WaterSubject

Irrigation delivers major benefits in food security and human development. Irrigation also leads to waterlogging and salinity which threaten the sustainability of irrigated agriculture and pose major socioeconomic and environmental risks. The issue can be addressed by limiting net recharge to groundwater such that the water and salt keep natural equilibria. Often the information on net recharge within catchments is unavailable, particularly at lower spatial scales such as the farm or paddock; this offers little guidance for on-farm land and water management decisions—basic decisions that ultimately impact regional net recharge and waterlogging and salinity dynamics. This paper develops a cross disciplinary framework based on the concept of net recharge for setting paddock scale targets and to link these to the regional targets and community’s goals for sustainable irrigation management. A management model, cast in a dynamic programming format to integrate a detailed hydrological model with an economic model was applied to estimate the productivity, profitability and sustainability of irrigated agriculture in a region of the Murray Darling Basin in Australia. SWAGMAN^® Farm model was used to determine paddock scale net recharge. This interactive model enables an individual farmer to choose a profit optimizing crop mix while lowering net recharge; this in turn leads to a win-win outcome for all farmers. The net recharge metric can be used for the conversion of diffuse source groundwater recharge to a point source recharge at paddock scale, enabling the definition of private property rights to a common pool problem and assigning individual responsibilities for its management—a vexing issue and a new concept for the commons literature. Net recharge shows significant spatial and temporal variation which warrants a targeted/zonal approach to address the issue. Regional and targeted strategies and actions to address the issue are identified. Apart from its applied and action research orientation, the development of paddock scale net recharge metric is perhaps the most significant conceptual contribution of this research which can lead to shared management of groundwater aquifers.

During the second half of the 20th century the global food system has responded to the doubling of world population by more than doubling food production. Crop yield growth and intensification have been the dominant factors, and irrigated agriculture has played a major role. About 40% of the global harvest came from just 20% of croplands that are irrigated. Irrigation has delivered multifaceted social benefits, virtually shielding irrigated communities against the episodes of hunger and famine. The benefits of irrigation are many: irrigation boots productivity and improves food security, generates employment and income, improves nutrition, health and education and fosters human development and enhances equity in favor of the poor households particularly where they have good access to land and water resources and related rural infrastructure and services.

Irrigation delivers major benefits in food security and human development. Poorly managed irrigation can also have unintended environmental consequences and social disbenefits. About 1/3rd of global irrigated land have lower productivity due to poorly managed irrigation causing waterlogging and salinity. Annually about 10 million ha (Mha) are lost to salinisation of which about 1.5 Mha are irrigated lands. Estimates differ widely for various irrigation systems. Cumulative global productivity loss due to land degradation over three decades has been estimated at 12% of total production from irrigated, rainfed and rangeland or about 0.4% per annum. National costs of dryland salinity in Australia are estimated at AUD$ 130 million per annum in lost agricultural production, AUD$ 100 million per annum in infrastructure damage and AUD$ 40 per annum in the loss of environmental assets. Data spanning 1971–93 from India and Pakistan Punjabs show that intensification of land and water resources caused resource degradation, slowing overall productivity growth. For Pakistani Punjab these data show that resource degradation has reduced over all productivity growth from technical change, education and infrastructure investments by 1/3rd. Estimates for the left bank main canal of Tungabhadra project in south west India show that land degradation alone accounted for about 15% of the system’s productive potential. Many of the high-potential irrigated areas such as Punjabs in India and Pakistan and parts of the Yellow River basin in China are now experiencing signs of stagnation in crop productivity growth, over-use of water resources, pest infestations, and buildup of toxic salts—the menace that had undermined the stability of several ancient irrigation societies and that places ours in jeopardy as well. China, India, Iran, and Pakistan are among the countries where a significant share of the irrigated land is now jeopardized by scarce river water, groundwater depletion, a fertility-sapping buildup of salts in the soil, or some combination of these factors, threatening rural industries and livelihoods of millions.

Shallow groundwater salinity induced by irrigation can be managed by keeping irrigation applications below the net recharge—a concept that links the evapotranspiration needs of crops, leaching requirements of soil, and water movement within underlying groundwater systems. Often net recharge within a catchment/basin is unknown or existing knowledge is incomplete, particularly at lower spatial scales such as the farm or paddock; this offers little guidance for on-farm land and water management decisions that together impact basin wide net recharge and waterlogging and salinity dynamics.

This paper developed the concept of net recharge for setting paddock scale recharge targets and to link these to the regional targets and community’s goals for sustainable irrigation management. A management model, cast in a dynamic programming format to integrate a detailed hydrological model with an economic model was applied to estimate the productivity, profitability and sustainability of irrigated agriculture in a region of the southern Murray Darling Basin in Australia. SWAGMAN^® Farm model was used to determine paddock scale net recharge. This interactive model enables an individual farmer to choose a profit optimizing crop mix while lowering net recharge through better irrigation management. A unified action by all irrigators across farms within the catchment can lead to a win-win outcome for all farmers.

The results of biophysical and hudrogeological modelling reported in this study provide evidence of considerable spatial variations in net recharge across different zones within the study area, requiring a spatially targeted approach to address the recharge issue. This implies that achieving overall recharge targets for the irrigation area as a whole alone would be ineffective—one size fits all approaches would be a blunt policy tool that would do more harm than the good, say be eliminating some beneficial recharges/return flows. It also implies that sustainable irrigation management requires a more informed and science-based policy response for effectively targeting the issue. Thus, recharge reduction strategies offer promise in those zones where net recharge occurs; limiting discharge would be desirable for the net discharge zones; and different salinity mitigation strategies would be called for in zones of shallow groundwater and high salt concentration. The modelling results show that on-farm land and water management actions offer the greatest promise to mitigate salinity and waterlogging.

To guide on-farm land and water management decisions, an explicit attempt was made to determine paddock scale net recharge targets. SWAGMAN^® Farm model was used for this purpose. This model enables farmers to determine paddock scale net recharge targets while optimizing profits. But alternatively irrigators optimize productivity and profitability subject to a zero net recharge constraint, promoting sustainable irrigation management. The paddock scale targets can then be linked to overall recharge targets at catchment scale to meet community’s vision for sustainable irrigation management.

The development of paddock scale net recharge metric is perhaps the most significant contribution of this study. The management of paddock scale net recharge target across farms and optimization of profits achieves a win-win outcome for all farmers. The recharge management metric enables the conversion of diffuse source recharge to point source recharge at paddock scale, enabling the conversion of private property rights to a common pool issue and assigning individual responsibilities for its management. To help refine this work, future research should calibrate the model in other irrigation systems facing waterlogging and salinity issues as well as cover additional dimensions such as:

• Hydro-economic ranking of on-farm and regional options for regional groundwater management.

• Hydrological assessment of downstream environmental benefits of net recharge management emanating from promising on-farm and regional groundwater management options.

• Design and testing of market based instruments for sharing environmental costs and benefits among the stakeholders.

This way forward would entail building enduring partnerships between biophysical scientists, economists and other professional as well governments, irrigators and wider community to help promote the development of sustainable irrigation management tools and practices.