Vegetation Line Point Intercept Data, Owens Valley, CA USA 1984-2013

Each year, the Inyo County Water Department monitors vegetation conditions on the floor of the Owens Valley. The primary goal of this monitoring, according to the technical appendix to the Long-Term Water Agreement (Green Book) are to detect any “significant decreases and changes in Owens Valley vegetation from conditions documented in 1984 to 1987”. Vegetation live cover and species composition documented during the 1984-87 mapping effort were adopted as the baseline for comparison with each annual reinventory according to the Agreement. The reference measurements collected within individual areas mapped with similar vegetation (parcels) are referred to as baseline.

The Green Book details certain decreases and changes in vegetation community types that must be avoided under the Agreement. Baseline vegetation communities in which evapotranspiration exceeds precipitation were classified as groundwater-dependent communities and are referred to as Types B, C, and D. These phreatophytic communities are dependent on shallow groundwater to maintain plant populations, as precipitation alone is inadequate to meet the water demand of evaporation and transpiration (Sorensen et al. 1991, Steinwand et al. 2006). For these parcels, according to the Green Book, “the goal is to manage groundwater pumping and surface water management practices so as to avoid causing significant decreases in live vegetation cover” and to prevent a significant amount of vegetation from changing to a “vegetation type that precedes it alphabetically (for example, Type D changing to either C, B, or A vegetation).” The goal is to manage the effect of pumping on the depth to groundwater to maintain baseline plant communities.

To determine whether significant decreases and/or changes in vegetation have occurred, three criteria need to be met that are described in the Green Book: (1) measurability of vegetation change, (2) attributability of vegetation change to LADWP groundwater pumping or surface water management and (3) degree of significance defined by the magnitude, extent, duration and permanency of the change along with other factors including air quality, human health, impact to species of concern, etc. To avoid confusion, it is noteworthy to highlight that the standard dictionary definition of “measurability” is the degree to which something can be measured. Vegetation of course can be measured and thus the first criteria for determining significant decreases or changes in vegetation may seem to the reader unfamiliar with the Green Book, unnecessary. However, the Green Book assigns a different definition to the term “measurability”—defined in part as a change that is statistically significant. Thus the first criteria, measurability, can be used interchangeably with statistical significance in the context of the technical appendix (Green Book) to the Agreement. A main objective of the vegetation annual report is to evaluate the statistical significance (measurability) of vegetation change compared to baseline. The second criterion, evaluating whether a statistically significant change in vegetation is caused by water management (attributability), is beyond the scope of this report owing to the need for a comprehensive analysis on a case by case basis for each vegetation parcel. Another source of confusion may arise with the third criterion which is the “degree or significance” of environmental change. For this criterion to be met, statistical significance is necessary but not sufficient. As described above, there are several other factors in addition to statistical significance that must be demonstrated to evaluate the degree of significance for the third criterion. For an example of a comprehensive evaluation of all three criteria for an individual parcel, see the report “Analysis of Conditions in Vegetation Parcel Blackrock 94” (available at www.inyowater.org).

A large proportion of groundwater-dependent parcels were mapped during baseline as Type C alkali meadows (61%), and the Agreement seeks to prevent these meadows from changing to shrub-dominated communities (Type B), a state change that is associated with increased depth to groundwater. Alkali meadows are of special concern because small increases in depth to groundwater can decouple the groundwater from the root zone of grass species (Naumberg et al. 1996, Elmore et al. 2006). Alkali meadow comprises 0.1% of the vegetation community types in California and 80% of alkali meadow communities are located within the Owens Valley (Davis et al. 1998). Local management of this community type will determine the likelihood these ecosystems persist in a changing environment.

Vegetation change across the Owens Valley was evaluated at both the valley scale and for each of 111 individual parcels sampled in 2013. First, at the valley-wide scale we evaluated plant community cover and composition in parcels affected by groundwater pumping and for parcels that were relatively unaffected by groundwater pumping during the period of maximum pumping rate (1987-1993). Second, we assessed whether transpiring vegetative cover differed over time for locations influenced by pumping compared to locations not influenced by groundwater pumping. Third, we quantitatively assessed the divergence of these groups of parcels from the baseline cover values recorded from 1984 to 1987. Fourth, we assessed whether vegetation composition in wellfield or control groups had changed from baseline values. Lastly, since determination of significant impacts attributed to water management by LADWP is made on a case by case basis at the individual parcel scale, we (a) quantified the magnitude of change in perennial vegetation cover over the twenty-three year reinventory period, (b) assessed whether the relative proportion of woody vegetation (hereafter shrub), gramminoid vegetation (hereafter grass) and non-gramminoid herbaceous vegetation (hereafter herb) has changed compared to baseline and (c), quantified the temporal trends of vegetation composition for each parcel and evaluated what environmental factors explained the observed patterns in vegetation change.