Salinity is a major cause of land and water degradation and economic losses in Thailand and Australia. In Australia dryland salinity is a national problem, affecting more than 2.3 million ha with a further 12.3 million at risk. In northeastern Thailand it is estimated that an area of 6 million hectares (or 34 per cent of arable land) are affected by salt, and the problem appears to be escalating. In Lao PDR it is of minor importance, but has the potential of becoming equally serious.
The reason for the spread of salinisation is the large-scale clearing of trees, leading to gradual increase in surface soil salinity and stream salt loads. The main difference between the dryland salinity in Australia and Thailand is the source of salt. In Australia the salt is stored in a metastable state in the soil until water tables rise and bring the dissolved salt to the surface. In Thailand the source of salt is the dissolution of rock salt in the Mahasarakham Formation that underlies much of north eastern Thailand and parts of Lao PDR.
The economic implications of soil salinisation are potentially very large, as seen in parts of Australia and Thailand. Yields of rice and economic returns from salt-affected soils are about one third of those achieved in unaffected areas. Rising salt can affect bores and drinking water quality.
In Lao PDR there are signs of salinity to the northeast of Vientiane and around Savannakhet. A low-technology salt-making industry is found in both these areas, as in northeastern Thailand. The research studied how salinity could be aggravated by irrigation and deforestation plans in Lao PDR.
The project sought to contribute to agricultural sustainability of salinised land in parts of northeastern Thailand, Lao PDR and southeastern Australia.
The first aspect of the research was to determine the potential for surface salinisation of land and water in response to major land use changes in the Suvannakhet area of Lao PDR. Researchers investigated the groundwater flow, recharge and salinity characteristics of this part of the Mekong Plain, and studied the close relationship with groundwater in northeastern Thailand, which has the same geological formations (including rock salt).
The second objective was to correlate tree water use with plantation behaviour as a means of reducing recharge in Thailand (Khorat, Lam Pao and Khon Kaen), Australia (Upper Macquarie Valley) and Lao PDR. Finally, hydrogeological, economic and social models were developed and applied at catchment and other scales to evaluate the impact of revegetation in Thailand (Lam Pao and Khorat) and Australia (Liverpool Plains and Upper Macquarie Valley). Revegetation strategies were planned for each project site.
New modelling approaches were applied in Thailand following the identification of limitations in existing socioeconomic and biophysical models. The existing SMAC model was restructured by updating the hydrogeological and socioeconomic modules to include an agroforestry component. Application of the new model to the Upper Macquarie Valley (NSW) successfully assessed the potential role of agroforestry as an alternative land use and salinity management tool. Modelling and optimisation results were then transferred to stakeholders including Landcare and DIPNR.
SMAC was rewritten as an EXCEL spreadsheet for Thai conditions as a catchment model capable of optimising land use for maximising economic return and minimising land salinisation. This Isaan Catchment Hydrological and Agricultural Model (ICHAM) was transferred to the Land Development Department (LDD) of Thailand whose staff members are further developing the user interface to include Thai language and to be compatible with models already in use. Restructuring the model format has increased the likelihood of user uptake. The joint development of ICHAM introduced new research techniques to Thai project scientists.
Researchers took field measurements of tree sapflow and compiled tree water use datasets (TWUs) to calculate the water consumption of various tree species in Australia and Thailand. These measurements were complemented at each site with data on rainfall, temperature, soil moisture and groundwater levels for statistical correlation with the TWUs. TWUs were compiled for Casuarina spp., Eucalyptus camaldulensis, Acacia ampliceps and Azadirachta indica (Neem) and others.
Water use by salt tolerant grasses, including Sporobolus virginicus (commonly known as Dixie grass) was also measured at a site near Khorat. These data have been used in other LDD projects rehabilitating saline land. Other sites in Khorat province are being successfully revegetated using selected species to manage local recharge-discharge areas. Data collected on water consumption by vegetation helped in compiling realistic estimates of water balance components and assisted the model development.
Knowledge of salinity processes in Australia and Thailand allowed a scoping of the salinity risk from irrigation expansion in areas of Lao PDR similar to northeast Thailand. Research focused on identifying the potential for a salinity problem by outlining the physical drivers and conceptual processes whilst increasing awareness amongst stakeholders. Initially, efforts focused on information review and data collection and a significant GIS database was established to identify the potential salinity risk areas, based on irrigation developments and land use change. Two key risk areas were identified in Vientiane and Savannakhet Provinces where salinity was likely to be derived from the underlying evaporite (rocksalt) formations, as in neighbouring Isaan, Thailand.
An investigation of the Champone District in Savannakhet showed that the active groundwater flow systems occur in the surficial, alluvial deposits of the Champone River and the catchment-rim silty gravel terraces. These deposits overlay a 40 m thick claystone, possibly fractured in places, which is above the rocksalt. Brine from the rocksalt, pumped locally for small-scale salt making, appears to be the source of the salinity. Groundwater modelling suggests that the rate of supply is extremely low, but proposed irrigation developments should avoid inadvertent augmentation of shallow groundwater systems such that flow comes into contact with the underlying claystone. If this happens there is potential for mobilisation and transport of the salt, leading to soil salinisation.
The research has successfully defined the salinity processes, enabling the identification of those land-use practices driving salinity. Given the wide scale of potential land salinisation and the rapid pace of agricultural development in the region, significant effort was made to increase awareness of salinity amongst government departments at both the central and regional scale. Numerous extension activities were conducted including meetings, workshops and study tours, introducing government officers to the processes causing salinity and highlighting the potential economic, social and environmental impacts. Particular success was noted through the involvement of Thai scientists and demonstration of the real impact experienced in neighbouring Thailand.
Links:
[1] http://www.aciar.gov.au/country/Laos
[2] http://www.aciar.gov.au/country/Thailand
[3] http://www.abc.net.au/rn/science/ss/features/suitcase/thai_laos/default.htm
[4] http://www.aciar.gov.au/programarea/Land and Water Resources