ICWD Home > Research and Reports > Annual Reports > 2005-2006 Monitor >

Status of Groundwater Dependent Vegetation with Regard to the Drought Recovery Policy
by Sally Manning, Vegetation Scientist, May 2006

The Inyo County Water Department (ICWD) continues to monitor vegetation conditions to identify and evaluate changes that may be caused by water table fluctuations. During 2005, ICWD staff collected data on 70 vegetation parcels that, since the late 1990s, have been monitored to track progress in achieving the goals of the Drought Recovery Policy (DRP). To assess vegetation conditions in these parcels, ICWD compares data collected in the field during the current year with data collected by LADWP during the baseline vegetation inventory and mapping period (1984-87, exact time dependent on location in the valley). Both ICWD and LADWP employed the "line-point" transect technique to characterize parcel vegetation. However, because ICWD aims to achieve an unbiased representation of current vegetation cover and species composition throughout the entire area of the vegetation parcel, ICWD randomly locates the transects using a sophisticated GIS application in conjunction with GPS, and ICWD runs several more transects than were employed by LADWP during the mid 1980s.

ICWD devised criteria for annually classifying parcel status with regard to the DRP (see for example, Inyo County Water Department staff 1999; Manning 2002; Manning 2006). The criteria incorporate the critical components of the DRP, which calls for an evaluation of water table depth, degree of water table recovery, soil moisture and type, vegetation conditions, and information on vegetation recovery, prior to establishing each year's annual pumping program. The goal of the DRP was to recover enough water within the phreatophytic root zone to protect the vegetation and avoid decreases and changes. Each year, ICWD collects data, then analyzes its data along with other available data to assess conditions with regard to DRP guidelines. For each vegetation parcel, current vegetation conditions and recovery to baseline perennial cover levels are assessed. ICWD uses LADWP data on depth to water table to derive an estimate of average water table depth beneath each parcel as well as degree of water table recovery to levels which occurred during the baseline period. Soil water content is measured many times per year at the 33 permanent monitoring sites in the valley. These measurements have provided insights into the mechanisms by which the quantity of water necessary for vegetation health is replenished from rising water tables and, to a lesser extent, precipitation. However, it is not possible to quantify available soil water at the much larger vegetation parcel scale. Because the DRP calls for recovery of soil water within the vegetation root zones -- 2m for grass-dominated and 4m for shrub-dominated parcels -- depth to water table, degree of water table recovery, and evidence of vegetation recovery (i.e., an increase in perennial cover) in conjunction with measured water table recovery are used as a surrogate for assessing the amount of water available to parcel plants.

The 70 parcels monitored in 2005 were all originally characterized as being dominated by groundwater dependent species. After applying the DRP criteria, each was classified into one of the following DRP categories:

Between 1995 and 2001, conditions in some wellfield parcels met the criteria ICWD devised for release from management constraints of the DRP. Since 2001, however, water tables have generally declined beneath Owens Valley wellfields, and as a result, no additional parcels have been designated DRPfree in recent years. As of 2005, 29 of the 70 monitored parcels were still assigned to the DRP category (that is, never recovered); 21 were DRPfree; and 20 of the 70 served as Control parcels.

Total perennial plant cover in the 70 parcels was calculated for 2005 and compared with baseline levels. Perennial cover is evaluated with regard to precipitation and water table conditions. During the 2005 water year, valley floor precipitation exceeded average (see related article). From 2004 to 2005, water tables generally rose somewhat in Control areas but remained stable or declined in wellfield areas. Average perennial cover relative to baseline is graphed in Figure 1.

Within the three groups, results for 2005 were consistent with cover trends since 1991 and with expected vegetation responses to changes in hydrologic conditions.

Control Group. Average cover in the Control parcels exceeded baseline levels in 2005 (Figure 1). Over the period from 1985 - 2005, water tables have fluctuated minimally beneath Control parcels (Figure 2a). ICWD transect data, collected 1991-2005, have shown that, on average, perennial cover in Control parcels equals or exceeds baseline levels, and the years in which cover exceeds baseline are high-precipitation years.

DRP Group. Water tables beneath the DRP parcels were drawn down during the 1987-1992 drought. Since that time, water tables beneath these parcels have failed to fully recover (Figure 2b). ICWD transect data, collected 1991-2005, have shown that, on average, perennial cover in DRP parcels remains significantly below baseline levels in most years. In 1995 and 1998, two high-precipitation years, perennial cover averaged for the DRP parcels equaled baseline, but in 2005 (a high precipitation year), average perennial cover for the DRP group was significantly below baseline (Figure 1). Unlike the Control group, cover in the DRP parcels has never exceeded baseline during the period ICWD has been monitoring.

DRPfree Group. Water tables beneath the DRPfree group of parcels declined during the 1987-1992 period, and, on average, made a brief recovery to baseline levels during the mid-late 1990s (Figure 2c). Since about 2000, water tables have been declining beneath these parcels. From 1998-2001, average perennial cover in the DRPfree group exceeded baseline levels. Thus, in those years, cover trend in the DRPfree parcels mimicked the pattern of the Control group relative to baseline. However, in 2002, a dry year, average perennial cover in the DRPfree group dropped significantly below baseline whereas the Control group average cover equaled baseline. Since 2001, perennial cover in the DRPfree group has been below or equal to baseline. In 2005, a wet year, average cover in the DRPfree group was statistically indistinguishable from baseline (Figure 1).

A more detailed analysis of parcel conditions, and data for each monitored parcel, can be found in "Status of re-inventoried vegetation parcels according to the Drought Recovery Policy, 2005" by Sara J. Manning, May 9, 2006.

Figure 2. Parcel-scale depth to water (DTW) estimates were judged reliable all years for some of the parcels ICWD monitored in 2005 (see Harrington 2003; Manning 2006). For those parcels with sufficient reliable data, parcel DTW values were averaged for all parcels within the management groups (Control, DRP, and DRPfree) and within vegetation type (meadow or scrub). The number of parcels in each group is shown in parentheses. Failure of the DRP group to achieve full water table recovery since the baseline period may account for the finding that perennial vegetation cover was significantly below baseline in 2005 (Figure 1), even though 2005 was a high precipitation year for Owens Valley. The agreed upon rooting zone for meadow is 2m, and for scrub, it is 4m.

References Cited:

Harrington, Robert F. 2003. Estimated of depth-to-water beneath vegetation reinventory parcels, 1985-2003. Inyo County Water Department report. December 2003.

Inyo County Water Dept. staff. 1999. Condition of selected vegetation parcels and assessment according to the Drought Recovery Policy. Inyo/LA Technical Group Report, March 2, 1999.

Manning, Sara J. 2002. Classification of re-inventoried vegetation parcels according to the Drought Recovery Policy, 2001. Inyo County Water Department report. February 19, 2002.

Manning, Sara J. 2006. Status of re-inventoried vegetation parcels according to the Drought Recovery Policy, 2005. Inyo County Water Department. May 9, 2006