SPDSS_Task93_MappingHistoricLandUse_20070109

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MEMO 93 6.1 Historic Diversion Structure The first step in assigning the water source to historic irrigated parcels was to identify diversions for each historic period. A list of non-current diversions was obtained from the SPDSS CU contractor that provided information on irrigation diversion structures whose diversion records ended before 2001. Each diversion structure record on the list provided a start and end date. The start date describes the start of diversion records, not the start of diversions, thus the start date was ignored. The end date is the year the diversion ceased diverting water and was used to create the diversions layer for each historic period. The diversion list was queried for the current diversions in each historic period. For example, to query 1987 records the non-current diversions were queried for end date >= 1987. The result was then merged with current diversions that were still active in 2001 to form a complete diversion list for each historic period. 6.2 Historic Ditch Service Areas 6.2.1 Create New Service Area Boundaries The 2001 service areas were used as a starting point to begin mapping the 1987 service areas, likewise 1987 service areas were used to begin mapping 1976 service areas, and 1976 service areas were used to begin mapping 1956 service areas. Then, historic ditch service areas were used to identify historic diversion structures that were active in the historic year but not in 2001, and thus not mapped in 2001. Service areas were delineated for these diversions using principles outlined in SPDSS Technical Memorandum 91, Section 2.2.1. In this step the term "new service areas" refers to a service area polygon boundary that was created by manually digitizing it or copying and pasting an existing service area polygon. New service areas were only created around parcels if it seemed possible they were irrigated by the diversions. In many cases the corresponding ditch was identified in the ditch layer or the orthophoto. A set of guidelines was developed for delineating new service areas. These guidelines helped to identify how and where new service areas should be created. The guidelines were as follows: • Create a new service area around new irrigated parcels that do not have a service area around them. • Create a new service area around new irrigated parcels that are already within a service area. Create service areas on top of the existing service area or cut it out of the existing service area. Service areas were cut out if they had their own ditch diverting from the river and appeared separate. • Create a new service area by copying an existing service area. Existing service areas were copied when no new parcels were present, the diversion was in or near the existing service area, and when the diversion did not have a separate ditch on the river. The parcels within both service areas received the water diversion identification (WDID) of both diversions. • Create new parcels and a new service area. When no parcels were near the new diversion the NDVI was checked to see if irrigated parcels could be added. Parcels were added when appropriate. • Label diversion with note "No irrigated acreage nearby." A comment was added to the diversion when there were no irrigated parcels in the vicinity and the NDVI imagery did not show any irrigated lands. 6.2.2 Identify Irrigated Parcels not in a Service Area After the service area boundaries were edited, the irrigated parcels were evaluated to determine which parcels were not within an irrigation service area. Most of these parcels were served by groundwater only and should not be in a service area. However, some of these parcels were new parcels or edited parcels and should be in a service area. The service areas were edited to include new and edited parcels following some general guidelines. The guidelines were as follows: Page 20 of 59 ,Rrvera~Ge Feel~n81'agy, ioe.