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Groundwater model chosen for
Back Country study
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Data gathering underway
in local communities
By Christy Scott
The Alpine Sun
BACK COUNTRY — The groundwater modeling equation that will govern a comprehensive groundwater study for the entire Back Country was presented at a recent meeting between the county hydro-geologists, various Back Country planning group members, and the Campo/Lake Morena Planning Group Groundwater Subcommittee.
“It looks like we’re making some headway in this study,” said Campo/Lake Morena PLanning Group groundwater subcommittee Chairman Larry Johnson. “The calibration for the model is nearly complete.”
According to San Diego County Department of Planning and Land use Hydro-Geologist, Jim Bennett, the study model for the Back Country is based on a model that the county has used for years to calculate groundwater depletion and replenishment. However, for this particular study, the model is being adjusted to more appropriately address conditions in the Back Country and to better respond to specific area data input.
“We’ve been really happy with how well the model is responding to data,” Bennett said.
Using a model for the Lee Valley area, located near Jamul, county hydro-geologists are gathering information from various Back Country wells belonging to residents who have volunteered to allow officials onto their properties. They are looking for more volunteers to allow testing of active and dry wells on their properties.
Data is being incorporated from more than 600 well logs, with more than 1,000 to be reviewed in total.
At the February meeting, the methodology to be used for the study was outlined for representatives from Descanso, Pine Valley and Campo by San Diego County Hydrogeologist Jim Bennett.
Step one is to identify the study boundaries.
The GP 2020 process spans the entire county, however for this specific study, only Back Country areas are being looked at.
“The Back Country areas are going to be investigated on a sub-basin by sub-basin approach,” Bennett said. According to the study, each sub-basin will be broken down into 300-foot by 300-foot grids in a Geographic Information Systems (GIS) computer model.
Step two is to estimate the groundwater recharge for the area.
According to the study, groundwater recharge will be estimated for each grid by feeding the numbers from wells and other sources into this equation: Recharge = Precipitation — Potential Evapotranspiration — Runoff — [Soil Moisture Capacity - Current Soil Moisture].
Evapotranspiration is the process by which water is discharged to the atmosphere as a result of evaporation from the soil and transpiration from plants. The transpiration aspect of evapotranspiration is essentially evaporation of water from plant leaves.
“We currently have information in our computer system for precipitation, evapotranspiration, soil types, and runoff,” Bennett said.
According to Bennett’s presentation, when testing runoff, the study will consider the different types of land use and soil types. The effects on runoff are much different, for example, between a residential lot, agriculture land or park space. In San Diego County alone, there are nearly 250 different soil types, according to the USDA.
Step three is to estimate the groundwater that currently exists.
“We will estimate groundwater in storage by looking at the various aquifer types throughout the county by reviewing geological maps, well logs, and other sources of information we have in each sub-basin,” Bennett said.
According to a study by the State Department of Water Resources in 2003, the Campo Valley Basin is mainly an alluvium aquifer.
“The alluvium in this basin consists of gravel, sand, silt, and clay,” reads the state report. “The alluvium ranges in thickness from a few feet to roughly 100 feet. The average thickness is estimated at 55 feet.”
The calculations for the county study show that for every 10 feet of depth in the alluvial aquifer, there is one foot of water.
According to the state report, the storage capacity for the Campo Valley Basin is estimated to be 63,450 acre-feet.
Step four is to calculate the water demand from the proposed GP 2020 land use designations.
According to Bennett, current water demand and water demand under maximum build-out of proposed GP 2020 land uses will be calculated.
“Land use maps and other sources will be used to provide the amount of current groundwater demand and the maximum build-out groundwater demand,” he said in his presentation to planning group members. The other sources include population speculations and current numbers, and the reality on the ground compared to current general plan designations.
According to the study outlines, groundwater demand is being quantified for various types of build-out. For example, a single-family home is expected to use about 163,000 gallons of water per year. Agriculture land consumes up to 1.3 million gallons per year, and for a golf course, groundwater demand can be higher than 1,000 acre-feet per year: the equivalent of 2,000 single-family residences.
The final step is to develop and run a computer model to see how the build-out will affect groundwater.
“A computer model will be run to compare existing and GP 2020 groundwater demand with available groundwater demand in order to identify areas where groundwater demand exceeds groundwater availability, thus creating a potentially significant impact to groundwater resources,” Bennett said. “The results of the study will be used in guiding land use densities for GP 2020.”
“The goal of study is to ensure that there will be an adequate and sustainable supply of groundwater for proposed build-out under GP 2020,” Bennett said. According to Bennett, the study is expected to be completed within the next few months.
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Christy Scott
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