Climate Change Effects and Water Vulnerability in the Molalla Pudding River Basin, Oregon, USA

<p> Enhanced warming and increasingly ice-free Arctic seas affect Arctic precipitation. We investigate increased Arctic precipitation due to declining sea ice by relating variations in moisture sources to stable isotope compositions of precipitation. We develop a novel method for deriving moisture sources using condensation profiles derived from cloud radar measurements to formulate initial heights for air mass back trajectories. This method was used to locate the moisture sources of seventy Barrow, AK storm events between 2009 and 2013. Trajectories were calculated by NOAA's HYSPLIT, using GDAS reanalysis wind fields. We demonstrate that the moisture source migrates with season, from distal in winter to proximal in summer. Moisture source dew point exhibits a semiannual cycle, with summer and winter maxima. The spring minimum reflects the reintroduction of the Arctic source. The autumn dew point minimum reflects pre-ice ocean cooling locally. 36% of isotopic variation is statistically explained by a combination of the moisture source dew point and trajectory cooling. Transport distance and path both influence the best descriptor of isotopic composition. For local events, dew point is the stronger influence on isotopic composition, explaining 21% of variance. For distal events, the effects of trajectory cooling supersedes the moisture source signal. The orographic effect of the Alaskan and Brooks ranges account for the influence of trajectory path on isotopic composition. Local moisture events during transition seasons were slightly enriched relative to distal events. If we measure further isotopic enrichment during future transition seasons, it may reflect increased contributions from the Arctic source and thus precipitation increase. Deuterium excess reflects various combinations of latitude, sea surface temperature and relative humidity. Moisture source dew point significantly but weakly predicts storm-specific d-excess. Similar analyses can be performed across the Arctic if reanalysis data can generate reliable condensation profiles. To evaluate the efficacy of condensation profiles produced by reanalysis data, we compared the condensation profiles derived from cloud radar to those from reanalysis. On average, reanalysis produced condensation profiles with mean cloud height 1.4 times higher than those from cloud radar. The greater elevation bias translated into a more distal, and thus warmer and drier, moisture source.</p>

<p> Landscape-scale forest restoration treatments are planned for four national forests in Northern Arizona: the Coconino, Kaibab, Tonto, and Apache-Sitgreaves National Forests. The first analysis area comprises 900,000 acres on the Coconino and Kaibab National Forests where the U.S. Forest Service is proposing restoration activities on approximately 600,000 acres over a ten year period pending acceptance of an Environmental Impact Statement. These forest restoration treatments are intended to accomplish a number of objectives including reducing the threat of catastrophic wild fire and subsequent flooding and to restore forest health, function, and resiliency. Previous studies suggest that in semi-arid, ponderosa pine watersheds there was a possibility to increase surface water yields 15-40% when basal area was reduced by 30-100%. Because of these results, there is considerable interest in the amount of increased water yield that may recharge from these activities. </p><p> The objectives of this study were to 1) examine the state of knowledge of forest restoration thinning and its hydrological responses and to evaluate the quality and type of related references that exist within the literature and 2) simulate possible changes in recharge and aquifer response following forest restoration treatments and climate change. A systematic review process following the guidelines suggested by the Collaboration for Environmental Evidence was conducted to examine literature relevant to this topic. The Northern Arizona Regional Groundwater-Flow Model was used to simulate the changes expected from forest restoration treatments and climate change. </p><p> The systematic review returned 37 references that were used to answer questions regarding tree removal and the associated hydrological responses. Data from individual studies suggest that forest treatments that reduce tree density tend to increase surface water yield and groundwater recharge while reducing evapotranspiration. On average, there was a 0-50% increase in surface water yield when 5-100% of a watershed was treated. Groundwater results were less conclusive and there was no overall correlation for all studies between percent area treated and groundwater recharge. A majority of studies (33 of 37) reported statistically significant results, either as increases in water yield, decreases in evapotranspiration, or increases in groundwater table elevation. Results are highly variable, and diminish within five to ten years for water yield increases and even quicker (&lt; 4 years) for groundwater table heights. </p><p> Using a groundwater-flow model, it was estimated that over the ten-year period of forest restoration treatment there was a 2.8% increase in annual recharge to aquifers in the Verde Valley compared to conditions that existed in 2000-2005. However, these increases were assumed to quickly decline after treatment due to regrowth of vegetation and forest underbrush. Furthermore, estimated increases in groundwater recharge were masked by decreases in water levels, stream baseflow, and groundwater storage resulting from surface water diversions and groundwater pumping. These results should be used in conjunction with other data such as those recovered from paired-watershed studies to help guide decision-making with respect to groundwater supply and demand issues, operations, and balancing the needs of both natural and human communities. </p>