Land Degradation & Rainfall

Transcription

1 Land Degradation & Rainfall Leo Stroosnijder WAGENINGEN UNIVERSITY Soil Science Center Erosion and Soil & Water Conservation Group

2 CONTENTS Land degradation (African bias) Rainfall analysis for LD assessment Rain water balance (at meeting point) Drought mitigation concepts Conclusions Recommendations

3 Less pictures More Food for Thought (based on a large number of PhD s)

4 Land Degradation from a farmers perspective

5 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

6 HOW TO DEFINE? LAND DEGRADATION DESERTIFICATION SOIL DEGRADATION EROSION

7 LAND DEGRADATION IS SERIOUS! DOOM PAPERS papers based on hard facts: RS, GIS and computer models

8 1976 UN Conference on Desertification, Nairobi 1984 Lester Brown 1986 Parkan 1990 Stoorvogel & Smaling 1991 Oldeman (ISRIC) 1992 Some et al Pimentel et al. (in Science) 1997 UNEP 1997 Smaling et al Scherr (IFPRI) 2000 FAO 2006 Pimentel

9 STATE RATE IMPACT Water erosion Wind erosion Chemical degradation Physical degradation TOTAL World soil degradation (millions ha)

10 STATE RATE IMPACT Soil loss Europe, Australia USA Africa, Asia, S-America Soil formation rate t ha -1 y

11 STATE RATE IMPACT Abandoned land: Soil is lost Productivity loss: 10 million ha y times faster than renewal 8% per year David Pimentel (Cornell), 2006 in Environment, Development and Sustainability

12 BUT: HOW SERIOUS IS LAND DEGRADATION? 1994 Tiffen et al. (1803 Malthus) (1965 Boserup) Biot er al. Leach & Mearns Fairhead & Leach

13 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 Warren et al Mazzucato & Niemeijer 2001 Lambin et al Lal 2002 Wolde-Amlak & Stroosnijder 2002 Niemeijer and Mazzucato 2007 Fleskens and Stroosnijder (olives)

14 HOW TO EXPLAIN THIS? DIFFERENT SCALES with DIFFERENT PROXIES FOR LD

15 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

16 Regional proxy: vegetation cover Rasmussen et al. (2001) Studied dunes Burkina ( ) = 15 yrs degradation, > 1985 reverse Arid environments highly variable, rather than disturbed equilibrium

17 Regional proxy: vegetation cover Lomborg (2001) 33 % of world is forest. World cover has not changed in 50 yrs.

18 Regional proxy: vegetation cover Conclusion: vegetation follows rainfall: no evidence for permanent vegetation cover decline.

19 Regional proxy: productivity Burkina Faso,

20 Regional proxy: productivity Mazzucato & Niemeijer (2000) No decline but a steady increase of 2.5 % y -1! In spite of decrease in rainfall!

21 Regional proxy: productivity Conclusion: forty years Sahel: no evidence for productivity decline

22 Watershed proxy: land use changes Land use types Natural forests Cropland Grazing land Plantations Bare land Beressa watershed. Ethiopia (Aklilu Amsalu et al., 2006)

23 Watershed proxy: land use changes Conclusion: Land use dynamics: considerable transitions occurred within the land use types

24 Watershed proxy: gully formation The gullied area under cropland declined by 36 % while those under grassland increased. (Ginchi watershed) Conclusion: no strong evidence

25 Farmers field proxy: fertility

26 Farmers field proxy: fertility Temporal fertility: IIED paper No fertility decline over 30 years!

27 Point scale proxies: Soil organic matter Physical soil qualities

28 For West Africa: Pieri (1989) Point proxy: SOM Natural reduction

29 Point proxy: infiltration capacity

30 Point proxy: structural crust

31 Point proxy depositional crust

32 Point proxies Decrease in soil organic matter Decrease in physical soil qualities CONCLUSION: degrading soil qualities

33 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!

34 HOW TO DEFINE LD? Is Land Degradation contextual??? (Warren, 2002)

35 What scientists want At UN headquarters

36 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.

37 Rainfall Analysis for Land Degradation Assessment

38 FIVE TEMPORAL SCALES Annual Monthly Decadal Daily Minute

39 AT EACH SCALE: Average (does not exist in real world) Spatial variation (village, social network) Temporal variation (risk)

40 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!

41 Annual rainfall: probabilities = wet year = dry year

42 Monthly rainfall: Long & short rains Monthly rainfall for a dry, normal and wet year, Kwalei

43 Decadal rainfall: Length of growing season Type of year Normal Dry Runoff 50 % Runoff 0 %

44 Daily rainfall: size classes

45 Daily rainfall: Probability of non-exceedance defines return period needed for design

46 Daily Rainfall: long-term modeling Runoff (% of rain) No Till Till Year yrs effect of till no-till on runoff

47 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, Hudson (1965) Dijk et al. (2002) RUSLE-2 (2003) Rain intensity (mm h-1) Threshold erosive showers: mm h -1

48 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

49 Rain Water Balance

50 Land Degradation and Rainfall meet at the soil surface: the rainfall is divided over several hydrological compartments.

51 Importance Soil Physics knowledge Rain water balance: Infiltration water balance: Soil water balance: P = INT + I + R I = S + D S = E + T

52 Soil physical/hydrological processes at deteriorating physical soil qualities Runoff Evaporation Drainage below root zone Plant available water Transpiration

53 Stored and available water: TAW = RD * 0.9(FC - WP)

54 For farmers: land degradation = coping with dry spells

55 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

56 GREEN WATER USE EFFICIENCY mm transpiration mm of rain In Africa GWUE =10-20 %, Great Plains USA > 55 %

57 Combating land degradation = dry spell mitigation

58 Three approaches to mitigate dry spells Reduce Runoff Improve TAW Improve GWUE

59 Basket of Land Management Practices

60 FANYA JUU TERRACE

61 Progressive terracing

62 COMBINATION = AREA + LINE MULCH STONE BARRIER

63 Water Harvesting

64 Conclusions

65 Conclusion 1 Be critical on hard evidence of LD from RS, models and measurements.

66 Conclusion 2 Analyse LD at multiple spatial-temporal scales.

67 Conclusion 3 Analyse rainfall also at multiple scales.

68 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.

69 Conclusion 5 Africa s Green Water Use Efficiency is too low.

70 Conclusion 6 What farmers perceive as drought is the effect of deteriorating physical soil characteristics.

71 Recommendations

72 Recommendation 1 Combine model studies with empirical studies at farm and village level.

73 Recommendation 2 Improve the monitoring of land degradation and rainfall.

74 Recommendation 3 Make available rainfall data better accessible: present situation is a shame!

75 Recommendation 4 Invest in GWUE improvement.

76 Recommendation 5 Research as well as education have become too disciplinary. There are hardly professionals that oversee the complexity of land management.

77 Recommendation 6 Develop a system of timely transfer of rainfall information to farmers.

78 Recommendation 7 Learn farmers how to adapt their land management.

79 Thanks for your attention

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