Water Department
by Aaron Steinwand, Soil Scientist
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 changed from On to Off status in July and October 2005. The large number of sites changing to Off-status is not unusual during a period of declining water tables and following a winter with ample rainfall on the valley floor. In October, only three sites were in On-status, the lowest number since the Green Book program was initiated in late 1989. These three sites along with the wells not subject to the Green Book management had an annual pumping capacity of approximately 110,500 ac-ft. Four sites entered On-status during the winter 2005-06 due to the combination of water spreading, above-average precipitation, and rising water table. Annual pumping capacity allowed by the Green Book in May, 2006 was approximately 137,000 ac-ft.
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 clearer picture of the conditions underground that are affected by pumping. 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. Nearly all wellfield sites have an intermediate zone where soil water contents change very little that separates lower depths affected by water table from upper depths affected by infiltration. Infiltration for several sites often extends to about 40 inches (1 m) deep which is the middle of the root zone in grass-dominated sites. In years like 2005-06 with above normal precipitation and rising water tables, it can be difficult to identify the source of water when infiltration and capillarity above the water table overlap. Most sites in the coupled and weakly connected category have relatively moist soil throughout the profile at the beginning of the 2006 growing season.
1. Disconnected: No recharge from lower depths is occurring in the root zone. Eleven sites occur in this category. Sites TA5 and BG2 have retained soil water available to plants. 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. Three sites occur in this category.
3. Connected: Water table fluctuations cause soil water changes in the top half of the root zone. Ten sites occur in this category. Water spreading in Laws and Thibaut-Sawmill wellfields in particular produced sufficient water table rise to wet the soil at several monitoring sites into the upper parts of the root zone.
The above average runoff and associated water spreading along with greatly decreased pumping in 2005-06 caused water table increase at all but two monitoring sites. Consequently, the number of sites where soil water responds to water table fluctuations has increased. As of May 2006, the water table was supplying water to the root zone at fourteen monitoring sites located in wellfields (see map). This compares to about eight sites last year. Soil water at eleven sites did not increase at depth after plant transpiration ceased in the fall 2005 and/or did not respond to rising water tables during the winter suggesting that the water table and root zone were disconnected.