Land Degradation & Rainfall Leo Stroosnijder WAGENINGEN UNIVERSITY Soil Science Center Erosion and Soil & Water Conservation Group
CONTENTS Land degradation (African bias) Rainfall analysis for LD assessment Rain water balance (at meeting point) Drought mitigation concepts Conclusions Recommendations
Less pictures More Food for Thought (based on a large number of PhD s)
Land Degradation from a farmers perspective
HOW TO DEFINE? LAND Soil + Water + Vegetation + Fauna LAND DEGRADATION The decline in the extent at which land yields products useful to local livelihoods COMPLEX NOTIONS: Too many (bad) definitions
HOW TO DEFINE? LAND DEGRADATION DESERTIFICATION SOIL DEGRADATION EROSION
LAND DEGRADATION IS SERIOUS! DOOM PAPERS papers based on hard facts: RS, GIS and computer models
1976 UN Conference on Desertification, Nairobi 1984 Lester Brown 1986 Parkan 1990 Stoorvogel & Smaling 1991 Oldeman (ISRIC) 1992 Some et al. 1995 Pimentel et al. (in Science) 1997 UNEP 1997 Smaling et al. 1999 Scherr (IFPRI) 2000 FAO 2006 Pimentel
STATE RATE IMPACT Water erosion Wind erosion Chemical degradation Physical degradation TOTAL 1100 600 500 100 2300 World soil degradation (millions ha)
STATE RATE IMPACT Soil loss Europe, Australia USA Africa, Asia, S-America Soil formation rate t ha -1 y -1 5-10 6-17 30-40 1
STATE RATE IMPACT Abandoned land: Soil is lost Productivity loss: 10 million ha y -1 10-40 times faster than renewal 8% per year David Pimentel (Cornell), 2006 in Environment, Development and Sustainability
BUT: HOW SERIOUS IS LAND DEGRADATION? 1994 Tiffen et al. (1803 Malthus) (1965 Boserup) 1995 1996 1996 Biot er al. Leach & Mearns Fairhead & Leach
1997 Crosson 1998 Scoones & Toulmin 1998 Fairhead & Leach 2000 Mazzucato & Niemeijer 2000 Lindert 2001 Lomborg 2001 Mortimer & Adam 2001 Scoones 2001 Benjaminsen 2001 Rasmussen et al. 2001 Warren et al. 2001 Mazzucato & Niemeijer 2001 Lambin et al. 2001 Lal 2002 Wolde-Amlak & Stroosnijder 2002 Niemeijer and Mazzucato 2007 Fleskens and Stroosnijder (olives)
HOW TO EXPLAIN THIS? DIFFERENT SCALES with DIFFERENT PROXIES FOR LD
FOUR SPATIAL SCALES Regional: proxies: vegetation cover & productivity Watershed: proxies: land use changes & gully formation Farmers field: proxies: soil fertility Point: proxies: SOM & physical soil qualities
Regional proxy: vegetation cover Rasmussen et al. (2001) Studied dunes Burkina (1955-1994) 1970-1985 = 15 yrs degradation, > 1985 reverse Arid environments highly variable, rather than disturbed equilibrium
Regional proxy: vegetation cover Lomborg (2001) 33 % of world is forest. World cover has not changed in 50 yrs.
Regional proxy: vegetation cover Conclusion: vegetation follows rainfall: no evidence for permanent vegetation cover decline.
Regional proxy: productivity Burkina Faso, 1960-2000
Regional proxy: productivity Mazzucato & Niemeijer (2000) No decline but a steady increase of 2.5 % y -1! In spite of decrease in rainfall!
Regional proxy: productivity Conclusion: forty years Sahel: no evidence for productivity decline
Watershed proxy: land use changes Land use types 1957 1984 2000 Natural forests 14.8 6.8 2.4 Cropland 54.8 56.6 60.6 Grazing land 8.5 18.7 11.6 Plantations 1.8 1.2 10.6 Bare land 16.3 12.8 8.0 Beressa watershed. Ethiopia (Aklilu Amsalu et al., 2006)
Watershed proxy: land use changes Conclusion: Land use dynamics: considerable transitions occurred within the land use types
Watershed proxy: gully formation The gullied area under cropland declined by 36 % while those under grassland increased. (Ginchi watershed) Conclusion: no strong evidence
Farmers field proxy: fertility
Farmers field proxy: fertility Temporal fertility: IIED paper No fertility decline over 30 years!
Point scale proxies: Soil organic matter Physical soil qualities
For West Africa: Pieri (1989) Point proxy: SOM Natural reduction
Point proxy: infiltration capacity
Point proxy: structural crust
Point proxy depositional crust
Point proxies Decrease in soil organic matter Decrease in physical soil qualities CONCLUSION: degrading soil qualities
THE PUZZLE LD at point scale seems proven LD at higher scales NOT WHY? Delayed effect? Compensation effect? Aggregation problem? This makes up- en downscaling difficult!
HOW TO DEFINE LD? Is Land Degradation contextual??? (Warren, 2002)
What scientists want At UN headquarters
What farmers think The indifference of many Sahelian farmers to high rates of soil loss may reflect their recognition that erosion does not seriously damage productivity in the short term.
Rainfall Analysis for Land Degradation Assessment
FIVE TEMPORAL SCALES Annual Monthly Decadal Daily Minute
AT EACH SCALE: Average (does not exist in real world) Spatial variation (village, social network) Temporal variation (risk)
Annual rainfall: trend analysis Burkina: mean over 80 years = 800 mm Decrease from 1100 (1950) to 700 (1990) This is 400 mm (50% of mean) over 40 years!
Annual rainfall: probabilities = wet year = dry year
Monthly rainfall: Long & short rains Monthly rainfall for a dry, normal and wet year, Kwalei
Decadal rainfall: Length of growing season Type of year Normal Dry Runoff 50 % 75 53 Runoff 0 % 108 93
Daily rainfall: size classes
Daily rainfall: Probability of non-exceedance defines return period needed for design
Daily Rainfall: long-term modeling 50 45 40 Runoff (% of rain) 35 30 25 20 15 10 5 No Till Till Year 0 1950 1955 1960 1965 1970 1975 1980 1985 35 yrs effect of till no-till on runoff
Minute scale: erosivity of rain 0,35 W&S (1958) Kinetic Energy (MJ ha-1 mm-1) 0,30 0,25 0,20 0,15 0,10 0,05 0 20 40 60 80 100 Hudson (1965) Dijk et al. (2002) RUSLE-2 (2003) Rain intensity (mm h-1) Threshold erosive showers: 10 25 mm h -1
Erosivity INDEX of a shower Wischmeier & Smith: KE * I 30 correlates best with erosion (splash + overland flow + rill) I 30 is the maximum intensity during 30 minutes
Rain Water Balance
Land Degradation and Rainfall meet at the soil surface: the rainfall is divided over several hydrological compartments.
Importance Soil Physics knowledge Rain water balance: Infiltration water balance: Soil water balance: P = INT + I + R I = S + D S = E + T
Soil physical/hydrological processes at deteriorating physical soil qualities Runoff Evaporation Drainage below root zone Plant available water Transpiration
Stored and available water: TAW = RD * 0.9(FC - WP)
For farmers: land degradation = coping with dry spells
Number of days overcoming a dry spell Good soil: 80 cm * 0.10 = 80 mm/4 = 20 days Degraded soil: 60 * 0.08 = 48 mm/4 = 12 days NOT a change in rainfall BUT a change in the soil
GREEN WATER USE EFFICIENCY mm transpiration mm of rain In Africa GWUE =10-20 %, Great Plains USA > 55 %
Combating land degradation = dry spell mitigation
Three approaches to mitigate dry spells Reduce Runoff Improve TAW Improve GWUE
Basket of Land Management Practices
FANYA JUU TERRACE
Progressive terracing
COMBINATION = AREA + LINE MULCH STONE BARRIER
Water Harvesting
Conclusions
Conclusion 1 Be critical on hard evidence of LD from RS, models and measurements.
Conclusion 2 Analyse LD at multiple spatial-temporal scales.
Conclusion 3 Analyse rainfall also at multiple scales.
Conclusion 4 Dryland ecosystems are adapted to the magnitude and frequency of dry periods and these are instrumental in controlling the long term functioning of these systems.
Conclusion 5 Africa s Green Water Use Efficiency is too low.
Conclusion 6 What farmers perceive as drought is the effect of deteriorating physical soil characteristics.
Recommendations
Recommendation 1 Combine model studies with empirical studies at farm and village level.
Recommendation 2 Improve the monitoring of land degradation and rainfall.
Recommendation 3 Make available rainfall data better accessible: present situation is a shame!
Recommendation 4 Invest in GWUE improvement.
Recommendation 5 Research as well as education have become too disciplinary. There are hardly professionals that oversee the complexity of land management.
Recommendation 6 Develop a system of timely transfer of rainfall information to farmers.
Recommendation 7 Learn farmers how to adapt their land management.
Thanks for your attention
Leo.Stroosnijder@wur.nl