Arid rain

Acid rain primarily affects sensitive bodies of water, that is, those that rest atop soil with a limited ability to neutralize acidic compounds (called buffering capacity ). Many lakes and streams examined in a National Surface Water Survey (NSWS) suffer from chronic acidity, a condition m which water lias a constant low (acidic) pH level. The survey investigated tlie effects of acidic deposition in over 1,000 lakes larger than 10 acres and in thousands of miles of streams believed to be sensitive to acidification. Of the lakes and streams surveyed in the NSWS, arid rain has been determined to cause acidity in 75 percent of the acidic lakes and about 50 percent of tlie acidic streams. Several regions in the U.S. were identified as containing many of the surface waters sensitive to acidification. They include, but are not limited to, the Adirondacks. the mid-Appalachian highlands, the upper Midwest, and the high elevation West. 

There also is a concern about the impact of arid rain on forest soils. There is good reason to believe that long-term changes in the chemistry of some sensitive soils may have already occurred as a result of acid ram. As acid lain moves through the soils, it can strip away vital plant nutrients through chemical reactions, thus posing a potential threat to future forest productivity,... 

Horn, K. Changes in Global F.nvimmnent-Arid Rain, KendalldTmit Publishing Company, Dubuque, IA, 1997. 

Your facility is located in a semi-arid region of the United States which has an annual precipitation (including snowfall) of 12 inches of rain. (Snowfall should be converted to the equivalent inches of rain assume one foot of snow is equivalent to one inch of rain.) The area covered by your facility is 42 acres (about 170,000 square meters or 1,829,520 square feet). The area of your facility is 50 percent unimproved area, 10 percent asphaltic streets, and 40 percent concrete pavement. 

The rainfall regime in arid areas is characterized by low, irregular and unpredictable precipitation, often concentrated in a few rainstorms, creating humid conditions in the soil for a short period and over a limited area. In many arid areas, several years may elapse between successive rainfalls. The moisture supplied to the soil from rain is offset by evaporation, that is related to air temperature, air humidity and intensity of solar radiation. Because of the irregular rainfall distribution, mean precipitation values have little meaning, if not also the range of variation is indicated. 

Most deserts and (semi-) arid regions occur between 10° and 35° latitude (e g. Sahara desert, Kalahari desert), in the interior parts of continents (e g. Australia, Gobi desert) and in rain shadow areas in fold belts (e.g. Peru, Nepal). Large parts of the arctic tundra receive less than 250 mm precipitation per annum and qualify as arid regions too (FAO, 2001). 

During the Late Pleniglacial, between 20,000 and 13,000 BP, some 25% of the land surface became covered with continental ice sheets (versus some 10% today). With so much water stored in ice sheets, the sea level dropped to about 120 meters below the present level and large parts of the world became extremely arid. The Amazon rain forest dwindled to isolated refugia, European forests disappeared but for small sheltered areas, and large parts of the globe turned to tundra, steppe, savannah or desert. 

South America has two major areas, separated by the Andes Mountain ridges, where arid and semi-arid zones dominate. In the barren, coastal Atacama desert of Peru and northern Chile, precipitation is extremely limited and comes as winter mists or drizzles. In the rain shadow east of the Andes in Argentina, arid zones are widespread. South America apparently has only a slightly greater proportion of dry zones than North America. 

Because of the irregular rainfall distribution, mean precipitation values have little meaning in the (semi)-arid zone, if not also the range of variation is indicated. This variability refers to both temporal and spatial variability. Temporal variability affects not only the onset and duration of the rains in the year, but plays also a role in year-by-year differences. The variability is highest in the hyper-arid zone, where the mean precipitation value is composed of a few intensive rainstorms. When these fall on a heated barren surface - as is often the case in the arid zone - a part of it is immediately evaporated and lost for soil processes. High rainfall intensity results on the other hand in a rapid saturation of the surface layers and creates lateral runoff and erosion, in particular on sloping land. Many arid and semi-arid soils show therefore features of gully and sheet erosion. 

Field studies have shown that the first 5-6 mm of rain falling on a heated desert surface evaporate almost immediately, whilst single storms with more than 20 mm rain lose a major part of it by lateral runoff. Hence, it is estimated that from the already low rainfall in the arid zone an important part is lost for weathering and soil formation. The importance of this runoff... 

Due to the spatial irregularity of the rains and the occurrence of lateral runoff concentrating additional water in (micro) depressions, soil profile development may vary quite considerably from one location to another in arid zones. This phenomenon might, to some extent, explain the occurrence of A-Bt-C profiles, e g., with clear clay illuviation features, at some isolated locations in desert zones. 

Overland Runoff The fraction of rainfall or irrigation water that flows over a land surface from higher to lower elevations, known as overland runoff, is an additional pathway for contaminant transport. Runoff occurs when the amount of rain or irrigation water is greater than the soil infiltration capacity. The formation of a crust on the soil surface is a major contributor to runoff formation in arid and semiarid zones, because it decreases the infiltration capacity. The soil crust is a thin layer (0-3 mm) with a high density, fine porosity, and low hydraulic conductivity compared to the underlying soil. This skin forms as a result of falling raindrops or sodification of soil clays. 

It is well known that dogs track better in humid air. Rodents find buried seeds better in wet soil. This is important in arid climates. After rains, yellow pine chipmunks, Tamias amoenus, and deer mice, Peromyscus maniculatus found experimentally buried seeds of Jeffrey pine, Pinus jeffreyi, and antelope bitterbrush, Purshia tridentata, better than in diy soil. The recovered number of seeds increased 27- and 15-fold, respectively. In wet soil, seeds take up water rapidly and emanate volatile organic compounds that the rodents exploit. By extension, variations in humidity in arid environments may have profound effects on olfaction-dependent behaviors such as finding food, social interactions, preying, and predator avoidance (Vander Wall 1998). 

Tropical rain forest, grassland and savanna, scrub and deciduous forest The tropics and subtropics are comprised within 30° of latitude on both sides of the equator. Various ecosystems are represented the tropical rain forest (Jacobs 1981), grassland and savanna, scrub and deciduous forest, and arid land (AY 1979) (Fig. 6.1). 

In both the experimental study and the field surveys, foliar diseases were not important on irrigated tomatoes (mostly by furrow irrigation) in the semi-arid climate of California. Only occasionally, diseases such as bacterial spot Xanthomonas campestris) occurred when it rained early in the season (Clark etal. 1998). Virus symptoms were also seldom observed. There were no differences in foliar disease incidence and severity between organic and conventional farming systems. Root diseases were quite common and sometimes severe in conventional tomato fields, but were absent or only slight in low-input and organic fields. 

Aridity is of interest in paleoelevation reconstruction for two reasons (1) in extreme aridity cases, the 818Osc values are dominated by evaporation, and realistic estimates of paleoelevation are probably not obtainable, no matter the sampling density, and (2) aridity develops in rain shadows, and thus may provide qualitative evidence of orogenic blockage of moisture. 

EPTC (S-ethyl di-N,N-propylthiocarbamate) is about the most volatile of the common herbicides, and evaporation limits its persistence under normal conditions and makes incorporation rather than surface application desirable. Nevertheless, it can be reactivated by rain many weeks after application under arid conditions. 

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