COMPUTER-ASSISTED MAPPING OF IRRIGATION WATER REQUIREMENTS FOR VANCOUVER ISLAND BASED ON SOIL AND AGRICULTURAL CAPABILITY SURVEY DATA

Abstract

The methodology is presented for computer assisted mapping of volumetric irrigation water requirement, as applied to the east coast of Vancouver Island, British Columbia. Soil and agricultural capability maps at 1:20 000 scale were digitized and the corresponding field and laboratory data were stored in a computer mapping system. An algorithm generates the volume of water required for each polygon by multiplying an area by a depth. The area is the number of arable hectares in the polygon. The depth is calculated as a weighted average of the individual irrigation depths for the arable soils within the polygon, where arability is tied to the attributes of each soil as compared to agricultural capability criteria. Using a water requirement model, the irrigation depth for a given soil is defined as a function of available water storage capacity, estimated to an assumed rooting depth for various crops, or to a root restricting layer; level of risk; and depth to the water table. On the resulting maps, polygons needing water are displayed, with a label depicting the required volume. Tabular summaries are provided on a polygon and mapsheet basis. The relative effects of using different definitions of arability (either agricultural capability classes 1 through 4 or agricultural capability classes 1 through 5), different rooting depths (0.5 or 1.0 m, depending upon proposed crops), and different levels of risk (one of 10, 20, or 50%) can be assessd by developing a set of scenarios. Key words: Irrigation, computer mapping