Page 32 - Beaumont Basin Watermaster

Beaumont Basin Watermaster
2013 Reevaluation of the Beaumont Basin Safe Yield 3-Apr-15

area. The subsurface underflow at Smith Creek was estimated using the values published in
Rewis et al. (2006).

4.1.5 Return Flow

Return flow rates were estimated for each of the land use categories shown on Figure 7 and listed
in Table 3. The unit return flow rates were applied to the areas for each land use condition to
estimate a total return flow for any given period. As the areas for each land use changed over
time, the total return flow for that land use condition also changed accordingly. The
methodology for estimating the unit return flow rates for each land use condition is described in
the following subsections.

4.1.5.1 Return Flow from Irrigated Agriculture
Return flow from irrigated agriculture was estimated based on estimates of applied water for
crops and trees historically grown in the Study Area (see Figure 7). Crop types were grouped
from land use maps into two categories: grains and trees. Total applied water for grains was
based on DWR, 1986. Total applied water for trees was based on an analysis of delivered water
records for parcels identified from aerial photographs as having small (i.e. less than 5 acres)
orchards. Consumptive use (water that is utilized by the plant, transpired, or evaporated) is
assumed to be 75 percent of the total applied water. Thus, 25 percent of total water demand from
irrigated agriculture is assumed to become groundwater return flow. Native and non-irrigated
agriculture is assumed to have no return flow.

4.1.5.2 Return Flow in Low-Density Residential Areas

The Cherry Valley community is a low-density residential area within the northern portion of the
Study Area, which discharges wastewater through individual household septic systems (see
Figure 7). Return flow was estimated for the area of individual septic systems based on parcel
size and average annual delivered water as reported by BCVWD (see Table 4). For the smallest
parcels (0.25-acre and smaller), 50 percent of the delivered water (0.17 acre-ft/year/parcel) was
assumed to be used indoors and ultimately discharged through the septic system. The other 50
percent was assumed to be used for outdoor landscape irrigation. For parcels larger than 0.25
acres, the indoor water use was fixed at 0.17 acre-ft/year and the balance of delivered water
assumed to be used outdoors. For low-density residential areas, 100 percent of indoor water use
is assumed to become groundwater recharge and 25 percent of outdoor water use is returned to
the groundwater. Thus, 62 percent of delivered water to low-density residential areas with
individual septic systems is assumed to become groundwater recharge.

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