17:15 〜 18:30
[HGG01-P04] 中央ケニア半乾燥地域の土壌浸食と採砂活動を取り巻く土地環境および社会環境条件
キーワード：採砂、土壌浸食（土壌侵食）、ガリー、半乾燥、ケニア、環境地理学
The objectives of our presentation are to clarify land environmental condition, mainly on the basis of geomorphological and physical geographic aspects, and social environmental one, surrounding soil erosion and people's sand harvesting in the semi-arid pastoral area, Laikipia North sub-county, central Kenya, as a continuation of our presentation in 2020 JpGU.
The investigated area, the Il Polei sub-location (N 0°21'56", E 37°04'32"), has an altitude of 1,750 to 1,850 m. According to previous literatures, a mean annual rainfall at the Mukogodo Station, close to the study area, is 362 and/or 371 mm; tree coverage is extremely low, which comprises sparse woods and shrub consisting mainly of Acacia genus. The area is underlain by Proterozoic gneiss, migmatite, quartzite, and schist, belonging to the Mozambique Belt. Geomorphologically, inselberg-pediment systems are regionally identifiable with widespread distribution of pediplain.
Below piedmont angle of the system, near the center of sub-location, 1.5- to 2-km long gullies exist on the pediment. According to our topographic measurements since March 2015 until February 2020, the maximum annual retreat rates range from 1.1 to 25.6 m/yr. Sheet erosion extensively predominates in the whole area, related to the gullying. Recent precipitation during the period, approximately six months, from the earliest September 2019 to latest February 2020, including one of the two rainy seasons a year, reaches 218.3 mm from our observation. It turned out that there were seven principal, relatively intense rainfall events in this period, attaining a total of more than 10 mm within each event, in which sheetwash and rill wash were presumed to occur. Particularly, the four principal events, accompanied by the most intense (33.4 mm in 11 hours) and other small, frequent ones, occurred during a relatively short period, from late November to mid-December, and the ground conditions continued, which were abundant in soil moisture and prone to retreat of gully heads.
In this area, local people manage sand harvesting for construction demands in urban cities, while building cooperated society based on group ranches. Our data show that the amount of sand harvesting has increased sharply since the beginning of 2018, and has doubled or tripled since before 2017. Although cash is distributed within the group ranches by the sand harvesting permit, the rapid increase in the amount of cash invested locally may have disturbed the balance with the household income from original major livelihood, livestock and farming, especially in recent years.
At present, it seems that sand harvesting has not significantly contributed to gully erosion; however, direct mining at gully bottom and river floor due to the rapid increase in construction demands, as well as increasingly serious illegal mining, can be linked to land degradation. In addition, this presentation will show information on the state of the cooperated society and the maintenance process of sand harvesting management.
The investigated area, the Il Polei sub-location (N 0°21'56", E 37°04'32"), has an altitude of 1,750 to 1,850 m. According to previous literatures, a mean annual rainfall at the Mukogodo Station, close to the study area, is 362 and/or 371 mm; tree coverage is extremely low, which comprises sparse woods and shrub consisting mainly of Acacia genus. The area is underlain by Proterozoic gneiss, migmatite, quartzite, and schist, belonging to the Mozambique Belt. Geomorphologically, inselberg-pediment systems are regionally identifiable with widespread distribution of pediplain.
Below piedmont angle of the system, near the center of sub-location, 1.5- to 2-km long gullies exist on the pediment. According to our topographic measurements since March 2015 until February 2020, the maximum annual retreat rates range from 1.1 to 25.6 m/yr. Sheet erosion extensively predominates in the whole area, related to the gullying. Recent precipitation during the period, approximately six months, from the earliest September 2019 to latest February 2020, including one of the two rainy seasons a year, reaches 218.3 mm from our observation. It turned out that there were seven principal, relatively intense rainfall events in this period, attaining a total of more than 10 mm within each event, in which sheetwash and rill wash were presumed to occur. Particularly, the four principal events, accompanied by the most intense (33.4 mm in 11 hours) and other small, frequent ones, occurred during a relatively short period, from late November to mid-December, and the ground conditions continued, which were abundant in soil moisture and prone to retreat of gully heads.
In this area, local people manage sand harvesting for construction demands in urban cities, while building cooperated society based on group ranches. Our data show that the amount of sand harvesting has increased sharply since the beginning of 2018, and has doubled or tripled since before 2017. Although cash is distributed within the group ranches by the sand harvesting permit, the rapid increase in the amount of cash invested locally may have disturbed the balance with the household income from original major livelihood, livestock and farming, especially in recent years.
At present, it seems that sand harvesting has not significantly contributed to gully erosion; however, direct mining at gully bottom and river floor due to the rapid increase in construction demands, as well as increasingly serious illegal mining, can be linked to land degradation. In addition, this presentation will show information on the state of the cooperated society and the maintenance process of sand harvesting management.