Figure 7. Four-year simulation of water table recovery to 2.0 meters at monitoring well 419T, TA wellfield. Second, third, and fourth year's pumping is held at 3,000 acre-feet/year. 2.0 meters is the approximate rooting zone depth at the site.

The multiple regression models use the relationship between the measured water table fluctuations at monitoring wells and records of pumping and runoff to predict the response of the water table to a given amount of pumping and recharge. ICWD hydrology staff looked at 170 shallow monitoring wells for suitability as indicator wells. The periods of record for the modeled monitoring wells ranged from 6 to 23 years. Multiple regression models were developed for each well and, based on a set of statistical tests, each of these wells was classified as (1) affected by both pumping and runoff, (2) affected by pumping, (3) affected by runoff, or (4) unaffected by either pumping or runoff. Wells affected by pumping and runoff or affected by pumping were regarded as useful for evaluating pumping plans. Other non-statistical criteria, such as distance from LADWP wellfields or proximity to future Lower Owens River Project rewatering areas resulted in a final culling of the set down to 37 wells that appeared useful as indicator wells. ICWD hydrology staff incorporated statistical methods into the models that provide both predictions of the likliest result and estimates in the uncertainty in that prediction. In the future, we can use such estimates of prediction uncertainty to examine various pumping scenarios, and to evaluate the probability of certain events occurring. For example, when evaluating a pumping plan's effect on vegetation health, we may want to know the probability that the water table will recover to the root zone as well as the likeliest water table elevation. Figure 7 shows the results of simulations of shallow monitoring well 419T, in the Taboose-Aberdeen (TA) wellfield. During the first year of the simulation, various rates of pumping from the TA wellfield are shown along the bottom axis; during the subsequent three years, TA pumping was held at 3,000 acre-feet per year to allow the water table to recover. The vertical axis shows the likelihood that the depth to water (DTW) in the well will not recover to 2.0 meters by the end of the runoff year (starting from a DTW of 1.4 m, approximately the water level extant in April, 1998). The significance of the 2.0 m DTW is that it is the approximate depth of the rooting zone of the vegetation at the site (an alkali meadow). It is clear from Figure 7 that a single year of high pumping from the TA wellfield is likely to affect this site for several years.

In the future, ICWD hydrology staff plans to integrate the multiple regression models with soil water balance models and evapotranspiration models, and to compare them with other hydrologic modeling tools. Integration with soil water balance and evapotranspiration models will be useful for projecting how long a given supply of soil water can sustain healthy native vegetation. A comparison of regression and other hydrologic models will allow ICWD hydrology staff to choose the best predictive tool at a given location for assessing the effects of groundwater pumping.