Water Department
The Monitor 1998
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Vegetation Conditions Sally Manning, Vegetation Scientist This issue of the Monitor provides a timely opportunity to discuss what has been observed, measured, and learned about the Owens Valley vegetation. Since the vegetation was inventoried and mapped in the mid-1980s, the Owens Valley environment has progressed through a full cycle, from a wet period, to drought, and back to a wet period. This cycle has allowed the Inyo County Water Department scientists to assess a wide range of vegetation responses to conditions imposed not only by the weather, but also due to pumping during this period. Results will be summarized in general terms; details of the data can be found in the reports listed at the end of this issue.  Figure 3. Annual LADWP pumping 1970-1998.Water tables were generally high throughout the valley when the Los Angeles Department of Water and Power performed the baseline vegetation inventory during the mid-1980s. Vegetation conditions during that period were therefore the result of a series of wet years (1982-86) and sufficient water provided from groundwater. Other factors influencing the baseline vegetation conditions included site substrate properties (for example soil type), relatively recent land use history, and effects of herbivores (organisms that feed on plants). As the inventory was being completed in 1987, there was a dry year and LADWP pumped nearly 210,000 acre-feet. The following year, 1988, was also dry, and pumping again exceeded 200,000 acre-feet (Figure 3). In the fall of 1989, Inyo County and Los Angeles began to apply newly developed groundwater management techniques at then-existing permanent monitoring sites. Because several of the monitoring sites were in soil water deficit at that time, pumping was reduced to 155,000 acre-feet. The heavy pumping during these years, combined with the drought period that lasted well into the 1990s, drove the water tables to significant depths in the wellfield areas. In most of the wellfields, water tables were substantially below baseline levels and plant root zones by 1991. Because of the drought, LADWP and Inyo County adopted a Drought Recovery Policy. The policy recommended that water tables, soil moisture, and vegetation be allowed to recover. As a result, from 1990 through 1998, annual groundwater pumping ranged between 50,000 and 90,000 acre-feet (Figure 3). Precipitation in 1993 was above normal, and from 1995 onward, the drought gave way to a series of wet years. Water tables responded to the reduced pumping and increased recharge by rising. By 1998, water tables had returned to baseline (mid-1980s) levels in portions of some wellfield areas. However, although water tables had risen regionally, they remained below baseline levels in large portions of most wellfields. Patterns of Water Table and Vegetation Response Before analyzing the vegetation data set for a given year, the best available information on depth to the water table (DTW) is assembled. Methods for assessing the water table conditions beneath a given unit of land area have improved since employing Geographic Information System (GIS) techniques (see past issues of the Monitor). Using data from over 150 test wells located throughout the valley, a spatial statistics routine is run to develop an estimation of the water table depth for all areas being monitored. With this technique, the average DTW beneath a selected vegetation parcel in April of any year from 1985 onward can be estimated. Graphing the estimated DTW during the past 14 years shows a picture of water table fluctuation beneath the parcel, and adding the vegetation data to the same graph allows certain trends in vegetation to be compared with water table conditions. |