DRYLANDS “The development of conservation strategies for the responsible human use and management of arid lands is probably the worlds most pressing problem in landscape management.” Mollison: Designers Manual. The Arid Landscape has a square profile carved away by wind and rare rainfall events. Arid lands are areas where direct evaporation exceeds rainfall, and where annual precipitation averages are below 80cm and as low as 1cm (sometimes only dew). All desert areas are extending; many dryland areas are being created and antecedent plant and animal species are thereby brought to extinction. Some features of desert are
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Plants produce copious seed with long viability, often wind-dispersed. Termites and ants are more effective than worms as soil aerators and decomposers. Rain may fall in mosaic patterns, so that vegetation is also a varied mosaic of fire and rain, and ephemeral patterns at different response stages from growth to decay are evident. New generations of shrubs may experience favorable seedling conditions as rarely as every 7-20 years. This becomes the period of recruitment of new forests or shrub lands. Much of the water run-off system may end not in rivers but in inland saltpans or basins (endorheic drainage) from which all water eventually evaporates. Normal erosion is by wind, but rare cloudbursts shape the main erosion features and move vast quantities of loose material from the hills in turbulent stream flows. Wind transports materials locally in dust storms. Animals burrow, seek shade, or are nocturnal in order to conserve water; many are highly adapted for water conservation. Plant associations may be very varied in response to changes in long-term aspects such as slope, soil depth, salinity, browsing intensity, ph, and rock type.
Classification of Deserts: Semi Arid has steppe (perennial grasslands, often treeless, established during periods of ice cap retreat) scrub, and low forest vegetation. Arid
has steppe and scattered low shrubs.
Desert has little but oases and ephemeral (short lived) vegetation appearing after the rare rains. Deserts have a rainfall classification of • Hyperarid: 0-2cm annual average (e.g. Atacama, Namib desert, central Sahara) • Extremely Dry: 2-5cm annual average • Arid: 5-15cm annual average (e.g. Mohave, Sonora, Sahara margins). • Semi Arid: 15-20cm maximum 40cm (much of the Australian desert, Asian deserts, Kalahari) Rainfall is not dependable in arid areas, and potential evaporation can range from values of 700cm/year in hyperarid areas to 100cm in steppe. Pollution can be a problem as breakdown of most organic substances can be very slow in the absence of water. Pollution from biocides is a particular problem as it is the (missing) aquatic plants that break these down most effectively. In Israel, (New Scientist 13.10.77) sewage and agricultural pollution rise through the sands in summer, and nitrates can be included in bore water pumped for domestic use. Winter rains again carry pollutants down, which must then be kept clear of water pumped to reservoirs. Many desert waters already have high levels of dissolved salts, additional stress from nitrates may cause kidney malfunction. 47% of aboriginal outstation hands using bore water do suffer kidney damage, subsequent high blood pressure and excessive intercellular water – a form of dropsy. It is easy to go wrong in deserts and put whole populations at risk. Water in particular must be frequently analysed for pollution and salts and deep well or bore water tested for excessive mineral and radioactive contaminants. A paradox in arid areas is humid environments caused by exotic rivers, which flow in from forested areas or better-watered areas, oases, and underground waters (aquifers), which should only be used with great caution as these resources can be locally exhausted (i.e. Jordan) and aquifers can be depleted over vast areas by immodest use. This can cause widespread subsidence (along Dead Sea), collapse of the aquifer and permanent disappearance of the oasis. Our focus should be on increasing the input of water into aquifers, soils and streams and to re-humidify desert air by planting trees and protecting existing vegetation. Trees and shrubs transpire rather than evaporate water keeping desert salts from evaporating at soil levels. When evaporation alone operates, capillary action quickly brings sub-surface salts to the surface and we can no longer establish vegetative cover.
Important desert strategy is to have many small systems going; all designed to catch and store water. Water must be stored in ground or underground. • • • • • • • • • • •
Placement of human habitation, animal shelter, manurial flow. Plant after animals Use of sunny cliff sites for cave housing, water storage. (Page 320-321 manual) Check dams; flood flow irrigation; holding banks stable (page 160 manual) Road run-off techniques Mulch-traps in desert Floodplain treatment in deserts (Navajo and zuni techniques) waffle gardens. Slopes and run-of. Catchment off anything solid including rocks. Use of basketry and woven mulch, fences Evaporation loss and moisture barriers. Plastic sheet 1m for vegetables and 3m for trees. Shade and shadehouse. Shade is crucial in desert. Lots of trellising Special treatment of showers, water run-off
Where overgrazing has not been the dominant feature of land use, a desert may present the eye-level appearance of a low-crowned forest with shrub understorey. Some areas may be richer in plant species than a more humid forest environment. However, in detail, there will be areas of bare soil between plant groups. Closer inspection may reveal a fungal-algal-lichen (cryptogam) crust on bare areas, depending on the frequency of hoofed animals. This crust is a critical and delicate feature preventing wind erosion. Its preservation is essential for soil preservation. A profound question concerns our basic uses of deserts. Livestock herding in arid areas has been traditional – devastating as a maximum intensity cash crop system.. Examples are Australia, Peru, Africa, Middle East and Tibet. In Australia 73% of land use is for pastoralism with 27% as reserves and conservation. However, income is only 15% from pastoralism and 76% from tourism associated with reserves and conservation areas. It is not possible to count on permanent cropping or herding in areas that experience one good year in 4-9 years. But it is possible to carefully develop core settlements setting out hardy plants along favourable “corridors”, taking advantage of rare rains to establish a wide biological resource for dry years. The most affective and cheapest strategy for desert revegetation is the exclusion of browsing animals from headwaters so the after a few years thousands of young plants may establish.
Precipitation: There are natural deserts e.g. Namib and Atacama where offshore winds and cold currents equal no rain. In these areas dew condensation is critical for plant growth. Dew traps of stone, scattered shrubs or vertical metal screens to 1m (as per Moroccan foresters). Precipitation responses: 8-12mm produces run-off sufficient for headwater stream flow. Tribes people and mobile fauna respond with a sequence of migration and breeding patterns. Walk-about is an appropriate and planned response. It is part of our strategy to collect some of the 88% of run-off and evaporation from these events. In natural conditions as little as 0.8% infiltrates to recharge aquifers. Soils Normally alkaline in waterways. Acidic sands around deep weathered granites. As a consequence of pH, low mineral trace element availability Indicators: Interveinal colour loss, leaf thinning, tip curl in severe cases (Mn in citrus). Severe leaf yellowing â€“ Zn deficiency (often associated with Mn deficiency) Copper deficiency, giant leaves, gum pockets in citrus branches, multiple budding or trunking in trees. Special problems: Non-wetting sands (caused by a fungus). Use bentonite or humus, swales and raised beds with high edges to prevent water run-off. High salt levels in water or soil â€“ humus production, ponding (with algae production and water crop), and salt tolerant crops. Free draining sands can be irrigated with water to 1500ppm salt. As spray irrigation, this would not be tolerated by plants. Despite these problems, we can usually establish home gardens and adapted tree crop systems and many selected areas especially near scarps, rivers, hills, or ranges will grow excellent fruit, vegetable and tree crop with appropriate water run-off harvesting. Perhaps most important, remember natural systems in desert are fragile. Good management and constant appraisal are essential. Use small systems, especially when run-off is harvested Avoid broad-scale or grand trials until system capacity especially of water resources is assessed. Fertilizers should be used sparingly if at all apart from humus and limited animal manures. Excessive green growth in plants creates drought stress. Magg suggested using 2 plastic bags of slow release fertilizer with pinholes in them, in the hole with valuable trees. These last the life of the tree. Gardeners get generally good results with leaf and bark mulch, manure and compost. Some sulphur if pH is high. Sewerage water gives good results with fuel wood trees
(check salt levels as water gains 300ppm salt on way through towns). Most people do not add potash as these and other salts may be plentiful in arid soils. Analysis of soil and leaf is best. Check for nitrate levels of leaf and water if children are eating. Landscape features in deserts Designers need to note: Process: whether wind, water, or infiltration is active Rock and soil type: these decide local response to process and produce characteristic landforms. Aspect: even slight shading by hills changes opportunities and promotes growth. Fire: frequency and time since last fire Last heavy rain: (more than 12mm) which may have been the trigger for a specific age group of plants (recruitment of species). In drylands, erosion landform are more significant, more conspicuous and more numerous in type than in humid areas. We see mountain ranges with complex long valleys and sheer sided gorges draining them. Inselbergs – isolated or grouped granitic domes rising steeply out of desert plain. Complex scarp and pediment landscapes of fault lines, mesas and wadis (box canyons). Badlands of complex softer eroded sediments, Gullies or gulches – sharp sided and many branched – develop on steep slopes. Basin and range topography from series of folds between desert and mountains Desert landscapes are angular and actively eroding. Humid areas especially when vegetated have softer and more rounded outlines with rare cliff faces at recent fault lines or shorelines. Water Storage and Use in Scarp Landscape Start on top erosion surface. Stone and cement dams across generally gentle valleys of upper plateau. Fit base pipes or plugs for periodic cleaning. Just before going over wadi walls, water often scours out holes. Some can be enlarged into cisterns, all are important to wildlife, esp. birds. Cut gutters away from main valley head to spill into side valleys as required. Deep sands and silt on protected wadi floor offer main (or only) cultivation opportunities. Erect strong rock walls across floors and let silt fields build up behind. In minor wadis whole width may be dammed with spillway cut in pediment rock. In wide wadis leave central channel open with fields built to side fed by side valleys. Once flood spreads out on lower plain, we may form broad earth and stone banks stabilized with unpalatable shrubs and enclosing soakage fields. Most efficient wadi development studied to date is ancient Nabatean in Negev. (Ancient Masters of the Desert (Evenari & Koller 1956). These are partly restored, but thousands of hectares of fields are still abandoned. This system holds 80-90% of runoff behind wadi dykes, spreading this to lower dykes in the plains. Water is spread to orchards, with every irrigated hectare served by 20 Ha catchment. The idea is to let minimal water escape as run-off, absorbing a maximum in fields or salt beds. We can hold all water with a lot of people or a few machines - even in heavy rainsalthough outer fields may seldom flood in normal rains. We aim to absorb about 0.5 – 1
m of water in each field. This gives enough to grow grain crop or keep palms and fruit alive. Upper reservoirs can be released soon after torrents cease to top up the system. Even without all this work, a few people can live in a wadi and establish productive tree crops by seeking deep silts in shady sites. Houses in scarp base get water gravity fed from cisterns above. Windmills can pump it up from wells near scarp. Fencing in a wadi is minimal, and apart from peneplain crusts, rock is generally soft. Fig 11.8 p320 & 11.9 p321.