Climate moderates potential shifts in streamflow from changes in pinyon‐juniper woodland cover across the westernU.S.

Abstract

Pinyon‐juniper (PJ) cover has increased up to 10‐fold in many parts of the western U.S. in the last 140+ years. The impacts of these changes on streamflows are unclear and may vary depending on the intra‐annual distribution and amount of precipitation. Given the importance of streamflow in the western U.S., it is important to understand how shifts in PJ woodland cover may produce changes in streamflow across the region's diverse hydroclimates. To this end, we simulated the land surface water balance with contrasting woodland and grassland cover with the Hydrologiska Byråns Vattenbalansavdelning (HBV) model at a 4‐km resolution across the distribution of PJ woodlands in the western U.S. We used shifts in evapotranspiration (ET) between woodland and grassland cover as a proxy for potential changes in streamflows. Comparison of HBV model results with paired catchment studies indicated the model reasonably simulated annual decreases in ET with changes from woodland to grassland cover. For the northern and western ecoregions of the PJ distribution in the western U.S. where precipitation predominantly occurs in the winter, HBV simulated a 25 mm (37%) annual decrease in ET with conversion to grassland from woodland. Conversely, in southern ecoregions of PJ distribution with prominent summer monsoons, annual differences in ET were only 6 mm (19%). Our results suggest that only 29% of the PJ distribution, compared to an estimated 45% based on precipitation amount alone, has the potential for meaningful increases in streamflow with land cover change from woodland to grassland.

Spatial and Temporal Variability of the Impacts of Pinyon and Juniper Reduction on Hydrologic and Erosion Processes Across Climatic Gradients in the Western US: A Regional Synthesis

“…Annual precipitation in the pinyon and juniper zones of the Colorado Plateau and southwestern US ranges from about 200 mm to 700 mm (Figure 2a) depending on elevation and aspect [3,5]. Mean annual air temperature is highly variable across these diverse regions [3], ranging from about 4 • C to 16 • C depending on elevation, and summer season air temperatures can exceed 38 • C. The seasonality of precipitation and temperature regimes in general have been postulated to have important ecohydrological ramifications on pinyon and juniper expansion and on PJ woodland responses to tree reduction treatments, disturbances, and climate change [1,8,11,31,41,42,50,115,134,136,138].…”

“…Wilcox [190] further noted that evapotranspiration is the dominant water-loss mechanism on southwestern US PJ woodlands, that streamflow from these woodlands is typically ephemeral, and that the seasonality of runoff for these landscapes is strongly related to the precipitation regime, with winter flows more common on snowy uplands and high summer flows occurring following intense summer thunderstorms. Collectively, the Wilcox [190] and Kormos et al [60] studies characterize watershed-scale runoff responses common to PJ woodlands at the annual time scale spanning the snow-dominated, mixed-phase, and rain-dominated precipitation regimes [56,57,59,136,166,201]. Although streamflow amounts to only a small portion of the annual water budget for these systems (<10% to~20%) [60,166,190], the patchy structure of PJ woodlands, particularly where degraded, exhibits limited buffering capacity to the most intense storms and can be subject to extreme runoff events [202].…”

“…Tree removal practices are being implemented on PJ woodlands across the western US to reduce fuel loading and risk of fire, and to mitigate negative ecological and economic impacts associated with woodland encroachment and infilling [22,28,29,38,41,42,45,46,[49][50][51]108,109,[121][122][123][124][125][126][127][128][129][130][131][132][133]. These broadly occurring and continuing climate trends, altered disturbance regimes, and management actions on PJ woodlands throughout the western US have important ecohydrologic implications across spatial and temporal scales that vary regionally with climatic gradients [134][135][136][137][138].…”

Process-based flood frequency analysis in an agricultural watershed exhibiting nonstationary flood seasonality

“…Riverine floods, among the most common natural disasters worldwide, are the product of complex interactions between heavy rainfall, watershed and river channel morphology, and antecedent (i.e., initial) conditions including soil moisture and snowpack. Their impacts are projected to increase in the future due to hydrometeorological factors (e.g., Hyndman, 2014) and increased human development in flood-prone areas (e.g., Ntelekos et al, 2010;Ceola et al, 2014;Prosdocimi et al, 2015). Estimating the relationships between flood likelihood and severity is central to flood risk management and infrastructure design; these relationships are typically represented by flood frequency distributions (or curves), while the broad family of procedures used to derive them is termed flood frequency analysis (FFA).…”

Process-Based Flood Frequency Analysis in an Agricultural Watershed Exhibiting Nonstationary Flood Seasonality

“…HBV has been widely used to study hydrological response in United States (Vis et al, 2015;Niemeyer et al, 2017) and other 5 regions of the world (Harlin and Kung, 1992;Osuch et al, 2015;Seibert, 2003;Chen et al, 2012). The "HBV-Light" ( henceforth referred to as HBV; Seibert and Vis, 2012) version is used in this study, and consists of four main routines: snowpack, soil moisture, catchment response, and runoff routing.…”