In a surprising new study, researchers have found that drylands are not getting drier. The researchers discovered that soil moisture exerts a negative feedback on surface water availability in drylands, which offsets some of the anticipated decline.
What are Drylands?
- Drylands are areas which face great water scarcity. They cover over 40% of the earth's land surface, and are home to more than two billion people.
UNEP on drylands
- The United Nations Environment Programme (UNEP) defines drylands according to an aridity index (AI). Drylands are lands with an AI of less than 0.65.
- Aridity Index is the ratio between average annual precipitation and potential evapotranspiration.
- Drylands are further divided, on the basis of AI, into:
- hyper-arid lands
- arid lands
- semi-arid lands
- dry sub-humid lands
- Characteristics: Drylands are characterised by:
- low, erratic, and infrequent rainfall
- limited water resources
- low soil moisture
- high evapotranspiration which results in water deficit
- They are highly adapted to climatic variability and water stress, but also extremely vulnerable to damaging human activities such as deforestation, overgrazing and unsustainable agricultural practices, which cause land degradation.
- Evapotranspiration is the process of transferring moisture from the earth into the atmosphere.
- Put simply, evaporation occurs when water vapor leaves the soil or a plant’s surface.
- Transpiration involves the passage of water through a plant, from its roots through its vascular system.
- The sum of evaporation and transpiration is evapotranspiration (ET).
Distribution of drylands
- Drylands are found on all continents, and include grasslands, savannahs, shrublands and woodlands.
- Geographically dryland agriculture area in India includes:
- the north western desert regions of Rajasthan
- the plateau region of central India
- the alluvial plains of Ganga Yamuna river basin
- the central highlands of Gujarat, Maharashtra and Madhya Pradesh
- the rain shadow regions of Deccan in Maharashtra
- the Deccan Plateau of Andhra Pradesh
- the Tamil Nadu highlands
Dryland degradation assessment
- Land degradation or desertification is the loss of the biological or economic productivity of land.
- Desertification reduces agricultural output, contributes to droughts and increases human vulnerability to climate change.
- The loss of biodiversity in drylands, including bacteria, fungi and insects living in the soil, is one of the major causes and outcomes of land degradation.
- The main processes of soil degradation associated with desertification may be summarised as follows:
- increased land pressure leads to local loss of vegetation cover and increased area of bare patches. Removal of crop (residues) for fuel or fodder reinforces this trend.
- direct exposure of top soil to solar radiation increases soil temperature and the rate of organic matter decomposition.
- loss of organic matter causes soil structural degradation (porosity, aggregate stability), reduces water holding capacity, decreased infiltration and increased runoff.
- decline in organic matter content decreases nutrient storage properties. Nutrients are lost by percolation and the efficiency of chemical fertilisers is reduced.
- impact of rain and sun on bare topsoil results in crusting; water infiltration is further reduced, and percentage of runoff increases.
- sediments are carried away by erosion. Effective soil depth accessible to plant roots decreases, leaving exposed restrictive soil layers or bare rock.
- exposed soil is eroded by wind, crops are destroyed by dust bearing winds (off-site effects), and dunes may encroach on arable land.
- in the worst-case scenario, gradually degraded patches link up to form extended areas of bare and degraded land. At this stage, reclamation becomes virtually impossible.
Effect of climate change on drylands
- Drylands are particularly affected by climate change through changing rainfall patterns and land degradation, which reduces the ability of species and people to cope with dryland conditions.
- About 20-35% of drylands already suffer some form of land degradation, and this is expected to expand significantly under different emission scenarios.
- Soil erosion is one of the more significant causes of land degradation in drylands, resulting in the loss of soil organic carbon present in roots and woody components of the soil, and the subsequent loss of land productivity.
The false assumption
- Scientists have thought that global warming will increase the availability of surface water — freshwater resources generated by precipitation minus evapotranspiration — in wet regions, and decrease water availability in dry regions.
- This expectation is based primarily on atmospheric thermodynamic processes.
- As air temperatures rise, more water evaporates into the air from the ocean and land.
- Because warmer air can hold more water vapor than dry air, a more humid atmosphere is expected to amplify the existing pattern of water availability, causing the “dry-get-drier, and wet-get-wetter” atmospheric responses to global warming.
Key-findings of the Study
- The study discovered that soil moisture exerts a negative feedback on surface wateravailability in drylands, which offsets some of the anticipated decline.
- Soil moisture’s influence on evapotranspiration and wind patterns could help to ease the loss of surface water in arid areas.
- The study provides an exception to the “dry-get-drier, and wet-get-wetter” rule.
- This new study is the first to show that long-term soil moisture changes and feedbacks between soil moisture and the atmosphere play an important and previously underestimated role in these predictions about the future of drylands.
- The researchers found that long-term soil moisture helps to regulate atmospheric circulation and moisture transport.
- These effects largely ameliorate the potential decline of future water availability in drylands.
- Although drylands will continue to become drier with climate change, the effect would be much worse without the feedbacks (soil moisture).
Why are drylands important?
- Support population: Drylands are home to more than a third of the world’s population – many of whom are the poorest of the poor. Whilst drylands also support some of the world’s biggest cities, such as Mexico City and New Delhi.
- Support biodiversity: Drylands support an impressive array of biodiversity.
- This includes wild endemic species and cultivated plants and livestock varieties known as agrobiodiversity.
- Biodiversity in drylands also includes organisms which live in the soil, such as bacteria, fungi and insects, known as soil biodiversity, which are uniquely adapted to the conditions.
- Food and water provision: Low precipitation and prolonged dry seasons in drylands can lead to water scarcity, and limit agricultural productivity and output. Drylands biodiversity maintains soil fertility and moisture to ensure agricultural growth, and reduces the risk of drought and other environmental hazards.
- Climate change mitigation and adaptation: Biodiversity in drylands has adapted over millennia to the seasonality, scarcity and variability of rainfall, and can be useful in helping people adapt to climate change.
- Important commodities: Drylands also produce a number of globally important commodities such as gum arabic, frankincense, and cashmere.
This study’s findings underscore the urgent need to improve future soil moisture predictions and accurately represent soil moisture-atmosphere feedbacks in models, which are critical to providing reliable predictions of dryland water availability for better water resources management.