Date post: | 27-May-2015 |
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Examining the coupled effects of land use and
climate change on watershed functions:
experiences from Asia and AfricaJohn M. Gathenya Xing Ma
Jianchu Xu Meine van Noordwijk
Flow at watershed outlet is determined by multiple factors
Land cover / vegetationLand managementSoils and GeologyRainfall Climate / WeatherTopographyDrainage patternWatershed shape
• Climate change Land use changeLess reliable
rainfallLonger dry spells
More ‘extreme events’More demand and less supply of ‘buffer
functions’ in our watersheds; increasing buffer capacity will reduce vulnerability and enhance
adaptation
Increased drainage
Soil compactionLess macropores
Watershedfunctions
1. Transmit water
2. Buffer peak rain events
3. Release gradually
4. Maintain quality
5. Reduce mass wasting
Site cha-
racteristics
• Rainfall• Land
form• Soil
type• Rooting
depth (natural vegetation)
Relevantfor
• Downstream water users,
• esp. living in floodplains & river beds,
• esp. without storage
• or purification
• at foot of slope
Definitions• Buffering capacity of a watershed is
its ability to reduce variation in streamflow relative to rainfall
• Flow persistence is the fraction of flow on the previous day that can be expected as minimum volume of river flow on a given day
Role of RHA tools• Builds on the concept of PRA but
incorporates the use of computer-based hydrological models to:– Find how severe problems are and their
relationship to land use– Find specific land use practices that can
reduce the problem – Establish the potential for RES to
support beneficial land uses– Compare LEK and MEK– Model scenarios of land use
GenRiver modelGeneric river flow model.Distributed process-based model spatial scale: 1-10km2, temporal scale: dailyIs a patch level representation of daily water balance.From each patch, the flows are routed to the outlet.Can be used to study the relationship between land use/management and flows.
FlowPer model• Models, even simple ones like
Genriver are over-parameterized• FlowPer can serve two functions:
– (1) summarize the key parameters that downstream stakeholders can observe on the flow pattern,
– (2) serve as a parsimonious (parameter-sparse)“null model”
– Extract as much information about upstream conditions from the flow data
FlowPer model
Qt+1 = fp Qt + Qadd
Where Qt and Qt+1 are river flows on subsequent days, fp is flow persistence factor (0<fp<1) and Qadd is a random variate reflecting input from recent rainfall
∑Qadd i = (1-fp) ∑Q
Ideally buffered: fp=1, Qadd = 0
Poorly buffered, erratic: fp=0
Can fp be used to indicate watershed quality?
Case Study: Jangkok sub-watershed, Lombok, Indonesia
Gura river
FAO Land cover
SOTER Soils database version 1
4AD01 is 441.9 km2.Rainfall and flow data 6 years 1970-1975 Mean monthly ET values
Genriver / FlowPer results - Gura river
Simulated and observed flows at 4AD01
Flow persistence from FlowPer model
Flow hydrographs from GenRiver model
Gura FlowPer results
High quality flow data required for application of FlowPer
GenRiver application in Mara River basin
Satellite image analysis showed forest in MRB has declined by almost 60% over the 25 years between 1975 and 1999
Amala and Nyangores
Using Genriver, two scenarios were tested in Amala and Nyangores: 1. base case 2. complete forest coverResult: Restoring forest cover may not necessarily increase water yield
Mara river basin
SWAT and FlowPer application Nyando basin, W Kenya
3587 km2
Rain: 800-160030 yrs records
Mean ET from Kericho and Kisumu met
Flow simulated with SWAT model
18 Scenarios
Flow persistence, base case
For Nyando basin, the inter-annual variability in rainfall causes a lot of variability in flow persistence
Flow persistence for changed rainfall and land use conditions – R. Nyando
Mean flow and Flood frequency River Nyando
Climate change, land surface infiltration and mean flow
Land use impact on flood frequency
+10% rain, land use and flood frequency
Flow persistence under changing rainfall
Flow persistence increases with rainfall
Flow persistence higher for improved surface infiltration
When surface infiltration is low, of flow persistence does not seem to respond to change in rainfall
GenRiver for Kapingazi river catchment
• On the southern slopes of Mount Kenya, Area 61 km2
• Is in Upper Tana River Basin• Average annual Rainfall ranges
between 1200 mm and 1800mm• Rainfall data 1976-1994, some
limited flow data for the same period, water permit records.
• Mean monthly ET for Embu KARI station
• SOTER soil DB, digital DEM, satellite derived land cover
Kapingazi ...in good times
Supplies Embu town with a portion of the water consumed and a number of community and individual irrigation and domestic water projects
History of drying up 1984, 2000, 2011
Decreased flows attributed to decreased rainfall, increased water demand, planting of eucalyptus, poor water allocation, farming on river banks and illegal extractions
Kapingazi cacthment
Rainfall at Embu town and Irangi Forest
No discernible trend in mean annual rainfall totals 1978-2008
Farmers say that the rainfall patterns are changing
1st rain season totals decreasing, 2nd rain season totals increasing, annual means remaining constant
Genriver model results – Kapingazi hydrograph 1976-1994
45 flow measurements spread over the period were used to guide the calibration, some points fit, others do not
Statistic Discharge, m3/s
Qmean 1.457
Normal flow, Q80 0.461
Reserve flow, Q95 0.241
In some extreme dry years, e.g. 1984, actual abstractions exceed low flows at outlet. This explains why the flow in Mar/April 2011 also reached zero
Kejie Watershed
• The Kejie watershed occupies a total area of 1755 km2
• The watershed provides ES to Baoshan Prefecture in Yunnan and to Myanmar and Thailand downstream
• Land use classes: forest, grassland, cropland, settlement, barren and water
Kejie watershed
Kejie watershed
• Elevation oranges from 963 to 3076 m.
• In the 40 years, dramatic change in land cover, gradual increase in temperature, no trend in mean annual rainfall, change in rainfall for some months
Kejie watershed Scenarios
• Temperature: T+1, T+2, T+3, T+4 • Rainfall: P-10, P-5, P+5, P+10 • Land cover: forest+, grassland+, crop+,
urban+
Land use change and water balance
Climate change and Q and ET
Land use change and Water balance
Land use change and climate change
ConclusionsIssue to be analysed SWAT GENRIV
ERFLOWPER
Water flows and land use √√√ √√√
Water flows and Agroforestry √ √√
Water flows and land management practices
√√√ √
Soil erosion and land use / management √√√
Peak flow buffering / flow persistence √ √ √√√
User friendly (input data, running), rapid
√√√ √√√
Large complex watersheds √√√GenRiver a good hydrological model for rapid assessment of small watersheds for PES
Conclusions• Impacts of climate change on
watershed functions interact with those of land use change: They may reinforce or weaken each other.– Both should be considered together in
order to identify interactions • Single effects of land cover change
tend to be greater than single effects of climate change...in some cases, effect of inter-annual variability of rainfall is even greater
Reference• van Noordwijk M, Widodo RH, Farida A,
Suyamto DA, Lusiana B, Tanika L and Khasanah N. 2011. GenRiver and FlowPer: Generic River Flow Persistence Models. User Manual Version 2.0. . Bogor. World Agroforestry Centre - ICRAF, SEA Regional Office. 119 p.
•http://www.worldagroforestry.org/sea/publication?do=view_pub_detail&pub_no= MN0048-11
A frog likes water...but not
when it is boiling