After studying this section you should be able to understand:
- that deserts are remarkable areas, and if managed correctly can be very productive
- the importance, and effects, of irrigation in the desert realm
The effects of people on the arid and semi-arid lands may be seen at three
overlapping cultural/technological levels.
- Small groups of people, like the bushmen of the Kalahari and the Australian aborigines, live a semi-nomadic food-gathering and hunting life, and have adapted to their environment with remarkable efficiency. Over the years, the hunter-gatherers have had little lasting impact on their environment.
- By contrast, pastoral nomads seasonally cultivate selected areas. Pastoral nomads and shifting cultivators have greatly affected the natural flora and fauna. They respond to physical conditions through their mobility.
- Many millions live a settled life in a relatively moist environment within arid zones near oases, on a flood plain or delta, or in an area irrigated by water brought from afar. Irrigation, with its associated land-use and settlement, has changed entire ecosystems, and ecological repercussions have been felt far beyond the irrigated areas.
Irrigation: a chronology
Benefits and problems caused by irrigation
Traditionally the people who settled at oases used simple technology to raise and distribute water locally through aqueducts and underground channels or qanats. Evaporation losses from this system are minimal, though some water is lost by infiltration.
Other methods were to use shallow, gravity-fed channels, blocked by mud packing as required; cisterns; underground caverns; depressions dug to catch run-off and retain water moving by gravity through the sub-soil; and the creation of lakes during the rains by blocking stream beds with boulders and earth dams.
Today electric or diesel pumps raise groundwater from greater depths; massive dams create deep lakes stretching back hundreds of kilometres. Metal pipelines of decreasing diameter carry water from reservoir to the field, and release it through nozzles, or tiny drip feeds, to the plants.
Problems such as water logging and saline accumulation. Water not used by crops, lost by evapotranspiration, or drained away, accumulates as rising groundwater. In desert conditions soils rapidly acquire salt from the evaporation of dilute saline irrigation water. Water acquires more salt as it slowly moves up through the ground. The land eventually becomes too saline for crops to tolerate.
Pakistan: an ‘irrigated’ country
In Pakistan, the whole hydrological system of a huge river, the Indus, has been transformed, as engineering systems have evolved progressively to control and distribute its waters.
Most of the flow in the Indus system is from catchments in the Himalayas and its foothills. Monsoon breaks in June and rivers reach peak flood-levels in the foothills in July–August, causing a flood wave to pass downriver. From September the river levels fall, then snowmelt brings another rise in March.
Irrigation has brought some problems for Pakistan.
- Dams and diversions, have caused floodplains to be deprived of alluvium and its nutrients, and to need expensive fertiliser.
- There is increased danger from infections like bilharzia.
- Where year-round cultivation replaces cropping with a dry fallow period, crop pests may thrive on perennial food sources.
- There are also many instances of plant pests and diseases being carried along water channels.
Despite these problems through the 19th and 20th centuries, irrigation projects have continued apace. Inundation canals were built to carry water from cuts in the bank to land parallel with the river. Gated barrages were constructed to raise the water level and allow accumulations of alluvium to pass downstream. Water could now be diverted throughout the year. Large canals were built to transfer water from the western rivers to supplement those further east. More recently, high dams in the mountains have provided storage and hydro-electricity. Perennial canals now serve most of the cropped land. Despite the aridity, over half Pakistan’s workforce is engaged in agriculture.