Abstract
The continued provision of water from rivers in the southwestern United States to downstream cities, natural communities and species is at risk due to higher temperatures and drought conditions in recent decades. Snowpack and snowfall levels have declined, snowmelt and peak spring flows are arriving earlier, and summer flows have declined. Concurrent to climate change and variation, a century of fire suppression has resulted in dramatic changes to forest conditions, and yet, few studies have focused on determining the degree to which changing forests have altered flows. In this study, we evaluated changes in flow, climate, and forest conditions in the Salt River in central Arizona from 1914–2012 to compare and evaluate the effects of changing forest conditions and temperatures on flows. After using linear regression models to remove the influence of precipitation and temperature, we estimated that annual flows declined by 8–29% from 1914–1963, coincident with a 2-fold increase in basal area, a 2-3-fold increase in canopy cover, and at least a 10-fold increase in forest density within ponderosa pine forests. Streamflow volumes declined by 37–56% in summer and fall months during this period. Declines in climate-adjusted flows reversed at mid-century when spring and annual flows increased by 10–31% from 1964–2012, perhaps due to more winter rainfall. Additionally, peak spring flows occurred about 12 days earlier in this period than in the previous period, coincident with winter and spring temperatures that increased by 1–2°C. While uncertainties remain, this study adds to the knowledge gained in other regions that forest change has had effects on flow that were on par with climate variability and, in the case of mid-century declines, well before the influence of anthropogenic warming. Current large-scale forest restoration projects hold some promise of recovering seasonal flows.
Highlights
Mountain environments provide about half of the world’s population with water [1], and this reliance is acute in semi-arid regions including the southwestern United States (US) where both human and natural communities rely heavily on freshwater derived from upland forests
We summarized estimates of these parameters across the study watershed from the Integrated Landscape Assessment Project (ILAP), a continuous raster layer (30x30m pixels) of modeled data built with Forest Inventory and Analysis (FIA) plot data and 2006 Landsat imagery [89]
We evaluated 20th century trends in streamflow, climate and forest conditions in the Salt River watershed in central Arizona and found that precipitation trends were variable across the century, temperatures rose, forest densities increased, and streamflow declined most consistently in the 1st half of the century
Summary
Mountain environments provide about half of the world’s population with water [1], and this reliance is acute in semi-arid regions including the southwestern United States (US) where both human and natural communities rely heavily on freshwater derived from upland forests. This region is vulnerable to current and projected shortages in water supply [2], a concern that has been heightened by recent changes in snowpack, which stores water in winter months and delivers it in spring and summer months. Whereas precipitation in the region fluctuated between wet and dry periods [13,19,20], temperature change has been identified as the persistent change agent that will cause future declines in snowpack and streamflow levels as well as earlier timing of spring runoff [21,22]
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