This law fixed a northern limit to dry farming, but it was never applied; in fact, millet cultivation spread farther and farther north, to the 10 in (250 mm) and 8 in (200 mm) isohyets, even in the major drought that began in 1969.
132 The retreating isohyets and decrease of the flooded areas of the inland delta of the Niger.
The ecological region known as the Sahel lies south of the Sahara, between the 4 in (100 mm) and the 24 in (600 mm) annual rainfall isohyet. It occupies a strip 373 mi (600 km) wide (from north to south) and 3,107 mi (5,000 km) long (from west to east) and thus occupies roughly 3 million [km.sup.2].
The GIS was used to generate mean annual temperature isotherms of 0.25 [degrees] C intervals and mean annual precipitation isohyets
of 1-cm intervals for the region.
Yet toward the south, in the region transitional to the Brazilian shield, the mean annual isohyets
are around 2,000mm (79in) (Sioli 1985).
The sandplain landscape of the northern wheatbelt of Western Australia lies between 28.3[degrees] and 30.7[degrees]S and 114.7[degrees] and 116.8[degrees]E, and between 300 and 400mm mean annual rainfall isohyets
. The area is characterised by deep, well-drained sands (Tenosols), yellow sandy loams and loamy sands (Kandosols), and clays (Vertosols and Dermosols) according to the WA Soil Group Classification (Schoknecht 2002) and Australian Soil Classification (Isbell 1996).
Perhaps the best example of zonality in savannah soils is in western Africa, where the isohyets
, the basic vegetation zones (Guinea savannah, Sudan savannah, and Sahel savannah), and the main soil types follow the same latitudinal gradient from the rainforest in the south to the desert in the north.
Climatic data were obtained from the existing meteorological stations in the area while the rainfall isohyets
for the area were determined (Pagounis, et al.).
The rainfall in the study area from the Bureau of Meteorology isohyets
is between 650 mm and 600 mm.
In the early twentieth century, Thomas Griffith Taylor, a prominent early geographer, in a major work on Australian rainfall and the climatic factors limiting agriculture and settlement, had perceptively described the line as an 'ecological isopleth' (8), and the historian Archibald Grenfell Price had seen in it a 'vegetation line' (9), but the explanation that had become widely accepted and entrenched in the first half of the twentieth century was that the line approximated an isohyet
of average annual rainfall.