Semi-arid climates

Differences in the prevailing land use and management of arid and semi-arid areas are determined in part by climate. Arid areas generally receive too little rainfall to support dryland agricultural or domestic livestock grazing. In contrast, in semiarid areas adequate moisture is usually available at some time during the year to produce forage for livestock, and there are some years when dryland crop production is successful (Heath et al., 1985; Penman, 1963). However, both climates are characterized by extreme variability, with commonly occurring droughts and infrequent periods of above-average rainfall.

Most arid areas of the world occur along two wide belts at approximately 30° latitude north and south of the equator (Lydolph, 1985). In these subtropical belts, the winds generally descend and are dry much of the time. Semi-arid areas associated with the arid deserts generally occur north and/or south of the deserts (in Africa, Asia and Australia) or inland and at slightly higher elevations (in North America, South America, the Middle East, Africa and Asia). On a more localized scale, a combination of terrain and prevailing wind direction can cause ‘rain shadow’ effects, resulting in arid and semi-arid areas downwind of major mountain features (Oliver and Fairbridge, 1987).

More than a third of the world's land surface is either arid, generally receiving less than 250 mm of annual precipitation, or semi-arid with between 250 mm and 500 mm of annual precipitation. More precise definitions of desert and semi-arid areas are given in climatic classifications based on precipitation, temperature and their seasonal distributions. For example, following Trewartha and Horn (1980), and based on extensive classifications of Köppen (1931), upper and lower mean annual precipitation limits defining semi-arid climates are as follows.

Semi-arid climates, for regions where annual precipitation is not strongly seasonal, are defined by equations linking mean annual values of precipitation, R, in mm, and temperature, T, in degrees C. The upper limit for semi-arid climates, in terms of mean annual precipitation given a specific value of mean annual temperature, is defined by

The corresponding lower limit that separates arid and semiarid (or alternatively desert and steppe) is defined as half the value of the upper limit from the above equation, or

Temperature and precipitation data from selected locations in arid and semi-arid areas of the world were used to classify climate based on the above criteria and a designation for hot (h, with 8 or more months of the year with average temperature above 10°C) and cold (k, with fewer than 8 months of the year with average temperature above 10°C). The general classification for dry climates is B and the specific classification for semi-arid is S. Thus, BSk represents a cold semi-arid climate and BSh is a hot semi-arid climate. These classifications and the upper and lower precipitation limits for each location are listed in Table S1.

Table S1 Location and summary of climatic classifications for selected locations in arid and semi-arid areas of the world
Full size table

The occurrence, frequency, and magnitude of precipitation events vary widely in semi-arid areas. Annual potential evapotranspiration significantly exceeds precipitation in such areas and can be accurately predicted with a number of techniques. In contrast, actual evapotranspiration is nearly equal to precipitation and is difficult to calculate under field conditions. Although actual evapotranspiration differs little from precipitation in magnitude on an annual basis, these differences are crucial, as by and large they determine annual soil moisture status, runoff and groundwater recharge.

Vegetation in arid and semi-arid areas is adapted to lack of moisture, extreme variations in precipitation and temperature, and soil characteristics (Hilgard, 1906), as described below. Seasonal distribution of precipitation and temperature also play a dominant role. Three deserts in North America illustrate the interactions of soil—water—plant relationships with climate, soil, and topography (Fuller, 1975). The Mojave Desert is characterized by the dominance of winter precipitation and the absence of precipitation during the hot summer. Vegetation varies strongly with elevation, and thus with temperature and precipitation from the desert to the adjacent semi-arid areas. At higher elevations yucca species are common and Larrea—Franseria associations dominate at lower elevations. The Sonoran Desert is the most floristically diverse of the three deserts with an abundance of cacti, shrubs, riparian trees, and desert to semi-arid grasslands dependent upon elevation, precipitation, and temperature. Precipitation tends to be more bimodal in the Sonoran Desert and thus there are distinct summer vegetative responses usually absent in the Mojave Desert. The Chihuahuan Desert is generally cooler than the Mojave and Sonoran Deserts and is much more dominated by summer rainfall. As a result of the different seasonal variation in precipitation and less variation in elevation, plant communities in the Chihuahuan Desert are less complex than those in the Sonoran Desert.

Terrain

Bedrock geology, describing the form, properties and dimensions of the rock units and how they are locally and regionally related, predetermines the basis of what we see in the landscape. Geologic processes that shape the Earth's surface can be endogenous or exogenous and together they provide the forces shaping the landscape. The existing landscape is also the result of interactions between the atmosphere and the Earth's surface. Because on geologic time scales climate is not constant, paleo-climatology and paleohydrology must be considered in describing the evolution of the landscape.

The terms watershed, catchment and drainage basin (q.v.) have similar meanings and definitions. The term watershed means an area above a specified point on a stream channel enclosed by a perimeter. The watershed perimeter defines an area where surface runoff will move into the stream or tributaries above the specified point. In arid and semi-arid regions, vegetative sparseness and normally clear, dry air make these features most evident. Thus a striking feature of arid and semi-arid landscapes is the stream channels, and thus watersheds, which combine in complex patterns to produce the channel networks and inter-channel areas. These features in turn control the movement of water and sediment when precipitation causes runoff. The runoff and the associated erosion and sedimentation processes combine with wind erosion and deposition to modify the evolving geomorphic features we see at any particular time (Cooke et al., 1993).

There are direct links between the form and structure (geomorphic features) of the terrain we see in a landscape and the processes that shape them. Geomorphic features influence the movement of wind and water and thus the erosion and sedimentation processes which are in turn modifying the geomorphic features (Doehring, 1977). Key questions in the earth sciences today deal with the interaction and feedback of form and structure with processes at various temporal and spatial scales (Fairbridge, 1968). That these interactions and feedbacks are functions of time, space, and process intensity scales make the’ scale’ problem a central focus of much scientific research.

For example, soils in arid and semi-arid regions are notable for their variations with respect to topographic features (Dregne, 1976). Perhaps as striking as their nonhomogeneity in space is their usual close relationship with the parent material due to their thinness, the lack of moisture, and the slowness of the soil-forming processes. The ‘better’ soils are often formed on alluvial deposits or deposits of loess.

In extensive semi-arid areas, vertical differentiation of soil profiles is lacking, due to weak chemical activity resulting from the dryness. A typical exception to this generalization is the forming of calcrete by deposition and leaching of calcium, in the form of calcium carbonate, and other soluble salts. Variations in soil properties from undifferentiated profiles to calcrete formations have a significant influence on the water balance of semi-arid areas and thus affect hydrologic processes, erosion and sedimentation, biological productivity, and ultimately land use and management (Hudson, 1987).