Las Virgenes Municipal Water District

 

Calculating Water Budgets

When LVMWD updated its billing system, discussed in Billing Design » , prior to BBR implementation, it used the following calculations for BBRs billing.

Potable Water- Single Family Residential, Multi-family Residential, and Irrigation Customers Factors in BBR calculation:

  • P – number of persons (3 or value stored for the customer)
    • Single Family Residential Customers: 3 or value stored for the customer
    • Multi-family Residential Customers: 3 times the multi-family dwelling unit factor or value stored for the customer
  • GPCD – gallons per person per day (55, changeable)
  • M – number of days in billing period
  • Ia – interior adjustment (gal/day)
  • Di – interior drought factor (1, changeable)
  • LA – landscape area (square feet) (see below for how LVMWD calculates the irrigated area for each customer)
  • Oa – outdoor adjustment (gal/day)
  • ETo – daily evapotranspiration rate (summed over billing period, inches)
    • LVMWD has 374 different climate zones defined and each zone has a daily ETo value stored. The ETo for the billing period will be the sum of daily ETos for that zone and date range.
  • mKc – monthly Kc factor (0.85, changeable)
  • lKc – monthly Kc factor for new (constructed after July 1, 2015) single family customers (0.65,
    changeable)
  • Do – outdoor drought factor (1, changeable)

Equations:

Indoor Budget (ccf) = ( (P × GPCD) + Ia × M × Di) 748(gal ccf)
Outdoor Budget (ccf) = ( (ETo × Kc × LA × 0.6234) + (Oa × M) × Do) 748(gal ccf) where
Kc is either mKc for existing residential customers or IKc for new residential customers

The total water budget for residential (both single family and multi-family) customers is the indoor budget combined with the outdoor budget. The residential BBR has four tiers with widths and rates as shown in Examples of Budget-Based Rates Tier Structures . Penalties may be applied for water usage greater than two times the total water budget.

The total water budget for irrigation customers only includes the outdoor water budget and uses the same calculation above, setting Kc at 0.8 originally. It has three tiers and penalties may be applied in the same way as for residential customers.

Potable Water- Commercial Customers

Commercial water budget is based on a 3-year rolling average for the same billing period. LVMWD currently has monthly billing, though the history is bimonthly. To calculate a rolling average using past bimonthly billing periods, a bimonthly billing period close the current read dates is used to calculate the daily usage. For commercial accounts with less than 3 years of history, all available history is used. For commercial accounts with no history, a daily average is assumed by LVMWD and used in the calculation until there is history.

Factors:

  • Dc – commercial drought factor (1, changeable)
  • M – number of days in billing period
  • Ca – commercial adjustment (ccf/day)

Equations:
Commercial Daily Average (ccf/day) = (usage over billing periods)/(days in billing periods)
where 0.22 ccf/day is used if the commercial daily average is less than 0.22 ccf/day (6.6 ccf/30 days)
CommercialWaterBudget(ccf)= (CommericalDailyAverage+Ca)×M×Dc

The four tiers of the commercial rate structure are included in the rate structure design examples in
Examples of Budget-Based Rates Tier Structures ».

Recycled Water
Recycled water’s BBR definition is identical to potable water irrigation customers, so only includes the outdoor water budget calculation. The rate structure design for both is shown in the table in Examples of Budget-Based Rates Tier Structures ».

Development of Irrigated Area Data

LVMWD used four-band (red, green, blue, and near infrared) aerial photography with six-inch pixel resolution imagery to develop irrigated area data for water budgets. The imagery was used to determine pervious and impervious land cover. All four bands were used to analyze several recognition elements of air photo interpretation: pixel values (tone or color), shape, size, pattern, shadow, texture association, spectral signature, and site. The acquisition date of the imagery is critical for analyzing color-infrared photography. Capturing land cover in mid to late summer helps differentiate non- irrigated landscapes that may appear irrigated earlier in the season due to precipitation. The consultant who provided the imagery used proprietary techniques to provide the analytical data requested by District.

Land cover was categorized as follows:

1. Impervious surfaces
2. Swimming pools
3. Irrigated non-turf
4. Irrigated lawn
5. Non-irrigated/natural vegetation
6. Open water
7. Potentially irrigated

Issues unrelated to the quality of the aerial imagery data and analysis arose during development of parcel- specific data. Appropriate time and resources should be allocated to address these conditions when preparing budgetary estimates and timelines for budget-based water rates implementation. The amount of work involved may be significant enough to impact data quality and project completion.
Conditions to consider when determining irrigated area:

  1. Property lines may not line up with aerial photography (see example)
    In many instances, parcel property lines do not coincide with physical lot boundaries or features such as fences or change in landscaping between adjacent properties. When customers disagree with data based on parcel property lines, physical boundaries are used to determine the extent of irrigated areas. The magnitude is oftentimes not significant, but using physical boundaries makes more sense to customers.
  2. City or county right-of-way extends into front yards (see example)
    The right-of-way extended six to ten feet into the front yards of about 7,500 parcels, resulting in the under estimation of irrigated areas. For some areas, the right-of-way included parkways, which are the landscaped strips between the sidewalk and the road. Three options were considered to account for these additional irrigated areas:
    A. Increase the irrigated area by a%
    B. Add a constant area factor for each property
    C. Perform a separate parcel-specific, manual analysis to determine the additional area
    LVMWD chose option C to get the most accurate information, but the effort required additional consulting cost.
  3. Aerial photography abruptly ends at the service area boundaries (see example)
    The abrupt end of aerial photography at the service area boundaries prevents analysis of some properties because the service area boundary lines do not always coincide with parcel physical boundaries. It is recommended that aerial photography limits be extended to a reasonable buffer outside the service area boundary (e.g. quarter mile) to allow further analysis where needed.
  4. Individual units in multi-family dwellings have incorrect property lines (see example)
    Property lines for multi-family dwellings do not accurately define the boundaries of each unit. Although not necessary for dwellings that do not have outside irrigation, manually laying out these lines can be a very tedious, labor-intensive process that sometimes requires field visits.
  5. Single family residential customers may irrigate outside of their property lines (see example)
    Some single family residential customers who responded to the LVMWD questionnaire on water budgets indicated that they irrigate other areas outside of their property lines; this was generally done by property owners adjacent to open space for purposes of fire protection or erosion control. Inspection of historical aerial imagery often indicated that this practice was going on for years with some customers. The District did not check whether customers had permission to do this with the landowner.
  6. Some meters for multi-family dwelling complexes also serve irrigation for common areas (see example)
    In addition to serving individual units in multi-family dwellings, some meters also provide irrigation to common areas. The lack of dedicated irrigation meters to irrigate common areas is often a good indicator that this is occurring. Data for this situation is best coordinated with the homeowners association or a landscape maintenance company familiar with the development.
  7. Coverage of meters within right-of-way or common areas is not known (see example)
    Since most right-of-way irrigation is performed by cities or homeowners associations, it is best to coordinate the extent of coverage of these meters with those responsible for day-to-day maintenance of these areas. Irrigation services can cover center medians and/or parkways on either side of the street. This information is best determined by having the landscape maintenance company manually operate the sprinklers.
  8. Some properties are irrigated with recycled and potable water (see example)
    Properties with irrigated water service are best handled and coordinated with staff in charge of the recycled water program. It may be necessary to review coverage of irrigation meters with the designated site supervisor.
  9. Some areas appear to be irrigated but have natural vegetation (potentially irrigated) or vice versa (see example)
    Some areas of properties are placed in a “potentially irrigated” category because it is difficult to determine whether the vegetation is natural or irrigated. To determine the correct classification, historical water use for the properties are reviewed. When the analysis indicates there is low water use to support the vegetation, these areas are reclassified to “non-irrigated” natural vegetation and vice versa. When the analysis is inconclusive, site visits are conducted to make a determination.