rainfall shows a well-defined north–south gradient: Mildura averages only per year but Hopetoun in the south receives around . Variability, however, is quite high : in 1973 the Mallee averaged as much as but in 1982 only . Temperatures in summer are generally very hot: during the Early 2009 southeastern Australia heat wave, Hopetoun reached and the average maxima in January and February are . In July, maximum temperatures average , but minima average and frosts are very common. In his 1939 paper on the physiography of North-Western Victoria, ES Hills stated that: "the chief factors involved in the distinction between the Mallee and the country south and east of it are precipitation, evaporation, rock type, porosity of the superficial deposits, drainage away from high areas and supply of water from streams that rise in regions of higher rainfall." Population At the 2011 census, the Mallee had a population of , most of which live close to the Murray River in the district's two cities, Mildura and Swan Hill. These and other settlements within the municipalities based on these centres contain over two-thirds of the region's population. Ouyen and Murrayville are the main settlements of the central Mallee, while Kerang, Cohuna, Hopetoun and Birchip are the major towns in the south. Economic development The Mallee is a primarily agricultural region: apart from possible mineral sands in the west and salt from certain ephemeral lakes there are no mineral deposits of value and industry is generally on a small scale and confined to food processing. Wheat and barley are widely grown on the less infertile soils in the southeast of the region, but fertilizers, especially superphosphate are essential for success and yields only a quarter of those in most of Europe or North America - in drought years, they can be as little as one twentieth of European wheat yields. Fruit growing along the Murray River exceeds grain crops as the Mallee's major source of revenue. Oranges and grapes are particularly important and the region produces a large part of Victoria's wine though most of it is inexpensive cask wines. Dairying on irrigated pastures is important in the south of the region, but is threatened by its high water demand and the extreme scarcity of water in the Murray-Darling Basin. Tourism is also significant industry. Mildura and Swan Hill contain a number of tourism focused businesses, including a number of specially