Fig 1.
Seasonal and temporal changes to 20th century streamflow in the Salt River, Arizona that were hypothesized to be the result of (a) forest management and (b) warming. Increases in forest density due to forest management policy of fire suppression [44] assumed to occur across full century. Increases in temperature due to human activities assumed to affect hydrological processes since mid-century [5,17,18,51]. Blue solid lines represent unaltered flow regimes whereas red dashed lines represent altered flow regimes. Left panels show seasonal changes; right panels show temporal departures from flows normalized for climate variability.
Fig 2.
Map showing the study watershed in grey along with neighboring portions of Salt and Verde watersheds in central Arizona [58]. Extent of ponderosa pine forests that store snowpack in winter months and are a major source of streamflow shown in green, from US Geological Survey Gap Analysis Program [59]. Locations of study flow gage, US Geological Survey streamgage Salt River near Roosevelt (#09498500) [60], National Weather Service climate station near McNary (NWS Station #025412) [61] and other stations [61,62] considered are also indicated.
Fig 3.
Flow and climate characteristics.
Monthly averages from 1914–2012 of (a) precipitation, (b) streamflow, (c) potential evapotranspiration, and (d) temperature and in the Salt River watershed in central Arizona. Streamflow data from US Geological Survey stream gage Salt River near Roosevelt (USGS station 09498500) [60]. All other parameters derived from PRISM model [66] clipped to study boundary.
Table 1.
Detailed study hypotheses.
Table 2.
Conceptual flow regimes.
Table 3.
Historical forest and human water demand.
Fig 4.
Historical changes in forest conditions.
Historical and current estimates of ponderosa pine forest (a) basal areas, (b) stem densities, and (c) canopy cover in or near Salt River watershed. Lines bisecting boxes represent median values; lower and upper box borders represent first and third quartile values; whiskers extend 1.5 interquartile ranges below and above boxes. Outlier points are not displayed for visual clarity. Historical basal area and stem density from sites in the Southwest, representing conditions from 1867–1925 [86]. Historical canopy cover from sites in northern Arizona adjacent to study area, representing conditions from 1873–1874 [85]. 1950s basal area from sites in the Salt River watershed from 1951–1952 [71]. Current conditions from US Forest Inventory and Analysis (FIA) program plots [87,88] and the Integrated Landscape Assessment Project Model (ILAP) extracted within in the Salt River watershed boundary [89].
Fig 5.
Estimates of historical forest management and water demand.
Qualitative summaries of 20th century (a) forest management activities and (b) human water demand in Salt watershed in central Arizona in 5-year increments. Activities in (a) are shown as watershed areas affected by logging/thinning [90], prescribed fires [90,91] and wildfires [90–92] in hectares and as percentage of ponderosa pine forests in the study watershed. Annual groundwater use, surface water use and surface water diversions shown in million m3/year and as percentage of mean annual flows of Salt River near Roosevelt [50,93]. Vertical line in middle of panels denotes rough break between 1914–1963 and 1964–2012 study time periods. See Table 3 and methods for detailed description of available data and data gaps.
Fig 6.
Changes in precipitation and temperature.
Estimated magnitudes of change in (a) monthly precipitation (mm) and (b) monthly temperature (°C) across three study periods in Salt watershed, Arizona based on data from McNary weather station near Pinetop, Arizona (NWS Station #025412) [61]. Monthly changes derived from Sen’s slope estimate multiplied by number of years in period. Significance levels (* p-value < = 0.05, + p-value < = 0.1) based on Mann-Kendall test.
Table 4.
Trends in snowpack, 1964–2012.
Fig 7.
Trends in annual, February–May streamflow.
Time series and trends in (a) annual/monthly flows and (b) annual/monthly flows adjusted for variation in precipitation and temperature in the Salt River near Roosevelt, Arizona. February to May monthly flow estimates are shown. Vertical axes are units are log m3 s-1. Dots in (a) are unadjusted flow values and (b) are residuals (+ mean) from regression model built from full dataset (1914–2012). Lines represent trends across three study periods using Sen’s estimate of slope for (a) data and (b) model residuals (+ mean) from each corresponding time-period. Trend significance based on Mann-Kendall test. See Table 5 and methods for detailed information on regression models used to derive climate-adjusted flows and trends.
Fig 8.
Trends in monthly streamflow, June–September, November.
As in Fig 7, but for monthly flows (log m3 s-1) in June–September and November. Data and trends for July, August, and November represent 75% of available data to remove serial correlation of model residuals.
Fig 9.
Changes in monthly streamflow.
Monthly trends from Figs 7 and 8, shown as changes of magnitude in streamflow (million m3) in Salt watershed near Roosevelt, Arizona from (a) 1914–2012, (b) 1914–1963, and (c) 1964–2012. Bars show magnitude estimates for unadjusted flows (blue), flows adjusted for precipitation and temperature using McNary weather station (red) and PRISM model (grey). Monthly changes derived from Sen’s slope estimate multiplied by number of years in period and converted to volumes. Significance levels (* p-value < = 0.05, + p-value < = 0.1) based on Mann-Kendall test.
Table 5.
Climate-flow regression models.
Table 6.
Trends in streamflow adjusted for precipitation, 1964–2012.
Fig 10.
Changes in timing of peak spring flow.
Changes in timing of peak streamflow during spring months in Salt River near Roosevelt, Arizona, across two study periods. Lines in (a) represent Sen’s slope estimate of trends for date at which 50% of flows from January–June was reached. Significance level based on Mann-Kendall test. Lines in (b) represent median daily flows smoothed by 10-day average in 1914–1963 (black) and 1964–2012 (red).