What Is Special about Desert Climates

Life in desert is limited by the scarceness of water. Secondary limiting factors are correlated to the main factor: the dearth of nutrients for producers and food energy for consumers and for both - at least temporally - high heat stress. Precipitation typically is so low that water becomes the controlling factor for biological processes. Precipitation is also highly variable throughout the year and typically occurs in infrequently defined events (discontinuous input). To make things even worse, precipitation varies randomly between years and is therefore not predictable. The coefficient of variation between years in arid areas is typically larger than 30% of the long-term average (and ranges in some extreme deserts to 70%). For comparison, temperate zones and tropical areas typically have coefficient of variation of less than 20%. Individual precipitation events in deserts can be tremendously large (for instance, 394 mm in a single rainstorm in the Peruvian desert that receives a long-term annual precipitation average of 4 mm) and due to surface runoff, large-scale flash flooding can occur (Figure 8). Even though such sudden floods will replenish needed water to desert systems, erosion and direct damage to desert plants are the

Figure 8 A flash flood obstructs traffic on a desert road. Depressions and stream beds quickly flood after strong rainfall events due to the high surface runoff in deserts. As in the case here, the precipitation source of the water can be remote and floodwater travels far distances. Due to high stream velocity and carried erosion material, such sudden floods can be disruptive to biotic communities and dangerous to humans. Sedom, Israel, March 1991. Photograph by C. Holzapfel.

Figure 8 A flash flood obstructs traffic on a desert road. Depressions and stream beds quickly flood after strong rainfall events due to the high surface runoff in deserts. As in the case here, the precipitation source of the water can be remote and floodwater travels far distances. Due to high stream velocity and carried erosion material, such sudden floods can be disruptive to biotic communities and dangerous to humans. Sedom, Israel, March 1991. Photograph by C. Holzapfel.

consequence. Because of the small and temporally highly variable rainfall amounts, deserts have been described by Noy-Meir as ''water-controlled ecosystems with infrequent, discrete, and largely unpredictable water inputs.''

Adding to and interacting with the pronounced temporal variation is the high spatial variation of rainfall in deserts. This variation is caused by: (1) oreographic features (e.g., increase with altitude), (2) differences in degree and direction of slopes, and (3) the typically small size of precipitation fronts (often less than1 km in diameter).

Depending on the latitudinal geographical location and the origin of rain fronts, deserts receive either precipitation in the cooler season (cyclonic/frontal rainstorms) or in the warmer season (tropical, convective rainstorms). Some transitional desert regions receive both. The season-ality of rainfall is of great bioclimatological importance as evapotranspiration is larger during the warmer season and rainfall therefore tends to have a smaller biological effect. On the other hand, cold deserts receive precipitation during the cold season mostly in the form of snow and biological activity is then limited both by low temperatures and aridity. Snowmelts in spring create deep-reaching wetting fronts that will hold water available for plant uptake during the warmer growing season. Locally important other water inputs are condensations of atmospheric moisture as dew. These are crucial for plant production in the coastal fog deserts that otherwise do not receive direct precipitations. It is less common inland but can be noticeable in high desert areas as well (for instance in the Negev Desert of Israel). Fog water inputs are directly usable for cryptogamic organisms (e.g., lichens) and many arthropods. Foliar uptake of fog by vascular plants has been demonstrated but its relative importance in the water balance remains controversial. Water vapor tends to move along temperature gradients and can be important in dry soils with strong diurnal radiation. An upward movement of water vapor at night causes formation of dew close to the surface. Such water might sustain germinated plants until they produce roots long enough to reach deeper and wetter soil depths.

Deserts usually experience an extreme diurnal temperature range, with high daylight temperatures (up to 50 ° C, the highest temperature recorded in Death Valley was 56.7 °C), and extremely low nighttime temperatures (often dropping below 0 °C). This is caused by very dry air that is transparent to infrared (heat) radiation from both the sun and the ground. Thus during daylight all of the sun's heat reaches the ground. As soon as the sun sets, the desert cools quickly by radiating its heat into space. Clouds reflect ground radiation and desert skies are usually cloudless, thereby increasing the release of heat at night. With intense sun radiation, surface temperatures can be extreme and depending on the color and type of surface can exceed 80 °C.

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