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Understanding impacts of sustainable land management interventions using SWAT

Hydrological Model

Birhanu Zemadim (IWMI) and

Emily Schmidt (IFPRI)

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

• Brief overview of previous research on Sustainable Land and Watershed Management (SLWM)

• Landscape level investments in SLWM• Hydrological simulations of watershed

investments • Implications of SLWM• Conclusions and Upcoming Research

Outline of presentation

Overview of previous research– Chemoga 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)

Cntd.. Overview of previous research• Soil 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 Sites: in the Blue Nile Basin of Ethiopia

Dapo watershed 18 km2

Mizewa watershed 27 km2

Meja watershed 96 km2

Simulation of watershed landscape-level investments

Slope gradient ShareUnder 5 0.135-20 0.65>20 0.22

Root Zone

Vadose (unsaturated)

Zone

ShallowAquifer

Confining Layer

Deep Aquifer

Evaporation andTranspiration

Location of Watershed Outlet

Watershed Flooding

Watershed drought

Photo taken in March 2013

Households Using Sustainable Land Management (SLM) on Private Land

DistrictPercent of

district

Year of first Community

Program

Most common activity on private

land (percent)Alefa 50% 1990 soil bund (64.2)Fogera 54% 1983 stone terrace (65.8)Misrak Estie 54% 1977 stone terrace (36.1)Gozamin 21% 1988 soil bund (40.9)Dega Damot 82% 1986 soil bund (42.8)Mene Sibu 7% 1992 soil bund (89.8)Diga 32% 2000 irrigation canal (2.9)Jeldu 2% na stone terrace (24.0)Toko Kutaye 79% 1989 soil bund (33.7)

SLWM Investments

Soil Bunds Wood check dam

Stone terraces Stone check dam

Perceived Most Successful SLWM activities (% of households)

Simulation of landscape-level investments• Investment decisions are simulated to take into

account tradeoffs in labor and land investment and terrain type:

1. Terracing on steep hillsides

2. Terracing on mid-range and steep hillsides

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

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

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

Labor

Land

a) Newly constructed Fanya Juu terrace /bund

b) Fanya Juu after five years of construction

Source: IWMI Africa Rainwater harvesting diagram

Terraces 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 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 following season (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: 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

Calibration ValidationENS R2 ENS R2

Weekly .72 .73 .61 .69Monthly .93 .94 .71 .81

Model simulation• Assume 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 bund

Residue mgt. (all)

Residue mgt. and

terraceSurface flow 45.0 -15% -45% -50% -17% -26%Lateral flow 200.3 1% 3% 3% 1% 2%Groundwater flow

72.2 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 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

Implications

• Average 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)

Implications• Residue 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.

Conclusions• Households 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 costs

– A mixture of strategies may reap quicker benefits

• May be necessary to think of a landscape / watershed approach

– Understanding differences in agro-ecological zones, slope and soil variations in order to plan most effective interventions

– Weigh 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.)

Cntd..Conclusions

• Decreases 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.

Cntd..Conclusions• Although 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 infrastructure• Off farm labor opportunities• Land 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.

Cntd..Conclusions

• HH 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.