M´ etodo de ondas planas aumentadas linealizadas con potencial total (FP-

A soil crust is defined for the purposes of this study for practical purposes facilitating

identification in the field as a layer at the top o f the soil horizon which has a distinct appearance when the soil is dry and which is associated with reduced infiltration during water application. A soil crust is normally discernible to the trained eye, as it has a characteristic appearance in spite of having many particular forms. A distinction is made between a crust and sealing. Sealing is

p - | LOAM Y SO IL S |--- SANDY SOUS INITIAL STAGE S e e d b e d I STRU CTU RA L C R U S T ] S l a k n g i . Sieving DEPOSITIONAL CRUST Runotl I EROSION C R U ST

(counterclockwise from upper right) a) Accelerated degradation by cattle

trampling around a borehole; 2 = erosion crust, 3 = ‘aeolian’ type surface (micro dunes), 4 = gravel crust

(exposed b horizon, and in this case linear erosion).

b) Implications in terms of runoff of such a transformations; possibly sometimes advantageous from the point-of-view of water harvesting.

c) Processes responsible for an idealised crust evolution sequence in a time and slope dependent model.

d) As (c), for sandy and loam soils; a ‘universal’ model combining work in France and the Sahel.

(a Casenave and Valentin 1989, b Valentin 1992, c van der Watt and Valentin 1992, Valentin and Bresson 1992)

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H E AOGREGATE E D MIXED SAND SHI CEM ENTED MIXED SAND r ' 1 FINE SA N D G 3 CO A RSE SA N D PLA SM IC LAVER K B C O A R SE FRAGM ENT GRAVELLY a HORIZON DEPOSITIONAL C R U ST ionnOefl w oiar I 100 80 P a v e m e n t c ru s t (4) E rosion c ru st (3) Stru ctu ral

cru st (2) 60

Drying c ru st (1)

W o rm s

0

Tim e (id e a lised ) T erm ite & W orm s (3) Illustrations of c ru st: G \

defined here as the actual effect o f reducing infiltration during the course of water application. A seal is defined for the purpose of this study as a layer with a substantially lower infiltration rate and hydraulic conductivity than soil underneath during water application.

It should be noted that crusts are not simply dry seals, as is often assumed in the literature (Bristow et al, 1994), as seals represent only a partial expression of hydrological processes that may influence soil surface morphology and properties; crust formation also involves processes of drainage and evaporation. Gush et a l (1994) demonstrated the little known fact that during drying crusting soils undergo processes of structural transformation which may result in even lower porosity and pore continuity, and hence infiltration, than the seal. The influence of wetting and drying cycles will be returned to in the results section, as a simple, non destructive technique for monitoring seasonal changes in relative infiltration / runoff potential has been devised for this study. It should also be noted that crusts, due to their much greater thickness, have greater mechanical strength than seals.

The study of sealing involves assessing the functional impacts of this process on water

movement, an aspect prioritised in this review, as this is the source of interest in crusting soils, whilst crusts have principally been studied with respect to their micromorphology and

(mechanical) strength. As sealing cannot be ascertained directly from field scale observation (requiring greatly magnified depictions o f the surface profile), it is precisely the relationship between sealing and crusting (where the latter is readily apparent, and has been defined as such, above, for the purposes o f this study) which is of interest. The 'method-dependence' and 'definition-dependence' of the results o f the studies o f crusting soils reviewed here, which is inevitable given the differences in scales and processes involved in sealing and crusting, as well as the complex nature of these phenomenon, will serve as an optic with which to examine the literature on crusting, as this present study is particularly interested in the ‘tradeoffs’ in accuracy between various possible crust assessment methods in a real-world environmental survey context, on criteria of cost, speed and scale dependence. This will be discussed in more detail in Chapter 4.

In practice, studies of crusting soils often address the soil both during a rain event and in a dry state, with little distinction made between the seal (affecting the results of the former) and the crust (the phenomenon being observed in the latter, but resulting from the former). As mentioned aheady, however, there may be a change in the soil structure between wet and dry states; thus.

any relationship established, for example, between infiltration characteristics of the seal and the mechanical properties of the crust, may be misleading in terms of prediction of infiltration rates.

Therefore, in order to estimate the error involved in estabhshing 'one-off relationships between crusts and sealing, a longitudinal study of crust development was carried out for one widely occurring crust type in the study area which will be reported in Chapter 7. Relationships between crust properties when dry on the one hand, particularly their morphology at a scale discernible to the naked eye, and the ejfects of sealing on infiltration on the other hand, are at the heart o f this study, and serve as a basis for the ‘catalogue’ o f surface types in the Sahel of Casenave and Valentin (1989), which describes each surface in terms of its observable and distinctive characteristics from simple field observations, and supphes associated hydrological parameters (with a typical range for each) elucidated from a rainfall simulation campaign, such that the latter can be deduced from the former.

The definitions chosen for crusting and sealing for the purposes of this study are intentionally functional, as the object of this study is to relate the occurrence of crusting to its implications in terms of runoff potential and hence infiltration. The assumption is made, which will be

investigated in this study, that crust morphology is related to crust function (there are multiple functional effects of cmsting, but this study is only concerned with the degree to which it reduces infiltration), and that morphology, at least at a certain level, is subject to useful field investigation by ocular observation. If this relationship can be established, then clearly the assessment of runoff would be greatly simplified.

The work of Casenave and Valentin (1989), as described above, attempts to systematise this relationship for commonly occurring crust forms in the Sahel, and the extrapolation of this system, directly or indirectly, to another semi-arid environment, in lowland Baringo, Kenya, attempted in the present study, without pretending to develop a systematic set of relationships for ‘East African drylands’ or whatever, as the Sahel programme was a heavily fimded, and long term research effort involving scores of individuals, whereas the present work merely involves one researcher and two field assistants and the part time assistance of one undergraduate student. This is, however, a realistic scenario in the absence of the politically enabhng research

environment from which the Orstom group in the Sahel benefited, and as such is a useful example of the possibihties of local development driven research to identify and a d ^ t to local conditions relevant approaches and findings of the well funded long term research programmes.

The issue o f the definition o f a crust is important for this study, as the assessment and mapping o f crusted soils is contingent upon defining what a crust is and defining the bases on which to distinguish between various forms o f crusting. In other words, to create some form of

classification system. The section will therefore review the most relevant literature on crusting soils in the light of an interest in relating the mechanisms o f crust formation to the effects on infiltration and to their properties, as identifiable in the field. As such the review of the literature will be organised principally on the basis of the method(s) employed, as it is hypothesised, as implied above, that the results are highly method-dependent, and furthermore that the method chosen pre-defines the concept of a crust for any particular investigation. Not only this, but as the present work is interested in identifying the most appropriate techniques for the purpose of rapid, cost-effective but accurate assessment of the runoff potential o f various surface units, and as such the question o f methodology is primordial.

(3.6.2)