BOX I.C.1.a.ii Transects for Monitoring Vegetation Response to Pumping

Automated Data Processing and Statistical Report

Author

Inyo County Water Department

Published

January 9, 2025

1 Introduction

Vegetation in Owens Valley is monitored annually to quantify change in groundwater‑dependent plant communities and evaluate potential responses to pumping. This report implements Green Book Section I.C.1.a, using line‑point transect data to compare current conditions to the 1984–1987 baseline, contrast Control and Wellfield parcels, and apply standardized statistical tests for measurable change in cover and composition.
Reference: Green Book Section I.C.1.a. Pumping management context: Soil‑Plant Water Balance and Groundwater.

2 Methods

Field methods are documented in Green Book Box I.C.1.a.ii (see reference link above). In summary, joint ICWD/LADWP line‑point transects are re‑measured annually at georeferenced locations to compare live cover and species composition to the 1984–1987 baseline and to detect measurable change over time. Methods follow the standing committee update (Feb 22, 2017) and include a consistent field protocol with annual calibration and shared data review.

2.1 Data Processing (ETL)

The annual line‑point transect data are extracted from ICWD Google Sheets and LADWP CSVs, standardized, and compiled into a single analysis‑ready dataset. The core steps are:

  1. Extract raw parcel sheets (or cached snapshots) and LADWP cover files.
  2. Transform to long format, harmonize species codes, and attach functional traits.
  3. Validate transect coverage, remove duplicates, and track entity provenance.
  4. Merge ICWD and LADWP datasets and append to the master dataset.
  5. Export long and wide deliverables for QA/QC, reporting, and archiving.

In 2025, ICWD measured 144 reinventory vegetation parcels (Google Sheets) and LADWP measured 145 parcels, using 1761 line-point intercept transects in total. LADWP delivers a long-format cover CSV and information file to ICWD each year. The merged master contains 145 canonical reinventory parcel IDs because LADWP’s extra parcel, BIS082, is included in the joint dataset even though ICWD did not field it. Statistical tests compared current-year vegetation cover to the 1984-1987 baseline for Wellfield parcels (pumping-affected) and Control parcels (no pumping).

LADWP parcel names TIN067 and UNW037 are alternate labels for single transects at BLK006 (transect 1) and MAN038 (transect 23), per the LADWP Information file (Parcel2 / Long_PCL fields); they are not separate parcels. Beyond the ICWD sheet extract, LADWP also measured BIS082 (reinventory data included in the merged master).

We compare parcel-level cover in the 1984-1987 baseline (Nominal Year 1986) to the current year (2025) for Wellfield and Control parcels using the paired subset shown below. Results summarize differences in total cover, grass, and shrub cover between baseline and current-year conditions; statistical significance is shown on the plot for each cover type (ns = not significant; * indicates p < 0.05).

2.2 Figure 1: Interactive Map (parcels sampled this past year)

Parcels sampled in 2025 are colored by proportional grass change (pGrass_delta = current year pGrass minus baseline pGrass). Red outlines indicate Cover_sig.counter > 5 or Grass_sig.counter > 5. Zoom and pan to explore; click a parcel for details.

Figure 1: Interactive map: parcels sampled this past year, colored by grass proportion change (current vs baseline). Red outlines indicate Cover_sig.counter > 5 or Grass_sig.counter > 5.

3 Results

  • Statistically below baseline:
    • 57 wellfield parcels are below baseline this year (cover and/or perennial grass). Of these, 16 have been below baseline for 5+ consecutive years. Filter Cover_Yrs or Grass_Yrs > 0 in the table below to view below-baseline parcels in cover or perennial grass.

4 Parcel Summary (Combined)

Table 1 summarizes current‑year parcel status for Wellfield parcels. Columns show baseline deltas (Cover_D, Grass_D, Shrub_D), years below baseline (Cover_Yrs, Grass_Yrs), chronic flag, cover types below baseline, and the last year at/above baseline (Last_Base, derived from the significance counters). Proportions (pGrass, pShrub) and current cover values (Cover, Grass, Shrub) are included for context, along with 2025 depth to water (DTW) and its percentile within the parcel’s DTW time series (DTW_pct, higher = shallower water).

4.1 Table 1: Parcel Summary (Combined)

Table 1

Table 2 lists control and other parcels (all parcels not in the wellfield group): Type = Control (C) or Type NA/blank. Same columns as Table 1.

4.2 Table 2: Control and other parcels (non-wellfield)

Table 2

4.3 Below vs. above baseline by group

Below baseline = cover and/or perennial grass below baseline in the current year (Cover.Delta < 0 or Grass.Delta < 0).

Table 3
Group N N below baseline N at/above baseline % below % above
Control / other 50 28 22 56 44
Wellfield 95 57 38 60 40

5 Wellfield vs. Control Group Comparison

Figure: Baseline (1986) versus current year cover by parcel type. Boxplots summarize total cover, grass, and shrub for control and wellfield parcels; significance labels indicate baseline vs current-year differences.

Control parcels (no pumping) show cover 35% baseline vs 32.1% current (Δ -2.8%), grass 22.1% baseline vs 16.7% current (Δ -5.4%), and shrub 6.5% baseline vs 9.8% current (Δ +3.3%).

Wellfield parcels (pumping-affected) show cover 28.8% baseline vs 32.8% current (Δ +3.9%), grass 15.4% baseline vs 12.6% current (Δ -2.8%), and shrub 8.8% baseline vs 13.7% current (Δ +4.9%).

Values reflect median parcel cover within each group. Significance labels on the figure summarize baseline vs current-year tests for each cover metric within parcel type (ns = not significant; * indicates p < 0.05).

Figure: Cover by parcel type for baseline (1986), previous year, and current year. Boxplots summarize total cover, grass, and shrub for control and wellfield parcels.

7 Parcel Profiles (QA/QC)

Parcel profiles are now provided on a dedicated page: Parcel Profiles — QA/QC (All Parcels). This page includes all parcels organized by wellfield from north to south — Laws, Control, Big Pine, Taboose-Aberdeen, Thibaut Sawmill, Independence Oak, Symmes Shepherd, Bairs Georges — with control parcels distributed throughout the valley just outside the zone of pumping influence.

8 Discussion

Vegetation in Owens Valley is monitored annually to detect and attribute changes potentially caused by groundwater pumping. This approach follows Green Book Section I.C.1.a, which sets out procedures for determining whether decreases in perennial cover are measurable (statistically significant).

8.1 Green Book and Baseline

  • Green Book
    Provides criteria for evaluating measurable vegetation changes, including the use of baseline data from 1984-1987 and statistical tests.

  • 1984-1987 Baseline
    LADWP inventoried and mapped vegetation across 2,126 parcels. Groundwater-dependent parcels (Types B, C, D, E) serve as focal points for assessing pumping-induced drawdown effects.

8.2 Interpretation

Direction of trends (with statistical significance): Control parcel group average shows no statistically significant linear trends in cover, grass, or shrub. Wellfield parcel group average shows statistically significant increasing linear trends in cover and shrub; wellfield perennial grass shows no statistically significant trend within this subset. P-values are reported in the “Parcel Trends Over Time” section above.

Flagged parcels: Flagged parcels indicate measurable (statistically significant) deviations from baseline. Whether these changes are attributable to pumping vs. other factors (e.g., drought, land use) are evaluated under Green Book Section I.C.1.b.

9 Appendix

9.1 Parcel time series (PDF)

Downloadable stacked time-series plots (cover, DTW, NDVI, precipitation) for reinventory parcels: Parcel_TimeSeries_2025.pdf.

9.2 References