Lesotho (from Stromquist et al. 1985)

The small country of Lesotho in southern Africa has historically been intensively grazed which has caused accelerated levels of soil erosion. Surface and gully erosion rates in a 4.9 km² watershed in Lesotho were documented with aerial photographs and ground measurements. Gully eroded areas increased by 260,000 m² between 1951 and 1980. Over that period gully erosion caused (1) a roughly 5% reduction in productive land area of the watershed, and (2) significant contributions of sediment to a downstream reservoir. The sediment delivered to a downstream reservoir was 300,000 t and 80,000 t, respectively from gully and surface soil erosion. The trapped sediment within the reservoir had a corresponding volume of about 267,000 m³.

Morocco (from Tayaa and Brooks 1984, Tayaa and Bazza 1992, 1993)

Mean annual precipitation averages less than 250 mm/yr in Morocco, but ranges from less than 50 mm/yr in the far south and southeast to more than 1,000 mm/yr in the highest elevations of the Rif, and the Middle and High Altas mountains. Nearly 90% of the country receives less than 250 mm/yr. Because of this uneven distribution of precipitation, reservoirs have been constructed near these high elevation zones to provide irrigation water for the drier areas.

Existing and planned reservoir projects, however, are threatened by excessive soil erosion in many parts of Morocco. Land use has a major effect on soil erosion and the resulting sedimentation rates to reservoirs, as illustrated below.

Characteristic	Watershed/Reservoir
	Loukos	Tleta	Nekor	Nakhla
Area (km2)	1,820	178	780	110
Percent area in --Agriculture --Range --Forest	47 32 21	48 42 2	45 55 0	32 27 41
Reservoir capacity (x 106 m3)	710	43	45	32
Sedimentation at reservoirs --Average rage (t/ha/yr) --% of the total capacity	43 10	220 15	79-285 43	62 36

It is estimated that excessive soil erosion and sedimentation, as illustrated above, threatens the storage capacity of the 34 existing large reservoirs in Morocco. These projects have a total storage capacity of 10 billion cubic meters which provides irrigation water for 600,000 hectares and generates 660.4 MW of hydroelectric power. It is estimated that 50 million m³/year is being lost (0.5 % of the total design storage capacity of the existing dams) corresponding to an average loss of 6000 ha of irrigated area each year. Under the prevailing sedimentation conditions, it is expected that 60 million Kwh of hydroelectric power and 40 million m³ of drinking and industrial water will be lost by the year 2010. To date, a total of more than 820 million m³ of the total storage capacity have been lost.

In addition to the impacts on reservoirs, excessive soil erosion in many parts of Morocco is causing the productivity of soils to decline. Annual loss of soil nutrients in agricultural areas located northwest of the Rif mountains is equivalent to $US20/ha/yr. This represents about 90% to 180% of what local farmers invest yearly in fertilizers.

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