by Aaron Steinwand, Soil Scientist
August 2005

The Water Agreement established procedures to determine which LADWP pumping wells can be operated based on soil water and vegetation measurements. Staff from ICWD 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 2004, six sites were in On-status. Three sites entered On-status following December/January rains and another in February due to precipitation and rising water table. Four sites went into Off status on July 1, 2005.

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, 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.

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 6.6 feet; shrub sites are assigned a root zone of 13.1 feet.

The wellfield monitoring sites (including three monitored but not used for pumping management) 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 recharge from lower depths is occurring in the root zone. This is primarily observed during winter or in response to rising water. Seventeen sites occur in this category. Sites L2, TA1, TA5, TS2, and IO1 have retained soil water available to plants that is taken up during the summers of 2004 and 2005. 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. Five sites occur in this category, and four (L1, TA2, TS3, TS6) have a considerable amount of retained soil water. TA4 has little retained water and the amount of recharge from below is very small.
3. Connected: Water table fluctuations cause soil water changes in the top half of the root zone. Three sites occur in this category.

The relatively low runoff and increased pumping from 2003 and 2004 caused water table and soil water decline at most monitoring sites from the relatively shallow water table conditions in 1999-00. Consequently, the number of sites where soil water responds to water table fluctuations has decreased. As of July 2005, the water table was supplying water to the root zone at eight monitoring sites located in wellfields (see map). This compares to about 20 sites with groundwater recharging the root zone in 2000. Soil water at slightly more than half of the sites (17) did not increase at depth after plant transpiration ceased in the fall 2004 and/or did not respond to water table increases over the following winter suggesting that the water table and root zone are disconnected.