Water Department
R. Harrington, Hydrologist
A. Steinwand, Soil Scientist
October 2005
Mary Austin aptly called the Owens Valley 'The Land of Little Rain' - about six inches of precipitation falls on the valley floor each year, an amount typical for a desert - yet the floor of the Owens Valley supports large areas of vegetation that require more water than the meager amount that falls as precipitation. These plants survive by getting most of their water from groundwater - water that percolates through the sand, gravel, and clay that fills the valley. Areas of groundwater-dependent vegetation can exist because groundwater flows from areas of groundwater recharge, such as the mountain front of the Sierra Nevada, to the arid valley floor. The depth of the water table beneath groundwater-dependent vegetation affects the availability of water to the vegetation - if the water table is too deep, water is not accessible to the plants' roots.
A central goal of the Long-Term Water Agreement (LTWA) is that LADWP groundwater wells will be managed to avoid harming groundwater dependent vegetation. The mechanism specified in the Green Book (the technical appendix to the LTWA) for managing groundwater wells uses 22 vegetation and soil moisture monitoring sites located from Laws to George Creek, near Manzanar, to determine if sufficient soil moisture is present in the plant root zone to sustain the plants at the site. Each monitoring site has a number of nearby LADWP production wells associated with it. If the soil moisture is sufficient, the associated wells can be operated; if the soil moisture is insufficient, the associated wells are not operated to allow the soil moisture to be replenished from the water table. A flaw in this management strategy is that pumping can lower the water table to levels far below the root zone before soil moisture is depleted to the point where wells are turned off. By the time soil moisture is depleted, even if pumping stops, the water table may not recover in time to prevent vegetation harm. In the early-1990's, the Inyo/Los Angeles Standing Committee adopted the Drought Recovery Policy, in recognition of the need to promote water table and soil moisture recovery in order to achieve the vegetation protection goals of the LTWA. The Drought Recovery Policy resulted in less groundwater pumping than would have occurred under the management mechanism specified in the Green Book.
To remedy the deficiencies in the soil-moisture-based management strategy, the Water Department has examined the possibility of managing pumping based on the level of the water table. In two reports produced by the Water Department, it was shown that (1) if groundwater had been pumped during the 1990's to the maximum amount allowed under the Green Book's well management mechanism, water table recovery would have been substantially less than the recovery achieved under the Drought Recovery Policy, and (2) the probability of recovery to water table targets within three years was low for many monitoring sites because the initial water table was below the target level, and because of the infrequency of large recharge years. The primary tools for carrying out these evaluations were regression models, which are mathematical models based on the relationship between water table fluctuations, pumping amounts, and amounts of groundwater recharge. The model results are expressed as probabilities of recovery to water table targets because of the uncertainty in amounts of future runoff. These reports are available on the Water Department's web site.
In the first report, Water Table Fluctuations Resulting from Management Under the Drought Recovery Policy and the Green Book, 1989 to 2000, by Aaron Steinwand and Robert Harrington, the amount of pumping that would have been allowable under Green Book management was applied to the regression models to estimate how the water table would have responded had pumping been managed under the Green Book from 1989 through 2000. Figure 1 shows the observed hydrograph for well 419T, a shallow monitoring well near the LA Aqueduct intake east of Aberdeen. Also shown is the modeled hydrograph for the amount of pumping that actually occurred under the Drought Recovery Policy, and the modeled hydrograph for the amount of pumping that could have occurred under the Green Book well management mechanism. It is apparent that water levels in this area would have been about 10 feet lower under the Green Book management. It is also apparent that the model reproduces the observed hydrograph fairly accurately.
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In the second report, Simulation of Water Table Fluctuations at Permanent Monitoring Sites to Evaluate Groundwater Pumping, by Aaron Steinwand and Robert Harrington, the regression models were used to evaluate the probability that the water table would achieve water table targets. First, water table targets were developed for monitoring sites based on the rooting depth, soil moisture data, or historical water levels at the site, and then water levels were simulated to determine the probability of recovery to the target water level for each monitoring site. For most sites, unless the water table was near the target level, the probability of recovery was low even at minimum pumping for a three-year duration. Simulations were also done to determine the level at which water tables would stabilize with many years of average runoff and reduced levels of pumping. In these model runs, the only pumping conducted was for uses such as fish hatcheries, irrigation that can't be supplied by other sources, and supply for towns. These simulations showed that at many sites, the water level stabilizes at levels deeper than the water table target, highlighting the importance of high runoff events in producing water levels comparable to the mid-1980's.
These modeling efforts show the need for revising the procedures in the Green Book, yet many hurdles remain in the development of water table based management of groundwater pumping. The water table targets described above were based variously on water levels during the mid-1980's and on the root zones described in the Green Book; it is not certain these targets will actually achieve the vegetation goals of the LTWA. Furthermore, to base management on water table fluctuations, it's necessary to know how long various groundwater dependent plant communities can tolerate disconnection from the water table.
For groundwater dependent vegetation in the Owens Valley, groundwater pumping results in a land of less than little rain, because pumping-induced reductions in groundwater levels threaten to remove the main source of water for these vegetation communities. Basing management of groundwater pumping directly on water table fluctuations would provide a more reliable strategy of managing groundwater pumping to achieve the LTWA's vegetation goals.