Land Degradation as a Natural Hazard

This type of land degradation occurs because of topographic (changes that occur in the Earth structure) or climatic factors.

Land degradation through soil: Much arable land has lost its productive capacity because of soil degradation, which has two components; 1) soil erosion, which is the physical wearing away of the fertile soil surface attribuTable to the combined affect of wind and water and 2) a decline of soil fertility. Other major soil degradation factors include the improper use of marginal quality (saline/brackish) water for irrigation purposes leading the productive soils to become marginal (saline and saline-sodic). Soil degradation is a process that is related to

biophysical activities and is aggravated by socioeconomic and political factors for different reasons (Lal 2001). Once the soil is degraded, it requires high inputs for crop production and hence leads to a high cost/benefit ratio.

Soil erosion by water: The disruption of the soil surface through rain drop splash impact and the subsequent removal of the soil by flowing water is an example of rain-induced water erosion. In arid and semi-arid regions, rain plays an important role in agriculture; however, its intensity and distribution throughout the year is important in crop production. World Meteorlogical Organization explains that high intensity rainfall creates problems for cultivation and causes severe floods that wash away good fertile soils; therefore, rainfall and its intensity plays a vital role (WMO 2005: 12). Water erosion is common in some countries, e.g., in Nepal, heavy rains have adverse effect on rain-fed slopes (Acharya, Tripathi and Donald 2002). Water erosion is of three types: sheet, rill, and gully erosion. Sheet erosion is common in flat plains, whereas rill and gullies are common in sloping landscapes.

Soil erosion through wind: Wind erosion is common in sandy desert areas, such as when landscapes are composed of sand sheets and sand dunes. Abdelfattah (2009: 1) has described the situation in desert-containing part of the Arab Emirates, and in Dubai, this type of soil erosion is the main reason for land degradation.

Soil Fertility: Because of intensive agricultural activities and poor replenishment of plant nutrients, soils sometimes become infertile and decline in their production capacity. The loss of productive surface soil through erosion also reduces the biological activities of soil, and the loss of organic matter ultimately affects soil fertility. Soil slope, texture, and surface and subsurface features play important roles in soil reclamation and the rehabilitation of degraded lands. Some aspects of land degradation are easily reversible, and some are difficult to return back to their original capacity. For example, the removal of the total top soil cover through wind or water erosion is irreversible, whereas a shortage of some nutrients can be recovered (Coxhead and Oygard 2007: 2).

Waterlogging: When the ground water Table becomes so high that excess water stagnates on the surface of land or in the roots of the plants, this is known as waterlogging. This occurs in those areas where water cannot penetrate deeply because of an excess amount of subsurface water or the presence of some hard subsurface layer (clay pan, hardpan) that restricts downward water movement. Waterlogging is evident in low lying areas where hydrological flow causes water logging in depressions and ponds are created, or in those areas that are

prone to intermittent floods. Waterlogged conditions are not conducive for agricultural activities and are deadly for plants. According to the strategic plan 2007-16 of Forum of Agriculture Research in Africa (FARA) most of the large scale irrigation systems established to enhance the productivity of the agriculture sector have failed to give the required results;

they are even contributing some serious natural hazards such as waterlogging and are the cause of the damage of agricultural land.

Salinity: This is a measure of the concentration of all the soluble salts in soil or water. Arid and semi-arid zones receive inadequate and irregular precipitation to accomplish the leaching of salts originally present in the soil profile. Normally, when the precipitation is more than 1000 mm per annum, salinity should not develop. This is not the case in arid zones; therefore, salts accumulate in soils. Salts building up in concentrations detrimental to plant growth is a constant threat in irrigated crop production. In arid and semi-arid regions, evapotranspiration is higher than the total annual rainfall. Therefore, rainfall contributes insignificantly to groundwater recharge, and hence, there is general shortage of fresh quality water to offset the total agriculture water demand in these countries. The shortage of fresh water necessitates the use of marginal quality ground water, such as brackish and saline water, for irrigation purposes. This is highly demanded in water-scarce regions. The improper use of saline/brackish water in irrigated agriculture often introduces salinity and sodicity problems, and the soil, if not properly managed, can reach a condition in which it cannot be exploited to its full production capacity. Under such conditions, irrigated agriculture has faced the challenge of sustaining its productivity for centuries; in particular, soil and water salinity, poor irrigation, and drainage management continue to plague agriculture, especially in arid and semi-arid regions (Tanji1996). If soil becomes saline and sodic, its quality becomes poor creating plant- and soil-related problems, with many plants either failing to grow in saline soils or their growth being retarded significantly; therefore, soil salinity often restricts options for cropping in a given area. Australia suffers from this kind of degradation as sixty eight percent of its total land is affected by this white plague (WMO 2005: 8).

Sodicity: This is a measure of sodium ions in soil or water relative to calcium and magnesium ions (Richards 1954). It is expressed either as the Sodium Absorption Ratio (SAR) or as the Exchangeable Sodium Percentage (ESP). If SAR of the soil is equal to or greater than thirteen or ESP is equal to or greater than fifteen, the soil is termed sodic (Richards 1954: 4).

Accumulation of excess Sodium on the soil exchange complex causes adverse effects on soil

structure and enhances concentration of Hydrogen ion in soil (pH) and soil erosion. High ESP also affects plant growth because of imbalances in plant nutrition, causing Na-induced nutrient deficiencies of several nutrients (Qadir and Schubert 2002: 276). Soil sodicity is a major constraint to Pakistani agriculture where sixty per cent of the salt-affected soils are affected by various levels of soil sodicity. The reclamation of sodic soils is a laborious, time consuming, and costly task. In Pakistan, the reclamation of sodic soils is usually performed with gypsum as a supplement. The gypsum on dissolution introduces calcium, which replaces Sodium from the soil exchange complex and reduces ESP levels. India has severe problem of soil sodicity. Morocco, in the southern Mediterranean region, has limited growth of vegetation and crop yield attribuTable to this natural problem in its agricultural land, mainly because of the over-utilization of ground water and land for more agricultural output (Bannari et al.

2008).

Soil Burial: In some countries in which floods are common, fertile soils are covered by new sediment brought by the floods; however, in many cases, these sediments bring good quality material, such as clay and silt, which improve soil structure and nutrient holding capacity. In sandy desert conditions, wind also plays a role in the burial of soil, and sand can deluge grazing land (UN/FAO 1994).

Impact of climate change (CC) on land degradation and agriculture: Climate change will affect rainfall amounts, frequency, patterns, and duration (rainfall becomes less reliable) leading to increased floods, hurricanes, storms, and drought (leading to water and food shortages). The green-house effect (GHE) will increase evapotranspiration, and thus crop water demand will definitely increase, leading ultimately to changes in cropping patterns and declines in yields. Immediate impacts will be on dryland farming in Africa, specifically in Ethiopia where less than one per cent of the total cultivated lands are irrigated, and the rest is rain-fed; therefore, the dry areas are likely to become even drier and will be too hot for certain crops. By 2020, yields from rain-fed agriculture in some African countries are projected to decline up to fifty percent, thereby increasing food insecurity and hunger. Seventy five to two hundred and fifty million people are predicted to be exposed to water stress attribuTable to climate change. In sub-Sahara Africa, the combination of historical crop production and weather data into a panel analysis has predicted a decline in the yield of maize, sorghum, millet, groundnut, and cassava by 22, 17, 17, 18, and 8 percent, respectively, by 2050 (Burke

et al. 2009: 4).