Gully and soil and water conservation structure densities in semi‐arid northern Ethiopia over the last 80 years

Gullies have been a common phenomenon in semi‐arid northern Ethiopia for the last centuries. However, soil and water conservation (SWC) structures have been implemented for a long time to curb soil erosion. Though, like most of the affected areas worldwide, density and distribution of gullies and SWC structures, their causes and interrelations are poorly understood. The aims of this study were to develop a technique for mapping these densities of gullies and SWC structures, to explain their spatial distribution and to analyze changes over the period 1935–2014. Aerial photographs from 1935 to 1936 and Google Earth images from 2014 of the 5142 km² Geba catchment were used. Transect lines were established to count gullies and SWC structures in order to calculate densities. On average, a gully density of 1.14 km km^?2 was measured in 1935–1936 of which the larger portion (75%) were vegetated, indicating they were not very active. Over 80 years, gully density has significantly increased to 1.59 km km^?2 with less vegetation growing in their channel, but 66% of these gullies were treated with check dams. There was c. 3 km km^?2 of indigenous SWC structures (daget or lynchets) in 1935–1936 whereas a high density (20 km km^?2) of introduced SWC structures (mainly stone bunds and terraces) were observed in 2014. The density of gullies is positively correlated with slope gradient and shrubland cover and negatively with cropland cover, whereas the density of SWC structures significantly increased with increasing cropland cover. Density maps of gullies and SWC structures indicate sensitive areas to gully formation and priority areas for the implementation of SWC structures in Geba catchment. The obtained results illustrate the feasibility of the methods applied to map the density of gullies and SWC structures in mountainous areas. Copyright © 2018 John Wiley & Sons, Ltd.     

Evaluation of soil physical properties of long-used cultivated lands as a deriving indicator of soil degradation,north Ethiopia

Soil degradation causes low land productivity. To tackle soil degradation, soil management practices have been implemented in the study area. However, less attention has been given to the management of physical soil quality. Hence, the objective of this study is to evaluate soil physical properties of long-used cultivated lands. Twelve Land Mapping Units (LMUs) were identified by overlaying slope and soil maps. Twelve composite and 12 undisturbed soil samples were collected from the 12 LMUs, and soil physical properties analyzed. Soil bulk density varied from 1.22 g cm^?3 in LMU3 to 1.68 g cm^?3 in LMU4. Available water capacity ranged from 0.09 in LMU4 to 0.17 in LMU3. Stability index (SI) values ranged from a low of 3.58 at LUM10 to 62.5 at LMU3; stability quotient (SQ) values ranged from 79.4 at LMU9 to 2782.8 at LUM3. Highest and lowest soil crust index values were found to be 1.53 in LMU5 and 0.29 in LMU9. This study indicated that poor soil management practice in the study area has caused soil physical degradation. Therefore, this study provides insight into improved land management of long-used cultivated land in the semi-arid region of the study area and other similar environments.     

Use of the revised universal soil loss equation (RUSLE) for soil and nutrient loss estimation in long-used rainfed agricultural lands,North Ethiopia

This study was conducted to quantify agricultural land degradation in the Ruba Gered watershed, Ethiopia. The watershed was divided into 12 land mapping units (LMU) after superimposing maps of soil, slope, land use/cover, and elevation. Subsequently, cultivated land was delineated to assess degradation types and severity based on standard approaches. Sheet erosion was estimated using the revised universal soil loss equation. Composite soil samples were collected from each LMU to quantify key soil nutrients (OM, total nitrogen, available phosphorus, and available potassium) lost by sheet erosion. The annual average soil loss due to sheet erosion was estimated to be 17.4 t ha^?1 yr^?1, with average annual nutrient losses estimated as 246.5 kg ha^?1 organic matter, 12.4 kg ha^?1 total nitrogen, 0.1 kg ha^?1 available phosphorus, and 1.6 kg ha^?1 available potassium. The study revealed that substantial quantities of soil and nutrients are lost every year in the study area due to severe sheet erosion. This amount of nutrient loss severely degrades soil and reduces soil fertility.