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ECOLOGY AND MANAGEMENT essentially a monospecific shrub land that is primarily composed of single lifeform patches. The shrub land has very lo~ obstruction value and a large percentage of the rainfall leaves the unit as overland flow or is transported in rills to ephemeral stream channels (Fig. 3). Run-off water from this unit has high sediment loads as a result of soil detachment by raindrop impact. The quantity of suspended sediment and textural composition of the sediment is dependent upon variables such as rain killetic energy, soi I detachabi Iity, characteristics of soi I surface, depth of surface water layer, and the influence of slope on detachment (Abrahams et af., 1988; Torri and Poesen, 1992). Healthy, function ing watersheds support a diverse fauna that contribute to soil - porosity, affect soi I bulk density, affect soil fertility and transport sub-soil to the surface where it is susceptible to incorporation into rLlIl-off as suspended sediment. Animal species that affect several soi I properties have been considered keystone species in the ecosystems in which they occur (Whitford, 1997). Several invertebrate taxa that are widespread in the arid regions of the world have been described as playing a keystone role by affecting the properties and processes of ecosystem s (Wh itford, 2000). Ants and term ite affect pedogenesis by transporting large quantities of subsoil to the surface. Because the soil used in the construction of foraging galleries of some species of term ites includes term ite feces, the surface gallery soil is enriched in some nutrients like nitrogen and phosphorus. Termites and ants have been reported to transport between OF ARID ZONE WATERSHEDS 269 I 22 and 4000 kg ha- i' of subsoil to the surface in construction of nests and/or foraging galleries (Whitford, 2000). The quantity of soil turnover by ants and termites is a function of landscape position, soi I and vegetation. Ants and termites produce soi I macropores (continuous tubes or voids in the soil) that transport flowing water to the deeper parts of the soil profile faster than predicted by infi ltration models of soils of different particle-size distributions (Phillips et af., 1989). Other invertebrate taxa that produce soil macropores include ground-nesting spiders, cicadas, earthworms, and isopods. Subterranean and/or epigeic termites are widespread and relatively abundant in soils of arid and semi-arid regions that are not subjected to periodic inundation. Because termites process such a large fraction of the dead plant material in the ecosystems in which they occur, they have a significant effect on the soil nutrient patterns of arid and semi-arid landscapes. In the Chihuahuan Desert of North America and watersheds in Tanzania, termites have been shown to be responsible for most of the variation in soil organic matter, soil organic carbon and associated nutrients (Jones, 1989; Nash and Whitford, 1995) and to playa critical role in nutrient cycling (Schaefer and Wh itford, 1981 ). Many species of desert mammals affect soi I properties thereby affecting vegetation spatial patterns, species composition of plant communities and the hydrological properties of ecosystems (Whitford and Kay, 1999). Burrowing animals eject large quantities of soil in the construction of burrows. The ejected soils have lower bulk density and 270 WllITrORD different texture and chem istry in comparison to surface soils. Digging by various species of mammals increases water infiltration in the area of the excavations. Some mammal species occupy the same burrows or burrow complexes for several generations. Soils around the burrows and burrow complexes of central place foragers frequently become enriched in several soil nutrients over time (Whitford and Kay, 1999). The ecosystems of healthy watersheds will support a number of species of invertebrates and vertebrates that affect soi I properties and processes thereby affecting the spatial patterns and structural features of the vegetation. In arid regions, sediment loss over time frequently results in soil surface with varying cover of rock fragments. The rock fragments and rock cover of the soil surface are the lag materials remaining following the loss of sediment. Rocks and rock fragments have different effects on infiltration and run-off, depending on their position in or on the soil. Rocks and rock fragments resting on the soil surface increase infiltration and decrease run-off volume. Rock fragments that are well embedded in the top-soil:tayer reduce infiltration and run-off generation is proportional to the percentages of rock cover (Poesen el 01., 1990). The rock lag on the soil surface of low obstruction value vegetation units contribute to the high volume of water and sediment transferred from these units into downslope units. On portions of watersheds or watersheds with slopes of one per cent or less, vegetation may be organized in bands or stripes oriented across the slope (Mabbutt and Fanning, 1987). The vegetation bands divide a slope into narrow contours of vegetation that serve as run-on areas or sinks separated by barren or sparsely vegetated areas that serve as rLlI1-off or source areas. Banded vegetation develops on fine-textured soils on slopes 0 as small as 0.5 (Dunkerley and Brown. 1995). The fine textured soils have low infiltration rates and most water falling on the sparsely vegetated areas runs off. The upslope edges of the vegetation are called interception zones (Tongway and Ludwig. 1996). The upslope pOltions of the vegetation bands are areas of high biological activity that results in high porosity of the interception zone soils. Interception of run-off water in the vegetation bands provides several times more water per unit area of vegetation than is available from rainfall. The width of the vegetation stripes is a function of the average area needed for infiltration of rainfall plus the run-off from the bare zone. Despite the large areas of un vegetated soil, banded vegetation represents a healthy, functioning watershed. Functional water sheds with banded vegetation have been described in arid and semi-arid regions of Australia. Africa, the Middle East and North America (White, 1971) and are probably present in other arid and semi-arid regions of the world. DcseJ'tified Watersheds Deseltification of watersheds in arid and semi-arid regions around the world varies in degree of degradation as a function of the cultural traditions of the people inhabiting a region. Land-use patterns frequently change as a result of changes in the socio-economic systems of the human inhabitant. Deleterious human impacts are exacerbated by cl imatic drought. which is a recurring condition in arid and semi-arid environments. Since most of the arid and semi-arid lands are used primarily for the production of domestic ECOLOGY AND MANAGEMENT livestock, over-stocking and inadequate herd management strategies are the most widespread agents of desertification. Desertification initiated by Iivestock impacts may include: (1) reduction or loss of desirable forage plant species, (2) reduction in total plant cover, (3) increased size of bare soil patches, (4) soil compaction, (5) loss of microbiotic soil crusts (6) increased bulk density of soils, (7) reduced plant species diversity, (8) changes in abundance and species composition of animals, and (9) local extinction of keystone species (Greenwood e·t 01., 1997; Whitford, 1995, 1997). Decreased plant cover and increased size of bare soil patches result in increased exposure of soil surface to solar radiation, increased so i Item peratu re, increased raindrop splash erosion, decreased infiltration rates, increased run-off and sediment loss, and decreased aggregate stability and soil cohesion (Greenwood et 01., 1997). The reduction in vegetation cover allows run-off water to traverse large distances with no obstruction to flow to reduce the erosive energy of the water (Fig. 4). Soil compaction by grazing animals reduces soil macropores and total pore space~ which contributes to decreased infiltration, increased run-off, and reduced soil water storage. Vegetation that survives on areas that are greatly impacted by livestock may have less resistance to drought (higher mortality) and lowered resilience in recolonizing these areas following drought (Whitford et 01., 1999). All of these changes affect the health of desertified watersheds and severely compromise the capability of the watersheds to provide needed goods and services to the human inhabitants. OF ARID ZONE WATERSHEDS 271 While domestic livestock production is the primary use of arid and semi-arid lands. Iivestock are notthe only source of deleterious impacts on arid and semi-arid watersheds. In many parts of the world, portions of watersheds have been modified to support run-off agriculture. Changes made to watersheds in order to develop run-off agriculture include but are not limited to: clearing vegetation from run-off slopes, construction of dikes and small dams, clearing rocks from run-off surfaces, and excavation of channels connecting productive patches (Evenari et 01., 1971). Run-off agriculture areas may be abandoned if and/or when crop production falls below what is required. The changes imposed on portions of watersheds for rllll-off water crop production are not reversed by natural means. Road construction or the development of roads from frequent traffic by animals or motorized vehicles is a major source of watershed degradation. Roads or vehicle tracks that fail to follow the contours of watersheds may become head-cutting gullies within a few decades. Gullies that develop on roadways are major sources of sediments that bury the soils at the base ofthe watershed. Roadways that cut across slopes interfere with the natural flow patterns of water from the upper slopes of a watershed. Interference with natural patterns of overland flow and/or rill and channel flow has numerous deleterious' effects on the hydrology of the downslope portions of the watershed. Mining produces other serious impacts on the health of watersheds. Mine tailings may cover several hectares on portions of watersheds. Mine tailings typically are leached by rain water and are a source of toxic chemical 272 WHITFORD 1m Fig. -I. II desrtiJied grassland WI/lOll (/ desert walershed with arrows depictillg water flow patterns that are essentially unidirectional. loads in run-off water on watersheds (Jones et al., 2000). Watershed RestorationlRehabilitation Because a large fraction of the arid land areas of the world have been degraded by human activities, many watershed functions have been compromized (Whitford, 1995). Degraded, desertified watersheds present difficult management challenges because some management options are lost as watersheds are degraded. While it may be possible to easily restore one or a few of the functions of disturbed watersheds, restoration of one function may eliminate the possibility ofrestoring another function. An example of the loss of management options as a result of restoring an ecological function of an arid watershed can be seen in the case study of the Santa ECOLOGY AND MANAGEMENT Rita Mountain Arizona. watersheds Large-scale south of Tucson, livestock drought during the last decades grazing and of the 19th century resulted in marked decrease in grass cover, increased cover of shrubs, especially velvet mesquite (Prosopis velutina), cholla and prickly a large (Medina, pear cacti (Opuntia spp.) and increase in bare, unvegetated water run-off increased dramatically flooding soil 1996). As a result of these changes, in the city causing of Tucson. The most important watershed function of the Santa Rita watersheds that had to be restored was the hydrological function. Restoring the hydrological function required restoring vegetation cover that provided obstruction to overland flow, reducing the velocity of overland flow and increasing small scale, high-infiltration patches. Grass tussocks are the most effective structural features of watersheds in terms of causing water flow to be tortuous with tLissocks and hummocks absorbing water in transit (Fig. I; Tongway and Ludwig, 1997). Numerous native grass species and grass species acquired from other parts of the world were tested for large-scale revegetation efforts. The most effective grass species for rapid establishment and survival on the disturbed landscapes was Lehmann's lovegrass (Eragrostis lehmallllialla), a species from the arid rangelands of South Africa (Roundy and Biedenbender, 1995). The Lehmann lovegrass - mesquite savanna that now dom inates the Santa Rita watersheds has largely restored the hydrological function of the piedmont landscapes. However, using the option of an introduced alien species in restoring hydrological function has compromised the potential for restoring OF ARID ZONE WATERSHEDS 273 animal biodiversity and has replaced nutritious forage grasses with less nutritious Lehmann lovegrass for Iivestock forage (Bock et al., 1986; Whitford, 1995). This example from southern Arizona, USA, clearly demonstrates that restoration - rehabilitation of arid watersheds cannot be achieved by restoring only one of the features of a healthy watershed. Restoring the health of watersheds requires an understanding the ecosystem processes and watershed properties as the basis for developing strategies for restoring these processes and properties from the patch scale to the landscape scale (Whisenant, 1999). Because desertification affects not only vegetation but also soil properties and processes, it is not possible to restore vegetation without restoring soil. Because arid and semi-arid soils are patchy with respect to variables that affect water infiltration and storage and also soil nutrients, initial restoration efforts should focus at the patch scale. Successful rehabilitation of soils and vegetation at the patch scale does not necessarily require massive expenditure of energy and resources. An inexpensive approach to restoring patch dynamics of soils on a desert watershed was described by Tongway and Ludwig (1996). After analyzing the soils, vegetation, and landscape scale hydrological processes of a degraded Acacia spp. woodland in Australia, Tongway and Ludwig (1996) devised a simple but effective method of restoring productive soil patches on an unproductive watershed. They demonstrated that establishing brush piles oriented with the long axis 90° to the slope, obstructed 274 WHITFORD overland flow, captured sediment, increased water infiltration, increased soil microbial populations as measured by soil respiration, and increased soi I ferti Iity ( nitrogen, organ ic carbon, avai lable phosphorus, and cation exchange capacity). The brush pi les ameliorated the soil environment under the piles thereby allowing higher rates of decomposition of trapped organics. Litter trapped with the sed iments provided food sources for soil fauna such as termites that changed the macroporosity of the soil. The changes in soil properties provided habitat patches for establ ishment and growth of perennial grasses. The fertile, water holding soil patches allowed high plant survivorship during drought thus providing stress resistant patches in the landscape (Ludwig and Tongway, 1996). Using brush piles to restore resource-rich soil patches on a watershed is not the only approach that can be used to restore appropriately structured patches on a watershed. Combinations of mechanical intervention to produce ridge, mound and basin l1)icrotopography and the application of appropriate mulches have been successful in restoring soil patches. This type of intervention may be necessary on severely degraded watersheds where water and wind erosion have effectively eliminated the natural 111 icrotopography and where upper soil horizons have been stripped away. There are a variety of mechanical means for producing micro-catchments on a watershed. Land imprinters, root plows, deep rippers, and roller-choppers can be employed to develop a desirable micro- topography on a degraded watershed (Whisenant, 1999).The selection of mulch materials to be distributed in the depressions is extremely important. Degraded soils may not support the essential soil macro- and meso-fauna that produce macropores, and regulate the rates of decomposition and mineralization of organic materials. Mulch materials must provide the necessary carbon source to support a diverse assemblage of soil bacteria and fungi. In arid soils, most of the mesofauna feed directly on the soil bacteria and fungi (Whitford, 1996), therefore establ ishment of an abundant and diverse m icroflora is an essential first step in soil restoration. In arid regions where termites are a significant component of the soil fauna, the mulch materials must also provide suitable materials to support termites because termites play a keystone role in arid ecosystems by affecting numerous soi I processes and properties (Whitford, 2000). Research on restoring resource-rich patches on open-cast mine spoils in New Mexico showed that waste materials from lumber mills (wood chips, bark, small stems, and sawdust) was the most effective mulch for restoring soil biota in a short period of time (Whitford and Elkins, 1986). A variety of organic materials have been used with varying degrees of success, e.g., straw, hay, peat moss, shredded bark, corncobs, sewage sludge, crop residues, manure and plastic (Whisenant, 1999). Restoring resource-rich soil patches on arid watersheds may require a combination of several types of organic materials to provide the necessary substrate for the biota and to ameliorate the soil m icrocl imate. Restoration of resource-rich soi I patches is only the first step in restoring a watershed to a functional landscape. Re-establishment of vegetation with the appropriate structure for retention of water and sediments may require more than waiting for seeds of the ECOLOGY AND MANAGEMENT plants that remain on the watershed to be dispersed to the soi I patches, germ inate and become established as viable plants. The germ inants of many plant species may be susceptible to herbivory by the animals that thrive on degraded watersheds. In the Chihuahuan Deselt, rodents kill a large percentage of mesquite (Prosopis glandlilosa) shrub germinants, creosotebush (Larrea /riden/ala) germinants, tarbush (Flollrensia cernu(1) germinants and grass germinants (Whitford, unpublished data, Wh itford e/ al., 200 I). In order to insure the establishment of desirable shrubs and grasses on resource-rich soil patches, it may be necessary to grow plants in containers that can be planted in the desired locations. Containerized shrubs may provide protection from herbivory and an amel iorated subcanopy microclimate, i.e., functioning as nurse plants that allow grass plants to establish (Franco and Nobel, 1989; Livingston el al., 1997). Selection of plant species that lare to be established on watersheds that require restoration interventions in order to re-establ ish all of the ecosystem functions of a healthy watershed must be based on knowledge of the life histories of the species and their spatial distributions on watersheds. While it may be more difficult to establish native species and require a longer time period than is needed for establ ishment of al ien species, the potential deleterious effects of established al ien (exotic) plants (see above) could seriously compromise the benefits to be gained from restoring a watershed to a healthy condition. Restoration may also require that certain landuses be suspended until the vegetation has achieved sufficient vIgor to withstand use. This is especially .or ARID 275 ZONE WATERSHEDS true of grazing by domestic livestock. We experienced th~ loss of most of the newly establ ished vegetation on open-cast mine spoils because of the grazing, browsing and trampling by cattle, sheep, and horses that entered the area through a hole in a fence. Limiting or eliminating human use of the resOlirces for a period of time in order to insure restoration of watershed function is only one of the costs associated with watershed restoration. In arid regions of the world where increasing human populations require the water resources and other resources provided by healthy watersheds, it will be necessary for the users to make short-term sacrifices in order for the population at large to receive the benefits of healthy watersheds. References Abrahams, A.D., Parsons. A.J. and Luk, S. 191\1\. Hydrologic and scdiment responses to simulated rainfall on desert hillslopes in southern Arizona. Catena Bock, 15: 1(l3-117. e.E., Bock, .I.H., .Icpson, K.L. and Ortega. .I.e. 191\6. Ecological effects of planting African lovegrasses in Arizona. National Geographic Research 2: 456-463. Brandt, C.J. 1989. The size distribution of through fall drops under vegetation canopies. Catena 16: 507-524. DeSoyza, A.G., Whitford, W.G., Martinez-Meza. L and Van Zee . .I.W. 1997. Variation i"n creosotebush (Larrea tridentata) morphology in relation to habitat. soil lCrtility and associated annual plant communities. American Midland Naturalist 137: 13-26. DeSoyza. A.G., Whitford, W., Turner. S ..I., Van Zee, .I.W. and Johnson. A.R. 2000. Monitoring ecological condition in the Western United States. In Assessing and Monitoring the lIealth of Western Rangeland Watersheds (Eds. S.S. Shandu, B.D. Melzian. E.R. Long. W.Ci. Whitford. and 13.'1'. Walton). pp. 153-1 CJ() Kluwer Academic Publishers. Boston. 276 WHITFORD Dunkerley, D.L. and Brown, K.I.J. 1995. Runoff and runon areas in patterned chenopod shrubland, arid western New South Wales. Australia: characteristics and origins. Journal of Arid Environments 30: 41-55. Elkins, N.Z., Sabol, G.V .. Ward, 1'.1. and Whitford, 1986. The influence of subterranean termites on the hydrological characteristics of a Chihuahuan desert ecosystem. Gecologia 8: 521-528. Evenari, M., Shanan, L., Tadmor, N.H. and Aharoni, Y. 1971. The Negev: The Challenge of a Desert. Harvard University Press, Cambridge, M assach usetts Franco, A.c. and Nobel, P.S. 1989. Effects of nurse plants on the microhabitat and growth of cacti. Journal of Ecology 69: 883-896. Greenwood, K.L., MacLeod, D.A. and Hutchinson. K. J. 1997. Long term stocking rate effects on soil physical properties. Australian Journal of Experimental Agriculture 37: 413-419. Jones, 1.A. 1989. Environmental influences on soil chemistry in central, semiarid Tanzania. Soil Science Society of America Journal 51: 1748-1758. Jones, K.B., Heggem, D.T., Wade, T.G., Neale, A.C., Ebert, D.W., Nash, M.H., Mehaffey, M.H., Hermann, K.A., Selle, A.R., Augustine, S., Goodman, I.A., Pedersen, J., Bolgrien, D., Viger, .I. M., Chiang, D., Lin, C. .I., Zhong, Y. Baker, J. and Van Remortel, R. D. 2000. Assessing landscape condition relative to water resources in the western United States: A strategic approach. Environmental Monitoring and Assessment 64: 227-245. Livingston, M., Roundy, B.A. and Smith, S.E. 1997. Associations of overstory plant canopies and the native grasses in southern Arizona. Journal of Arid Environments 35: 441-449. Ludwig, J.A. and Tongway, D. J. 1996. Rehabilitation of semiarid landscapes in Australia. II. Restoring vegetation patches. Restoration Ecology 4: 398-406. Luk, S., Abrahams, A.D. and Parsons, A.J. 1993. Sediment sources and sediment transport by rill flow and interrill flow on a semi-arid piedmont slope, southern Arizona. Catena 20: 93-111. Mabbutt. lA. and Fanning. P.c. 1987. Vegetation banding in arid Western Australia. JOllrnal of Arid Environments 12: 41-59. Martinez-Meza, E. and Whitford. E. 1996. Stem flow. throughfall and channelization of stemllow bv roots iR-three Chihuahuan Desert shrubs. JOllrn(;1 of Arid Environments 32: 271-288. Medina, A.L. 1996. The Santa Rita Experimental Range: History and Annotated Bibliography (/903-1988). General Technical Report RM-GTR-276. US. Department of Agriculture Forest Service, Rocky Mountain Forest and Range Experiment Station, Ft. Collins, CO. 67 p. Nash. M.S., Jackson, E. and Whitford, W.G. 2002. Soil microtopography on grazing gradients in Chihuahuan Desert grasslands. Journal of Arid Environments (In Press). Nash, M.H. and Whitford, W.G. 1995. Subterranean termites: Regulatiors of soil organic matter in the Chihuahuan Desert. Biology and Fertility of Soils 19: 15-18. Phillips, R.E., Quesenberry, V.I., Zeleznik. J.M. and Dunn, G.H. 1989. Mechanisms of water entry into simulated macropores. Soil Science Society of America Journal 53: 1629-1635. Poesen, 1., Ingelmo-Sanchez, F. and Mucher. I I. 1990. The hydrological response of soil surfaces to rainfall as affected by. cover and position of rock fragments in the top layer. Earth Sw/ace Processes and Landforms 15: 651-671. Reid, I. and Frostick, L.E. 1987. Flow dynamics and suspended sediment properties in arid zone flash floods. Hydrological Processes I: 239-253. Roundy, B.A. and l3iedenbender. S.II. 1995. The desert grassland. In Revegetation in the Desert Grassland (Eds. M.P. McClaran and T.R. Van Devender), pp. 265-303. University of Arizona Press, Tucson. Schaefer, D.A. and Whitford, W.G. 1981. Nutrient cycling by the subterranean termite. Gnathamitermes tubiformans in a Chihuahuan Desert ecosystem. Gecologia 48: 277-283. Sharma, K.D., Choudhari, J.S. and Vangani, N.S. 1984. Transmission losses and quality changes along a desert stream: the Luni Basin in N. W. India. Journal of Arid Environments 7: 255-262. Tongway, D.J. and Ludwig, J. A. 1996. Rehabilitation of semiarid landscapes in Australia. I. Restoring ECOLOGY AND MANAGEMENT productive soil patches. 4: 388-397. Restoration Ecology Tongway, D . .!. and Ludwig, .I.A. 1997. The conservation of water and nutrients within landscapes. In Landscape Ecology: Function and Management (Eds. LA. Ludwig, D. Tongway, D. Freudenberger, .I. Noble and K. Hodgkinson), pp. 13-22. CSIRO Publishing, Collingwood, Victoria, Australia Torri, D. and Poesen, .I. 1992. The effect of soil surface slope on raindrop detachment. Catena 19: 561-578. Van Elcwijck, L. 1989. Influence of Icaf and branch slope on stemtlow amount. Catena 16: 525-533. Verstraete, M.M. 1986. Defining desertification: review. Climate Change 9: 5-18. a Whiscnant, S.G. 1999. Repairing Damaged Wildlands.' Cambridge University Press, Cambridge, United Kingdom White, L.P. 1971. Vegetation stripes on sheet wash surfaces. Journal of Ecology 59: 615-622. Whitford, W.G. 1995. Desertification: Implications ai1d limitations of the ecosystem health metaphor. In Evaluating and Monitoring the Health of Large-Scale Ecosystems. (Eds. OJ. Rapport, Gaudct, C.L. and P. Calow), pp. 271-293. Springer- Verlag, Heidelberg Whitford, W.G. 1996. The importance ofbiodivcrsity of soil biota in arid ccosystems. Biodiversity Conservation 5: 185-195. or ARID ZONE WATERSHEDS 277 Whitford. W.G.- 1997. Desertification and animal biodiversity in the desert grasslands of North Amcrica. Journal of Arid Environments 37: 709-720. Whitford, W.G. 2000. Keystone arthropods as webll1asters in desert ecosystems (Eds. D.C. Coleman and P.F. Hendrix), pp. 25-41. Invertebrates as Webmasters in Ecosystems. CAB International, New York. Whitford, W.G. 2002. Ecology of Desert Systems. Academic Press, London Whitford, W.G. and Elkins, N.Z. 1986. The importance of soil ecology and the ecosystem perspcctivc in surface-mine reclamation. In (Eds. C.C. Reith and L.D. Potter), pp. 151-187. Principlcs and Methods of Reclamation Science. University of New Mexico Press. Albuquerque, New Mexico .. Whitford, W.G. and Kay, F.R. 1999. Biopedturbation by mammals in deserts: A review. Journal of Arid Environments 4 I: 203-230. Whitford, W.G., Nielson, R. and DeSoyza, A.G. 200 I. Establishment and effccts of establishment of crcosotebush, Larrea tridentata, on a Chihuahuan Desert watershed. Journal of Arid Enviroments 47: I -10. Whitford, W.G., Rapport, OJ. and DeSoyza, A.G. 1999. Using resistance and resiliencc mcasurements for 'fitness' tests in ecosystcm hcalth. Journal of Environmental Management 57: 21-29.