Assessing mechanisms of climate change impact on the upland forest water balance of the Willamette River Basin, Oregon

Abstract

Projected changes in air temperature, precipitation, and vapor pressure for the Willamette River Basin (Oregon, USA) over the next century will have significant impacts on the river basin water balance, notably on the amount of evapotranspiration (ET). Mechanisms of impact on ET will be both direct and indirect, but there is limited understanding of their absolute and relative magnitudes. Here, we developed a spatially explicit, daily time‐step, modeling infrastructure to simulate the basin‐wide water balance that accounts for meteorological influences, as well as effects mediated by changing vegetation cover type, leaf area, and ecophysiology. Three CMIP5 climate scenarios (Lowclim, Reference, and HighClim) were run for the 2010–2100 period. Besides warmer temperatures, the climate scenarios were characterized by wetter winters and increasing vapor pressure deficits. In the mid‐range Reference scenario, our landscape simulation model (Envision) projected a continuation of forest cover on the uplands but a threefold increase in area burned per year. A decline (12–30%) in basin‐wide mean leaf area index (LAI) in forests was projected in all scenarios. The lower LAIs drove a corresponding decline in ET. In a sensitivity test, the effect of increasing CO2 on stomatal conductance induced a further substantial decrease (11–18%) in basin‐wide mean ET. The net effect of decreases in ET and increases in winter precipitation was an increase in annual streamflow. These results support the inclusion of changes in land cover, land use, LAI, and ecophysiology in efforts to anticipate impacts of climate change on basin‐scale water balances.

Finding water scarcity amid abundance using human–natural system models

“…Evapotranspiration of forests (described in ref. 26) adapted the Penman-Monteith approach. Spatially explicit irrigation and municipal water rights were fully represented.…”

“…In mid-to late summer, however, soil water limits ET so that warmer temperatures, when combined with less snow, lead to a longer summer dry period. Given the prohibitive cost of transporting water from reservoirs to forests, the anticipated result is increased water scarcity resulting in a 200-900% increase in forest wildfires across the three climate scenarios (26). The likely outcome will be increased costs for fire suppression, the transition to new forest types, and reduced availability of forestland for timber harvest (26) (SI Appendix, section 4.2).…”

“…This, in turn, allows more precipitation falling in forest zones to be routed downstream. This means that climate change could actually lead to increases in the runoff ratio despite increasing evaporative demand (26). In our reference case scenario, there is a projected decrease in forest water use of 315 million m 3 for April-July between the 2010s and 2090s due mainly to the effects of projected wildfires (SI Appendix, section 4.2).…”

Climate Change and Curtailment: Evaluating Water Management Practices in the Context of Changing Runoff Regimes in a Snowmelt-Dominated Basin

“…Flow operates on contiguous collections of IDUs that behave in a similar hydrological manner. Each collection of IDUs is referred to as a Hydrologic Response Units (HRUs) [14,32]. We created the HRU coverage by grouping contiguous polygons with identical land cover at the intermediate level (i.e., middle column) level in Figure 2, identical elevation class, and were located in the same HUC-12 catchment.…”

Climate Change and Curtailment: Evaluating Water Management Practices in the Context of Changing Runoff Regimes in a Snowmelt-Dominated Basin

“…The hydrologic model in Envision applies algorithms to Hydrologic Response Units (HRUs, Jin and Sridhar, 2011;Turner et al, 2016), which are an aggregation of IDUs that would theoretically behave hydrologically similar. To create the HRU coverage, we grouped polygons that had the same intermediate land cover (Figure 2), identical elevation class, and were located in the same HUC-12 catchment.…”