These images show changes in the IJsselmeer, a lake on the coast of the Netherlands. (This lake, or meer, is named after the IJssel River and is pronounced EYE-ssel-mare.) The lake appears blue/black, and vegetation appears as red. Highly reflective areas like pavement or bare soil appear light blue or blue-green. Amsterdam can be seen in the lower left of the images.

Sea to lake to land

Till 1932, this area was the Zuiderzee (pronounced ZIGH-dr-zee and meaning Southern Sea), simply a saltwater inlet of the North Sea. By 1968, the Dutch had transformed 764 square miles of the Zuiderzee into blocks of usable land, called polders. Here is how that typically happened:

• In 1932 the Dutch completed a dike across the mouth of the Zuiderzee, creating the IJsselmeer. This dike can be seen in the upper middle portion of the zoom-out. The freshwater from the IJssel River flushed out the saltwater creating a lake.
• Between 1930 and 1968 dikes were built around five portions of the IJsselmeer.
• The polders were drained using pumps.
• Reeds naturally grew on the former sea bottom. To help dry out the soil, the Dutch let the reeds grow. Transpiration moves water into the air faster than evaporation alone would.
• When the soil had dried the reeds were cleared, and colza was planted. Colza is related to cabbage and turnips.
• The colza was cleared, and grain crops were planted.
• The polders were cultivated for up to five years before the land was ready to produce commercially.
• Land was then leased to commercial farmers, or towns were built.

Changes in Southern Flevoland and Markerwaard

Southern Flevoland, the southernmost of the polders, was the last to be diked and drained. It can barely be seen in the 1964 image, diked but undrained. By 1973 it had been drained and the soil was being cultivated to make it suitable for commercial agriculture. The 1987 image shows Southern Flevoland covered with active farming.

Markerwaard was diked even later, and appears in the 1973 image as light-toned water. The dike separating Markerwaard from the rest of the IJsselmeer is partially completed in 1973 and complete by 1987. The Markerwaard was not drained, but used as a freshwater reservoir and a buffer against flood waters.²

Question

The 1973 image has light blue areas which do not appear in the 1987 image. What are the blue areas? Hint: Consider the time of year.

(See the answer below.)

Footnotes

1. Ministry of Transportation and Public Works, no date, Facts and Figures: The IJsselmeer Polders, Lelystad, Netherlands: Directorate Flevoland RWS Information Department, p. 2. Van Lier, Hubert, 1982, A Review of the Zuiderzee Reclamation Works: An Example of Dutch Physical Planning, Landscape Planning v. 9, p. 35-59. Ministry of Transportation and Public Works, no date, A Profile: The IJsselmeer Polders Development Authority, Lelystad, Netherlands: IJsselmeer Polders Developement Authority, p. 23.

2. Ministry of Transportation and Public Works, p. 17.

Satellite images

LM1214023007308290 (Landsat MSS, 23 March 1973)

LM5198023008718690 (Landsat MSS, 5 July 1987)

DS09066A011MC055 (Argon satellite photograph, 21 August 1964)

Maps

Defense Mapping Agency, 1988 (revised as of 1985), Operational Navigation Chart E-2: edition 19-GSGS, scale 1:1,000,000.

The blue areas represent bare soil, and they are mostly polder land, which is good for planting crops. The March 1973 image was taken between harvest and the growth of the next crop, while the July 1987 image was taken during the growing season. The non-polder lands are not as good for planting crops and are better used for grazing, so the plant cover is present year-round.