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Monitoring hydrologic water quality in pasturelands through spatial overlay analysis


Much of the grazing in the state of Oregon occurs on federal lands. Grazing areas are divided into allotments by federal agencies. They issue permits or leases to ranchers for individual grazing allotments.

Studies by the state of Oregon Department of Environment Quality (DEQ) indicate that streams located in grazing areas are majorly polluted by sediments and animal waste. This is a substantial concern as it causes degradation of water quality, threatening human and ecological health. The department thus wants to inspect the effect of livestock grazing on the state’s water quality.

While federal agencies manage the grazing lands by allotments, the state's biologists monitor water quality by watersheds, or hydrologic basins (as the hydrologists refer to them). If a basin has water quality issues, then biologists who monitor water quality for watersheds or hydrologic basins could identify all grazing allotments that are in that basin. They can then work with federal agencies who manages the grazing allotments to ensure that permit holders are conforming to best practices.

Grazing allotments and hydrologic basin boundaries. Many allotments fall in more than one hydrologic basin.

Since grazing allotments were not created with basin boundaries in mind, an allotment can fall completely within a hydrologic basin, or can cross basin boundaries, falling in two or more basins.

This sample uses ArcGIS API for Python to find out which watershed, or watersheds, each grazing allotment falls in, for water quality monitoring.

It demonstrates using tools such as overlay_layers to identify allotments in a particular basin. This will assign each allotment to the hydrologic basins it falls within.

Moreover, in order to successfully identify the source of pollution in each basin, each basin is assigned the grazing allotment name and the number of streams within. This will help identify which allotment each segment of each stream passes through. If field tests find a water quality issue with a particular stream, biologists can link back to the federal database and get a report on each suspect allotment (the type and number of livestock, the owner information, the administrating office, and so on). The information will help them determine the source of pollution.