Desertification - Concept, Causes and Amelioration

Desertification is a condition of human-induced land degradation that occurs in arid, semiarid and dry sub-humid regions (precipitation/potential evapotranspiration or P/ETP 0.05 to 0.65) and leads to a persistent decline in economic productivity (> 15% of the potential) of useful biota related to a land use or a production system. Climatic variations intensify the decline in productivity, restorative management mitigates it. Drylands or territories susceptible to desertification occupy 39.7% (~5.2 billion ha) of the global terrestrial area (~ 13 billion ha). The highest concentration of drylands occurs in Africa, Asia and Australia. Two out of every three hectares of drylands suffer from land degradation of one kind or another. Barring 78 M ha which are irreversibly degraded, the remainder area -affected by desertification -is reclaimable at a price. Desertification is caused primarily by over-exploitation of natural resources beyond their carrying capacity. Solutions to combat desertification lie in the management of the causes of desertification. However, there are no easy options to combat it. While managing demographic pressure should receive priority, the solutions to combat desertification involve local action, guided by land use and climatic conditions and in harmony with local needs and people’s expectations. Drylands are used as rangelands or as croplands, with the latter either irrigated or rainfed. Integrated data on land and soil degradation and on the socio-economic environment within which it occurs are the basis to formulate strategies for reclamation and proper use of drylands. Rangelands constitute the dominant land use (est. 88%) in the territories susceptible to desertification. Of the 3333 M ha rangeland area affected by land degradation 757 M ha are severely affected., 72 M ha irreversibly. Within rangelands, vegetation degradation is the primary cause of desertification –it represents 72% of the total area desertified worldwide (2576 M ha out of 3592 M ha). Overgrazing by excessive numbers of low productivity livestock and fuel wood extraction by man are the principal causes of vegetation degradation. Centralized management of common rangeland resources and insecure tenancy laws stand in the way of communities and herders adopting a long-term view to conserve and invest in range improvement measures. Inadequate dissemination of knowledge on vegetation improvement methods is another cause of rangeland degradation. Five suggestions are made to assure sustainability and effectiveness of rangeland management programs: (1) shifting to community management of rangelands that have been nationalized, (2) granting formal rights to individual transhumance herders that have been settled, (3) providing education and training on range management and improvement, (4) introducing elite breeds of livestock for high productivity, (5) implementing programs for harnessing alternative sources of energy for cooking (solar and biogas).Rainfed croplands occupy an area of 457 M ha, 216 M ha of which have degraded soils. Some 4 M ha suffer from irreversible degradation. Of the remainder, 29 M ha and 183 M ha are, respectively, affected by severe (reclaimable with engineering works) and moderate degradation. Soil constraints in rainfed croplands arise primarily from their vulnerability to erosion, which leads to loss of organic matter, fertility and rooting depth. Eroded soils are structurally unstable and are prone to crusting and compaction. Risk arising from drought susceptibility and poverty limit the adoption of restorative management. Rainwater conservation to minimize risk is not adopted due to insecure tenancy and centralized management of government supported programs. A lack of adequate knowledge and skills of efficient use and storage of rainwater allow degradation processes to proceed unchecked. The imperatives to succeed are: (1) land tenure policies towards freehold ownership; (2) community participation in the management of rainwater, (3) efficient use of harvested water supported by high value land use options built on indigenous knowledge and (4) government support to facilitate the development of rainfed agriculture. Irrigated croplands occupy an area of 145 M ha. Of this, 2 M ha are affected by irreversible degradation and 41 Mha suffer from reversible degradation, mainly from salinity and waterlogging. The mechanisms of salinity development differ and so do the solutions when canal or underground water is used for irrigation. With canal water irrigation, three key development options are suggested to remove excess salts and water and to minimize conveyance and application losses of water: (1) effective drainage, (2) properly lined or closed water conveyance systems and efficient irrigation techniques, and (3) participatory management of irrigation systems. The costs of installing drainage and leak-proof conveyance systems are high, but so are the economic and ecological gains. With underground water use, salinity develops as the water reserves are depleted due to over-extraction. While efficient methods of irrigation can help in postponing the occurrence of salinity, sustainable solutions lie in balancing the water withdrawals with recharge. Efforts should therefore be made to promote groundwater replenishment through runoff harvesting. Although it is not always possible to recharge the deep aquifers with the limited quantities of runoff produced by the low annual precipitation, still, the use of harvested runoff for irrigation can save groundwater. Once water-efficient systems are operational, cropping systems that maximize productivity per unit of water can be introduced. The entire strategy of reclaiming desertified land revolves around water, the reestablishment of the vegetation of rangelands, the rejuvenation of the productivity of rainfed croplands, and the halting of loss of irrigated farmlands. Humans play a central role in that strategy; desertification begins and ends with human action. Unless it ends, the estimated 900 million people affected today will grow to billions tomorrow.