October 11, 2000

Introduction

The Long Term Groundwater Management Agreement ("Agreement") between Inyo and Los Angeles depends on soil water information to govern pumping and surface water management practices. The goals of this management are to protect the vegetation of the Owens Valley from specified impacts and to provide a reliable supply of water for use in the valley and for export to the City of Los Angeles. Recently, Inyo County and Los Angeles put forth differing recommendations regarding the operation of the McNally canals in Laws during this runoff year, and also differing recommendations regarding the long-term operation of the canals. In support of the County's position, this report presents the relevant results from soil water monitoring conducted in Laws and also presents a synopsis of general observations of the soil water/groundwater relations from monitoring conducted throughout the Owens valley.

Inyo County Water Department staff routinely visit 22 monitoring sites located in wellfields to collect soil water data needed to determine the operational status of LADWP pumping wells according to the management scheme described in the Agreement and Green Book. Eight additional sites have been established outside areas potentially affected by groundwater pumping to serve as controls. Each site is equipped with a 100 m long vegetation transect to collect plant leaf area measurements, a monitoring well to observe groundwater fluctuations, and multiple access tubes to monitor soil water conditions (before 1995, however, only a single access tube existed at the sites). The procedures to collect the measurements in the field and to analyze the data to determine the operational status of nearby pumping wells (On/Off status) is described elsewhere and will not be elaborated upon here. (see the Agreement section V.A-C, the Green Book Sections I and III, and Steinwand (1996, 1997). Data analyses procedures and field procedures describing neutron soil water gauge methods were adopted by the Technical Group at its April 10, 1996 meeting and were discussed at the Standing Committee at its November 7, 1996 meeting. The Green Book has not been officially revised to reflect those changes.)

Figure 1. Simplified configuration of soil water/groundwater relations under desert phreatophyte plant communities.

 Soil water/ground water relations

The simplest example showing the relationship of groundwater to soil water is an unsaturated zone occurring at the surface and a saturated zone occurring at some depth (Figure 1). The boundary between the zones is referred to as the water table. It occurs at the depth where water will drain freely from the soil pores and enter a well open to the atmosphere. Above the water table, water flows upward from the saturated zone due to adsorption on soil particles and capillary forces present in the unsaturated soil. This simplified example of a groundwater discharge area represents conditions common under desert phreatophytes. The quantity and rate that groundwater flows upward is highly variable temporally and spatially depending on soil hydrologic properties and antecedent soil water and groundwater conditions. Generally, the soil is wetter nearer the water table and water content decreases upward. The moist unsaturated zone above the water table is called the capillary fringe. Although this term has been assigned several definitions, the one used here serves the purposes of this discussion adequately. The capillary fringe usually extends higher above the water table in finer textured soil (silt and clay) than in coarser textured soil (sand and gravel). The depth and thickness of the capillary fringe fluctuates as the water table fluctuates.

Monitoring using the neutron soil water gauge was initiated at 21 wellfield monitoring sites in October, 1990 through March, 1992. Monitoring at one site began in 1994. Increased pumping in 1987-1989 lowered the water table substantially at 17 wellfield sites (data from three sites were inconclusive), and soil water within the root zone was approaching or at the lower limit available for plant uptake at these sites when monitoring commenced. Pumping was curtailed during the 1990's in most wellfields and water tables generally have been increasing steadily since then. As a result, we have observed numerous examples of the rewetting of the soil as water tables rose (Figures 2-5). For brevity, examples from four monitoring sites are presented here to demonstrate the linkage between soil water conditions and water table fluctuations. The sites span the length of the Owens Valley and exhibit various soil properties, dominant vegetation, and groundwater conditions. In Figures 2-5, the lower graph shows the sum of available soil water occurring within the root zone, assumed to be 2m for grass-dominated sites and 4m for shrub-dominated sites. The available soil water values presented include a small reduction to account for decreasing water availability for plant uptake at greater soil depths (Steinwand, 1996). Before 1995, a single location at each site was monitored approximately every three months, hence the often abrupt changes in available soil water evident in the lower graphs. The upper graph presents the hydrograph of water table fluctuations measured in the test well located at the site and three or four examples of the volumetric soil water content (%) with depth measured at a single location along the 100 m vegetation transect. The entire record of available soil water and depth to groundwater is presented, but for the sake of figure clarity, only a few representative soil water content profiles are shown. The inset graphs are positioned approximately when the data were collected. Water content increases to the right on all inset graphs. All soil water/groundwater data are available at Inyo County and LADWP for inspection upon request.

Not surprisingly, the monitoring sites vary considerably in the depth to water table, depth to capillary fringe, and soil water content in the capillary fringe. To summarize the soil water/groundwater relations, the sites are grouped into categories as to whether the capillary fringe is connected or disconnected to the plant root zone. As of October, 2000 increases in soil water due to rising water table (or nearby spreading) have been observed at 17 of the 22 wellfield monitoring sites. The root zone at two sites has not been disconnected from the water table; three sites require further water table recovery to connect the capillary fringe with the root zone.

One additional observation is evident from Figures 2-5. At all four sites in1990-1995, groundwater input to the soil water was negligible and the inputs from precipitation were easily detected. Additions from precipitation are reflected in the increase in available soil water over successive winters. In nearly all instances, however, winter precipitation was not carried over from year to year. Without inputs from groundwater, the soil water typically was exhausted by plant uptake and evaporation by August or September as evident from the repeated return an approximate lower baseline each fall. Precipitation undoubtedly promotes increased plant growth to some degree, but these observations suggest that soil water from precipitation is ephemeral and that water derived from a shallow water table dominates the soil water balance. Thus, recovery of the water table seems necessary to promote recovery of soil water following water table decline; precipitation seldom persists from year to year to recharge the soil water in appreciable quantities.

 


























Figure 2. Changes in depth to groundwater and available soil water since 1987 at monitoring site Laws 2. Four representative profiles of soil water content (%, volumetric basis) with depth are inset on the depth to water graph to show the dependence of soil water on depth to groundwater.

 
























Figure 3. Changes in depth to groundwater and available soil water since 1987 at monitoring site Taboose-Aberdeen 4. Three representative profiles of soil water content (%, volumetric basis) with depth are inset on the depth to water graph to show the dependence of soil water on depth to groundwater.

 
























Figure 4. Changes in depth to groundwater and available soil water since 1989 at monitoring site Independence-Oak 1. Three representative profiles of soil water content (%, volumetric basis) with depth are inset on the depth to water graph to show the dependence of soil water on depth to groundwater.

Figure 5. Changes in depth to groundwater and available soil water since 1987 at monitoring site Symmes-Shepherd 4. Four representative profiles of soil water content (%, volumetric basis) with depth are inset on the depth to water graph to show the dependence of soil water on depth to groundwater.


Soil Water Deficit In Laws

The Inyo County Water Department routinely monitors the soil water conditions/groundwater levels at three sites located in the Laws wellfield. One site, Laws 2, is located in or near parcels where the water table and vegetation have not yet recovered to baseline conditions and thus is relevant to this discussion. Laws 2 is located in the northern portion of parcel Laws 85 and within 300 meters from the southern boundary of Laws 82. The available soil water and vegetation water requirements determined according methods described in Steinwand (1996 and 1997) and in the Green Book Section III are shown in Table 1 (L2). Presently, this site has insufficient soil water to meet the expected transpiration demand of the vegetation; 12.5 cm water required by the vegetation compared with 6.9 cm available soil water. Wells linked to Laws 2 last entered Off-status on July 1, 1996, and the available soil water content has not exceeded the vegetation water required at the time wells were turned off (8.8 cm). Thus, the wells remain in Off-status according to the well turn-on/turn-off provisions of the Agreement and Green Book (Figure 6).

 References

Steinwand, A.L, 1996. Protocol for Owens Valley neutron probe soil water monitoring program. Report to the Inyo/Los Angeles Technical Group, August 6, 1996. 17pp.

Steinwand, A.L. 1997. Protocol for Owens Valley neutron probe soil water monitoring program. Addendum: Conversion to CPN gauge use. Report to the Inyo/Los Angeles Technical Group, April 27, 1997. 5pp.


Table 1. October 1, 2000 available soil water, vegetation water requirement and status of permanent monitoring sites.

H: These values of soil water required for well turn-on were derived using calculations based on %cover that were routinely performed in the past. The values have not been updated to conform to the Green Book equations in section III.D.2, p. 57-59.

Figure 6. Monitoring site Laws 2 soil-plant water balance data and resulting On/Off status of linked pumping wells. Data collected and analyzed according to Green Book methods and soil water methods described by Steinwand (1996, 1997). Before May 1996 available soil water was measured using psychrometer methods described in the Green Book, Sec. III. Depth to groundwater is shown in the lower graph.