Assessment of the combined effects of land use/land cover and climate change on soil erosion in the Sile watershed, Ethiopian Rift Valley Lakes Basin

Figures & data

Figure 1. Location of the study area.

Table 1. The metrological stations and satellite data used in this study

Characteristics of the rain gauge stations
Station name	Latitude	Longitude	Elevation (meter)	Mean annual rainfall (mm)
Arbaminch	334487	666294	1,336	888
MirabAbaya	335240	672585	1,223	764
Gidole	309647	667316	2,248	1,257
Gato	327282	643820	1,771	857
Satellite images used in this study for LULC change analysis
Satellite image	Path/Row	Sensor*	Date of acquisition	Spatial resolution
Landsat 4–5 TM	169/056	TM	31 January 1987	30*30
Landsat 7ETM	169/056	ETM+	2 February 2003	30*30
Landsat-8 OLI/TIRS	169/056	OLI/TIRS	27 February 2020	30*30

*TM (Thematic Mapper), ETM+ (Enhanced Thematic Mapper Plus), OLI/TIRS (Operational Land Imager/Thermal Infrared Sensor).

Table 2. Description of the driving GCMs used in this study

Regional climate model	Model institute	Country	Driving GCM
Canadian Regional Climate Model version 4 (CanRCM4)	Canadian Centre for Climate Modelling and Analysis (CCCMA)	Canada	CCCma_canESM2
Rossby Center Regional Atmospheric Model (RCA4)	The Swedish meteorological and hydrological institute (SMHI)	Sweden	MOHC-HadGEM2-ES
Rossby Center Regional Atmospheric Model (RCA4)	The Swedish meteorological and hydrological institute (SMHI)	Sweden	CCCma_canESM2
Regional Atmospheric Climate Model Version 2.2 T (RACMO22T)	Koninklijk Nederlands Meteorologisch Instituut (KMNI)	Netherland	ICHEC-EC-EARTH

Table 3. Major LULC types and their descriptions (Bewket & Teferi, 2009; Gelagay & Minale, 2016)

LULC cover type	Descriptions
Water body	Covered by water and it may be reservoirs, lakes, large water harvesting ponds, etc.
Natural forest	Area covered by dense and tall trees both natural and plantations
Bare land	Areas that have little or no vegetation cover, mainly with classic gullies and exposed rocks
Shrub land	Areas covered with short shrubs and thorny bushes, usually stony with a very rugged relief
Banana land	Areas covered with banana plantation
Cultivation land	Land covered by annual and perennial crops and fallow lands
Grazing land	Areas with grass cover

Table 4. Land cover classes and assigned cover (C) factor values

LULC class	C-values	References
Banana Land	0.089	Angima et al. (2003)
Barren Land	1	Yesuph and Dagnew (2019); Girma and Gebre (2020)
Cultivation Land	0.15	Yesuph and Dagnew (2019); Girma and Gebre (2020)
Grazing Land	0.05	Yesuph and Dagnew (2019); Girma and Gebre (2020)
Forest Land	0.01	Hurni (1985); Girma and Gebre (2020)
Shrub Land	0.014	Wischmeier and Smith (1978);Girma and Gebre (2020)
Water Body	0.0	Girma and Gebre (2020)

Table 5. LULC and slope classes with assigned P-values

Land use type	Slope classes in percent (%)	P-factor
Agricultural land	0–5	0.1
	5–10	0.12
	10–20	0.14
	20–30	0.19
	30–50	0.25
	>50	0.33
Other land use	All	1

Source: Adapted from Wischmeier and Smith (1978).

Figure 2. Overall study framework used to estimate soil erosion.

Table 6. Classification accuracy assessment result for 1987, 2003, and 2020

Land use type	1987		2003		2020
	PA	UA	PA	UA	PA	UA
Water body	100	100	100	100	100	80
Cultivation	97.6	95.38	98.6	97.33	100	93.89
Barren land	91.7	91.67	93.3	90.32	100	100
Banana land	90.5	90.48	93	91.01	96.29	100
Shrub land	96.2	100	97	97.01	94.74	100
Grazing land	97	98.89	94.9	98.95	90.48	92.68
Forest land	95	96.04	94.6	96.43	88.68	100
Overall accuracy (%)	96.03		96.49		95.73
Kappa Statistics (%)	0.81		0.80		0.94

PA, producer accuracy; UA, user accuracy.

Figure 3. LULC map (a) 1987, (b) 2003, (c) 2020, and (d) 2050 in Sile watershed.

Table 7. Expected change of future LULC (2050) in study area

LULC category	LULC_2020		LULC_2050		Change (2020–2050)		Annual change
	Area (ha)	%	Area (ha)	(%)	Area (ha)	%	Area	%
Water body	208	1	198	1	−10	−5	−0.35	−0.17
Cultivation	12,793	46	14,562	53	1,769	14	59	0.46
Barren land	4,133	15	4,791	17	658	16	22	0.53
Banana land	1,257	5	1,631	6	629	50	21	1.67
Shrub land	6,539	24	4,711	17	−1,953	−30	−65	−1.00
Grazing land	1,386	5	990	4	−397	−29	−13	−0.95
Forest land	1,295	5	728	3	−697	−54	−23	−1.79
Total	27,610		27,610

Table 8. Ensemble RCM performance evaluation

Stations	NSE		RMSE		R^2
	RCP4.5	RCP8.5	RCP4.5	RCP8.5	RCP4.5	RCP8.5
Arbaminch	0.686	0.589	20.684	26.382	0.794	0.50
M/Abaya	0.627	0.756	12.647	12.271	0.723	0.802
Gidole	0.691	0.576	14.245	23.333	0.699	0.646
Gato	0.599	0.745	16.961	14.804	0.764	0.777

Figure 4. R factor map (A) 1987, (B) 2003, (C) 2020, (D) 2050 under RCP 4.5, and (E) 2050 under RCP 8.5.

Figure 5. K factor (A) and LS factor (B) map of Sile watershed.

Figure 6. Cover (C) factor map (A) 1987, (B) 2003, (C) 2020, and (D) 2050.

Figure 7. Practice (P) factor map for 1987 (A), 2003 (B), 2020 (C), and 2050 (D).

Figure 8. The mean annual soil loss: (A) 1987, (B) 2003, (C) 2020, (D) 2050-RCP4.5, and (E) 2050-RCP8.5.

Table 9. Soil erosion under different LULC classes in the study watershed

LULC	Soil loss 1987		Soil loss 2003		Soil loss 2020
	Annual loss (t yr^−1)	Mean (t ha^−1 yr^−1)	Annual loss (t yr^−1)	Mean (t ha^−1 yr^−1)	Annual loss (t yr^−1)	Mean (t ha^−1 yr^−1)
BC	1,028	2	2,626	3.5	2,639	1.9
GL	14,698	15.4	38,067	15.2	38,153	20
BL	31,571	27.9	214,178	35.5	534,664	45.6
CL	257,224	23.3	270,126	29.3	545,637	41.7
SL	51,556	5.1	73,559	7.9	48,466	9.3
FL	33,375	8.6	18,110	4.7	15,361	8.1
Total	389,456		616,669		1,184,920

CL = Cultivation Land, NF = Forest Land, GL = Grazing Land, BC = Banana Cover, BL = Bare Land, SL = Shrub Land.

Figure 9. Estimated soil erosion severity classes based on sub-watershed level in Sile watershed.

Table 10. Soil erosion severity in sub-watershed level and conservation priority class

Sub WS	Severity class (t-ha^−1 yr^−1) and its area coverage (ha)					Total soil loss (t/yr^−1)	%	Priority
	0–5 (Very slight)	5–11 (Slight)	11–30 (Moderate)	30–50 (Severe)	>50 (Very severe)
Sub_1	1,252	103	322	279	976	126,591	11	4
Sub_2	776	53	198	184	508	84,656	7	7
Sub_3	1,020	120	189	188	721	123,159	10	5
Sub_4	1,817	270	500	358	941	247,264	21	2
Sub_5	833	92	213	196	793	258,282	22	1
Sub_6	647	62	78	93	437	75,450	6	8
Sub_7	3,005	176	139	87	323	47,300	4	9
Sub_8	2,634	187	251	253	908	126,988	11	3
Sub_9	2,481	196	110	38	109	9,364	1	10
Sub_10	1,432	156	142	170	594	85,866	7	6
Total (ha)	15,897	1,414	2,143	1,846	6,310	1,184,920
Total (%)	58	5	8	7	22

SW = Subwatershed, (t yr⁻¹) = ton/year, (t ha⁻¹ yr⁻¹) = ton per hectare per year.

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