Hydrologic Connectivity of Sub Basins

Citation:

Summit Environmental Consultants Inc. (2013). Okanagan Hydrologic Connectivity Model: Summary Report. Vernon: Okanagan Basin Water Board.

PDF Link:

http://www.obwb.ca/newsite/wp-content/uploads/2013/09/ohcm_final_report.pdf

Summary:

The Okanagan Basin Water Board (OBWB) retained Summit Environmental Consultants Inc. (Summit) to develop a nimble and user-friendly model that can be used to demonstrate the hydrologic and legal linkages between water suppliers in the Okanagan Basin and to support water management decisions during drought conditions. Summit constructed the Okanagan Hydrologic Connectivity Model (OHCM) using the Water Evaluation and Planning software developed by the Stockholm Environment Institute. Three water supply and demand models developed in 2010 as part of the Okanagan Water Supply and Demand Project were also used in the creation of the OHCM. In this study, the OHCM was used to run scenarios investigating how water users are impacted by changes in upland storage, increased water demands, in-stream flow needs, their location in the Basin (and associated stream catchment), and licence seniority in the “first in time, first in right” licensing system under the British Columbia Water Act. This report presents an overview of how the OHCM was developed and the key outcomes of the scenarios.

Key Findings

Prioritizing instream flow needs versus water withdrawals

Under present-day water demands, giving instream flow needs for fish and other environmental values a higher priority would not likely significantly affect the ability of water suppliers to continue to provide water to their customers. If water demands increase (approaching the amount of licensed volumes), either IFN are frequently not met; or water suppliers frequently run short of water (depending on the priority attached to meeting IFN. Since we chose to assign a high priority to meeting minimum flow releases out of mainstem lakes and IFN in the Okanagan River than to keeping Okanagan Lake within its usual elevation range (which reflects current practice), the instream flow needs of Okanagan River were not impacted under any scenario. However, different choices concerning priorities could create different results.

Dealing with the simultaneous use of licences

Each utility’s water intake is associated with one or more water licences – most utilities have several licences. In OHCM terms, each water licence is referred to as a “demand site”. We identified individual “demand sites” in order to incorporate and evaluate water licence priorities (using the FITFIR principle) when running demand-supply scenarios.

Water utilities consider that their water allocation is the sum of their individual licences, and they generally manage their allocation as a single unit – they don’t manage extractions on the basis of individual licences. One of the biggest challenges during development of the OHCM was to assign the surface water demand at each intake and each time step to each of the licences. This was accomplished through a demand calculator tool developed for this purpose, which assumes that the licensed volume is spread out across the licensed usage period in the year, and that each licence is used at each time step until it is exhausted. This Okanagan Basin Water Board Executive Summary method appears satisfactory, in that it does not appear to cause conflicts with the current water management regime in the Basin. However, under future scenarios of increased demands and potentially a drier climate, water regulators could specify rules or guidelines for the way in which water use is assigned to licences. Various approaches to applying water withdrawals against licences could be simulated in a future version of the OHCM, such as specifying the use of junior licences before more senior licences.

Water storage

For some water suppliers, increasing storage in upland reservoirs would help to reduce the frequency with which their customer demands and IFN are not met. Since we chose to assign higher priority to meeting water users’ demands than to maintaining lakes and reservoirs within their normal operating ranges in this version of the OHCM; none of the water users that withdraw water from mainstem lakes or the Okanagan River (e.g. City of Kelowna, Town of Oliver) were found to be limited by supply. However, in a future model application, other priority choices could be made, which could influence these results.

Drawdown of Okanagan Lake

Because of our priority-setting choices in the current version of the model, the model satisfies all water demands from Okanagan Lake and the Okanagan River downstream of the lake at the cost of the lake’s useable storage. This occurs because water users and IFN requirements were assigned higher priority than meeting lake level targets. Under maximum demand scenarios (i.e., scenarios 5-8), Okanagan Lake becomes depleted during drier years and lake levels begin to approach the sill elevation of the dam (339.75 m GSC) at Penticton. Under this framework (which appears to reflect the actual practices and priorities of the provincial water managers who manage the mainstem lakes), FITFIR does not seem to influence water users that withdraw water from Okanagan Lake (or the other mainstem lakes) or the Okanagan River, since all higher priority demands are supplied at the cost of mainstem lake levels. However, the results of these scenarios could have been different if maintaining Okanagan Lake within its normal operating range had been assigned a higher priority in the model. The impact of choosing different priorities could be investigated further in future model applications.

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