EXPLORING IMPACTS OF CLIMATE CHANGE ON THE HYDROLOGY OF THE BASIN

Wendy S. Merritt and Younes Alila Department of Forest Resources Management, University of , 2424 Main Mall, Vancouver, BC, V6T lZ4, Canada

Mark Barton and Bill Taylor Environment Canada, 1200 West 73rd Ave, Vancouver, BC, V6P 6H9

Stewart Cohen Adaptation & Impacts Research Group, Environment Canada, Sustainable Development Research Institute, University of British Columbia, 2029 West Mall, Ponderosa Annex B Vancouver, BC, V6T lZ2

Abstract

Water resources in the Okanagan River Basin are under increasing stress due to intensive regulation for irrigation and urban water supply. The impacts of climate change on the basin's water resources, and the industries and ecosystems that rely on these resources, are far from certain. A multidisciplinary study has been established to develop scenarios of the possible impacts of climate change on water supply and demand in the basin, and to use these scenarios to expand the dialogue on adaptation with resource managers and stakeholders within the region. This paper details the on-going development of a basin scale hydrologic model for exploring the possible impacts of climate change scenarios.

Introduction

The Okanagan Basin is a major horticultural and agricultural center in British Columbia. Water resources in the basin are under increasing stress with rapid population growth and land use change. Areas of the basin are heavily regulated for urban water supply in addition to horticultural and other activities. How climate change will affect water resources is an issue of utmost importance for the region.

A multidisciplinary study has been established to provide scenarios of the impacts of climate change on water supply and demand in the basin, and to use these scenarios to expand the dialogue on adaptation with resource managers and stakeholders within the region. The 'Expanding the dialogue on Climate Change and Water Management in the Okanagan Basin, British Columbia' project has the following objectives • develop regional climate change scenarios • construct plausible land and water use scenarios • explore possible changes in the basin hydrology to the aforementioned scenarios • assess crop suitability and crop water demands for the whole basin, and conduct detailed case studies of Ellis Creek, Penticton Creek and Trout Creek • assess current and possible future regional water management contexts, including water demand scenarios • evaluate potential impacts of climate change and adaptation options on operational water management (e.g. flood control, water licensing, environmental flows, demand side management) • expand the dialogue on climate change and adaptation with regional stakeholders. This paper details the on·going development of a distributed hydrologic model of the entire Okanagan basin to be used to in exploring the impacts of land use and climate change scenarios on the basin hydrology. Plans for the adaptation component are outlined in Shepherd et at. (2003) elsewhere in these Proceedings.

The Okanagan River Basin

The Okanagan Basin is an approximately 8000 km 2 basin located in the Southern Interior Region of British Columbia (Figure 1). The basin has a semi·arid climate with a monthly average precipitation of roughly 30 mm. The annual distribution of precipitation is bimodal with a winter peak associated with migrating Pacific storms, and a summertime convective precipitation maximum (Table I). A rise in mean temperature and a gradual decrease in precipitation occurs moving north to south. This is reflected in the basin hydrology, with much of the discharge that exits the Okanagan Basin originating above the outlet of .

Elevation ranges from nearly 270 metres above sea level (masl) in the southern valley regions to in excess of 2100 masl on the plateaus. Pipes (1971) defined three major altitudinal zones in the basin: the valley floor (less than 610 mas!), elevations between 610 m and 1219 m, and the plateau regions above 1219 m. The valley floors, where summer evaporative demand exceeds summer rainfall, support sparse grassland and shrub vegetation. Up to 1219 m, the increased rainfall is sufficient to support forest vegetation and maintain growth throughout the hot summers. The plateau regions are characterized by sub-alpine vegetation and a water surplus. This zone contributes much of the basin's runoff during the spring freshet.

2 The main features of the basin are Okanagan Lake, with a surface area of 350 km , and five smaller main 2 stem lakes of Okanagan River with a combined surface area of approximately 87 km • These lakes greatly

2 impact the basin hydrology through evaporation from the lake and interactions between surface and groundwater systems. These lakes are heavily regulated with dams at the outlets of five of the six lakes.

Table 1. Mean monthly maximum temperature (OC), minimum temperature (OC), and precipitation at the Summerland Research Station. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec ~imum "mpe,",",e -5.3 -2.9 -0.1 3.3 7.3 11.4 13.9 13.7 9.3 4.4 -0.2 -4.3 ximum temperature 0.1 3.6 8.9 14.3 19.3 23.8 27.2 26.6 20.6 13.4 5.6 0.7 I Precipitation 31.9 19.6 17.1 23.9 31.8 33.6 26.6 28.9 21.8 16.6 26.8 36.3

Climate Change in the Okanagan Basin

Recent trends suggest that the basin is getting warmer and wetter, with minimum temperatures increasing at a rate greater than maximum temperatures, and frost-free days having increased by approximately 3.1 days per decade during the 20th century (Cohen and Kulkarni, 2001). In spite of increased water usage in the Okanagan basin, net inflow or streamflow has increased over the last 50-100 years by 0.3% to 0.5% per year, due largely to the increase in precipitation (cited in Obedkoff, 1994). Previous modeling in subwatersheds of the Okanagan basin suggests that an earlier onset of spring peak flows will occur under climate change, with these peaks having less volume than current peak flows (Cohen and Kulkarni, 2001).

Hydrologic model applications in the Okanagan Basin

Developing a hydrologic model of the Okanagan Basin is a complex task, due largely to the recognized challenges in modeling the hydrology of semi-arid regions, deficiencies in the meteorological network, and the high degree of regulation in the basin. In semi-arid regions, the timing and rate of snowmelt are more significant in the region than areas of water abundance (Pipes, 1971). If minimum temperatures are increasing at a rate greater than maximum temperatures, capturing this is of crucial importance given the temperature thresholds that determine the development and depletion of snowpacks. As with most mountainous basins, existing climate data is predominantly located at low elevations. In the Okanagan Basin, where a strong elevation gradient exists, in addition to the precipitation gradients running east to west and north to south, this further complicates the process of accurately representing the rainfall and snow processes in the model.

Hydrological modeling applications within the Okanagan Basin have generally been undertaken in selected subwatersheds (e.g. Cohen and Kulkarni, 2001) or as part of broad scale modeling of the Columbia River Basin (e.g. Hamlet and Lettenmeier, 1999). Obedkoff (1973) estimated mean monthly runoff for the extent of the Okanagan Basin with a 5 km2 grid cell resolution. Our study extends the

3 hydrologic modeling work detailed in Cohen and Kulkarni (2001) to provide basin wide estimates of the impacts of climate change scenarios on the discharge of tributaries entering the Okanagan River and the main-stem lakes, and changes in the seasonal water level of these lakes. A regional approach is used so as to provide outputs at a resolution appropriate for assessing the potential effects of these hydrologic changes on, for example, the major reservoirs in the system or on environmental flows (e.g. fish habitat). A daily time step is used so as to capture the development and depletion of snow packs.

N A

I) U." Z!o r-...... - ...-+­ ....., -+'~---i Figure 1. Location of gauged subwatersheds with greater than 5 years of unregulated record in the Okanagan Basin, and Environment Canada maintained climate stations with long-term records.

Distributed hydrologic models largely fit into the categories of either large-scale models that are routinely linked with Global Climate Models (GCM) and tend to be applied at coarse resolutions (e.g. Variable Infiltration Capacity [VIC] - Liang et ai., 1994), or models developed for application at finer resolutions. Neither of these broad model types is suitable for this application. Initial reviews suggested that VIC would be applicable. However, the finest cell resolution used in the literature is 1/8th degree, a resolution that would lump a number of tributaries in the Okanagan Basin together and fail to distinguish between the eastern and western sides of Okanagan Lake. Discussions with the model developer at the University of raised questions regarding the validity of model assumptions at finer resolutions, particularly the assumption of no transfer of soil moisture between cells other than through the channel (D. Lettenmaier - pers. comm.). On the other hand, a model such as the Distributed Hydrology Soil Vegetation Model (DHSVM - Wigmosta et aI., 1994) was designed for application to small watersheds in which the watershed is discretized into grid cells of less than ISO m x ISO m in size. These models

4 typically have large requirements for spatial data and often use sub-daily meteorological inputs. The available data and the scale at which model outputs are required for this study do not justify their use. A more suitable option is the use of semi-distributed conceptual models in conjunction with routing and reservoir models. Two examples are the UBC Watershed Model (Quick, 1995) and the HBV model (Bergstrom, 1995). The two models are similar in terms of their input requirements and the outputs that are simulated. HBV was used in the previous study (Cohen and Kulkarni, 2001), but only in several unregulated creeks. The longer history of application of the UBC watershed model in British Columbia compared with HBV, combined with the work done by Micovic and Quick (1999) to develop global parameter sets, makes it the more appropriate choice for this application.

Development of a Basin Scale Hydrologic Model

Developing the basin-wide model involves three steps; calibration and testing of a precipitation-runoff model on unregulated and gauged tributaries in the Okanagan Basin, development of regional parameter sets for ungauged tributaries, and construction of the basin network.

Precipitation-runoff modeling

UBC Watershed Model - The UBC Watershed Model (Quick, 1995) is to be used to model precipitation-runoff processes for subwatersheds of the Okanagan Basin. The model has been used extensively within British Columbia, and has been shown to adequately reproduce the hydrologic response of watersheds, and has previously been used in climate change studies (e.g. Morrison et aI., 2002). The model conceptualizes a watershed as a series of elevation bands. Meteorological data is distributed with elevation to each band, with the precipitation form at each elevation band estimated based on temperature. Snowpack accumulation is estimated based on temperature and elevation, whilst snow melt is modeled using a simplified energy balance approach. Snowmelt and rain distribution between the runoff response components (very slow, slow, medium, and fast) is controlled by the soil moisture model. The water allocated to each component off is subject to a routing procedure based on the linear storage reservoir concept. The quick and medium components use a set of reservoirs, while the slower components are represented by a single reservoir. Each component runoff is summed to produce runoff for each band, and for the watershed at each time step.

Data processing - Daily time series of maximum air temperature, minimum air temperature, and precipitation are required to drive the model. For calibration purposes, daily discharge data is also

5 required. Daily time series data for the Okanagan Basin has been provided by Environment Canada. Each elevation band is defined according to the following characteristics: band area, mean elevation, forested fraction, density of the forest canopy, impermeable fraction, and the orientation index. The band area, mean elevation and orientation index were determined from the TRIM 1:20000 positional files provided by the Base Mapping and Geomatic Services Branch of the Ministry of Sustainable Resource Management (SRM). The impermeable area for each band was determined from the primary land use in Baseline Thematic Mapping (Decision Support Services, SRM) and Aggregate Resource Potential maps (Bobrowsky et al., 1998). Ministry of Forestry forest cover maps were used to calculate the mean density of the forest cover canopy.

Model Calibration and Testing - Twenty-one stations in the basin have a sufficient length of record of unregulated streamflow to test the UBC watershed model (Figure 1). These stations will be used to verify the performance of the model and to develop relationships between the model parameters and watershed descriptors. As the model will be applied to explore climate scenarios, and will be applied on ungauged tributaries, we need to verify as much as possible that the model performs adequately under different climate regimes and between different subwatersheds. To establish model validity, model calibration will be split into two components, as suggested by Xu (1999). A proxy-basin test will be used to test the geographic transferability of the model. Jn this test, the model is calibrated on watershed A and validated on watershed B (and vice versa). To verify that the model has general validity under different climatic periods in the existing records, a differential split-sample test will be undertaken, whereby the discharge record is split into different climatic regimes on which the model is calibrated and tested.

Regionalization of Model Parameters to Ungauged Tribntaries The next step is to develop relationships between model parameters and watershed characteristics. Micovic and Quick (1999) used the UBC watershed model to develop regional parameter sets to predict streamflow in ungauged watersheds. Data from a number of basins in British Columbia was used to develop the parameter sets and test model performance using these parameters. The authors demonstrated that, as long as the precipitation inputs are sufficient and that the impermeable fraction of the watershed can be determined, the standard parameter set could be used to obtain reasonable model results for watersheds that were not used to generate the parameter sets.

6 Basin-wide Model Network

The Okanagan Basin model network is illustrated in Figure 2. Unregulated and smaller tributaries are linked directly with the lakes or river (e.g. subwatershed 5 in Figure 2b), whilst larger systems will be split into subwatersheds to which the hydrology model is applied (e.g. sub watersheds 49 in Figure 2b). Modeled runoff is then routed through to the next link in the network, and so on to the basin outlet. The regionalization relationships are to be developed for unregulated watersheds. To apply them to many of the tributaries entering Okanagan Lake or Okanagan River we need to account for the regulation. In the development of the basin model, only those major sources of regulation will be accounted for. Minor regulation, such as direct extraction from tributaries by individual licensees, will not be accounted for in the model development. This is considered not to be verifiable as there is little existing information on the extent of this type of regulation, let alone the timing and volume of abstractions. These sources of regulation may be accounted for in scenarios where the diversions on a tributary (or segment of a tributary) are summed together and treated as one diversion. A sensitivity analysis could be performed to explore the impacts of varying degrees of direct diversions from streams on model output.

Generation of Meteorological Inputs

The network of climate stations in the Okanagan Basin, as with most mountainous basins, has a low density of gauges in higher elevation zones. As the majority of the unregulated subwatersheds have their outlets in mid~to~high elevations, much of the existing network is not representative of these subwatersheds. A stochastic weather generator could be used to generate climate inputs that will drive the hydrology model. To provide input to the basin-wide hydrology model, the generator must have the capability to spatially interpolate generated climate series, as well as provide spatially correlated synthetic time series. Three different weather generators were investigated: LARS-WG (Semenov et aI., 1999), WGEN (Wilks, 1999), and GEM (Johnson et aI., 2000). While each has its own particular strengths and appeal, no one model was capable of interpolating in data sparse areas while also preserving the spatial correlation of time series generated at multiple sites. Clearly, an opportunity exists here for the development or enhancement of stochastic models capable of performing both of these functions simultaneously.

Climate Change Scenarios

Climate change scenarios for the Okanagan Basin are currently being developed based on the results of several GeM's available on the Canadian Climate Impacts Scenarios project web site:

7 http://www.cics.uvic.ca/scenarios/index.cgi. Due to the large uncertainties in climate modeling, the Intergovernmental Panel on Climate Change recommends the use of multiple scenarios covering a range of possible future climates rather than relying on one "best guess" scenario (lPCC, 1994).

As GCM's provide outputs at a resolution too coarse to be of use in this study (e.g. 3.75° latitude by 3.75° longitude for CGCM2 [Flato et aI., 2000]), regional climate models or downscaling techniques are required to link GCM outputs to regional climate impacts. The most common method of downscaling involves adjusting the daily values in the observed climate record by mean monthly differences of temperature and precipitation for the future as projected by the GCM's. This same technique can also be applied to synthetic time series produced by weather generators such as LARS-WG. While GCMs are unable to resolve the fine-scale features such as orographic forcing that influence the surface climate, the perturbation in the climate represented by the difference between the mean monthly baseline climate and a corresponding 30-year future time period is assumed to be relatively constant over an area comparable to the scale of the GCM. The variance in the daily climate record is assumed to remain unchanged in the future, an assumption that may not be valid.

8 A Figure 2. Preliminary model network for the Okanagan Basin (A) and the corresponding subwatershed boundaries (B) dMJ -UBC watershed model, ® - Routing Model, III] [in 2A] ­ Okanagan River main stem lakes)

Another approach to downscaling GeM output involves the application of statistical downscaling methods. These models are based on relationships between large-scale circulation features and surface variables such as daily temperature and precipitation. Since synoptic features are well resolved by the GeM, this relationship can be potentially used to estimate the future surface temperature and precipitation based on projected changes to circulation patterns. Investigation of the Statistical Downscaling Model (SDSM - Wilby et aI., 2001) produced satisfactory results for daily minimum and maximum temperatures. However, the correlation between the large-scale circulation parameters and daily surface precipitation was too weak to identify suitable predictor variables for the GeM.

9 Summary

This paper introduced a framework for the hydrologic component of the 'Expanding the dialogue on Climate Change and Water Management in the Okanagan Basin, British Columbia' project. A basin wide model of the Okanagan Basin is being constructed. Once developed, the model will be used to explore scenarios of climate change on the volume and timing of runoff from tributaries, and the availability of water resources for the various stakeholders in the basin.

Acknowledgments

Funding for this work is provided by the Climate Change Action Fund, Natural Resources Canada (Project A463/A433). The UBC watershed model was developed, and provided for use in this project, by Dr. M.C. Quick and the Mountain Hydrology Group, Civil Engineering, University of British Columbia.

References

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