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Hydrological modeling of sustainable land management interventions in theMizewa watershed, Blue Nile Basin Emily Schmidt (IFPRI ESSP-II) and Birhanu Zemadim (IWMI)

Sustainable Land and Watershed Management Interventions and Impact WorkshopMay 10, 2013Hilton Hotel, Addis Ababa

Brief overview of previous research

Landscape level investments in SLWMHydrological data collected by IWMI from 2011-ongoing

Hydrological simulations of watershed investments

Conclusions and Upcoming Research

Outline of presentation

Overview of previous researchImproved water infiltration and decreased runoff volume due to SLWM investmentChemoga watershed (Blue Nile basin): cropland expansion and overgrazing attributed to significant declines in dry season stream flow from 1960-1999. (Bewket and Sterk, 2005)

May ZegZeg catchment (north Ethiopia): stone bunds, check dams and abandonment of post-harvest grazing permitted farmers to plant crops in previously active gullies increased infiltration and decreased runoff volume (Nyssen, 2010)

On-farm experimental sites in diverse agro-ecological zones: SLWM investments reduced soil loss and runoff in semi-arid watersheds; however increases in agricultural yields did not outweigh the estimated costs of soil conservation. (Herweg and Ludi, 1999)

Overview of previous researchSoil loss due to erosion vary by location, which reflects the varying Ethiopian landscape and soil characteristics

Highlands test plots on cultivated land: 130 to 170 metric tons ha / year on cultivated land. (Hurni, 2008)

Medego watershed, North Ethiopia: 9.63 metric tons ha/year (Tripathi & Raghuwanshi, 2003).

Chemoga watershed in the Blue Nile Basin: 93 metric tons ha/year (Bewket & Teferi, 2009).

Borena woreda, South Wollo: ranged from 0 loss in the flat plain areas to over 154 metric tons ha/year in some areas. (Shiferaw, 2011)

Study site: Mizewa watershed, Fogera Woreda

Simulation of watershed landscape-level investmentsSlope gradientShareUnder 50.135-200.65>200.22

Root Zone

Vadose (unsaturated) Zone

ShallowAquifer

Confining Layer

Deep Aquifer

Evaporation andTranspiration

Simulation of landscape-level investmentsInvestment decisions are simulated to take into account tradeoffs in labor and land investment and terrain type:

Terracing on steep hillsides

Terracing on mid-range and steep hillsides

Terracing on mid-range and steep slopes with bund construction on flatter areas

Residue management on all agricultural terrain (.5 1 tons/ha of residue left on field).

Mixed strategy of terraces in steep areas and residue management on mid-range terrain

LaborLand

a) Newly constructed Fanya Juu terrace /bund

b) Fanya Juu after five years of constructionSource: IWMI Africa Rainwater harvesting diagramTerraces and bunds to slow runoff, increase percolation and decrease erosion

Residue Management to stabilize soil, trap sediment, decrease runoff Crop residues are important to stabilize soil, as well as replenish soil nutrients Restricted grazing on agricultural and pasture landMinimum tillage on agricultural land

Current practices (Terefe, 2011 Chorie, North Wollo) Crop residue used for:Stall feeding and stubble grazing (74-90%), Fuel (11-15%), Sale during extended dry season

Livestock graze on stubble in field until planting the following season (in some areas considered communal grazing)

Model setup and calibrationAugust of 2011 December 2012 (and ongoing)Network of data gages installed and collecting daily dataSoil moisture probesAutomatic and manual stream level gauges Automatic and manual weather stations and rain gaugesShallow ground water monitoring devices

Calibrate surface, groundwater and total runoff: Observed versus simulated

Calibrating the SWAT model requires adjusting a number of sensitive parameter values and their combinations, which in turn determine runoff behavior.

Model was calibrated at a daily, weekly and monthly time step

Calibration: observed and simulated stream flow CalibrationValidationENSR2ENSR2Weekly.72.73.61.69Monthly.93.94.71.81

Model simulationAssume future weather patterns will display similar trends to previous years, simulations utilizing Bahr Dar rainfall and weather data from 1990 2012. July and August experience the greatest rainfall and runoff volumes, and minimum runoff volumes occur between March and April

Average Annual Flow and Sediment yield (1990-2012)Base (mm)Terrace(>20)Terrace(>5)Terrace and bundResidue mgt. (all) Residue mgt. and terraceSurface flow45.0-15%-45%-50%-17%-26%Lateral flow200.31%3%3%1%2%Groundwater flow72.20%13%15%6%5%Stream flow317.6-1%-2%-2%-0.5%-1%Sediment (erosion)1.99-45%-83%-85%-19%-54%

Constructing terraces and bunds on different slope gradients provides the largest reduction in surface runoff and erosion. Increases groundwater flow by 15 %. However this intervention is very labor intensive (and pests may be an issue).

Terracing on only steep agricultural slopes (>20%) decreases surface flow by 15% and erosion by 45%.

Residue management at mid-range slope paired with terraces on steep slopes decreases surface flow by 26% and erosion by 54%

Average Monthly Surface Flow (1990 2012)Terracing on steep slopes similar to residue mgt. on all agricultural land

Terracing >5% slopes, and mixed terrace/bunds simulations : Surface flow reduced to 12.4 and 11.3mm (45% and 50%)

Terraces + Residue: decreases surface flow from 26mm to 16.8mm (-25%) in July

Average Monthly Sediment Yield (1990-2012)Terrace + Residue mgt.: Sediment yields decrease from 1.03 tons/hectare in the base simulation to .47 tons/hectare in the month of July (similar to steep terrace scenario)

Terraces >5% slope and terrace + bund produce very similar results

ConclusionsDecreases in average monthly runoff during the rainy season is the primary driver to decreased sediment yield and surface flow.

Simulated investments decrease surface runoff, AND increase groundwater flow due to improvements in percolation.

Groundwater flow is prolonged into dry months as well.Increased 8-32% in March Increased 13-52% in April

Increased percolation may extend the crop growing period which may have a direct effect on farmer livelihoods.

Conclusions and Upcoming ResearchHouseholds investments on individual plot land require at least 7 years of maintenance for significant benefits. Unlike technologies such as fertilizer or improved seeds, benefits may accrue over longer time horizons.

The longer one sustains SWC, the greater the payoff. However, the individual benefits of sustaining SLWM on private land may not outweigh the costsA mixture of strategies may reap quicker benefits

May be necessary to think of a landscape / watershed approachUnderstanding differences in agro-ecological zones, slope and soil variations in order to plan most effective interventionsWeigh benefits and costs of comprehensive SLWM approach, possible opportunities to phase-in investments (i.e. terraces on steep slopes first, then some residue management, etc.)

Conclusions and Upcoming ResearchHH survey calculated SLWM benefits of improved water capture and decreased erosion on private land investment implicitly

Hydrological model explicitly quantifies biophysical improvements to water balance processes within the watershed on agricultural land

The type and amount of investment in SLWM has different implications with respect to labor input and utilization of agricultural land at household and landscape level.

Conclusions and Upcoming ResearchAlthough simulations suggest that a landscape-wide approach may reap the greatest long-term benefits, it is important to understand the costs of such an investment.

The economic impacts of SLWM interventions may be more favorable in certain areas:

Simulate long-term effects of complex ecological-economic systems are necessary in order to inform policy decision and investments. Access to markets and infrastructureOff farm labor opportunitiesLand rental (agricultural and foraging rental)

Link the household survey data and hydrological simulations to model impact of different SLWM interventions, taking into account socio-economic drivers and climate scenarios.

Thank you

Calibration of Hydromodel

ImplicationsAverage monthly runoff during the rainy season is the primary driver to decreased sediment yield and surface flow.

Simulations decrease surface runoff from 15% (terraces >20) to 50% (terraces and bunds) and decrease erosion from 19% (residue mgt. on all ag. fields) to 85% (terraces and bunds)

Comprehensive investment of terraces and bunds maintained over the simulation period (1990-2011) would decrease surface flow 50%, increase groundwater flow by 15%, and decrease erosion by 85%. (However, can achieve similar effects from constructing terraces on slopes > 5% without bund construction)

ImplicationsResidue management also has a significant effect on surface flow and erosion in the Mizewa watershed. Average annual surface flow decreased 17% when adopting residue management on all agricultural land and 26% when implementing a mixed terracing and residue management.

Simulated investments decrease surface runoff, AND increase groundwater flow due to improvements in percolation.

Groundwater flow is prolonged into dry months as well.Increased 8-32% in March Increased 13-52% in April

Increased percolation may extend the crop growing period as well which may have a direct effect on farmer livelihoods.

Agriculture in the Blue Nile BasinLand degradation in Ethiopia continues to challenge agricultural development

Land degradation in some areas is estimated to decrease productivity by 0.5 to 1.1% per year. (Holden et al. 2009)

Moisture stress between rainfall events (dry spells) is responsible for most crop yield reductions (Adejuwon, 2005)