Hydrological modeling of sustainable land management interventions in the Mizewa watershed, Blue Nile Basin
1Hydrological modeling of sustainable landmanagement interventions in theMizewa watershed, Blue Nile BasinEmily Schmidt (IFPRI ESSP-II) and Birhanu Zemadim (IWMI)Sustainable Land and Watershed ManagementInterventions and Impact WorkshopMay 10, 2013Hilton Hotel, Addis Ababa
• Brief overview of previous research• Landscape level investments in SLWM– Hydrological data collected by IWMI from 2011-ongoing• Hydrological simulations of watershedinvestments• Conclusions and Upcoming ResearchOutline of presentation
Overview of previous research• Improved water infiltration and decreased runoff volumedue to SLWM investment– Chemoga watershed (Blue Nile basin): cropland expansion andovergrazing attributed to significant declines in dry seasonstream flow from 1960-1999. (Bewket and Sterk, 2005)– May ZegZeg catchment (north Ethiopia): stone bunds, checkdams and abandonment of post-harvest grazing permittedfarmers to plant crops in previously active gullies – increasedinfiltration and decreased runoff volume (Nyssen, 2010)– On-farm experimental sites in diverse agro-ecological zones:SLWM investments reduced soil loss and runoff in semi-aridwatersheds; however increases in agricultural yields did notoutweigh the estimated costs of soil conservation. (Herweg andLudi, 1999)
Overview of previous research• Soil loss due to erosion vary by location, which reflectsthe varying Ethiopian landscape and soil characteristics– Highlands test plots on cultivated land: 130 to 170 metric tonsha / 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 tonsha/year (Bewket & Teferi, 2009).– Borena woreda, South Wollo: ranged from 0 loss in the flatplain areas to over 154 metric tons ha/year in some areas.(Shiferaw, 2011)
Simulation of landscape-level investments• Investment decisions are simulated to take intoaccount tradeoffs in labor and land investment andterrain type:1. Terracing on steep hillsides2. Terracing on mid-range and steep hillsides3. Terracing on mid-range and steep slopes withbund construction on flatter areas4. Residue management on all agricultural terrain(.5 – 1 tons/ha of residue left on field).5. Mixed strategy of terraces in steep areas andresidue management on mid-range terrainLaborLand
a) Newly constructedFanya Juu terrace /bundb) Fanya Juu after fiveyears of constructionSource: IWMI Africa Rainwater harvesting diagramTerraces and bunds to slow runoff, increasepercolation and decrease erosion
Residue Management to stabilize soil, trapsediment, decrease runoff• Crop residues are important to stabilize soil, as well asreplenish soil nutrients– Restricted grazing on agricultural and pasture land– Minimum 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 followingseason (in some areas considered communal grazing)
Model setup and calibration• August of 2011 – December 2012 (and ongoing)– Network of data gages installed and collecting daily data• Soil moisture probes• Automatic and manual stream level gauges• Automatic and manual weather stations and rain gauges• Shallow ground water monitoring devices– Calibrate surface, groundwater and total runoff: Observedversus simulated– Calibrating the SWAT model requires adjusting a number ofsensitive parameter values and their combinations, which inturn determine runoff behavior.– Model was calibrated at a daily, weekly and monthly time step
Model simulation• Assume future weather patterns will display similar trends toprevious years, simulations utilizing Bahr Dar rainfall and weatherdata from 1990 – 2012.• July and August experience the greatest rainfall and runoffvolumes, and minimum runoff volumes occur between March andApril010203040506070MillimetersPrecipitation 1990-2012 (Bahr Dar)
Average Annual Flow and Sediment yield (1990-2012)Base(mm)Terrace(>20°)Terrace(>5°)TerraceandbundResiduemgt. (all)Residuemgt. andterraceSurface flow 45.0 -15% -45% -50% -17% -26%Lateral flow 200.3 1% 3% 3% 1% 2%Groundwaterflow72.2 0% 13% 15% 6% 5%Stream flow 317.6 -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 largestreduction 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 slopesdecreases surface flow by 26% and erosion by 54%
Average Monthly Surface Flow (1990 – 2012)0510152025JanFebMarAprMayJunJulAugSepOctNovDecSurfaceFlow(mm)BaseTerraces on >20 slopeTerraces on >5 slopeTerraces on >5 slope and bundon 0-5 slopeResidue management on allagricultural landResidue management (5-20slope) and terraces (>20 slope)• Terracing on steep slopes similar to residue mgt. on all agricultural land• Terracing >5% slopes, and mixed terrace/bunds simulations : Surfaceflow 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)00.20.40.60.811.2JanFebMarAprMayJunJulAugSepOctNovDecSedimentYield(Tons/Ha)BaseTerraces on >20 slopeTerraces on >5 slopeTerraces on >5 slope and bundon 0-5 slopeResidue management on allagricultural landResidue management (5-20slope) and terraces (>20 slope)• Terrace + Residue mgt.: Sediment yields decrease from 1.03 tons/hectarein the base simulation to .47 tons/hectare in the month of July (similar tosteep terrace scenario)• Terraces >5% slope and terrace + bund produce very similar results
Conclusions• Decreases in average monthly runoff during the rainy seasonis the primary driver to decreased sediment yield and surfaceflow.• Simulated investments decrease surface runoff, AND increasegroundwater 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 periodwhich may have a direct effect on farmer livelihoods.
Conclusions and Upcoming Research• Households investments on individual plot land require atleast 7 years of maintenance for significant benefits.– Unlike technologies such as fertilizer or improvedseeds, benefits may accrue over longer time horizons.• The longer one sustains SWC, the greater the payoff.However, the individual benefits of sustaining SLWM onprivate land may not outweigh the costs– A mixture of strategies may reap quicker benefits• May be necessary to think of a landscape / watershedapproach– Understanding differences in agro-ecological zones, slope andsoil variations in order to plan most effective interventions– Weigh benefits and costs of comprehensive SLWMapproach, possible opportunities to “phase-in” investments (i.e.terraces on steep slopes first, then some residuemanagement, etc.)
Conclusions and Upcoming Research• HH survey calculated SLWM benefits of improved watercapture and decreased erosion on private landinvestment implicitly• Hydrological model explicitly quantifies biophysicalimprovements to water balance processes within thewatershed on agricultural land• The type and amount of investment in SLWM hasdifferent implications with respect to labor input andutilization of agricultural land at household andlandscape level.
Conclusions and Upcoming Research• Although simulations suggest that a landscape-wide approach mayreap the greatest long-term benefits, it is important to understandthe costs of such an investment.• The economic impacts of SLWM interventions may be morefavorable in certain areas:– Simulate long-term effects of complex ecological-economic systemsare necessary in order to inform policy decision and investments.• Access to markets and infrastructure• Off farm labor opportunities• Land rental (agricultural and foraging rental)• Link the household survey data and hydrological simulationsto model impact of different SLWM interventions, taking intoaccount socio-economic drivers and climate scenarios.
Implications• Average monthly runoff during the rainy season is the primarydriver to decreased sediment yield and surface flow.• Simulations decrease surface runoff from 15% (terraces >20°) to50% (terraces and bunds) and decrease erosion from 19% (residuemgt. on all ag. fields) to 85% (terraces and bunds)• Comprehensive investment of terraces and bunds maintained overthe simulation period (1990-2011) would decrease surface flow50%, increase groundwater flow by 15%, and decrease erosion by85%. (However, can achieve similar effects from constructingterraces on slopes > 5% without bund construction)
Implications• Residue management also has a significant effect on surfaceflow and erosion in the Mizewa watershed.– Average annual surface flow decreased 17% when adopting residuemanagement on all agricultural land and 26% when implementing amixed terracing and residue management.• Simulated investments decrease surface runoff, AND increasegroundwater 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 aswell which may have a direct effect on farmer livelihoods.
Agriculture in the Blue Nile Basin• Land degradation in Ethiopia continues tochallenge agricultural development• Land degradation in some areas is estimated todecrease productivity by 0.5 to 1.1% per year.(Holden et al. 2009)• Moisture stress between rainfall events (dryspells) is responsible for most crop yieldreductions (Adejuwon, 2005)