Environmental Engineering Reference
In-Depth Information
Table 1-5 Water Balance in Southern California, 2000 (1,000 Acre-Feet)
Supply
Use (Demand)
Precipitation
7,500
Evapotranspiration
7,441
Imported
2,991
Consumptive use
1,819
Depletion of groundwater
1,245
Outflow to the salt sink
2,498
Total supply
11,752
Total use
11,758
Note: The runoff lost to the ocean is almost equal to the required import.
Could LID practices such as capture and reuse of stormwater runoff signif-
icantly reduce the need for importation of water supply to the region? These
practices offer the possibility of working to redress at least some of the water
cycle imbalances that confront southern California communities, but will require
a significant rethinking of existing stormwater and water supply system designs.
Central to the concept of watershed sustainability is the water cycle and its
balance, very roughly defined as the matching of water inputs (“supply”) to water
outputs (“demands”). Any watershed, physiographic region, or land area that can
be well defined in terms of water supply and use, both natural and human, can also
be evaluated in terms of water cycle. Analysis of the existing water balance for
southern California demonstrates that the natural water resources are insufficient
to meet the demands of the existing 19 million residents in the 11,000-square mile
region from Ventura to San Diego, with 5 million additional residents projected
to arrive by 2020 [12].
The deficit in natural water resources has been met over the years by three
aqueducts (Figure 1-10), which convey imported water hundreds of miles to the
region. Table 1-5 provided a simplification of this water resource balance for the
year 2000, with both supply and demand (use) summarized from more detailed
statistics. In 2000, rainfall provided only 72% of the total water supply use or
demand, again with the quantity of runoff to the ocean [2,498 thousand acre-feet
(TAF)] close to the water importation (2,991 TAF).
The depletion of groundwater is especially troublesome in this “balance.”
Although a number of large aquifer systems lie beneath the surface of the region
and are constantly being replenished by recharge from surface sources, their
capacity has been exceeded during most years. The Los Angeles area receives
over 40% of their current water supply from these aquifers, utilizing spreading
grounds to recharge both runoff and recycled effluent, based on the concept
of “conjunctive use.” However, year after year, the groundwater reservoirs are
further depleted, despite our plans.
The Energy Demand for Water in Southern California
It requires over 10,200 kilowatthours (kWh) for every million gallons (MG)
of water imported into southern California, 40 times greater than the national
average and 20% of total residential energy usage for the region, as shown in
Search WWH ::




Custom Search