Calibration and validation of the CONCEPTS model for predicting gully erosion and evaluating gully control measures in the sub-humid highlands of Ethiopia

Authors: ZEGEYE, ASSEFA - Amhara Regional Agricultural Research Institute; Langendoen, Eddy; STEENHUIS, TAMMO - Cornell University; AMARE, SELAMAWIT - Bahir Dar University; TILAHUN, SEIFU - (NCE, CECR)networks Of Centres Of Exellence Of Canada, Centres Of Excellence For Commercilization A

Submitted to: Blue Nile Journal of Agricultural Research (BNJAR)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2019
Publication Date: 6/1/2020
Citation: Zegeye, A.D., Langendoen, E.J., Steenhuis, T.S., Amare, S.D., Tilahun, S.A. 2020. Calibration and validation of the CONCEPTS model for predicting gully erosion and evaluating gully control measures in the sub-humid highlands of Ethiopia. Blue Nile Journal of Agricultural Research (BNJAR). 1(2):66-89.

Interpretive Summary: Gully erosion is a major cause of land degradation in northern Ethiopia. Classic gully erosion control measures such as check dams have been unsuccessful in halting gully formation and growth in the humid northern Ethiopian highlands. Scientists at the USDA, ARS, National Sedimentation Laboratory in collaboration with researchers at Cornell University and Bahir Dar University, Ethiopia, are using the USDA, ARS computer model CONCEPTS for developing more effective gully rehabilitation measures for humid northern Ethiopia. These models were applied to investigate the possibility to stabilize a rapidly expanding gully in the Debre-Mewi watershed, about 30 km south of Lake Tana through regrading and planting of vegetation. Reshaping the sidewall slope to a slope of 45 degrees reduced the bed erosion by 25% and sediment yield by 33%. Targeted bed and bank toe protection reduced sediment yield by 90% and sidewall erosion by 100%. When the groundwater table is lowered from 0.1 m to 0.7 m below the ground surface, the sidewall erosion reduced by 83%. The introduction of computational modeling technology of increasing complexity, such as CONCEPTS, could have major benefits to develop cost-effective erosion control measures in developing countries such as Ethiopia.

Technical Abstract: The objective of this study was to calibrate and validate the CONCEPTS model for simulating gully expansion and for evaluating the effects of protection measures on gullies in the sub-humid Debre Mawi watershed near Lake Tana, Ethiopia. The main CONCEPTS model inputs are: channel geometry, runoff entering the channel, and resistance to erosion of the boundary (bed and bank) materials. Erosion of gully bed and banks is a combination of fluvial scour by the flowing water and gravity-driven mass failure. The fluvial erosion resistance is parameterized using a critical shear stress and soil erodibility coefficient. Mass failure resistance is represented by the soil shear strength (cohesion and friction angle). We collected data on boundary material stratigraphy (four layers) and its associated grain-size distributions, bulk density, and shear strength. The model was calibrated and validated against the observed changes in width and bed elevation of a 389 m long, 23 m wide, and 4.7 m deep gully during the 2013 (calibration-period) and 2014 (validation-period) rainy seasons. The average values for calibrated erosion-resistance parameters for the four layers were: cohesion 12.6 kPa, which is about half of that measured; internal friction angle 20.8o, increased by 30% from that measured; soil critical shear stress 0.67 Pa; soil erodibility 1.22x10-7 m3(N·s)-1, and suction angle 10o. The simulation results showed that the model predicted the sidewall erosion more accurately (R2 = 0.99, NSE = 0.99) and fairly predicted the change in bed elevation (R2 = 0.88, NSE = 0.98). Based on the calibrated values, we used the model to evaluate the effect of gully erosion control measures. Considering the bank on one of the 35 cross sections of the gully, reshaping the sidewall slope to a slope of 45o reduced the bed erosion by 25% and sediment yield by 33%. Targeted bed and bank toe protection reduced sediment yield by 90% and sidewall erosion by 100%. When the groundwater table is lowered from 0.1 m to 0.7 m below the ground surface, the sidewall erosion reduced by 83%. In summary, the validation results show that CONCEPTS can successfully simulate gully erosion based on the calibrated values, which fall within the typical range expected for the soils surrounding the gully.