Introduction

tte pressure of increasing world population demands for higher crops yields from the finite area of productive agricultural lands. Meeting the needs especially in developing countries through more intensive use of existing agricultural lands and expansion into more marginal lands will substantially increase erosion, ttere is an urgent need to take preventive and control measures to mitigate the threat to global food security, ttese concerns are supported by a report by El Swaify (1994) that the annual rates of soil erosion can often range between 20 to over 100 t ha4, which results in about 15-30 per cent annual decline in the soil productivity. An estimated loss of about 6 million ha annually is estimated as a result of degradation by erosion and other causes (Pimental et al. 1993). tte data for these estimates are often selective from small scale studies conducted over short time periods, however, this does draws attention to the increasing problem of soil loss. In the western world the loss in productivity from erosion may be masked or compensated by increased costly and efficient management practices such as improved crop varieties, fertilizer, pesticides, and irrigation. Even under these management practices, soil erosion has continued and sediment loss has become a very costly factor in the overall picture.

tte processes of water erosion are closely linked to the pathways taken by water in its movement through vegetation cover and over the ground surface. During a rainstorm, part of the water falls directly on the soil, because there is no vegetation or because it passes through gaps in the plant canopy, tte rain falling directly on the soil surface can potentially produce rain splash erosion. Rain intercepted by the vegetation may either evaporate or drip down the plant to the soil surface, tte rain and intercepted water reaching the soil may infiltrate contributing to the soil moisture storage. However, when the soil is either saturated with water or surface conditions prevent infiltration, the excess contributes to runoff on the surface, resulting in erosion by surface flow causing rills and gullies, tte infiltration rates are controlled by the soil characteristics such as the water holding capacity and hydraulic conductivity, surface dryness, the rate of rainfall, land slope and soil management practices. Higher rates of runoff from eroded surfaces wastes valuable moisture—the principal factor limiting productivity in arid lands.

tte agrometeorological coping strategies would first require the study and evaluation of the causes of water erosion, runoff and soil loss for the area of interest, tte erosion process begins at the local level, but requires evaluation at the land scape and regional levels to adapt effective measures to minimize and control the erosion and soil loss. Soil runoff models are used to study the temporal and spatial extent of erosion and soil loss, tte EPIC model (Williams 1995) is one example used extensively in the U.S. and internationally to study the process and extent of erosion at local and watershed levels. Data from satellite remote sensing can help in defining the channel flow of surface water based on the digital elevations imagery maps and also other surface characteristics such as landuse and surface roughness.

Soil erosion is a three stage process: (1) detachment, (2) transport, and (3) deposition of soil. Different energy source agents determine different types of erosion. ttere are four principal sources of energy: physical, such as wind and water, gravity, chemical reactions and anthropogenic, such as tillage. Soil erosion begins with detachment, which is caused by break down of aggregates by raindrop impact, sheering or drag force of water and wind. Detached particles are transported by flowing water (over-land flow and inter-flow) and wind, and deposited when the velocity of water or wind decreases by the effect of slope or ground cover, ttree processes - dispersion, compaction and crusting - accelerate the natural rate of soil erosion, ttese processes decrease structural stability, reduce soil strength, exacerbate erodibility and accentuate susceptibility to transport by overland flow, interflow, wind or gravity, ttese processes are accentuated by soil disturbance (by tillage, vehicular traffic), lack of ground cover (bare fallow, residue removal or burning) and harsh climate (high rainfall intensity and wind velocity).

tte effects of erosion and soil loss on soil properties vary by soil series, management, landscape position and climate. In general, soil erosion affects the chemical properties by loss of organic matter, plant nutrients, and exposure of subsoil materials with low fertility or high acidity (Olson et al. 1999). tte changes in physical properties of soil, include structure, texture, bulk density, infiltration rate, rooting depth, and available water-holding capacity (Frye et al., 1982). tte mineralogical properties of soils are also affected by the thinning of the plow layer (Ap horizon) and subsequent mixing of the subsoil (B horizon) into the Ap horizon by tillage. Eroded soils are subject to higher temperatures, have lower porosity and microbial activity.

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