Hydrologic Basis for SLVWD Drought Management Plan presented to SLVWD Board of Directors March 6, 2014

Transcription

1 Hydrologic Basis for SLVWD Drought Management Plan presented to SLVWD Board of Directors March 6, 2014 by Nicholas M. Johnson, Ph.D., P.G., C.Hg. Principal Hydrogeologist, MWH

2 SLVWD Service Areas

3 SLVWD water sources

4 North System s drought cycle conjunctive use of stream diversions & groundwater wells 1 AF/Y = 0.62 GPM

5 North System s seasonal conjunctive use of stream diversions and groundwater wells Stream diversions Groundwater pumping

6 South System relies entirely on groundwater wells. Long term decline in groundwater levels and storage. Some recovery in wet years. Worsened during and following drought years drought 1998 El Niño

7 Felton System relies entirely on stream & spring diversions.

8 SLVWD Service Areas North South Felton Total Area (square miles) Population served (2010) 15,660 2,500 4,020 22,170 Service connections 5, ,335 7,310 % Residential 97% 100% 88% 96% Production ( ) acre-feet per year 1, ,490 million gallons per day (mgd) gallons per minute (gpm) 1, ,540 gallons per capita per day maximum monthly (mgd) (gpm) 1, ,100

9 Long-term rainfall record Prior & current droughts ? % of average 70% 60% 70% 60%* rainfall * to date

10 North System average production from diversions and wells. Reflects available diversions limited by demand and infrastructure Reflects demand unmet by diversions 300 gpm. Groundwater = Storage

11 North System average production from diversions and wells. Baseflow recession Reflects demand unmet by diversions 300 gpm. Groundwater = Storage

12 Dry period stream baseflow recession. North System Diversions

13 San Lorenzo River at Big Trees February 1 March 4, 2014 average

14 (most recent 18 months) 170 gpm Water year (WY) 2014 is 3 rd year of current drought

15 WY 1987 was 1 st year of 6 year drought 120 gpm Conservation

16 WY 1988 was 2nd year of 6 year drought 100 gpm Conservation

17 WY 1990 was 4th year of 6 year drought 160 gpm Conservation

18 WY 1991 was 5th year of 6 year drought March Miracle 160 gpm Conservation

19 Groundwater Production Limited by Aquifer Saturated Thickness and Well Construction

20 Quail Hollow Well Water Levels QH-4 & -4A QH-5 & -5A

21 Quail Hollow Wells rd year of 6 year drought <300 gpm* *also: & 1994

22 Olympia Well Water Levels

23 Olympia Wells 2007 Heavy use during WY with only 60% average rainfall <600 gpm

24 North System Drought Production Capacity (approximate) 2014 Prior to February Rains Source gpm Diversions 200 Quail Hollow Wells 300 Olympia Wells 600 Total 1,100 Potential Worst Case Source gpm example years Diversions Quail Hollow Wells , 1990, 1992 Olympia Wells Total 800

25 Needed Conservation North System gpm Average peak month, ,500 Recession peak month, ,200-1,400 Minimum supply 800-1,100 Average summer winter difference 500 (40%) Past drought conservation (1980s & 90s)* 10% - 20% Expected conservation needed* 20% - 30% Worst case conservation needed* 40% - 50% *Percentages apply to peak demand months.

26 Conclusions Northern Service Area peak demand must adjust to limited available drought supplies. Based on current conditions, the Northern Service Area is estimated to require 20% to 30% conservation during summer 2014, potentially achievable voluntarily and with use restrictions. Conservation of up to 50% could be required during subsequent years of continued severe drought, which likely would require mandatory rationing. Conservation is warranted in the Southern Service Area given the drought s exacerbation of long term supply limitations. Drought conservation is warranted in the Felton Service Area in light of diversion limitations.

27 Questions?