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Agriculture from 'Landsat Imagery: A Unique Resource'

Farmers have long known that not all areas in a field produce the same yield, yet the ability to measure and map this variability is a fairly recent arrival in agronomic management. The practice of measuring and mapping yield limiting variables throughout a field is generally referred to as zone mapping (Zhang and others, 2010). Zone maps are used to identify areas within a field that express a similar composition of one or more factors, including soil properties (structure, organic content, depth, and drainage), nutrient levels, topography (flatland, rolling hills), and historic crop growth and yield. A prerequisite to mapping these factors is the ability to accurately and consistently measure them at a high enough resolution to be able to rectify differences at the sub-field level. Multi-spectral sensors such as the Operational Land Imager (OLI) aboard Landsat 8 and the previous generation Enhanced Thematic Mapper Plus (ETM+) aboard Landsat 7 have proved exceptionally capable in this regard. Additionally, both the OLI and ETM+ sensors collect imagery at 30-meter resolution for multispectral bands, which is sufficient for mapping variability at the field level. By utilizing different combinations of bands, it is possible to measure different environmental factors, particularly soil properties and vegetation status.
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The Cropland Data Layer (CDL) is a product of the USDA National Agricultural Statistics Service (NASS) with the mission “to provide timely, accurate and useful statistics in service to U.S. agriculture” (Johnson and Mueller, 2010, p. 1204). The CDL is a crop-specific land cover classification product of more than 100 crop categories grown in the United States. CDLs are derived using a supervised land cover classification of satellite imagery. The supervised classification relies on first manually identifying pixels within certain images, often called training sites, which represent the same crop or land cover type. Using these training sites, a spectral signature is developed for each crop type that is then used by the analysis software to identify all other pixels in the satellite image representing the same crop. Using this method, a new CDL is compiled annually and released to the public a few months after the end of the growing season through the online CropScape data portal (fig. 1).
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The Foreign Agricultural Service (FAS) handles USDA international activities, including the monitoring and estimation of crop supply and demand across global markets. Estimates for foreign production, supply, and demand are developed primarily through agricultural attachés. Attachés are based in foreign embassies, primarily in countries representing potential markets for U.S. crops (fig. 1). FAS attachés are further assisted by the International Production Assessment Division (IPAD), which collects and analyzes global crop condition and production. IPAD crop production analysts and FAS Global Agricultural Information Network (GAIN) reports both provide crucial inputs for the monthly World Agricultural Supply and Demand Estimates (WASDE) report.
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Remote sensing, including Landsat satellite imagery, plays an important role in developing crop production estimates. In particular, the use of satellite imagery plays an important role in enhancing the accuracy and reliability of global crop production estimates (Vogel and Bange, 1999). Domestically, satellite imagery provides supplemental data to annual ground-based agricultural surveys. In addition to supplementing annual estimates, the ability of satellite imagery to provide near real-time production estimates of major crops is increasing, with significant strides occurring within the last decade. The task of estimating crop production is the responsibility of the National Agricultural Statistics Service (NASS) for domestic (U.S.) production and the Foreign Agricultural Service (FAS) for all global production (excluding the U.S.).
Landsat Thematic Mapper image taken June 2006 of portions of the Uncompahgre and Lower Gunnison Valleys in the Upper Colorado River Basin.
Monitoring consumptive water use is an important component of global agricultural monitoring (Curt Reynolds, USDA FAS, written commun. and oral commun., 2014), as a majority of global production relies at least in part on irrigation for crop production (fig. 1). Globally, irrigated agriculture supports production of about half of the world’s food supply (Thenkabail and others, 2010). In the United States (U.S.), agriculture accounts for around 80 percent of consumptive water, reaching upwards of 90 percent in many western states (USDA Economic Research Service, 2014).
NDVI (Normalized Difference Vegetation Index) maps derived from Landsat 5 imagery in the Wimmera Region, north-central Victoria, Australia. Courtesy of the Victoria Department of Environment and Primary Industries.

Australia’s agricultural industry has evolved significantly within the last decade. Change in agriculture, whether it is an increase in land used for production or the types of crops being produced, has an impact on land-cover soil properties and water availability. Landsat imagery is used in monitoring irrigation areas and dry land agricultural areas to detect changes in agricultural practices over time. Additionally, land cover is also used in benchmarking of water-use efficiency which supports irrigation water management at the catchment level.

Seasonal METRIC evapotranspiration along the Snake River, Idaho. Junior water rights affected by curtailment are highlighted in yellow. Courtesy of Idaho Department of Water Resources.
In 2009, Snake River Farm, a trout farm owned by Clear Springs Foods, Inc., in Snake River Canyon saw a decrease in surface water from springs and sought curtailment of junior groundwater pumping. In this case, the Director of IDWR found that Clear Springs was materially injured by junior groundwater pumping and ordered curtailment. Landsat imagery processed with METRIC was used to establish water budgets to assess depletions versus recharge. For recharge estimates, both evapotranspiration and surface-water diversion return flows were used. In response to the finding of material injury, groundwater irrigators in the affected area developed a mitigation plan, which was approved by IDWR, and they are no longer subject to curtailment.
Map of water sources in A&B Irrigation District (outlined in red) and adjacent land.A&B Irrigation District. Courtesy of Idaho Department of Water Resources.
In 2006, A&B Irrigation District (A&B), a senior groundwater user, claimed it was materially injured due to junior groundwater pumping. Landsat data processed with the METRIC model served as key evidence for the case. One way of determining if there was a shortage of water in A&B was to analyze three archived Landsat scenes. Water use was compared through the evapotranspiration measurements conducted in METRIC for groundwater and surface-water users in A&B and the surrounding groundwater users. The mean daily evapotranspiration chart did not show water shortage in the area in dispute.
Evapotranspiration for Tadla region, Morocco, November 2006. Courtesy of Riverside Technology, inc.

Morocco’s irrigation systems and water-allocation methods. Eighty-five percent of the water in Morocco is consumed by irrigated agriculture. Increases in population and drought seasons have made it difficult for Morocco to use reservoirs and irrigation infrastructure. Groundwater has been relied on extensively by irrigators to meet their water demand. The increase in groundwater pumping leads to a rapid decrease of the water tables.

False color composite Landsat image of the Shepparton Irrigation District, Victoria, Australia. Courtesy of the Victoria Department of Environment and Primary Industries.
Australia’s Murray-Darling Basin (MDB) produces the highest value and largest volume of irrigated products, including rice, cotton, dairy, horticulture, and viticulture, in Australia. The MDB represents over 60 percent of all irrigated agricultural area in Australia. In the 2010–11 production years, MDB had nearly 3 million acres (1.2 million hectares) of irrigated land. This large agricultural industry is responsible for 95 percent of all diversions of the Basin’s water resources and represents 75 percent of all water used for irrigation in Australia and over half of all water use in Australia.

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