The nexus between land use, land cover dynamics, and soil erosion: a case study of the Temecha watershed, upper Blue Nile basin, Ethiopia

At the current times, soil erosion is the major problem that affects land and water resources, especially in Ethiopia's highlands. Due to the dynamics of land use land cover change, land degradation, and soil erosion increase significantly and result in a loss of fertile soil every year and lead reduction in agricultural production. This study was therefore designed to explore the land use land cover (LULC) dynamics from 1986 to 2020, to estimate mean annual soil erosion rates and identify erosion hotspot areas from 1986 to 2020, and finally, to evaluate the impacts of land use land cover change on soil loss of 1986 to 2020. For this, Landsat imageries of 4 years from 1986 to 2020 were used. Maximum likelihood supervised classification methods were used to classify LULCs. The dynamics of LULC change were used as an input for measuring soil loss by employing the combination of geospatial technologies with the revised universal soil loss equation (RUSLE). The LULC maps of 1986, 1997, 2009, and 2020 were used for identifying crop management (C) factor and conservation practice (P) factor. Rainfall erosivity factor (R), soil erodibility factor (K), and slope length and steepness factor (LS) were also used as sources of data. Based on the five factors, soil erosion intensity maps were prepared for each year. Results showed that the annual soil loss in the watershed ranged from 0 to 3938.66 t/ha/year in 1986, 0 to 4550.94 t/ha/year in 1997, 0 to 5011.21 t/ha/year in 2009, and 0 to 6953.23 t/ha/year in 2020. The annual soil loss for the entire watershed was estimated at 36.889, 42.477, 47.805, and 48.048 t/ha/year in 1986, 1997, 2009, and 2020, respectively. The mean soil loss of 1986, 1997, 2009, and 2020 was higher in cultivated land followed by shrub land, grazing land, and forest land. Mean soil loss increased from 1986 to 1997, from 1997 to 2009, and from 2009 to 2020. This is because of the expansion of agricultural land at the expense of grazing land and shrub land. Therefore, urgent soil and water conservation practices should be made in hotspot areas.

Evaluating the Effectiveness of Best Management Practices On Soil Erosion Reduction Using the SWAT Model: for the Case of Gumara Watershed, Abbay (Upper Blue Nile) Basin

This study was conducted to evaluate the effectiveness of best management practices (BMPs) to reduce soil erosion in Gumara watershed of the Abbay (Upper Blue Nile) Basin using the Soil and Water Assessment Tool (SWAT) model. The model was calibrated (1995-2002) and validated (2003-2007) using the SWAT-CUP based on observed streamflow and sediment yield data at the watershed outlet. The study evaluated four individual BMP Scenarios; namely, filter strips (FS), stone/soil bunds (SSB), grassed waterways (GW) and reforestation of croplands (RC), and three blended BMP Scenarios, which combines individual BMPS of FS and RC (FS & RC), GW and RC (GW & RC), and SSB and GW (SSB & GW). Mean annual sediment yield at the baseline conditions was estimated at 19.7 t ha^-1yr^-1, which was reduced by 13.7, 30.5, 16.2 and 25.9% in the FS, SSB, GW, and RC Scenarios, respectively at the watershed scale. The highest reduction efficiency of 34% was achieved through the implementations of the SSB & GW Scenario. The GW & RC, and FS & RC Scenarios reduced the baseline sediment yield by 32% and 29.9%, respectively. The study therefore concluded that the combined Scenarios mainly SSB & GW, and GW & RC can be applied to reduce the high soil erosion in the Gumera watershed, and similar agro-ecological watersheds in Ethiopia. In cases where applying the combined scenarios is not possible, the SSB Scenario can yield significant soil erosion reduction.

Spatiotemporal variability of agricultural drought and its association with climatic variables in the Upper Awash Basin, Ethiopia

Drought is a serious threat to agriculture in Ethiopia. This study examined the spatiotemporal variability of agricultural drought and its association with climatic variables in the Upper Awash basin. Mann–Kendall (MK) trend test was employed to examine the drought trend while Sen’s slop estimator and pixel-based linear regression model were used to analyze the magnitude of drought changes. The association between agricultural drought and climatic variables was evaluated by the Pearson correlation coefficient (r). High spatiotemporal variability of drought was observed in Kiremit (June–September) and Belg (February–May) seasons. The Belg season spatial average vegetation condition index (VCI) trends were decreased insignificantly from 2001 to 2019 at a 5% significant level, whereas the spatial average VCI trends of Kiremit season were increased insignificantly. The return period of severe droughts during the Belg season was less frequent than the Kiremt season severe drought. The correlation between spatial average VCI and precipitation was positive for Belg and Kiremit seasons. Likewise, the correlation between average VCI and land surface temperature (LST) was negative in Belg and positive in Kiremit season. Moreover, the correlation between mean VCI and Pacific Ocean Sea Surface Temperature (SST) was positive for Belg and Kiremit seasons. The influencing factor of precipitation and LST on VCI during Belg season was higher than Kiremit season. The findings of this study are vital for decision-making systems and preparing plans to adjust sowing time, select drought-resistant crops, practice in situ water conservation, practice small-scale irrigation and diversify the income of smallholder farmers.

Evaluating potential impacts of land management practices on soil erosion in the Gilgel Abay watershed, upper Blue Nile basin

Assessing the potential impacts of different land management practices helps to identify and implement sustainable watershed management measures. This study aims to assess a change in soil erosion rate under different land management practices in the Gilgel Abay watershed of the upper Blue Nile basin, Ethiopia. The Revised Universal Soil Loss Equation (RUSLE) model that was adapted to the Ethiopian highlands context was employed to estimate the rate of soil erosion. The impact of land management practices on soil erosion was estimated for three scenarios, which were baseline, intensive cultivation, and extensive cultivation scenarios. At the baseline scenario, the mean annual soil erosion was estimated at ~32.8 t ha⁻¹yr⁻¹, which is equivalent to a loss of ~13.66 Mt yr⁻¹ from the entire watershed. While the rate of soil erosion reduced to ~11.3 t ha⁻¹yr⁻¹ during the implementation of intensive cultivation management practice, which reduced the total soil loss in the watershed by 65%. On the other hand, under the extensive cultivation scenario, the mean annual soil erosion rate increased to ~34.4 t ha⁻¹yr⁻¹. The findings suggest that implementing agricultural intensification management practices can significantly reduce soil erosion in the watershed.

Modeling the hydrological impacts of land use/land cover changes in the Andassa watershed, Blue Nile Basin, Ethiopia

Understanding the hydrological response of a watershed to land use/land cover (LULC) changes is imperative for water resources management planning. The objective of this study was to analyze the hydrological impacts of LULC changes in the Andassa watershed for a period of 1985-2015 and to predict the LULC change impact on the hydrological status in year 2045. The hybrid land use classification technique for classifying Landsat images (1985, 2000 and 2015); Cellular-Automata Markov (CA-Markov) for prediction of the 2030 and 2045 LULC states; the Soil and Water Assessment Tool (SWAT) for hydrological modeling were employed in the analyses. In order to isolate the impacts of LULC changes, the LULC maps were used independently while keeping the other SWAT inputs constant. The contribution of each of the LULC classes was examined with the Partial Least Squares Regression (PLSR) model. The results showed that there was a continuous expansion of cultivated land and built-up area, and withdrawing of forest, shrubland and grassland during the 1985-2015 periods, which are expected to continue in the 2030 and 2045 periods. The LULC changes, which had occurred during the period of 1985 to 2015, had increased the annual flow (2.2%), wet seasonal flow (4.6%), surface runoff (9.3%) and water yield (2.4%). Conversely, the observed changes had reduced dry season flow (2.8%), lateral flow (5.7%), groundwater flow (7.8%) and ET (0.3%). The 2030 and 2045 LULC states are expected to further increase the annual and wet season flow, surface runoff and water yield, and reduce dry season flow, groundwater flow, lateral flow and ET. The change in hydrological components is a direct result of the significant transition from the vegetation to non-vegetation cover in the watershed. This suggests an urgent need to regulate the LULC in order to maintain the hydrological balance.