Narrative Description of the Reference scenario

The Reference Case scenario is the base case, which we use as a comparator for all other scenarios.  It adopts mid-range assumptions about climate change, population and income growth, and also assumes that institutions such as water rights, the land-use planning system, reservoir operating rules, forest practices, and urban water pricing continue to operate in their present form. Here we describe key data sources and assumptions incorporated into the Reference Case Scenario.

 External Drivers

Climate - Future climate is based on projections made by the MIROC5 global climate model as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5, 2013) downscaled to 4-km resolution (Abatzoglou, 2013). This model projects regional changes that are in the middle of the range of changes predicted by a suite of global climate models determined to perform well for the Pacific Northwest (Rupp et al., 2013). The MIROC5 climate scenario projects that annual mean temperature for the Pacific Northwest in 2100 is about 5.5ËšC (10ËšF) warmer than the average observed between 1950 and 2005.

Population and Income - We model population growth to 2050 by allocating county-level projections (Oregon Office of Economic Analysis, 2011) in proportion to existing population within UGBs and areas zoned for rural residential use. Household income forecasts to 2040 come from Woods and Poole (2011; WP) county-level estimates. Given that the OEA and WP projections follow a nearly linear trend, a linear extrapolation is used to project both population and income out to 2100. By 2100, the projections suggest a basin-wide population of roughly 5.4 million people and a mean annual household income of $242,000 (in $2005). This projection assume a lower growth rate in income (1.13%) in the next 85 years compared to the previous 85 years (1.63%).

Assumptions that Affect Water Supply

We calibrated parameters in the hydrology sub-model with snow and streamflow discharge observational data from the recent past. We assume that the parameters do not change throughout the century – i.e., that the physics of the system is fundamentally the same throughout the period of simulation.

Reservoir Operations – We assume that reservoirs operate according to the rule curves that incorporate recommendations of the Biological Opinion implemented as of 2009, with the exception of the selective withdrawal structure at Cougar Dam. Other elements of the Biological Opinions (e.g., selective withdrawal at Detroit Reservoir) that were not implemented in 2011 ResSim model are not included in the Reference Case.

Water Rights – In the Reference Case we model water rights for irrigation, municipal and instream uses as recorded in the OWRD Water Rights Information System, as of July 2012. Water rights are satisfied in order of seniority among all rights within the Willamette Basin. We assume that water can be removed from the stream up to 90% of discharge, in order of water right priority date. Irrigation diversion rates are limited by water right and diverted to satisfy crop water demand -- in the Reference Case, diversion rates cannot exceed 1/80th cfs/acre. Duty (volume of water that can be diverted over a year) is limited by water right and in the Reference Case is limited to 2.5 acre-feet/acre. The threshold to deny a water right for the season is seven days -- If a water right cannot be met for for seven days, then it is “shut off” for the season. Pending incorporation of a valley groundwater sub-model, the Reference Case assumes that wells tap an unlimited water source and that water rights for wells cannot be shut off.

Assumptions that Affect Water Demand

Change in land use types affects water demand for an IDU. The major trajectory of land use change in the Reference Case is increased urban development -- rising population and income cause developed land values to increase and therefore the likelihood that forest and agricultural lands will be converted to urban uses. The Reference Case scenario assumes that all future development takes place within UGBs. The simulation begins with the existing UGBs in place, but includes a mechanism for future expansions of UGBs. In the Reference Case, when the ratio of private developed land inside a UGB to the total area of private developable land inside a UGB exceeds 80%, an expansion takes place to bring this ratio under 80%. IDUs are added to the UGB in a way that represents current state planning goals and processes. In particular, land zoned for Exclusive Farm Use (EFU) is avoided during UGB expansions.

Urban – Urban water use will increase in absolute terms as population and income grow, and as more areas are converted to urban land use. In the Reference Case , we assume that urban per capita water consumption rates change for the first 15 model years (2010 - 2025) at a rate of 1.5%/yr to cover the current backlog of infrastructure needs. After that time, prices change in the same proportion as average current cost (based on current expenditures). Because of scale economies as the population serviced by water utilities grows, average costs and therefore water rates decrease slightly after the initial 15 year price increase.

Agriculture – We assume that farmers continue to grow the same range of crops and make irrigation decisions similar to those observed in recent years. In the Reference Case lands in crops, grass seed and pasture, can transition between cover types from year to year, however, lands currently in orchards, vineyards, and tree farms remain in those land cover types and do not change over the century. In the Reference Case, we assume that external factors that affect the agricultural models, such as the price of wheat ($64/ton), grass seed ($5/bushel) and energy, stay the same in real terms and do not rise relative to the price of other factors affecting farm profitability.

Forestry – In the Reference Case Scenario, we initiate rates of forest harvest as equivalent to those observed since introduction of the Northwest Forest Plan. These rates were determined by time series analysis of Landsat satellite data for 1996-2011 (Kennedy et al., 2012). The harvest rate is set based on the observed rate (1985-2010) and responds to the availability of appropriately aged stands in the projection. Stands must be 60 years or older to be harvested. Initial rates for area burned per year are likewise from analysis of Landsat data. We start with a rate of 0.2% per year across all ownerships. As the climate changes, the rates of fire occurrence change, which can affect the availability of land for harvest.

Fish and Stream Temperature - To date, we have not included changes to stream temperature and fish communities in our analysis of the Reference Case.

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