Dryland salinity

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Dryland salinity is salinity that occurs in a landscape that is not irrigated, as distinct from irrigation salinity and urban salinity. This discussion provides a simple explanation of dryland salinity, the role that soil plays and raises some considerations from a catchment management perspective.


Contents

[edit] Overview

Salinity refers to the movement and concentration of salt in the landscape and its associated detriment to land and water resources; dryland 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. Understanding dryland salinity requires a look at the water cycle.

Water enters the soil from precipitation – this is called Infiltration; 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. 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).

Excessive recharge causes the upward and lateral movement of groundwater; when salty groundwater intersects the ground surface and discharges, this is termed saline discharge. Saline discharge manifests in such problems as: reduced agricultural production, degradation of natural environment, reduced water quality, damage to infrastructure including roads (DPI, 2007), as well as soil erosion and denudation of land.

Dryland salinity is a sign that the water balance of the associated catchment is skewed – with soils not able to regulate recharge by a normal regime of cyclic wetting and drying in association with plant activity. The balance is tipped by vegetation change in the landscape and by soil factors that affect its ability to absorb and contain moisture. For example, as Young & Young (2002) explain, in northern Victoria (Australia) a natural cover of mallee eucalypts and perennial grasses would normally intercept and transpire most incoming rainfall; but this cover has largely given way to crops and annual pastures – evapotranspiration has dropped and seepage has increased.

[edit] Management of Dryland Salinity

[edit] The role of soils in dryland salinity

Dryland salinity management often focuses on vegetation, yet it is the collective role of soils and vegetation that has an effect on the root cause of the problem, recharge. Soil health 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 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.

The role of soil in recharge is also subject to soil health and the effect this has on, for example, field capacity. Field capacity is increased by improving soil structure, through practices such as adding lime to soils, incorporating organic matter; and reducing physical disturbance.

Soils in groundwater discharge areas

The manifestation of dryland salinity is largely a problem of soils – 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

Soil management, as a way to address dryland salinity, needs to be looked at as an extensive treatment. That is, a treatment that produces catchment scale benefits from the cumulative effect of small changes made consistently over large areas.

Seldom are there funds within a catchment to resolve dryland salinity completely and if soils are to be targeted extensively, economically advantageous practices are called for. Fortunately, soil health improvements undertaken at a catchment scale bring many benefits, not the least of which is providing for increased agricultural and associated regional productivity – utilising water for production that otherwise would contributed to an environmental problem.

A further benefit of focusing on improvements in soil health for dryland salinity is that soils are not just the means to protecting regional assets (waterways, infrastructure etc) - soils themselves are a regional asset. It is often in the interest of land managers, at say the farm scale, to look at improvements in soil heath as a means to productive gain than for what they might regard as geographically detached, dryland salinity benefits.

[edit] See also

[edit] References