Semi-arid Niger Research Articles

Electrical imaging to decode the potential aquifer locations for water security in semiarid Niger, Africa

The UN Sustainable Development Goals for the millennium will be left incomplete without its focus on water availability and its sustainability in the semi-arid zone of North and Central Africa. While studying about underlying aquifers, it is necessary to map their extent for groundwater usage. For such purpose, surface hydro-geophysical methods are widely used in exploration of groundwater in 8 regions of Niger country. Out of 207 soundings carried out, only 136 Vertical Electrical Sounding data is interpreted here. Rest 71 datasets are not considered as they were very close to each other, within the distance of less than 50 m. Only one dataset from a cluster is selected for analysis. Data was interpreted in real time and was re-interpreted to be more confident about the deliverables. The procedure followed was to start the Clustering of raw datasets. The trend analysis was carried out on the identified clusters, after removing the outlier data from the trend. 1D Forward model was redefined by reducing the error and giving initial value based on the trend of the basic curve, which led to either increase or decrease in the number of layers. This brings out a layer cake model of the area with different depth slices. The lateral and vertical mapping of electrical resistivity of the unsaturated zone (by resistivity variations) allowed us to delineate preferential pathways for deep infiltrations through alluvial sandy fans. Thus, in this study, the usefulness of resistivity mapping for characterizing the geometry of aquifers clearly brings out the potential zones for recharge and the pathways for moisture to travel in the unsaturated zone.

Combined effect of trees and soil fertility management practices on millet yields in the Sahel

Tree/shrub species are used in farmer fields of sahelian agroforestry parklands to conserve and restore degraded lands, improve soil fertility and increase crop yield. A field study was conducted to investigate the effect of five tree species (Annona senegalensis, Balanites aegyptiaca, Faidherbia albida, Guiera senegalensis and Piliostigma reticulatum) on soil fertility and crop yield under different fertility management practices in semi-arid Niger. Three factors (tree species, positions to tree trunk and fertilizer management) and four levels of fertilizer microdosing management (DAP, NPK, manure and control) were investigated. We found that tree species improved soil fertility and crop yield differently. F. albida, improved the soil quality far better than other four tree species for all soil nutrients analyzed. The highest millet yield was recorded under P. reticulatum and F. albida for both grain (1011 and 1005 kg ha−1, respectively) and straw yields (3662 and 3786 kg ha−1, respectively). The yield was found to be two- to four-folds higher under direct tree crown compared to close neighborhood or treeless outside. The combined tree species and fertilizer effect were additive and the positive effect can extend to the outside crown. However, the combined effect of mineral fertilizer with tree species on yield was better than that of manure. Integrated tree species and moderate fertilizers was effective and can be used as a sustainable fertility management practice in Sahelian countries where organic amendment is frequently scarce and it is difficult for smallholder farmers to adopt high recommended mineral fertilizer for crop production. Further investigations are needed to assess the economic performance and the long-term sustainability of the integrated trees and soil fertility management in the Sahel.

Dynamic relationship of traditional soil restoration practices and climate change adaptation in semi-arid Niger

Climate change increases the vulnerability of agrosystems to soil degradation and reduces the effectiveness of traditional soil restoration options. The implementation of some practices need to be readjusted due to steadily increasing temperature and lowering precipitation. For farmers, the best practice found, should have the potential to achieve maximum sustainable levels of soil productivity in the context of climate change. A study was conducted in South-West Niger to investigate the use of the suitable practice, through (i) a meta-analysis of case studies, (ii) using field survey and (iii) by using AquaCrop model. Results showed that the effects of the association zaï + mulch on crop yield was up to 2 times higher than control plots depending on climate projections scenario RCP 8.5 under which carbon dioxide (CO2) concentrations are projected to reach 936 ppm by 2100. The practice appeared to be an interesting option for enhancing crop productivity in a context of climate change. Concerning its ability, it offers the best prospects to reverse soil degradation in the study area. In addition, the simulation showed that this strategy was suitable for timely sowing and therefore confirmed scholars and farmers views. Furthermore, this practice is relatively more effective compared to the others practices. These results show that association zaï + mulch could be considered as the best practice that can participate to a successful adaptation to reduce risk from climate change at the same time by reducing the vulnerability of farmers in Southwest of Niger for now and even for the future.

Genetic and geographic variation in growth of Balanites aegyptiaca in Niger: comparing results from provenance/progeny tests in the nursery and field

Some tree improvement programs in Africa use nursery tests to investigate genetic and geographic variation in growth, but do they lead to the same conclusions as field tests? We investigated this question using provenance/progeny tests (12 provenances, 108 families) of Balanites aegyptiaca from semi-arid Niger. The nursery test included treatments for time (12–16 months) and watering regime (reduced and normal). Family variation was significant for shoot diameter (Sdia), height (Sht), dry weight (Swt) and root dry weight (Rwt) in the nursery, and for tree height at 1 and 2 years (Fht-1, Fht-2) but not at 13.5 years (Fht-13.5) in the field. Provenance variation was significant only for root/shoot weight ratio (RSwt) in the nursery. Family mean Fht-1 and Fht-2 were positively correlated with all nursery growth variables except RSwt. Provenance mean Fht-2 was positively correlated with Sht, while provenance mean Fht-13.5 was negatively correlated with Swt and positively correlated with RSwt. Family mean survival at 13.5 years was positively correlated with Swt, Sdia, Fht-1 and Fht-2. Family mean Rwt, RSwt and Fht-2 increased from the more humid western to the drier eastern locations. Most correlations with nursery growth variables were stronger at 16 than at 12 months and in the normal compared with the reduced watering regime. Results suggest that Swt and Sdia may be useful for predicting family survival in the field, and Rwt and RSwt may be useful for investigating geographic variation in growth in the field. We recommend conducting both nursery and long-term field tests.

Optimizing Fertilizer Use by Smallholder Farmers and Economic Returns to Maize in Semi-arid Niger

Optimizing Fertilizer Use by Smallholder Farmers and Economic Returns to Maize in Semi-arid Niger

Evaluating surface and subsurface water storage variations at small time and space scales from relative gravity measurements in semiarid Niger

[1] The acquisition of reliable data sets representative of hydrological regimes and their variations is a critical concern for water resource assessment. For the subsurface, traditional approaches based on probe measurements, core analysis, and well data can be laborious, expensive, and highly intrusive, while only yielding sparse data sets. For this study, an innovative field survey, merging relative microgravimetry, magnetic resonance soundings, and hydrological measurements, was conducted to evaluate both surface and subsurface water storage variations in a semiarid Sahelian area. The instrumental setup was implemented in the lower part of a typical hillslope feeding to a temporary pond. Weekly measurements were carried out using relative spring gravimeters during 3 months of the rainy season in 2009 over a 350 × 500 m2 network of 12 microgravity stations. Gravity variations of small to medium amplitude (≤220 nm s−2) were measured with accuracies better than 50 nm s−2, revealing significant variations of the water storage at small time (from 1 week up to 3 months) and space (from a couple of meters up to a few hundred meters) scales. Consistent spatial organization of the water storage variations were detected, suggesting high infiltration at the outlet of a small gully. The comparison with hydrological measurements and magnetic resonance soundings involved that most of the microgravity variations came from the heterogeneity in the vadose zone. The results highlight the potential of time lapse microgravity surveys for detecting intraseasonal water storage variations and providing rich space-time data sets for process investigation or hydrological model calibration/evaluation.

Coping with Rainfall Variability: Dry Spell Mitigation and Implication on Landscape Water Balances in Small-scale Farming Systems in Semi-arid Niger

Rainfall variability and inherent dry spells are a reality with severe implications for smallholder agro-ecosystems in semi-arid Sahel. To increase both on- and off-farm biomass production and productivity is challenging with these climate-induced temporal and spatial variations of water. This paper tests the idea that increased vegetation through tree cover may impact water balance in a water-stressed landscape: South-east Niger. Local rainfall data, farming systems data and a landscape water-modelling tool (ArcSWAT) are used. Four production domains (conventional or fertilized combined with millet crop or millet crop plus trees) were assessed for long-term yield and landscape water balance impacts. The dry-spell analysis shows a frequency of dry spells less than 14 days is in the order of one to two dry-spell events per season in 7 years out of 10 years. The occurrence has increased between 1960 and 2004, despite a slight recovery of total annual rainfall amounts since the severe droughts of the 1980s. Results of modelled millet yields and landscape water balances suggest that options exist to enhance landscape productivity. With marginal inputs of fertilizer, millet yields increased fivefold to 2.0–2.4 t ha− 1, and water productivity improved from 6,000 to 12,000 m3 actual evapotranspiration (ETa) t− 1 grain, to an improved 1,700–3,000 m3 ETa t− 1 grain. In addition, 10% tree cover in combination with fertilized millet increased yield with marginal or no impact on water partitioning and flows in the landscape. The policy opportunities are complex and urgently needed in view of increased rainfall variability due to expected climate change. To develop sustainable pathways in these landscapes dominated by poor smallholder framers requires water managers to be more innovative and go beyond water resources alone.

Effect of termite mound material on the physical properties of sandy soil and on the growth characteristics of tomato (Solanum lycopersicum L.) in semi-arid Niger

This investigation assessed the effects of termite mound material (TMM) on the physical properties of sandy soil and on tomato (Solanum lycopersicum L.) growth characteristics and water use efficiency. TMM combined with organic manure, TMM combined with rice straw mulching and organic manure, organic manure alone (OM) and unamended (T0) were the treatments used. Results showed that soil treated with TMM had more clay sized particles and organic carbon content than T0 and OM. In TMM-treated soil, more water was being retained at both field capacity and permanent wilting point. The application of TMM did not affect the amount of plant available water. Saturated hydraulic conductivity also remained unaffected by the TMM application, but increased with the organic matter treatment. Tomatoes grown in TMM amended soils had greater plant height and more leaves, fruit and biomass. No specific rate of TMM application was better for all parameters being assessed. The amount of water used by the tomatoes was significantly correlated (P < 0.01) with fresh fruit yield (r = 0.82), leaf area index (r = 0.82) and total dry matter production (r = 0.68). While TMM did not specifically affect plant water-use efficiency, this parameter was generally improved in amended soils.

Understanding climate change adaptation: lessons from community-based approaches

Prelims (Preface, Acknowledgements) 1. Introduction: understanding community-based adaptation 2. Erosion and flooding in northern Bangladesh 3. Changing seasons and flash flooding in the foothills of the Nepal Himalaya 4. Desert and floodplain adaptation in Pakistan 5. Increasing paddy salinity in coastal Sri Lanka 6. Increasing drought in arid and semiarid Kenya 7. Multiple pressures on pastoralism in semi-arid Niger 8. Declining water resources in Sudan's Red Sea coastal belt 9. Extreme weather in the Peruvian high Andes 10. Conclusion: community-based adaptation in practice Back Matter (References, Index)

Contribution of geophysical surveys to groundwater modelling of a porous aquifer in semiarid Niger: An overview

Subsurface geophysical surveys were carried out using a large range of methods in an unconfined sandstone aquifer in semiarid south-western Niger for improving both the conceptual model of water flow through the unsaturated zone and the parameterization of numerical a groundwater model of the aquifer. Methods included: electromagnetic mapping, electrical resistivity tomography (ERT), resistivity logging, time domain electromagnetic sounding (TDEM), and magnetic resonance sounding (MRS). Analyses of electrical conductivities, complemented by geochemical measurements, allowed us to identify preferential pathways for infiltration and drainage beneath gullies and alluvial fans. The mean water content estimated by MRS (13%) was used for computing the regional groundwater recharge from long-term change in the water table. The ranges in permeability and water content obtained with MRS allowed a reduction of the degree of freedom of aquifer parameters used in groundwater modelling.

Contribution of magnetic resonance sounding to aquifer characterization and recharge estimate in semiarid Niger

To improve the knowledge of the regionally important Continental Terminal 3 (CT3) aquifer in south-western Niger, fifteen magnetic resonance soundings (MRS) were carried out in December 2005 in the vicinity of wells and boreholes. The output MRS geophysical parameters, i.e. water content and decay constants versus depth, were compared to hydrogeological characteristics, i.e. water table depth, total porosity, specific yield and transmissivity estimated from direct measurements, pumping tests and transient groundwater modelling. The MRS-determined parameters were then used to estimate the rates of groundwater recharge. Contained in poorly consolidated Tertiary sandstones, the CT3 aquifer's water table has continuously risen by 4 m in total over the past four decades. Additionally, a significant portion of this increase has occurred in the past decade alone, with an annual rise now ranging between 0.1 and 0.3 m depending on the monitored well. Increase in groundwater recharge due to land clearance and deforestation explains this situation. According to previous estimations, the pre-clearing recharge ranged from 1 to 5 mm per year in 1950–60 s, while more recent recharge rates (1990s–2000s) range from 20 to 50 mm per year. These recharge values are directly affected by estimated aquifer specific yield value, while the spatial variation of rates of water table rise can be attributed to large scale hydrodynamic heterogeneities in the aquifer. However, few field measurements were available to confirm these assumptions. The main results of this study are: (1) The water table depth and aquifer transmissivity are estimated from MRS output parameters with an average accuracy of ± 10% and ± 9% respectively. (2) The MRS-determined water content is linked to both the total porosity and the specific yield of the aquifer, but no quantitative formulation can be proposed as yet. (3) Using the average MRS-determined water content over the investigated area, i.e. 13%, the groundwater recharge rates can be estimated to be ~ 2 mm per year in the 1950–1960s (pre-clearing period), and ~ 23 mm per year for the last decade. (4) The variations in specific yield and transmissivity cannot explain by themselves the spatial variability of the rise of the water table. (5) The ranges in transmissivity and water content obtained from MRS are more realistic than the groundwater modelling outputs. Therefore, MRS could be used to better constrain the aquifer parameters in groundwater modelling with a dense site network. Finally, this work illustrates how MRS can successfully improve characterisation and transient multi-year groundwater balance of commonly found sedimentary aquifers, particularly when integrated with well observations and pumping tests.

Deep infiltration through a sandy alluvial fan in semiarid Niger inferred from electrical conductivity survey, vadose zone chemistry and hydrological modelling

In semiarid southwestern Niger, most of the groundwater recharge is indirect and occurs through endoreic ponds. Elsewhere in the landscape, there is no evidence of deep infiltration, with a possible exception for gullies and alluvial fans on sandy slopes. In order to verify this hypothesis, a detailed geophysical and geochemical survey was conducted on a large, representative mid-slope fan (6 ha). At this site, distributed hydrological modelling conducted over the encompassing endoreic catchment (190 ha) showed high losses of runoff water by infiltration. Electromagnetic mapping and 2-D electrical imaging survey were used to investigate the 35 m deep vadose zone; in addition, 8 boreholes were drilled following the geophysical survey to constrain the interpretation. Variations in apparent electrical conductivity measured in boreholes appear to be mainly linked with changes in the soil solution mineralization. An extrapolation throughout the area shows that apparent electrical conductivity of the ground is systematically lower below channels; this suggests localised leaching through the unsaturated zone. A physically-based, 2-D distributed hydrologic model was used to estimate the amount of surface water loss by infiltration for the 1992–2002 period. Depending on year, infiltrated volumes range from 1000 to 24 000 m 3. This represents between 5% and 16% of the runoff that reaches the final outlet of the basin, an endoreic valley bottom pond where recharge to the aquifer has been shown to occur. Because leaching of the vadose zone is observed down to a depth of 10 m below channels, episodic groundwater recharge through sandy mid-slope fans is highly probable during rainy years.

Soil organic matter, surface crusting and erosion in Sahelian farming systems based on manuring or fallowing

The cultivated soils of the Sahel are sandy, very poor in organic matter, and are prone to surface crusting and erosion. Farmers sustain pearl millet production through permanent cultivation with manure input or long-term fallowing. Due to population increase fallows are reduced bringing on low yields. This on-farm study (Banizoumbou, semi-arid Niger) examines the impact of clearing and weeding on soil surface processes, the decline in soil organic matter under prolonged cultivation, and whether manure inputs are sufficient to sustain millet production. Short-term effects were examined in a 3-year experiment with dung application (5 Mg ha−1) and weeding frequency (no, two and four weedings). Long-term effects were studied a 4-year experiment comparing three farming systems: (i) permanent cultivation with manure; (ii) cultivation after long fallows >15 years; (iii) cultivation after short fallows 3–5 years. Due to the difference in scale, different processes of erosion were studied, saltation and creep in the first experiment, suspension in the second. Dung deposits provided mechanical protection of the soil surface by reducing crusting and by trapping aeolian sand. A very low cover of dung, 0.5–5%, is enough to protect the soil effectively by forming barriers against creep and saltation. Suspension occurs during tillage. Manual cultivation, while disrupting aggregates and surface crusts, enhances wind erosion and this leads to loss of clay and silt from the top soil. Changing from long to short fallow periods and even to permanent cultivation, constitutes a trend towards more intensive land use and this, eventually, leads to extremely sandy top soils (from 890 up to 934 g kg−1) and coincidences with a soil surface becoming less crusted (from 49% of field surface affected down to 7%). Manure input cannot prevent the erosion of fine earth which seems to be an irreversible feature of long-term cultivation. Dung inputs, 0.5–1 Mg ha−1 per year, are insufficient to increase substantially millet grain yield (350 kg ha−1 in manured; 300 kg ha−1 in long fallow fields), or elevated soil organic matter (2.9 g kg−1 soil, in manured fields; 4.0 g kg−1 in long fallow fields, 0–20 cm). Yet grain yield remains stable under permanent cropping, while fields without manure need fallowing for long periods to restore fertility. Under short fallows without dung input soil organic matter drops to 2.4 g kg−1, and grain yields to 125 kg ha−1.

Nutrient availability and pearl millet production in Sahelian farming systems based on manuring or fallowing

Pearl millet production in the sandy soils of the Sahel is maintained by fallows and manures are the principal added source of fertility. Due to rapid population growth fallow periods are reduced, yields and nutrient availability decline and manure seems to be the only reasonable alternative for fallowing. This on-farm study (Banizoumbou, semi-arid Niger) investigates crop response to low and very low nutrient availability as well as within-season shortages and within field variations. Two sources of data were combined: a 3-year field experiment with dung application (5 Mg ha −1 ) and weeding frequency (no, two and four weedings); and a comparison of farming systems, n = 32, field × years (i) permanent cultivation with manure; (ii) cultivation after long fallows >15 years; (iii) cultivation after short fallows 3–5 years. The permanent cultivation of soils in the study area is achieved with extremely low levels of manure input 0.5–1 Mg ha −1 per year, adding about 10 kg N, 1.2 kg P and 5 kg K to the soil. Dung inputs allow grain yield (350 kg ha −1 ) and nutrient supply (ECEC 0.81 mmolc kg −1 , total N 151 mg kg −1 , 0–20 cm depth) to remain stable under prolonged cropping (10–17 years), while field without manure need fallowing for long periods (>15 years) to restore fertility. The trend from long to short fallow periods accords with decline in grain yield from 300 kg ha −1 yields down to 125 kg ha −1 and of a ECEC reduction from 1.10 to 0.72 mmolc kg −1 . The positive effect of dung application on crop growth is restricted to the first half of the season resulting in extra panicles and 50% extra crop residues. Other yield components remained unaffected and grain yield is but slightly improved because the initial good conditions do not continue. Very small variations in fertility—ECEC, NPK—induced by dung application, excessive weeding or due to soil surface conditions, had yet an impact on pearl millet growth and grain yield. The field experiment showed that nutrient levels and grain yield decline with excessive weeding but less so if dung is applied. Manuring is an alternative for fallowing rather than replacing it because additional constrains crop up with prolonged cultivation: the population of coprophage insects was reduced so some dung stayed unconsumed, and extremely sandy soils developed because wind erosion had carried off the most fertile particles from these rather bare soils making manure input no longer effective. © 2004 Elsevier B.V. All rights reserved.

Inter-annual variation of species composition of fallow vegetation in semi-arid Niger

To monitor inter-annual changes of species composition, ten permanent plots were investigated on six fallow sites on different soil types in Niger, West Africa, from 1994 to 1998. Based on the behaviour of species occurrence, four groups of plant species were distinguished for each site: (i) a group of species that occurred in one of the years but was absent in others, (ii) a group of species that occurred in all years but changed enormously in abundance from year to year, (iii) a group of species of constant occurrence over the years and (iv) a group of species that could not be attributed to any of the other groups because they either occurred in too small numbers or showed a contradictory behaviour between the different sites. In total, 43 species, most of them restricted to one or two sites, showed inter-annual fluctuations in occurrence. This constitutes a considerable problem for vegetation studies in semi-arid environments, because for phyto-sociological approaches as well as for the analysis of indicator species, species that are abundant in some years but absent in others cannot be used as character species or differential species for the distinction of plant communities. For this purpose we identified five groups of species, that, in relation to soil type and grazing intensity, can be used as indicator, character or differential species groups for vegetation studies in the Sahel.

Estimate of recharge of a rising water table in semiarid Niger from 3H and 14C modeling.

A hydrodynamic survey carried out in semiarid southwest Niger revealed an increase in the unconfined ground water reserves of approximately 10% over the last 50 years due to the clearing of native vegetation. Isotopic samplings (3H, 18O, 2H for water and 14C, 13C for the dissolved inorganic carbon) were performed on about 3500 km2 of this silty aquifer to characterize recharge. Stable isotope analyses confirmed the indirect recharge process that had already been shown by hydrodynamic surveys and suggested the tracers are exclusively of atmospheric origin. An analytical model that takes into account the long-term rise in the water table was used to interpret 3H and 14C contents in ground water. The natural, preclearing median annual renewal rate (i.e., recharge as a fraction of the saturated aquifer volume) lies between 0.04% and 0.06%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity between 10% and 25%), this implies a recharge of between 1 and 5 mm/year, which is much lower than the estimates of 20 to 50 mm/year for recent years, obtained using hydrological and hydrodynamic methods and the same aquifer parameters. Our study, therefore, reveals that land clearing in semiarid Niger increased ground water recharge by about one order of magnitude.

Influence of dry season supplementation for cattle on soil fertility and millet (Pennisetum glaucum L.) yield in a mixed crop/livestock production system of the Sahel

An experiment was conducted in 1996 and 1997 in semi-arid Niger, to determine the influence of supplementation (no supplement, supplemented with millet bran + simple superphosphate + blood meal) of cattle and mulching (0 or 3 t ha−1 of Aristida sieberiana straw) on soil fertility and millet yield. Manure was applied through corralling at a rate of 3 t faecal dry matter (FDM) ha−1 alone or associated to mulching. The residual effects of the treatments were measured on a second millet crop in 1997. Compared to control, the association of mulching and corralling of supplemented as well as non supplemented cattle increased soil pH (KCl) (P < 0.01), Bray1-P (P < 0.05) and NH4-N (P < 0.05); grain by 136% (P < 0.01) stover yeild by moer than 150% (P < 0.05); and N and P uptake (P < 0.01) during the two cropping seasons. The association of mulching and corralling increased soil NH4-N (P < 0.01) and soil pH (P < 0.01), compared to the sole corralling. The improvement of soil chemical properties resulted in grain yield increases of 54% (P < 0.01) and stover increases of 42% (P < 0.01). The effect of mulching and corralling association on grain and stover yeilds was higher when cattle were supplemented (67 and 50%) than when they were not supplemented (30 and 26%). The effects of the supplementation on grain and stover yields, and N and P uptake by millet, were restricted, when animals were corralled on bare soil (no mulching). The residual effects of supplementation were minimal.

Spatial and temporal rainfall variability in the Sahel and its effects on farmers' management strategies

The variability in rainfall in semi-arid Niger is great both spatially and temporally and is considered to be one of the most limiting factors in agriculture. To investigate these constraints more quantitatively, climate data at both survey and detailed scale were analysed. The results show that rainfall can vary considerably even within a few kilometres distance and on different time scales which means that crop yields are very unpredictable. Socio-economic surveys at two villages showed that smallholders have adapted a range of management strategies to ensure at least a minimum yield. Despite this, soil fertility is declining and further technologies need to be integrated into farming systems to make them sustainable.

Farmers' perception of vegetation changes in semi-arid Niger

Vegetation changes due to climate and human impact in Sahelian countries are rarely documented at species composition level. The decrease or disappearance of certain plant species reduces vegetation cover and enhances the exposure of soil surfaces to wind and water erosion leading to increased land degradation. Men and women in Niger were asked to note plant species and relate their numerical development to pre-defined observation criteria comparing the present and past. Plant species in decline or that had disappeared were reported more often than species that had increased or were newly introduced. The increasing species that were mentioned were often in use for different purposes and seem to be robust enough and well adapted to the semi-arid environment. But the interviewed farmers also stated the loss of many valuable species that they used for construction and hand tools, consumption, medicine or fodder. Those declining species are of socio-economic importance for individual households and the rural ecology. Men mentioned 18 species that had disappeared. Differences in the perception of men and women could be found. Women seemed to have a better perception for species which had increased or were newly introduced, whereas men perceived more a decrease or disappearance of species. Copyright © 2000 John Wiley & Sons, Ltd.

Incidence of soil surface crust types in semi-arid Niger

Soil water availability is most essential in the Sahelian agriculture but is hampered by several factors. Surface crusts or crust-like surfaces, which are characteristic of most Sahelian soils, have been shown to decrease water infiltrability and increase runoff. Their type and structure are influenced by soil texture, vegetation cover, erosion and deposition effects of wind and water. A soil and terrain survey in semi-arid SW-Niger was carried out to explain the patterns of soil surface crusts and the deterioration of the land. The soil surface crusts were shown to depend also on specific terrain factors including land use type and intensity, and terrain type and position. Chemical and physical soil factors such as organic carbon, soil colour and texture occurring with specific crusts indicate soil degradation, especially in sloping terrain, which increases runoff and soil erosion. For sandy soils, surface tillage is required to break up the crusts. Higher surface organic matter is recommended to enhance water infiltration in soils.