Drought is a critical lack in water supply or availability. Causes include deficiency in precipitation, dry seasons and El Niño, and human activities which degrade and exhaust the natural systems of the planet such as deforestation, over-farming and excessive water extraction. Deforestation, over-grazing and excessive irrigation exacerbate soil degradation via erosion, excessive salinity, panning, leaching, and defoliation, subsequently this can bring desertification - which is excessive or total vegetation loss and complete exhaustion of the soil.
Many plants have a high degree of natural drought resistance, most easily identified by physical features such as a waxy cuticle (thick leathery leaves), a reduced number of leaf pores (stomata) succulent leaves, or none at all - where the stem takes over photosynthetic functionality, notably thicker stems and deep or wide root systems. Drought resistant species can be employed to facilitate restoration agriculture techniques which include : re-vegetation using specific plant assemblages, keyline design and rotational and zonal grazing strategies. These approaches applied to an area of land, can act to restore it to a state where it can be agriculturally productive and maintain the resilience, stability and diversity of a natural ecosystem again.
Food producing, perennial plants and trees, with drought resistant properties are increasingly recognised for their suitability in increasing community resilence and sustainability in drought affected areas. This approach; employing ecosystem services and natural processes, is proving to support a greater diversity of food crops, compared to more industrial approaches (such as greenhouses, irrigation and chemical fertilisation) which can have long-term negative effects such as excess soil salinity and salt panning, and much higher water consumption, both contributing to drought, devegetation and desertification.
Permacultural techiniqes have been developed, tested and continue to be advanced in the Jordanian dessert by Geoff Lawton and his team, for instance the 'Greening the Desert' project sites. Governments in drought-affected regions increasingly adpot the same or simlar strategies and invest in developing revegetation strategies for areas severely affected by drought and desertification.
Water retention techniques, such as swales and a series of catchments can reduce surface runoff, whilst increasing abosorption of water into the soil. This has the effect of increasing the level of the water table and subsequently allows for a greater range of vegetation to propser. Building soil and soil cover, through mulching and vegetalisation acts to (re)establish natural ecosystemic resilience to the causes and effects of drought.
By reducing the exposure of bare soil to direct sunlight, the effects of drought are reduced, this is achieved by establishing a surface covering of mulch material, working to achieving 100% vegetation cover and developing a tree canopy cover - effectively speeding up biological succession. Establishing vegetative layers from ground-covers, up to and including trees, re-clothes the earth and prevents scorching and soil degradation.
By increasing the levels of organic content; creating humus in the soil, moisture retention is heightened enabling vegetation to be more resilient to extended periods of drought. The shelter (shading and windbreak) afforded by increasing the vegetation cover, reduces soil moisture evaporation, and reduces surface heating caused by solar gain and reflection. The tree cover creates shade allowing surface air to cool. Vegetation absorbs, deflects and prevents direct sunlight heating the ground, rocks and other stone and heat retentive structures, which contribute to continued heating and parching of the land. Windbreaks reduce the extent of wind evaporation. The combination of deliberate land and planting interventions acts to stabilise evapotranspiration, reduce the negative effects of runoff and improve groundwater recharge in drought-affected areas.