Dryland salinity
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Dryland salinity is a natural process for
soil Soil, also commonly referred to as earth or dirt Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty. Common types of dirt include: * Debri ...
, just like other processes such as
wind erosion Aeolian processes, also spelled eolian, pertain to wind activity in the study of geology and weather and specifically to the wind's ability to shape the surface of the Earth (or other planets). Winds may erode, transport, and deposit materials ...
. Salinity degrades land by an increase in soil salt concentration in the environment, watercourse or soil in unirrigated landscapes, being in excess of normal soil salt concentrations in dryland regions.


Overview

Salinity refers to the movement and concentration of
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
in the landscape and its associated detriment to land and water resources;
dryland Drylands are defined by a scarcity of water. Drylands are zones where precipitation is balanced by evaporation from surfaces and by transpiration by plants (evapotranspiration). The United Nations Environment Program defines drylands as tropical ...
salinity refers to salinity in unirrigated landscapes. Salinity processes extend from local to regional scales and are driven by imbalances in the water budget that result, primarily, from agriculturally driven landscape change. There are two types of salinity:


Types of salinity

There are two types of salinity. Primary salinity (natural salinity) and secondary salinity (induced salinity). (Nrm.qld.gov.au, 2013) Primary salinity naturally occurs in arid and saline environments such as salt lakes, marshes, pans and salt flats. Natural accumulation of salt in soils is an outcome from previous cycles of drainage, desiccation and sea winds. High levels of salt are often found in dry soils, more so than wet soils as it is diluted and washed through the
soil profile A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...
. (Barry and Holwell et al., 2012). Secondary salinity is a direct result of human interaction with the land, during
development Development or developing may refer to: Arts *Development hell, when a project is stuck in development *Filmmaking, development phase, including finance and budgeting *Development (music), the process thematic material is reshaped * Photograph ...
,
agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people t ...
and irrigation. Certain land practices have led to changes in the natural structure of the
biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...
resulting in excess salting of the land, waterways and soils; thus having detrimental effects on biodiversity and the lands'
productivity Productivity is the efficiency of production of goods or services expressed by some measure. Measurements of productivity are often expressed as a ratio of an aggregate output to a single input or an aggregate input used in a production proces ...
.


Salinity and water cycle

Factors such as
climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
, features of landscape, soils,
drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...
, aspect and the effects of human activities; all impact on the severity and occurrence of dryland salinity. Dryland salinity effects human and natural resources, such as native vegetation and crops, animals, infrastructure, agricultural inputs, biodiversity, aquatic ecosystems and water supply quality in the environment. Understanding dryland salinity requires a look at the
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly cons ...
. Water enters the soil from precipitation – this is called
Infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...
; water may remain indefinitely within the spaces or pores between soil particles as soil moisture. Soil moisture may be lost to the surface or atmosphere directly, or through plant uptake – this is called evapotranspiration. Soil moisture may also continue to move downward to join the groundwater—this is called
groundwater recharge Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs ...
. Recharge is most likely to occur when the amount of water that is available to the soil exceeds the soil’s capacity to store it (field capacity). Recharge may also occur by saturated flow when water bypasses the soil matrix as it moves to depth in macropores (e.g. root holes, fractures). Excessive recharge may raise the water table locally, or at a landscape scale. When brackish to saline groundwater intersects the ground surface and discharges, this is termed saline discharge. Areas of discharge are called saline seeps (when groundwater intersects the soil surface) or saline scalds (where water is lost by evaporation only). Groundwater discharge manifests in such problems as: reduced agricultural production, degradation of natural environment, reduced surface water quality, damage to infrastructure including roads, as well as soil erosion and denudation of land.


Reasons for dryland salinity

Dryland salinity is broadly the result of three processes:
Groundwater recharge Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs ...
, Groundwater movement and
Groundwater discharge Groundwater discharge is the volumetric flow rate of groundwater through an aquifer. Total groundwater discharge, as reported through a specified area, is similarly expressed as: :Q = \fracKA where :''Q'' is the total groundwater discharge ( 3·T ...
. (Environment.nsw.gov.au, 2013) Groundwater recharge occurs naturally, being a key process in the development of salinity, however land clearing accelerates this development as the once present deep-rooted plants do not use excess runoff, and it now seeps past the roots zone to enter the groundwater system. (Environment.nsw.gov.au, 2013)
Evapotranspiration Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
is reduced, as vegetation is lost, resulting in an imbalance in groundwater recharge and discharge, causing the water table to rise.
Capillary action Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of, or even in opposition to, any external forces li ...
brings salt to the surface initiating surface salt accumulation. (Barry and Holwell et al., 2012) Prevention and alleviation of dry land salinity is a simplistic concept in theory however complex in application. Ceasing the removal of deeply rooted vegetation in order to moderate unbalanced groundwater recharge and the replanting of deeply rooted vegetation such as Eucalyptus and salt tolerant species in regions where salinity is present, will start to alleviate the salt and ground water discharge problems. (Barry and Holwell et al., 2012) However establishing plants in salt laden areas is extremely difficult. Dryland salinity is a sign that the water balance of the nearby area of land or
catchment A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, t ...
has been altered. Clearing as little as 25% of a catchment can cause salinity to occur. In addition to adding extra recharge, salinity may also be caused if the aquifers discharge capacity has been exceeded. In many Australian landscapes, aquifer capacity may be several orders of magnitude below that of the altered recharge. Restoring the balance requires either the introduction of natural vegetation (e.g. mallee eucalyptus or perennial grasses), which intercepts and transpires most of the incoming rainfall; or by adapting agriculture to the increased area of shallow, saline groundwater.


Management


Role of soils

Dryland salinity management often focuses on
vegetation Vegetation is an assemblage of plant species and the ground cover they provide. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characte ...
, yet it is the collective role of
soil Soil, also commonly referred to as earth or dirt Dirt is an unclean matter, especially when in contact with a person's clothes, skin, or possessions. In such cases, they are said to become dirty. Common types of dirt include: * Debri ...
s and vegetation that has an effect on the root cause of the problem, recharge.
Soil health Soil health is a state of a soil meeting its range of ecosystem functions as appropriate to its environment. In more colloquial terms, the health of soil arises from favorable interactions of all soil components (living and non-living) that belong ...
cannot be ignored as a valuable and extensive activity for the management of dryland salinity – the multiple benefits of improving soil health are clear and can be motivated by the potential for local and regional economic and social gains. Soil is considered in two contexts when it comes to dryland salinity: Recharge and discharge. ;Soils in groundwater recharge areas Soils absorb and store water according to their water holding or
field capacity Field capacity is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. This usually takes place 2–3 days after rain or irrigation in pervious soils of u ...
and how dry they are to start with. In much of Victoria, under typical rainfall and natural vegetation cover, soils take on water during wet winters and dry out over summers as plants consume the water (Young & Young, 2001). The drier the soil when leading into winter, the more water can be stored that might otherwise leak to groundwater. To reduce recharge to levels that existed in pre-clearing conditions is generally infeasible in most agricultural landscapes. This is because there are too few profitable perennials that can replace crops or that can be adopted at the scale required. In most recharge areas that are at risk of future salinity, the goal is to minimise recharge. This can be done by planting wide-spaced trees (alley farming), areas of perennials on suitable soils, and by preventing soils from being left without a significant leaf area in winter and spring. In recharge areas remote from saline areas there is often little incentive for farmers to adopt low profit, low recharge systems. In addition, recharge is the source of fresh groundwater, and a trade off between consumptive use and salinity should be acknowledged. ;Soils in groundwater discharge areas The manifestation of dryland salinity is largely a problem of groundwater – however the accumulation of salt within the soil and at the surface due to proximity to or saturation by saline groundwater causes changes to the soil’s chemistry, structure and stability, and the plant life that it supports. ;Managing soils for dryland salinity in catchments In discharge areas, salinity can be managed by establishing salt-tolerant plants or by engineering systems. Engineering systems include deep open drains, pumps, siphons and various forms of surface water management. Engineering system involve the obvious discharge of salt and water. While saline areas also discharge salt and water, the abundance and timing will be changed. In most Australian states, farmers would be advised to seek advice before using engineering systems. Establishing salt-tolerant plants can improve salt discharge rates and improve soil health. Improvements undertaken at a
catchment A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, t ...
scale bring many benefits, not the least of which is providing for increased agricultural and associated regional productivity – using water for production that otherwise would contribute to an environmental problem.{{cn, date=May 2016


See also

*
Salinity in Australia Soil salinity and dryland salinity are two problems degrading the environment of Australia. Salinity is a concern in most states, but especially in the south-west of Western Australia.
*
Salinity control Soil salinity control relates to controlling the problem of soil salinity and reclaiming salinized agricultural land. The aim of soil salinity control is to prevent soil degradation by salination and reclamation of already salty (saline) soils ...
*
Soil salination Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization. Salts occur naturally within soils and water. Salination can be caused by natural processes such as mineral weathering or by the ...


References


Department of Primary Industries (DPI)
viewed June 2007. *Young, A & Young R 2001, Soils in the Australian landscape,
Oxford University Press Oxford University Press (OUP) is the university press of the University of Oxford. It is the largest university press in the world, and its printing history dates back to the 1480s. Having been officially granted the legal right to print books ...
,
Melbourne Melbourne ( ; Boonwurrung/Woiwurrung: ''Narrm'' or ''Naarm'') is the capital and most populous city of the Australian state of Victoria, and the second-most populous city in both Australia and Oceania. Its name generally refers to a met ...
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Dryland salinity – causes and impacts
ccessed: 20 Oct 2013
Dryland Salinity
ccessed: 15 Oct 2013
Types of salinity
ccessed: 15 Oct 2013
Factors contributing to salinity
ccessed: 20 Oct 2013 Soil science Natural resources Land management