Erosion modeling in the Upper Blue Nile Basin: the Case for Mizewa Watershed

Maru Alem Assegahegn Birhanu Zemadim(PhD)

Nile Basin Development Challenge (NBDC) Science WorkshopAddis Ababa, Ethiopia, 9–10 July 2013

Contents

1. Introduction2. Materials and Methods3. Result and Discussion4. Conclusion and Recommendation

1. Introduction1.1. Background and Problem statement Erosion is a universally accepted environmental

problem that threat man’s dev’t (Hudson, 1981)Ethiopia is described as the most soil erosion affected country in the world (Beyene, 2011)Annual soil loss ranges from16 to 300 t/ha Lake Tana Basin-experiences severe erosion-soil fertility & water quality degr’n, siltation, floodingCharacterization of erosion rates in a catchment

scale especially in the study area is rarely done

1.2. ObjectivesThe overall objective of the research is studying soil

erosion and its impact on water productivity in the study area.

Specific Objectives• To investigate hydrological processes and r/nships

between water budget and soil loss in Mizewa watershed

• To estimate magnitude of erosion spatially and temporally

• To identify erosion sensitive areas in the study area

2. Material and Methods2.1. Study Area Description

3.2. Data collection and AnalysisFlow data- was collected from IWMISS data-sampled from July 1 to Aug 31/2012 at the outlet and upstream on site 2 (2 times/day)o Simple grabbing technique was usedo Suspended sediment conc.(g/L) was analyzed o Sediment rating curve

Climatic Data-collected from NMSA and IWMI Landuse/land cover data - collected in the fieldDEM, Soil Map Data

3.3 Modeling- the SWAT ModelSWAT- WBESurface Runoff Volume- Peak Runoff rate or

PET- Penman Monteith Soil Water and Groundwater were simulated by SWAT Sediment- MUSLE- Sensitivity Analysis, Calibration & Validation- were done Model Performance test-

3.4 SWAT Model Setup- Watershed delineation, Landuse map, soil map, slope class, HRU

4. RESULT AND DISCUSSION4.1. Sediment Rating Curve

,

Calibration Result 10 flow & 5 sediment sensitive parameters - calibrated • Flow calibration

• Sediment calibration

Model Validation ResultFlow & sediment Validation

Simulated Results Flow simulation- the model captured both dry &

wet periods flow - the dry period slightly deviates negatively and wet period slightly deviates positively with 15% annual deviation

Simulated rate of average annual soil loss of the watershed is 40.91 ton/ha (high)

Simulated average annual sediment yield at the outlet is 12.78 t/ha & measured 11.45 ton/ha

The highest measured and simulated soil loss was recorded in July followed by August

Subbasin erosion simulation-temporal/spatial

Water budget and erosion r/nshipRelatively high average annual rainfall (1385mm

measured & 1374 mm simulated) responsible for high erosion in the watershed

From SWAT water balance simulation, surface runoff component, 335.70 mm (24%) of Rainfall is responsible for soil loss in the catchment (high erosion potential)

High soil water storage characterizes high runoff in turn high erosion

5. Conclusion and Recommendation Modeling was conducted with one year primary data Model calibration & validation were done with data of the

same time series but with varied spatial location The R2 and NSE

values (>0.5) (good model performance) and with fair agreement of observed Vs simulated result

High SSC(g/L) and soil loss recorded in July and Soil loss in subbasin 3> subbasin 2 > subbasin 1

Long duration time series primary data of the watershed is vital to minimize uncertainty in modeling

Catchment scale controlled modeling should be encouraged Continuous studies should be conducted to recommend

appropriate erosion control remedies at the watershed