Watershed area

Many hydrologic reservoirs can be further subdivided into smaller reservoirs, each with a characteristic turnover time. For example, water resides in the Pacific Ocean longer than in the Atlantic, and the oceans surface waters cycle much more quickly than the deep ocean. Similarly, groundwater near the surface is much more active than deep reservoirs, which may cycle over thousands or millions of years, and water frozen in the soil as permafrost. Typical range in turnover times for hydrospheric reservoirs on a hillslope scale (10-10 m) are shown in Table 6-4 (estimates from Falkenmark and Chapman, 1989). Depths are estimated as typical volume averaged over the watershed area. 

Longer turnovers associated with large watershed areas. 

In random samples of soil taken from five Alabama counties, only 3 of 46 soil samples contained methyl parathion. The concentration in these samples was <0.1 ppm (Albright et al. 1974). Aspartofthe National Soils Monitoring Program, soil and crop samples from 37 states were analyzed for methyl parathion during 1972. Methyl parathion was detected in only 1 soil sample, at a concentration of <0.1 ppm and taken from South Dakota, out of 1,246 total samples taken from the 37 states (Carey et al. 1979). In soil and sediment samples collected from a watershed area in Mississippi, methyl parathion was not detected in the soil samples. In three wetland sediment cores, however, measurable concentrations of methyl parathion were detected during application season (Cooper 1991). 

A study of estuarine fish in 21 coastal states conducted from 1972 to 1976 as part of the National Pesticide Monitoring Program detected a mean concentration of 47 ppb in 3.9% of the fish tissue samples collected (Butler and Schutzmann 1978). In another study (Cooper 1991), fish collected in a watershed area of Mississippi were analyzed for residues of methyl parathion. Methyl parathion was detected in seven species of fish, with white bass having the greatest mean concentration, at 15.96 ppm. Methyl parathion was found in 3 of the 32 fish samples collected before spraying of methyl parathion and in 12 of the 25 samples of fish collected after methyl parathion spraying. 

Percent wetlands in watershed area Intensive and cluster Once B... 

Curtail agricultural use of chlorpyrifos in watershed areas pending acquisition of additional data on its transport, fate, and effects, including data on chlorpyrifos flux rates from soils and sediments and its resultant bioavailability. 

Curtail agricultural use in watershed areas pending acquisition of additional data on chlorpyrifos toxicokinetics. 

Winston, A. W. and R. M. Rltty, 1971. "What Happens to Phenoxy Herbicides When Applied to a Watershed Area."... 

Restrict the use of chlorpyrifos for mosquito control in wetlands, estnaries, and waterfowl breeding areas because recommended treatment levels are demonstrably harmfnl to nontarget resident biota. Curtail agricultural use in watershed areas pending acqnisition of additional data on chlorpyrifos toxicokinetics. 

Hg flux and watershed area, the relative importance of direct deposition and catchment contributions can be assessed. Such interpretations assume that lake sediments are stratigraphically and quantitatively reliable archives of Hg inputs to aquatic systems. 

Many factors control whether a given water body will become acidified as a result of a given deposition regime. In addition to the deposition rate and the lake residence or turnover time, these include the ratio of water surface area to watershed area, the composition of the lake bottom, the residence time of incident precipitation en route through the watershed, and the buffering capacity of the watershed. The presence of organic material can also be important. 

The watershed areas (see Chap. 15) are particularly sensitive to ischemia in cardiac arrest, especially in the parietal lobe area in the borderzone between anterior, posterior and middle cerebral artery territories. 

The chlorophenoxy groups of herbicides includes 2,4-D, 2,4,5-T, and many other chemically related compounds. The chlorophenoxy compounds are primarily selective herbicides and comprise approximately half the total domestic herbicide market. Although 2,4-D is essentially insoluble in water, its esters are slightly water-soluble, and salts of 2,4-D are completely water-soluble. Several of these compounds are used not only for application to plant foliage and soil but also as aquatic herbicides (8). Each year hundreds of tons of these compounds are applied directly to lakes, rivers, and other surface waters for weed control. Approximately 100,000 pounds of 2,4-D granules are applied annually to the lakes in the TVA system alone (7). The herbicide 2,4-D may persist for several months in lake water whereas the esters of 2,4-D are usually broken down in a few days (I). When applied to watershed areas, the phenoxy herbicides are not likely to constitute a major water pollution hazard since the rate of bacterial degradation is sufficiently rapid to destroy them within a few days (26). However, a few of these compounds can remain in the environment for a year or more. 

Table 9.1 The largest 25 rivers (in order of decreasing annual discharge Meade 1996 Milliman and Meade 1983), river watershed areas, river discharge estimates, and NEWS-model-estimated DIN, DON, PN, and TN (Total exports from the 25 largest rivers and the percent of total global export for which these rivers account are also shown, Tg = 10 g)...

Figure 36.9 Fluxes of nitrogen (kg per km of watershed area per year) from the landscape to coastal oceans in rivers for contrasting regions of the world in the temperate zone. The natural background flux is based on the intercept of plots of nitrogen export vs. NANI (such as shown in Figs. 36.2 and 36.4B). Data from Flowarth el al. (1996,2002) Bashkin et al. (2002).

Figure 36.10 Comparison of anthropogenic nitrogen inputs and associated riverine N exports (kg per km of watershed area per year) in two contrasting water regions of the USA The northeast USA and the Mississippi River hasin. Modified from NRC (2000) based on data from Howarth et al. (1996).

The most prominent examples of element-based rates are those reported for annual stream or river discharge and normalized to the watershed area. The relatively straightforward nature of these measurements, coupled with a significant amount of watershed research related to land management issues, has produced a large database describing such fluxes. A tabulation of these data is beyond the scope of the present paper and the reader is referred to previous compilations (Dethier, 1986 Meybeck, 1979 Bluth and Kump, 1994 White and Blum, 1995). 

The vasoconstrictor action of pseudoephedrine can predispose susceptible patients to ischemic colitis, particularly in the watershed area of the splenic flexure. Perimenopausal women may be more susceptible (19). A group from Yale have described four patients, women aged 35-50 years, who developed ischemic colitis while taking pseudoephedrine (19). Three of them had taken the drug for about 1 week, the fourth for 6 months. In all cases there was ischemic colitis affecting predominantly the splenic flexure, and there was full recovery after drug withdrawal. 

There has been an increasing interest in the determination of selenium at trace levels in a wide variety of matrices. Selenium determinations of environmental samples have become quite important. Large-scale poisoning of water fowl has occurred in several watershed areas of central California. Selenium is washed or leached into these areas, helped through the widespread irrigation of selenium-containing farm soil. 

The last four rivers in Table 3 reflect the diversity of rivers with similar dissolved solid discharges. The Cunene, in Angola, is an arid river that discharges about as much water as the Citandy in Indonesia, but drains more than 20 times the watershed area. As such, total dissolved solid (TDS) values for the two rivers are similar (51 vs 62 mg 1 ), but the dissolved yield (TDS divided by basin area) of the Cunene is < 1 % that of the Citandy. In contrast, the Ems River, in Germany, has a similar dissolved load to the Citandy, but concentrations are roughly three times greater. 

---