The Owens Valley Monitor 2002-2003

Figure 5. Soil water/root zone
connection at permanent monitoring sites.

Soil Water Conditions
— Aaron Steinwand, Soil Scientist

     The Inyo/Los Angeles Water Agreement established procedures to determine which LADWP pumping wells can be operated based on soil water and vegetation measurements. Staff from the Water Department routinely monitors depth to groundwater and soil water content at 25 sites in wellfields and eight sites in control areas. Data from 22 wellfield sites visited each month are used to determine the operational status (On or Off) of nearby pumping wells. In October 2002, nine sites were in On-status. Three sites with low plant cover entered On-status following November storms and one site entered On-status in March due to water table recovery and precipitation.
     The purpose for the On/Off procedures is to manage pumping to protect plant communities that require periodic connection to the water table for long-term survival. Generally, the sites with On-status have wet soil and shallow water tables, and the Off-status sites have dry soil and deep water tables. The On/Off determination is affected by several other factors, however, and sometimes On-status sites are those with a deep water table and low plant cover. Conversely, a site with adequate water table depth may be in Off-status if the water table occurs just below the root zone and plant cover is high. These inconsistencies contradict the assumption that operating pumps in On-status will not adversely affect vegetation near the monitoring sites, but operating wells in Off-status will.
     We determine which monitoring sites are connected with the water table to give a clearer picture of the conditions underground that are affected by pumping. We rely on soil water and groundwater data because the water table depth necessary to provide water to the plant roots depends on the soil characteristics as well as water table depth. For example, the capillary rise above the water table in a silty soil is much greater than in a sandy soil. At the same water table depth, the plants may have access to groundwater if the soil is silty, but not if it is sandy. How well plant roots can take up groundwater also depends on the type of vegetation. In similar soils, a shallower water table is necessary to supply groundwater to grasses than shrubs because of the shallower roots of the grasses. For management purposes, grass-dominated monitoring sites are assigned a root zone of 2 meters; shrub sites are assigned a root zone of 4 meters.
     The wellfield monitoring sites were grouped into three categories to summarize the connection between the root zone and the water table. Brief descriptions of the three categories are given below.

1. Disconnected: No groundwater is recharging the root zone. Thirteen sites occur in this category. Sites L2, TA4, and IO1 have retained soil water in the root zone. Soil at the other sites is dry except at shallow depths affected by rain.
2. Weakly connected: Water table fluctuations cause soil water changes in the bottom half of the root zone. Seven sites occur in this category, and all have a considerable amount of retained soil water.
3. Connected: Water table fluctuations cause soil water changes in the top half of the root zone. Five sites occur in this category.

     The relatively low pumping and high runoff from 1995 to 1998 caused the water table and soil water recovery to increase at most monitoring sites from the decline induced by drought and large amounts of pumping in the late 1980s. Since 1999, however, the water table stopped rising or began to decline at most sites because of lower runoff, increased pumping, or because water tables approached shallow depths similar to pre-drought levels. Consequently, the number of sites where soil water responds to water table fluctuations has decreased. As of April 2003, the water table was supplying water to the root zone at twelve monitoring sites located in wellfields (Figure 5). This compares to about 20 sites with groundwater in the root zone in 2000. Soil water at slightly more than half of the sites (13) did not increase at depth after plant transpiration ceased in the fall of 2002 and/or did not respond to water table increases over the following winter suggesting that the water table and root zone are disconnected.

Return to the Table of Contents