Impact of Soil Conservation and Eucalyptus on Hydrology and Soil Loss in the Ethiopian Highlands

Abstract

The Ethiopian highlands suffer from severe land degradation, including erosion. In response, the Ethiopian government has implemented soil and water conservation practices (SWCPs). At the same time, due to its economic value, the acreage of eucalyptus has expanded, with croplands and pastures converted to eucalyptus plantations. The impact of these changes on soil loss has not been investigated experimentally. The objective of this study, therefore, is to examine the impacts of these changes on stream discharge and sediment load in a sub-humid watershed. The study covers a nine-year period that included installation of SWCPs, a three-fold increase from 1.5 ha in 2010 to 5 ha in 2018 in eucalyptus, and the upgrading of an unpaved to the paved road. Precipitation, runoff, and sediment concentration were monitored by installing weirs at the outlets of the main and four nested watersheds. A total of 867 storm events were collected in the nine years. Runoff and sediment concentration decreased by more than half in nine years. In the main watershed W5, we estimated that evapotranspiration by eucalyptus during the dry phase (November to May) increased approximately from 30 mm a−1 in 2010 to 100 mm a−1 in 2018. In watershed W3 it increased from 2 mm a−1 to 400 mm a−1, requiring more rainfall before saturation excess runoff began in the rain phase. The reduction in runoff led to a decreased sediment load from 70 Mg ha−1 a−1 in 2010 to 2.8 Mg ha−1 a−1 in 2018, though the reduction in discharge may have negative impacts on ecology and downstream water resources. SWCPs became sediment-filled and minimally effective by 2018. This indicates that these techniques are either inappropriate for this sub-humid watershed or require improved design and maintenance.

Hydrological Foundation as a Basis for a Holistic Environmental Flow Assessment of Tropical Highland Rivers in Ethiopia

“…The average cumulative rainfall required for a 20 mm runoff depth increased from 371 mm in the first period before intervention to 442 mm after interventions ( Figure 10). The result agrees with the finding of another recent study from the Ethiopian Highlands [47]. Therefore, using the satellite image data of MODIS, the dry season, the month of March, evapotranspiration over the Gumara was extracted and found to be increasing between 2001 and 2019 ( Figure 9).…”

“…studies in lake Tana Basin [46]. In addition, a study on the hydrological impact of a Eucalyptus plantation found that the cumulative rainfall required to generate 3 mm runoff was higher after a threefold expansion of the plantation area [47].…”

“…An increase in vegetation has increased the evapotranspiration, which, in turn, increased the amount of infiltration water need to saturate soils before runoff generation. This suggests that the hydrological process is highly influenced by tree plantations [47,48]. Research has shown that in the Ethiopian highlands where saturation excess runoff dominates, daily discharge is a function of daily amount of rainfall, not of the rainfall intensity [48][49][50].…”

“…An increase in vegetation has increased the evapotranspiration, which, in turn, increased the amount of infiltration water need to saturate soils before runoff generation. This suggests that the hydrological process is highly influenced by tree plantations [47,48].…”

The Effect of Hydrology on Soil Erosion

“…Another clear example of human disturbance is the research published by Mhiret et al [14] about land degradation and soil conservation measures carried out to reduce soil loss in the Ethiopian Highlands. This territory suffers from severe land degradation, including erosion, and in response, the Ethiopian government has implemented soil and water conservation practices considering the acreage of eucalyptus, which has been expanded.…”