Water Department
by Bob Harrington, Hydrologist
September 2005
When LADWP inventoried Owens Valley vegetation from 1984 through 1987, water tables were generally high throughout the valley because of a series of wet years (1982-86) and relatively low groundwater pumping. The vegetation mapped during 1984 through 1987, which became the baseline for management under the Inyo/Los Angeles Water Agreement (LTWA), reflected the high water table prevalent at that time. Following the inventory, during the first three years of a six-year drought, LADWP pumped large amounts of groundwater: approximately 210,000 acre-feet (1987), 200,000 acre-feet (1988), and 155,000 acre-feet (1989). In response to the stress of groundwater pumping, water tables declined in most wellfields to substantially below the plant root zones, and as a result, native groundwater-dependent vegetation declined.
In 1990, in recognition of the decline in water tables and vegetation, the Inyo/Los Angeles Standing Committee adopted the "Drought Recovery Policy." The policy requires that groundwater pumping be managed in a conservative manner to allow substantial recovery of water tables, soil moisture, and vegetation. Since then, LADWP's pumping has been lower than the pumping of the late-1980's. In response to both lower pumping and several high runoff years, water tables rose during the 1990's.
Figure 1a shows the difference between depth to water during the baseline period (1985-1987) and depth to water in areas of groundwater-dependent vegetation in 1992, the last of six consecutive dry years. Figure 1a represents the most depressed water levels since the baseline mapping period. Figure 1b shows the difference between baseline water levels and April 1999 levels. Figure 1b represents the highest water levels since the baseline mapping period. Figure 1c shows the difference between baseline levels and April 2004. Figure 1c represents recent water table conditions relative to baseline water levels. Figures 1a-c illustrate the decline from baseline from 1986 to 1992 due to pumping and drought, subsequent recovery to a peak in 1999, and a subsequent decline. Red areas indicate areas where the water table is below baseline; green areas are areas above baseline. LADWP production wells are generally arrayed along the western edge of the valley floor (indicted in figures 1a-c as blue circles), because this location situates the wells upslope of the LA Aqueduct. Figures 1a-c were developed by interpolating depth to water measurements from several hundred shallow groundwater monitoring wells throughout the Owens Valley. These maps showing how depth to water has changed over time in areas of groundwater dependent vegetation are used by the Water Department to relate changes in groundwater levels to changes in vegetation conditions.
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Figure 1a-c. Depth to water deviation from baseline water levels (feet) in areas of groundwater dependent vegetation. Red indicates areas where the water table is below baseline. Figure 1a represents the deepest water tables during the drought of 1987-1992; 1b shows the how the water table recovered during the mid to late 1990's, but remained below baseline in some areas; Figure 1c shows how the water table has declined since its high point in 1b. |
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Areas of greatest water table decline in Figure 1a coincide with the locations of highest groundwater extraction along the western edge of the valley floor. In Figure 1b, many of areas that remain furthest from recovery to baseline are also near areas where the greatest amount of pumping has occurred. Comparison of Figures 1a and 1b shows that some areas recovered during the 1990's in response to high recharge and pumping managed under the Drought Recovery Policy; however, areas near centers of pumping remain below baseline levels. Since 1999, low recharge due to low runoff and a steady increase in pumping has resulted in declining water levels, evidenced by more areas below baseline in 2004 than in 1999 (Figure 1c).
In the Laws area, north of Bishop, the water table responds dramatically to pumping and recharge from the McNally canals (e.g., well T492 in Figure 2). Water tables declined to over four feet below baseline between the mid-1980's and 1992 (Figure 1a). However, in 1999, several monitoring wells in the area were at baseline or above (Figure 1b). These high water table levels were the result of recharge induced by LADWP's operation of the McNally canals and water spreading in the Laws area during the summer of 1998, resulting in the water table rising over 10 feet in some wells. Water levels in the Laws area declined since 1999 in response to pumping and low recharge. Water was again diverted from the Owens River into the McNally canals in March of 2005, which will provide much needed recharge in the Laws area. LADWP's Annual Operations Plan for 2005-2006 called for pumping 12,350 af in Laws, however a recent court order has reduced LADWP's pumping and required spreading of 16,294 acre-feet of in the Laws wellfield for groundwater recharge. This will result in significant increases in the water table in Laws.
Pumping on the Bishop Cone and recharge from the extensive network of surface water conveyances balance to stable water levels in west Bishop (e.g. well T387 in Figure 2). Water tables in the area between Bishop and Big Pine are relatively stable due to the absence of pumping stress (e.g. T479 in Figure 2).
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The Big Pine wellfield has historically been subject to high levels of groundwater pumping by LADWP. Water table hydrographs near Big Pine show a typical pattern of a mid-1980's maximum, rapid decline in the late-1980's, gradual recovery to a level below the maximum level, and a gradual decline since the late 1990's (e.g. well T425 in Figure 3).
The Taboose-Aberdeen wellfield has undergone intermittent stress when wells have been operated during droughts. Some of LADWP's highest capacity wells are located in this wellfield. Water table hydrographs in this wellfield reflect large, pumping induced fluctuations (e.g., well T421 in Figure 3).
The Thibaut-Sawmill wellfield is subject to a constant pumping stress due to the Blackrock Fish Hatchery, plus additional stress from pumping for the LA Aqueduct. The shallow water table in this wellfield shows large pumping-induced fluctuations where it is not buffered by surface water conveyances (e.g., LA Aqueduct, Blackrock Ditch).
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The Independence-Oak wellfield is subject to sustained pumping due to a large number of wells that area exempt from the on-off provisions of the LTWA. As a result, the water table in the Independence area is depressed below baseline.
Pumping in the Independence-Oak wellfield also impacts the northern portion of the Symmes-Shepherd wellfield. The Symmes-Shepherd wellfield has seen large variability in the amount of water pumped. After nearly a decade of relatively modest pumping, pumping in this wellfield increased in 2003, resulting in a pumping-induced decline in the water table. A recent pumping induced decline was observed in well T401 due to the operation of well W075 (Figure 4).
The Bairs-Georges wellfield has a small pumping capacity, and has been pumped little in the past fifteen years, resulting in water levels fluctuating around their baseline levels (e.g., well T398 in Figure 4).
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Pumping in the Lone Pine wellfield has primarily been for town supply, Diaz Lake, and an irrigation enhancement/mitigation project east of town, thought LADWP has constructed a new production well west of town on Lone Pine Creek to supply the LA Aqueduct. LADWP and the County are currently developing a process and plan for testing this wells and implementing management to protect groundwater dependent natural resources and non-LADWP wells.
The recent court order curtailing LADWP's pumping in the Owens Valley to 57,412 af per runoff year for this year and the next is a reduction in pumping stress the last several years; since the baseline period of the mid-1980's, only one year's pumping (1998) has been below this level. Unless runoff conditions are very poor (i.e., light winter snow pack in the Sierra Nevada), this reduced pumping should result in increases in water table elevation.