Impacts of Land Use and Land Cover Change on Soil Erosion and Hydrological Responses in Ethiopia

Land use and land cover (LULC) dynamics, in general, and the conversion of the natural vegetation cover into cultivated land, in particular, are major human-induced problems in Ethiopia, which have played a significant role in increasing the rate of soil erosion and altering the hydrological balance in the country. )e main aim of this review was to view previous studies in Ethiopia that quantify the change in the rate of soil erosion and hydrological responses as a result of the change in land use and land cover in the country. From the past researches reviewed in this paper, the expansion of cultivated land at the expense of forest land, shrubland, and grassland in Ethiopia has increased the mean rate of soil erosion, sediment yield, surface runoff, mean wet monthly flow, and mean annual stream flow in the last four decades. On the other hand, the change has reduced the dry average monthly flow, groundwater recharge and groundwater flow, and evapotranspiration (ET) in the country. Future research works should pay more attention to the investigation of the impacts of land use and land cover change on groundwater hydrology and the prediction of future soil loss and hydrological imbalance under the changing land use and land cover in the country since little information is available from past researches on these issues. Research works are also required in lowland arid and semiarid areas in Ethiopia to effectively manage soil and water resources in all parts of the country.


Introduction
Land use and land cover (LULC) change triggered by the interaction between demographic and socioeconomic changes as well as biophysical conditions [1,2] is one of the main driving forces on global and local environmental changes [3,4]. It exerts multidimensional consequences on essential Earth's ecosystem functions and services at local, regional, and global scales [2].
In sub-Saharan African countries, land use and land cover change, in general, and conversion of the natural land cover into agricultural land, in particular, are major continuous phenomena mainly caused by anthropogenic activities [5]. Like other sub-Saharan African countries, human-induced conversion of the natural land cover into cultivated land is the major problem in different areas of Ethiopia where agricultural activity serves as the backbone of the economy [6,7].
Land use and land cover (LULC) change in Ethiopia is triggered by the interaction of various demographic, socioeconomic, institutional, and biophysical factors [1]. Studies have shown that population pressure [6,[8][9][10][11], widespread agricultural expansion [6,[10][11][12], expansion of settlement [10,12], rural poverty [9], inadequate management of common property resources, and land tenure insecurity due to institutional and policy reforms [3,8,9,11,13] and demand for fuel wood and construction materials [10][11][12] were recognized as the major drivers of land use and land cover change in the country. e level of land cover conversion is particularly higher in the highland areas of the country mainly due to demographic pressures and consequent expansion of croplands and household energy demands [6,14].
Different studies indicated that land use and land cover change triggered by the aforementioned factors in the country have led to severe soil erosion [6,11,15], loss of biological diversity [8,11,15,16], decline of agricultural production and productivity [8,[15][16][17], and decline of ecosystem service values (ESV) due to changes in individual ecosystem service functions such as erosion control, provision of raw material, nutrient cycling and climate regulation [18][19][20][21], and change of rural livelihood [11]. e problem of soil erosion is one of the major environmental constraints to agricultural sustainability and food security in Ethiopia particularly in the highlands [22,23]. e overall soil loss of the country is estimated at about 1.5 billion tons per year with a mean erosion rate of 42 t·ha −1 ·year −1 [9,24,25]. e rate of soil erosion has been accelerating in the country due to land use and land cover (LULC) change and inappropriate land use and management practices [22,26]. e change in land use and land cover in Ethiopia also played a substantial role in changing hydrological processes such as an increase in surface runoff volumes, reduction of infiltration, and reduction of groundwater recharge in the country [27][28][29][30]. Furthermore, LULC change is one of the factors responsible for the sedimentation or siltation of lakes in the country. For instance, the extinction of Lake Alemaya in the eastern highland of Ethiopia due to siltation caused by LULC change in the upper catchment area of the lake [31,32].
In recent years, the spatial and temporal change of land use and land cover (LULC) and its impacts on soil erosion and hydrological processes have gained increased attention in the country. Several previous studies [7,23,24,26,[33][34][35][36][37][38] have quantified the impact of land use and land cover dynamics on soil erosion at the watershed level through the integration of revised universal soil loss equation (RUSLE) with geographic information system (GIS) and remote sensing techniques. Other research works [1,[27][28][29][30][39][40][41][42][43][44][45][46][47] also investigated the impact of land use and land cover change changes on watershed hydrological processes in the country using Soil and Water Assessment Tool (SWAT), Hydrologiska Byråns Vattenbalansavdelning (HBV), and Water and Energy Transfer between Soil, Plants, and Atmosphere (WetSpa) hydrological models applied in GIS and remote sensing techniques.
As indicated in Figure 1, different studies on how LULC dynamics affect the rate of soil erosion and hydrological balance were undertaken in different areas of the country, particularly in the highlands. Despite comprehensive review and documenting of past studies on the change in soil erosion and hydrological processes due to the change in LULC is essential for future resource management, a comprehensive review of these past fragmented studies to aid future soil and water resource management is scarce in Ethiopia. Hence, the main objective of this review was to view previous studies in Ethiopia that quantify the change in the rate of soil erosion and hydrological processes as a result of the change in land use and land cover in the country.

Land Use and Land Cover Change in Ethiopia
Land use and land cover (LULC) change is the human modification of Earth's terrestrial surface from existing management of the land or land cover to new management of land or new land cover type [48]. e nature of LULC dynamics significantly differs from country to country as the drivers of LULC change and land management activities vary from place to place.
In Ethiopia, land use and land cover (LULC) change is mainly dominated by the conversion of natural vegetation cover to use for agriculture activities [9,42]. According to FAO [49], the forest cover in Ethiopia has decreased from 13.3% of the total area of the country (14.69 million ha) in 1993 to 11.4% of the total area (12.54 million ha) in 2016 with an estimated annual rate of change 0.8% (104,600 ha·year −1 ) ( Figure 2).
On the contrary, as shown in Figure 2, the agricultural land of the country has increased from 27.66% of the total area of the country (30.54 million ha) in 1993 to 32.83% of the total area (36.26 million ha) in 2016 [49].
Several research works [2,8,9,11,12,14,15,48,[50][51][52][53][54][55][56][57][58] were undertaken on the spatial and temporal land use and land cover change at watershed and district level in Ethiopia and have shown that there has been a significant LULC change in different parts of the country dominated by the expansion of cultivated land and built-up areas at the expense of natural vegetation cover, shrublands, and grasslands particularly in the highlands.
A study by Bewket and Abebe [9] reported the consistent expansion of cropland and settlement at the expense of forest and dense tree cover over the period between 1957 and 2001 in Gish Abay Watershed, Blue Nile Basin of Ethiopia. A similar study by Hassen and Assen [13] reported that farmlands and settlement areas were expanded at the expense of forest area, shrublands, and grasslands over the study period between 1957 and 2014 in Gelda catchment, Lake Tana Watershed of Ethiopia. Tesfaye et al. [57] reported a significant increase in cultivated land and settlement and a decrease in vegetation cover between 1976 and 2008 in Gilgel Tekeze Catchment, Northern Ethiopian Highlands. According to Shawul and Chakma [56], cropland and urban area replaced a large area of forest cover and shrublands in Upper Awash Basin, Ethiopia, over the years between 1972 and 2014. Dibaba et al. [15] also found an expansion of agricultural land and built-up area at the expense of forest land in Finchaa Catchment, Northwestern Ethiopia, over the period between 1987 and 2017.
Similarly, Aneseyee et al. [18] reported the expansion of cultivated land, built-up area, and bare land at the expense of forest land and shrubland between 1988 and 2018 in Winike Watershed of Omo Gibe Basin, Southwest Ethiopia. A study conducted by Demissie et al. [51] also reported the continuous expansion of cultivated lands at the expense of forest area and grasslands over 42 years  in Libo Kemkem District of Northwestern Ethiopia. An increase in farmland area at the expense of forest land was also observed in Bale Mountain Eco-Region of Ethiopia from 1985 to 2015 [48]. Another study by Wubie et al. [11] found a significant increase in cultivated land and settlement and a decrease in forest land, shrubland, grassland, and wetland over 48 years (1957-2005) in Gumara Watershed of Lake Tana basin, Northwestern Ethiopia. Yesuph and Dagnew [2] reported the consistent increase in farmland and settlement area at the expense of Afro-alpine and sub-Afro-alpine vegetation areas between 1973 and 2017 in Beshillo Catchment of Blue Nile Basin, Northeastern Highlands of Ethiopia.
A study conducted by Deribew and Dalacho [14] revealed that forest area was converted into agricultural land and barren land between 1957 and 2017 in the Central Highlands of Ethiopia. Ayele et al. [59] also found that farmland expansion was the major cause for the reduction of forest area between 2000 and 2015 in Delomena District, Bale Zone of Ethiopia. Belay [50] reported a significant conversion from natural vegetation cover to cropland was observed between 1957 and 1986 in Derekolli Catchment of the South Wollo Zone of Ethiopia. e expansion of farmlands at the expense of other land use and land cover types has occurred not only in the highlands of Ethiopia but also in lowland arid and semiarid areas of the country. According to Alemu et al. [12], the expansion of agricultural land and bare land and the reduction of woodland cover were observed between 1985 and    [8]. In the Northeastern Somali Rangelands of Ethiopia, grassland cover was converted into cultivated and settlement areas between 1985 and 2017 [61].
Contrary to the findings of the aforementioned studies in different parts of the country, Bantider et al. [62] reported the expansion of forest area and shrubland and the decline of the area of cropland since the mid-1980s in the Eastern Escarpment of Wollo, Northeastern Ethiopia. Asmamaw et al. [63] have also shown the expansion of forest land during the study period between 1958 and 2006 in Gerado Catchment of Northeastern Ethiopia.

Impacts of Land Use and Land Cover Change in Ethiopia
3.1. Impact on Soil Erosion. LULC change exerts negative impacts on ecosystem services, in general, and on biodiversity, climate, soil, water, and air, in particular [48]. Soil erosion is affected by LULC change despite other factors such as climate, soil characteristics, and topography. Land cover plays a significant role in controlling soil erosion by reducing the direct impacts of raindrops on the soil, enhancing the organic matter content in the soil, increasing the infiltration rate of water, reducing the velocity of runoff, and reducing the transportation of sediments on the surface [64,65]. Hence, a change in land use and land cover due to anthropogenic activities significantly affects the rate of soil erosion. Different studies (Table 1) undertaken in different parts of Ethiopia indicated the impacts of land use and land cover change on soil erosion. Among these, a recent study made by Woldemariam and Harka [26] at Erer Subbasin, Northeast Wabi-Shebelle Basin of Ethiopia, indicated that the expansion of cropland, bare land, and settlement from 47. Similarly, a study by Moges and Bhat [37] in Rib Watershed, Northwestern Highland of Ethiopia, reported that the expansion of cultivated land at the expense of shrubland and grassland was the most detrimental factor for the increment of annual soil loss in the watershed from 0 to 236.5 t·ha −1 ·year −1 in 1986 to from 0 to 807 t·ha −1 ·year −1 in 2016 and increment of average annual soil loss of the entire watershed from 40 t·ha −1 ·year −1 in 1986 to 68 t·ha −1 ·year −1 in 2016. A study in Didessa River Catchment, Southwest Blue Nile of Ethiopia, by Chimdessa et al. [24] has shown that the average soil loss of the river catchment increased by 9.6 t·ha −1 ·yr −1 , 11 t·ha −1 ·yr −1 , and 20.9 t·ha −1 ·yr −1 due to LULC change between 1986 and 2000, 2001 and 2015, and 1986 and 2015, respectively.
In general, Table 1 summarizes the results of previous studies in Ethiopia which were undertaken to estimate the impacts of LULC change on the mean rate of soil loss and sediment yield in different watersheds. From the results, a significant increase in the mean rate of soil loss and sediment yield with a decreasing trend in forest cover and shrublands

Impact on Hydrological
Responses. Watersheds hydrological processes are affected by a multitude of factors such as land use and land cover, climate, soil properties, geology of the land, and topography. Land use and land cover change mainly caused by anthropogenic interference modifies watershed hydrological processes by altering the balance between rainfall, evaporation, and runoff response of an area [24]. e change in LULC alters infiltration, groundwater recharge, surface runoff, and river flow within a watershed [45]. erefore, a better understanding of LULC change and its effect on hydrological processes in Ethiopia is highly indispensable for the management of water resources in the country. LULC change contributes to the change in the hydrological system in Ethiopia. Quantifying the effects of LULC change on watershed hydrological processes has recently been given much attention by the researchers, and previous studies (Table 2) in the country have quantified watershed hydrological responses as a result of the change in land use and land cover. A study in Gilgel Tekeze Catchment, Northern Highlands of Ethiopia, by Haregeweyn et al. [46] found that the increment of cultivated land by 15.4% and settlements by 9.9% at the expense of shrubland and grazing lands triggered the increment of annual surface runoff by 101 mm, reduction of groundwater recharge by 39 mm, and reduction of annual evapotranspiration by 91 mm over the period between 1976 and 2003. Similarly, Gashaw et al. [42] reported the continuous expansion of cultivated land and built-up area and diminishing of forestland, shrubland, and grassland, which have occurred from 1985 to 2015, had increased the annual flow by 2.2%, wet seasonal flow by 4.6%, surface runoff by 9.3%, and water yield by 2.4%. On the other hand, the observed changes had reduced dry season flow by 2.8%, lateral flow by 5.7%, groundwater flow by 7.8%, and evaporation and transpiration (ET) by 0.3% in the Andassa Watershed, Blue Nile Basin of Ethiopia.
Another study by Welde and Gebremariam [30] in Tekeze Dam Watershed, Northern Ethiopia, also reported that the increment of cultivated land by 8. Chimdessa et al. [24] reported that land use and land cover change which occurred between 1986 and 2001, 2001 and 2015, and 1986 and 2015 have increased the average monthly river flow by 4.9 m 3 /s, 5.7 m 3 /s, and 10.6 m 3 /s, respectively, in Didessa River Catchment, Southwest Blue Nile Basin of Ethiopia.
Another study on the hydrological impact of land use change by Getachew and Melesse [44] in Angereb Watershed of Ethiopia reported that the mean wet monthly flow for 2011 land cover has increased by 39% compared with the 1985 land cover, and the dry average monthly flow reduced by 46% in 2011 compared with the 1985 land cover due to the expansion of cultivated land and built-up areas and decline of forest and grassland areas over the years between 1985 and 2011. Fufa et al. [41] found that the mean wet monthly flow of the catchment for 2010 land cover had increased by 3.8% compared with the 1986 land cover, and the average monthly flow in the dry season decreased by 12.3% in 2010 compared with the 1986 land cover due to the rapid expansion of agricultural land and settlement and reduction of forest land and grassland in Ketar Watershed, Lake Ziway Catchment, Ethiopia. Addisu and Tolosa [27] also reported that the increment of cultivated land from 10.8% to 39.1% and settlement area from 12.8% to 30.8% and the reduction of forest cover from 32.5% to 9.4% and grassland from 20.9% to 12.3% in Weib Catchment of Ethiopia between 1986 and 2010 caused an increase in the mean wet monthly flow for the 2010 land cover by 40.7% compared with 1986 land cover, and the dry average monthly flow for the 2010 land cover and 1995 land cover decreased by 45.2% and 26%, respectively, when compared with that of 1986 land cover.
A study on the impact of land use dynamics on the base flow responses by Gessesse et al. [43] reported that the expansion of cultivated land by 29.6% and urban areas by 0.53% and reduction of shrubland by 11.9% and grassland by 18 As summarized in Table 2, the results of previous studies in Ethiopia on the impacts of LULC dynamics on watershed hydrological responses have shown a decreasing trend in the dry average monthly flow, groundwater flow and recharge, and evapotranspiration (ET) and an increasing trend in the surface runoff, mean wet monthly flow and mean annual stream flow with a considerable loss of forest cover and shrublands and expansion of cultivated lands in different watersheds. However, no significant change in hydrological components such as surface runoff, groundwater recharge, and evapotranspiration (ET) has been observed despite the significant expansion of cultivated land from 70% in 1986 to 82% in 2015 and reduction of forest land from 11% in 1986 to 5% in 2015 and grassland from 18% in 1986 to 10% in 2015 in Gumara Watershed, Blue Nile Basin of Northwestern Ethiopia [1].

Conclusions
In Ethiopia, land use and land cover changes are accelerating by anthropogenic factors, and the change intern affects humans and other natural resources in general and soil and water resources in particular. Hence, the main objective of this paper was to review the impacts of land use and land cover changes on the rate of soil erosion and hydrological processes in the country. From the past researches reviewed in this paper, the expansions of cultivated land at the expense of forest land, shrubland, and grassland in the country have increased the rate of soil erosion, sediment yield, annual surface runoff, mean wet monthly flow, mean annual stream flow, and water yield in the last four decades. On the other hand, the change has reduced the dry average monthly flow, groundwater recharge, groundwater flow, lateral flow, and evapotranspiration (ET) in the country.
From the review, it was understood that the past studies on the impacts of LULC change on hydrology did not sufficiently show the change in groundwater recharge and groundwater flow as a result of a change in LULC in the country. Hence, future research works should pay more attention to the investigation of the impacts of LUCC change on groundwater hydrology in the country.
Little information is available on the impact of land use and land cover change on soil and water resources in arid and semiarid areas of the country from past researches. Accordingly, long-term historical research on land use and land cover change and its impact on soil and water resources in lowland arid and semiarid areas of the country are required to manage the degradation of soil and water resources across all parts of the country.
From the review, it was also understood that previous studies in the country have concentrated more on the effect of past LULC change on soil erosion and hydrological processes and did not show the likely effect under the changing LULC in the future. Hence, future research works should pay more attention to forecast future soil loss and hydrological imbalance under the changing land use and land cover in the country.

Data Availability
e data used to support this review paper are from previously reported studies, which have been cited.

Conflicts of Interest
e author declares that there are no conflicts of interest.