Soil Water Conditions

The Water Agreement established procedures to determine which LADWP pumping wells can be operated based on soil water and vegetation measurements. Staff from ICWD regularly 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.  Seven sites were in On-status through the 2006-07 runoff year; no sites have changed status (either On or Off) since April 2006.

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 based on an incomplete accounting of the components of the soil water balance, however.  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 identify the monitoring sites where the root zone is connected with the water table to give a better picture of the conditions underground that are affected by pumping. Nearly all wellfield sites have an intermediate zone where soil water contents change little that separates lower depths affected by water table fluctuations from upper depths affected by precipitation.  Infiltration for several sites sometimes extends to about 40 inches (1 m) deep which is the middle of the root zone in grass-dominated sites.  In years like 2006-07 with little precipitation over the winter, it is relatively easy to distinguish soil water recharge from the water table. We rely on both 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. For 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 (2 m); shrub sites are assigned a root zone of 13.1 feet (4 m).

The wellfield monitoring sites (including three that are monitored but not used for pumping management) were grouped into simple categories to summarize the connection between the root zone and the water table.  Brief descriptions of the three categories are given below.  Sites in Laws and Big Pine wellfields experienced water tables sufficiently shallow to wet the soil profile due to spreading this past year.  The water table has since declined after spreading stopped.  Where the water table is still above the depth necessary to wet the root zone as of April 2007 (estimated when water table initially rose into the dry soil), the sites were considered weakly connected.  If the water table was deeper it was classified as disconnected.  In all these sites (L1, L2, BP1, BP2, BP3) the soil has retained water that will be available to plants this summer.   Most sites in the coupled and weakly connected category have relatively moist soil in much of the soil profile at the beginning of the 2007 growing season.

 1. Disconnected:  No recharge from lower depths is occurring in the root zone.  Eight sites occur in this category compared with eleven last year.  Sites BP1, TA5, and TS2 have retained soil water available to plants but the water table at the beginning of the 2007 growing season is probably too deep to recharge the root zone.  Soil at the other sites is dry.

2. Weakly connected:  Water table fluctuations caused soil water changes in the bottom half of the root zone.  Eight sites occur in this category.  Sites L1, L2, BP3, and BG2 have ample soil water stored in the soil profile.

3. Connected:  Water table fluctuations caused soil water changes in the top half of the root zone.  Nine sites were placed in this category.

The above average runoff and associated water spreading along with greatly decreased pumping in 2006-07 caused water table increase at 18 of the wellfield monitoring sites.  As of May 2007, the water table was capable of supplying water to the root zone at 17 monitoring sites located in wellfields (see map).  This compares to about fourteen sites last year.  Three sites are probably not receiving groundwater in the root zone, but have moist soil from higher water levels earlier in the 2006-07.  The remaining five sites have dry soil throughout the root zone.