Monthly rainfall totals for February and March were close to normal. Even still, the 2014 water year will likely go down as one of the driest in the City’s history, along with 1924 (10.85”), 1976 (13.88”) and 1977 (15.93”).
While the bulk of the wet weather season has now passed, the weather forecast calls for more showers throughout the central coast region over the next few days. These late season storms are certainly welcome, but any precipitation from now on likely will be too little and too late to have much impact on this year’s severe drought.
Monthly stream flow levels in the San Lorenzo River, the City’s primary source of water supply, have tracked far below normal all winter long (Figure 2). On average, mean monthly flow in the San Lorenzo River peaks during the month of February at just under 400 cubic feet per second (cfs). This February, even though rainfall was above average for the month, conditions in the watershed were so dry that the storm systems generated very little runoff. Mean monthly flow was 51 cfs in February (13 percent of average) and 50 cfs in March (17 percent of average).
Reservoir storage in Loch Lomond Reservoir currently stands at 1.93 billion gallons, or nearly 68 percent of capacity, which translates to 76 percent of average at the end of March. The lake level remains more than 17 feet below the spillway elevation.
Water Year Classification
Cumulative runoff from the San Lorenzo River for the year to date measures 9,409 acre feet, or 13 percent of average through the month of March (Figure 3). Accordingly, Water Year 2014 remains classified as “Critically Dry”. Cumulative runoff now slightly exceeds the amount recorded at this time of year during 1977, but remains well below the 29,000 acre-foot threshold of total annual runoff required for the water year classification to be upgraded to “Dry”.
As of March 25, 2014, all of Santa Cruz County remains designated as being in a condition of “Exceptional Drought”, according to the US Drought Monitor (Figure 4).
Projection of Water Supply Availability for 2014
With 2014 shaping up to be the third in a series of consecutive dry years, and the driest year experienced in possibly a generation, there is a lot of uncertainty involved in trying to forecast the City’s water supply availability for the season ahead. Water Department staff has developed and explored a number of different scenarios in order to determine a recommended end of season storage goal for Loch Lomond Reservoir and to establish the appropriate demand reduction goal for 2014. There is not a lot of guidance either from historical stream flow records or production volumes to draw on. Moreover, this forecast relies on a draft flow proposal that the City is making to the fishery agencies that is yet to be approved. Accordingly, it should not be characterized as a “final” forecast by any means, rather a draft that represent the best collective judgment of staff at this point in time.
Table 1 presents monthly production estimates for each source of supply, along with estimates for monthly water demand, and determines how much water would be needed from Loch Lomond Reservoir each month from April through October. The assumptions used for each line item are summarized in the footnotes beneath Table 1. The key factors considered by staff are highlighted below, by source of supply.
North Coast Streams
Gross production (i.e., the amount of water entering the system intakes at the source) from the north coast streams was conservatively estimated using actual production values obtained during the 1977 drought from Liddell Spring and Majors Creek, less 20 percent. No production is assumed from Laguna Creek, which is consistent with how the Department operated last year under a temporary arrangement with state and federal fishery agencies.
Net production represents the amount of water reaching the Coast pump station and available for treatment. For the season, the coast system is estimated to produce only 166 million gallons, which is 15 percent or about 30 million gallons less than the amount actually produced last year. The main uncertainty is how well Liddell Spring will perform after three dry years, which is the reason why the supply was estimated so conservatively. It can usually be counted on to contribute 1.0 mgd or 30 million gallons per month on a sustained basis during normal years.
San Lorenzo River
The San Lorenzo River is the City’s single largest water supply. Until recently, it was running so low and the weather was so dry that there was no reliable guidance for how the river might perform this year. Therefore, different scenarios were developed based on some percentage of flows recorded in 1977, which was the worst case on record.
The method to forecast water production from the San Lorenzo River starts with making a projection of mean monthly flow rate in cubic feet per second for the river at the USGS gauge in Felton using the table in Appendix C of the City’s Water Shortage Contingency Plan. Next a factor is added to account for any tributary inflow below the gauge in Felton. The sum of these flows is labeled “Flow at Tait St Diversion”. Subtracted from that figure is the proposed instream flow release that would be dedicated for fishery uses in the lower San Lorenzo River and is otherwise unavailable for diversion. Next, a safety factor is added representing an allowance for plant operators who are trying to balance the goals of optimizing water production at the Tait St. diversion with the need to stay above the fish flow target so that they don’t inadvertently violate the instream flow release requirement.
This remaining amount of flow is considered available for water production. The value is then converted from a flow rate expressed in cubic feet per second to a volume and expressed in million gallons per month.
All of these analytical inputs in Table 1 described above are estimates and subject to error.
Recent rains have bolstered seasonal totals and given some optimism that even though seasonal rainfall totals are lower this year compared to 1977, staff is forecasting that the river can be expected to run at levels equal to 100 percent of what occurred in 1977. Staff conferred with the state hydrologist at the California Department of Water Resources, who indicated that conditions statewide are shaping up to be the fourth or fifth driest year, but not the worst ever, and suggested comparing the flows in the San Lorenzo River measured at the end of 2013 with same flows at the end of 1976 as a way to read how the river will perform this year. As staff had used flows from 80 to 90 percent of 1977 flows in developing initial scenarios, this change makes a big difference in the monthly production from the river, and ultimately in the end of season reservoir storage.
A small amount of water production, about 8 million gallons per month is also provided by one the Tait Street Wells. The other Tait Well is considered to be in poor condition and too unreliable to count on for any supply this year. Engineering staff is examining options for rehabilitating this well, but it is unrealistic to expect a new well to be in service this year.
Live Oak Wells
Water production from the Live Oak Wells is the easiest of all sources to project based on a constant operating rate of 0.8 million gallons per day and a goal to extract 210 million gallons per year limit in critically dry years. The big unknown, however, is whether or not the surrounding water level along the coast can be maintained at least 2 feet above mean sea level all season long. Staff will be monitoring groundwater levels closely this year. It may be possible to bring the new Beltz 12 well, now under construction, online later this summer so that groundwater production could be shifted further inland away from the coast.
The 2014 water supply forecast includes two lines items for water demand. The first line is based on an average of 2012 and 2013 actual demand. These years were comparable in annual volume but differed slightly in the seasonality due to differences in weather patterns. Both years were shaped in part by the Stage 1 Water Shortage Alert and accompanying water restrictions in effect at the time, reducing demand by about 5 percent.
The second, lower line labeled “Curtailed System Demand” represents the estimated monthly demand associated with water rationing under a Stage 3 Water Shortage Emergency. It is based on the 2012/13 average, minus 20 percent, beginning in May. Staff consciously used the mid-point of the 15 to 25 percent range in Stage 3 rather than the upper end for forecasting purposes, for two reasons. First, there is no way to know precisely how customers will respond to water rationing and to say exactly how much demand will be reduced in advance. Overall demand for water has dropped in recent years, and demand hardening could make the customer’s ability to respond to water rationing this year more difficult to achieve. Second, it is assumed that 2012/13 level of demand on which the lower line is based already includes the up to 5 percent reduction due to restrictions that were in place at the time.
Loch Lomond Reservoir
The final part of the water supply forecast involves comparing water supply from the City’s flowing and groundwater sources against the curtailed demand to understand how much lake water will be needed each month to meet estimated system demand, and to project how the reservoir will respond over the coming dry season. This includes factoring in both evaporation from the reservoir surface and outflows for downstream flow requirements, the latter of which has been reduced to reflect the approval earlier this year by the State Water Resources Control Board of the City’s temporary urgency petition.
The results are illustrated in Figure 5. The reservoir begins in April at 68 percent of capacity. This projection assumes there will be no further inflow into Loch Lomond, so any additional rain and runoff after April 1 that helps improve the starting position for storage will similarly help bolster storage at year’s end. With water rationing in place, the reservoir is projected to decline to 47 percent of capacity at the end of October, leaving a little more than 1.3 billion gallons of water in storage. Figure 5 includes a line to show how the reservoir would be drawn down without water rationing in place this year, for illustration purposes. That would put the reservoir at 33 percent of capacity at the end of the season, leaving only 933 million gallons in storage.
The appropriate amount of carryover storage to target for 2014 is something that was carefully considered by staff. The City’s operations model uses 1.0 billion gallons as a planning guide in a worst case drought to balance the use of the reservoir in the current year with the goal of leaving some amount of storage in place in case of a subsequent dry year. However, with so much uncertainly, staff felt that it was better to set target a higher end of season storage goal of no less than 1.2 billion gallons (42 percent of capacity). This would provide 200 million gallons more to serve as a cushion in case some or all of the forecasting parameters miss their mark.
Accordingly, staff recommends that the decision made earlier this year by City Council declaring a Water Shortage Emergency and directing the Water Department to implement Stage 3 actions be upheld, with the caveat that if during the dry season the trend for reservoir storage goes off track negatively due to changes in either the availability of supply or level of demand then City Council should reconsider escalating the water supply emergency to Stage 4. The same would be said if requirements for instream flow differ markedly from the City proposed flow set for 2014.
Table 2 presents monthly production and reservoir level targets for the 2014 season based on this projection. These figures will be posted on the City’s drought webpage:
They will be updated and made available for the community to track as the season progresses.
 From a water supply perspective, rainfall that occurs in the watershed is more important than rain that is reported in the City limits. The Santa Cruz location, however, is used to best illustrate rainfall patterns because it is an official National Weather Service observation station with a long period of record.