host tree species. We mapped stand susceptibility to outbreaks of spruce beetle (SB,
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Abstract Root zone soil moisture (RZSM) is a dominant control on crop productivity, land-atmosphere feedbacks, and the hydrologic response of watersheds. Despite its importance, obtaining gap-free daily moisture data remains challenging. For example, remote sensing-based soil moisture products often have gaps arising from limits posed by the presence of clouds and satellite revisit period. Here, we retrieve a proxy of daily RZSM using the actual evapotranspiration (ETa) estimates from Surface Flux Equilibrium Theory (SFET). Our method is calibration-less, parsimonious, and only needs widely available meteorological data and standard land-surface parameters. Evaluation of the retrievals at Oklahoma Mesonet sites shows that our method, overall, matches or outperforms widely available RZSM estimates from three markedly different approaches, viz. remote sensing data based Atmosphere-Land EXchange Inversion (ALEXI) model, the Variable Infiltration Capacity (VIC) model, and the Soil Moisture Active Passive (SMAP) mission RZSM data product. When compared with in-situ observations, unbiased root mean square difference of retrieved RZSM were 0.03 (m 3 m −3 ), 0.06 (m 3 m −3 ), and 0.05 (m 3 m −3 ) for our method, the ALEXI model, and the VIC model, respectively. Better performance of our method is attributed to the use of both SFET for the estimation of ETa and non-parametric kernel-based method used to relate the RZSM with ETa. RZSM from our method may serve as a more accurate and temporally-complete alternative for a variety of applications including mapping of agricultural droughts, assimilation of RZSM for hydrometeorological forecasting, and design of optimal irrigation schedules.more » « less
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