Orange County Basin - Metropolitan Water District of Southern ...

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Chapter IV – Groundwater Basin Reports Orange County Basins approximately 70,500 AFY (OCWD, 2004). Active recharge in the Orange County Basin significantly increases the yield of the basin. For example, total recharge amounts (inclusive of natural incidental recharge and all active recharge) to the basin can vary between 300,000 AFY to 400,000 AFY and change annually depending upon the hydrology of the Santa Ana River, the amount of imported Metropolitan replenishment water that is purchased, the amount of water injected into the seawater barriers, and other incidental recharge. It is important to note that the Orange County Basin is not managed on a safe yield basis every year, but rather, as discussed below, is managed to maintain basin balance over the long-term. Precipitation falling on the watershed contributing recharge to the Orange County Basin varies between the low lands and the flanking mountains depending on elevation. As measured at the Santa Ana Fire Station, approximately in the center of the basin, the average yearly precipitation over the 1985 through 2005 period is about 14 inches. Figure 10-4 provides the historical precipitation data from the Santa Ana Fire Station over that period. Precipitation (inches) 35 30 25 20 15 10 5 0 1985/86 1986/87 Figure 10-4 Historical Precipitation in the Orange County Basin Average = 14.0 inches 1987/88 1988/89 1989/90 1990/91 1991/92 UC IPM, 2006 (SantaAna.C Station) 1992/93 1993/94 1994/95 FINAL IV-10-6 September 2007 1995/96 Fiscal Year Groundwater pumping is the primary outflow from the basin. Groundwater production from the basin is managed by OCWD. As described below, the amount of groundwater producers can use is set annually and changes depending upon the management goals at that time. In fiscal year 2006/07, pumping is expected to be about 330,000 AFY. Annual pumping is expected to increase with the development of the Groundwater Replenishment System to 380,000 to 400,000 AFY. 1996/97 1997/98 1998/99 1999/00 2000/01 2001/02 2002/03 2003/04 2004/05
GROUNDWATER MANAGEMENT Chapter IV – Groundwater Basin Reports Orange County Basin The following section describes how the Orange County Basin is currently managed. It includes a discussion of the governing structure within the basin and relationships with other adjoining basins. Basin Management The Orange County Basin is a managed basin. OCWD has managed the Orange County Basin since 1933 pursuant to a special act of the State legislature (West’s Annotated California Codes, Water Code Appendix Chapter 50 as amended and Deering’s California Codes Annotated Water Uncodified Acts.Act 5683). OCWD has managed the basin based upon the principle of seeking to increase supply rather than restricting access and to provide for uniformity of cost. Table 10-2 provides a list of other agencies that OCWD may interact with in carrying out its mission. The basin groundwater pumping is not operated on a safe-yield basis each year. Rather, the goal is to maintain an approximate balance over a period of several years. The amount of production from the basin is governed through financial incentives based on establishing an annual Basin Production Percentage (BPP), which is the percentage of groundwater production out of the total water demand for the Orange County Basin. Pumping up to the BPP is charged a fee on a per AF basis, i.e., the Replenishment Assessment (RA). Groundwater production above the BPP is charged the RA plus the Basin Equity Assessment (BEA). The BEA is typically set so that the cost of groundwater production above the BPP is similar to the cost of purchasing alternative supplies. Pumping agencies do not accrue individual storage rights if they pump less than the BPP, which is a major difference compared to most adjudicated basins. Additionally, agencies cannot transfer groundwater-pumping rights. The basin is managed to provide approximately three years of drought supplies for the region. The accumulated overdraft target of 100,000 AF was in part set to meet this goal. If Santa Ana River supplies decline and/or Metropolitan replenishment water is not available, OCWD can generally sustain high pumping rates by overdrafting the groundwater basin for a three-year period down to an accumulated overdraft of 400,000 to 500,000 AF. The 100,000 AF target also provides sufficient storage space to capture excess water supplies that become available during very wet winters. Figure 10-5 shows the historical BPP between 1985/86 and 2004/05. During this period, the BPP ranged from 66 percent to 80 percent. For the last 6 years of this period, Santa Ana River flows were significantly less than average. For the first four years of the drought (1998/99 through 2002/03) the BPP was maintained at 75 percent, which allowed for normal pumping levels. However, groundwater storage was reduced by approximately 230,000 AF during this period. Due to the continued drought conditions and seawater intrusion concerns, the BPP was reduced in fiscal year 2003/04 to 66 percent for two years and then lowered to 64 percent in fiscal year 2005/06. This lower BPP and heavy rainfall over the past few years has refilled the basin. As such, the BPP was raised to 69 percent for fiscal year 2006/07 and will be raised to 74 percent in fiscal year 2007/08. FINAL IV-10-7 September 2007
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Page 9 and 10: BPP (%) 85% 80% 75% 70% 65% 60% Cha
Page 11 and 12: Category Non-Irrigation Wells (larg
Page 17 and 18: Water Level (feet MSL) 200 150 100
Page 19 and 20: TDS Chapter IV - Groundwater Basin
Page 23 and 24: Groundwater Treatment Chapter IV -
Page 25 and 26: In-lieu Storage (AFY) 90,000 80,000

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