Water Erosion Risk Assessment in the Kenya Great Rift Valley Region

Abstract

The Kenya Great Rift Valley (KGRV) region unique landscape comprises of mountainous terrain, large valley-floor lakes, and agricultural lands bordered by extensive Arid and Semi-Arid Lands (ASALs). The East Africa (EA) region has received high amounts of rainfall in the recent past as evidenced by the rising lake levels in the GRV lakes. In Kenya, few studies have quantified soil loss at national scales and erosion rates information on these GRV lakes’ regional basins within the ASALs is lacking. This study used the Revised Universal Soil Loss Equation (RUSLE) model to estimate soil erosion rates between 1990 and 2015 in the Great Rift Valley region of Kenya which is approximately 84.5% ASAL. The mean erosion rates for both periods was estimated to be tolerable (6.26 t ha−1 yr−1 and 7.14 t ha−1 yr−1 in 1990 and 2015 respectively) resulting in total soil loss of 116 Mt yr−1 and 132 Mt yr−1 in 1990 and 2015 respectively. Approximately 83% and 81% of the erosive lands in KGRV fell under the low risk category (<10 t ha−1 yr−1) in 1990 and 2015 respectively while about 10% were classified under the top three conservation priority levels in 2015. Lake Nakuru basin had the highest erosion rate net change (4.19 t ha−1 yr−1) among the GRV lake basins with Lake Bogoria-Baringo recording annual soil loss rates >10 t ha−1 yr−1 in both years. The mountainous central parts of the KGRV with Andosol/Nitisols soils and high rainfall experienced a large change of land uses to croplands thus had highest soil loss net change (4.34 t ha−1 yr−1). In both years, forests recorded the lowest annual soil loss rates (<3.0 t ha−1 yr−1) while most of the ASAL districts presented erosion rates (<8 t ha−1 yr−1). Only 34% of all the protected areas were found to have erosion rates <10 t ha−1 yr−1 highlighting the need for effective anti-erosive measures.

A Review on Challenges and Opportunities in Management of Soils of Arid and Semi-Arid Regions of Kenya

“…Unfortunately, this has continued unabated, leading to formation of large gullies spread across many ASALs counties (Ongwenyi et al, 1993). These are Samburu, Laikipia, Tana River, Kajiado, Narok, Kilifi, Mandera, Kitui, Makueni, Garissa, Baringo, Wajir, and West Pokot (Sigunga et al, 2011;UNDP, 2013;Konana, 2017;Peterson et al, 2018;Watene et al, 2021). Despite erosion affecting both low (arid to semi-arid) and high potential (humid to sub humid) counties, it is more pronounced in the former (ASALs) (Kiome & Stocking, 1995;Ovuka, 2000, Mulinge et al, 2016Watene et al, 2021).…”

“…These are Samburu, Laikipia, Tana River, Kajiado, Narok, Kilifi, Mandera, Kitui, Makueni, Garissa, Baringo, Wajir, and West Pokot (Sigunga et al, 2011;UNDP, 2013;Konana, 2017;Peterson et al, 2018;Watene et al, 2021). Despite erosion affecting both low (arid to semi-arid) and high potential (humid to sub humid) counties, it is more pronounced in the former (ASALs) (Kiome & Stocking, 1995;Ovuka, 2000, Mulinge et al, 2016Watene et al, 2021). ASALs are said to be more susceptible to water erosion leading to loss of vegetation cover, hence contributing to increased desertification in Kenya (Watene et al, 2021).…”

“…Despite erosion affecting both low (arid to semi-arid) and high potential (humid to sub humid) counties, it is more pronounced in the former (ASALs) (Kiome & Stocking, 1995;Ovuka, 2000, Mulinge et al, 2016Watene et al, 2021). ASALs are said to be more susceptible to water erosion leading to loss of vegetation cover, hence contributing to increased desertification in Kenya (Watene et al, 2021).…”

“…In most cases, researchers opt for field measurements method which is expensive and labour intensive (Gitonga, 1994). Gully erosion could be contributing to massive soil loss compared to sheet and rill erosion which has been predicted to contribute to soil loss at the rate of 1 -40 t ha -1 yr -1 in Kenya, depending on soil type, slope, and vegetation cover (Kogo et al, 2020;Watene et al, 2021).…”

“…Gully erosion is accelerated by a multi-interaction of various factors such as unfavourable and ineffective policies regarding land ownership, sand mining, changes in land use and cover, road construction without appropriate drainage channels, inappropriate discharge of water from roads or buildings such as markets and schools, urban centres, deforestation and through indiscriminate tree logging or bush burning, vegetation loss through prolonged droughts or over-grazing, steep slopes, over-stocking, foot and livestock paths, heavy rainfall intensities and intensive farming (Rowntree, 1991;Gitonga, 1994;Ovuka, 2000;Jungerius et al, 2002;Waswa et al, 2002;Konana et al, 2017;Asuoha et al, 2019;Hassen & Bantider, 2020;Watene et al, 2021). In communally owned land especially group ranches, agro-pastoralist farmers are not motivated to engage in the expensive and time-consuming rehabilitation activities (Njoka et al, 2016).…”