The Open Watershed Overlay (OWO) designated on the Mariposa County Zoning Map, is an overlay district and shall be applied to lands utilized, or proposed to be utilized as watersheds for public or community surface water supply, as defined by the engineering studies for that project. Within any OWO, the specific policies, standards and regulations of the principal zone with which the OWO is combined, are modified in accordance with this chapter.

1. Development standards for the OWO. Development standards for the OWO shall be as follows:
   1. Uses:
      1. Permitted uses: Single family residences, home enterprise, agriculture, public parks, and the managed production and harvesting of timber and those applicable uses listed under Chapter 17.108 of this title.
      2. Conditional uses: None.
      3. Prohibited uses: All other uses not listed above are prohibited, except similar uses in compliance with section 17.08.120 and 17.108.030 of this title.
   2. Minimum parcel or lot size: No parcel of real property in the OWO shall be divided or split into two (2) or more parcels by voluntary transfer, court action, or other conveyance where any one (1) of the parcels so created will be less than twenty (20) acres or a legal one-half of a quarter-quarter section except, where larger parcel sizes are required in the principal zone shall apply.
   3. Density: Standards required in the principal zone or one (1) single family residence per twenty (20) acres, whichever is more restrictive.
2. Improvement standards for OWO. For the purposes of insuring adequate protection of water quality within an area designated OWO, the following improvement standards shall be required on all building permits, variances, or subdivision proposals:
   1. Land located within an OWO are designated as "erosion hazard areas" in accordance with provisions of this code with respect to grading.
   2. All development proposals shall require approved percolation tests.
   3. No septic system shall be installed unless such septic system is installed on a specific location wherein an approved percolation test has been conducted in accordance with this code.
3. Amendment standards. Wherever it can be proven that a  parcel, or part of a parcel, lies outside the watershed, the Open Watershed Conservation (OWC) shall be modified to exclude that parcel or part of a parcel. The burden of proof is upon the applicant and shall consist of an engineered survey of the watershed line and any information deemed necessary by the planning director. Also, whenever a lot line adjustment or land division is completed where the watershed becomes a surveyed line, the OWC shall be modified to follow the actual watershed boundary. Once the watershed becomes a surveyed line, the county shall initiate an amendment to reflect the modification of the OWC. The development policy and improvement standards of the OWC shall not apply to those lands outside the watershed during the land division or lot line adjustment application process.