Grafica XX. Situación de deuda de las viviendas financiadas
Mapa 8. Delimitación del Centro Histórico de la Ciudad de Puebla.
well as developing, or assessing the relevance of, the methodology used to address the major research questions posed by this study. The relevance o f the methodology is assessed with respect to and the major research questions posed in the context o f the overall objective o f how to best determine runoff potential of a study area in the context of a land evaluation exercise for water harvesting.
This chapter starts with an introduction to the major research questions, the methodology and sampling strategy employed, and then moves on to explain the importance o f water harvesting, why one might want to determine the suitability of an area for this soil and water conservation practice. This chapter then continues with an introduction to a key topic in this study, the
phenomenon of crusting as a characteristic feature of soils in semi-arid areas and then turns to the effects of this phenomenon on infiltration, and implications for runoff assessment and for the research design. This chapter then briefly addresses models of infiltration into crusted semi-arid soils and specifies the approach taken in this study and clarifies that this study is not about mathematical modelling of infiltration, which has been done in great detail by many other studies, but rather about the implications of crusting for determining runoff potential for assessing
suitability for water harvesting. Finally, this chapter ends with a summary of the research approach taken, the nature of the study area and a brief overview o f the thesis on a chapter by chapter basis as a way of making the structure of the thesis apparent to the reader as well as to introduce some of the key concepts employed in this study.
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Research questions addressed, methodology and sampling strategy employed in this
study
central problem of how to map runoff potential in a semi-arid landscape with soils which are prone to crusting. How to best determine runoff potential is the key challenge on the
environmental side o f the broader research question of how to identify suitable locations to implement to the soil and water conserving technique called water harvesting. The advantages of water harvesting, which multiphes the effective rainfall, for crop / range / reafforestation in semi- arid and arid areas are described in the next section. The importance of socio-economic factors in determining suitable locations for water harvesting is recognised, but not addressed in this study.
On the environmental side of a methodology for determining suitability for water harvesting the two key questions to be asked are:
(a) which areas in the landscape will produce runoff
(b) which areas in the landscape are best to receive and store that runoff for the purpose of vegetative production.
The latter question involves the assessment of ‘runon’ potential, which involves the assessment of the available water capacity and the fertility of the soil. This is recognised to be very important but is not the focus of this study. Secondary data on runon potential will be utilised in Chapter 9 to illustrate a GIS (Geographical Information System) based assessment of suitability for water harvesting for the study area, lowland Baringo, Kenya. The focus o f this study, however, is on the first question, how to determine which areas of the landscape in the study area have potential as runoff producing surfaces.
As such, this study is largely about the testing and assessment o f various techniques which are potentially relevant in the development o f a methodology for assessing suitability for water harvesting, and in particular for assessing runoff potential. In doing so, however, this study also engages the literature on the importance o f crusting and sealing as controls on the surface hydrology in semi-arid areas. This literature is reviewed in Chapter 3. A central precept of this study was that, on the basis of the review of the literature carried out prior to fieldwork, that there is most likely a surface state control on runoff production in the study area. This assumption was tested in various ways over the course of the fieldwork in order to confirm or deny it. Indeed, this is one of the major research questions addressed by this study, hsted below.
including both the dry and rainy season. Rainfall simulators were employed together with ‘simple’ or rapid, low cost tests to determine runoff potential of various surface types around the study area. The former were employed to generate a quantitative assessment of the effects of crusting on infiltration and a comparison of runoff potential between sites, the latter to generate a qualitative ranking of runoff potential between sites. These results fi*om the quantitative and qualitative assessments were then compared with each other in order to determine whether and under what conditions qualitative measures o f runoff potential can substitute for the much more time consuming rainfall simulations in terms of ranking relative runoff potential o f the sample sites. Indeed, this is one of the major research questions addressed by this study, listed below.
Related to this last research question is the issue of the relative importance of wetting and energy controls on sealing and therefore runoff potential in a semi-arid area. On the basis of the
literature review carried out prior to fieldwork, it was found that kinetic energy was generally considered to be the single most important parameter explaining crusting, which occurs in semi- arid areas in part because of the lack of vegetation to dissipate the energy of raindrops (Casenave and Valentin 1989). As such, the differences between the results o f the rainfall simulations, which reproduce up to 75% of the kinetic energy of natural rain events in the study area, and the essentially energy fi-ee water acceptance simple tests are potentially usefiil in assessing the
importance of kinetic energy on runoff production for crusting soils. The issue o f the influence of rainfall characteristics on runoff is important, and in particular a kinetic energy control on
crusting and sealing, and therefore runoff, is hypothesised in this study and tested using rainfall simulation at various drop heights. As a raindrop, however, can be conceived of as a ‘co-quanta’ of moisture and energy, the interaction between the wetting and impact effects on crusting and sealing, expressed in terms of runoff response, was also investigated.
A fundamental dilemma presents itself, however, in that the objective of ascertaining the effects of and interactions between raindrop energy and wetting effects implies a detailed study with a large number of replicates in order to establish a statistical confidence level regarding treatment effects, as does the objective of characterising the spatially distributed nature of the infiltration in the study area. As both objectives require a large number of rainfall simulations, it is beyond the scope of this research to both gain a systematic understanding of the rainfall-crusting interactions at a process level in the study area and to address the question of the spatial variability in
suitability assessment protocol, whilst keeping the former strongly in m ind.. The issue o f the relative importance of rainfall and surface characteristics in determining runoff places this
research within a wider debate on these themes within the soil science and hydrology o f semi-arid environments, but attention must be drawn to the typical lack o f rainfall (especially intensity) data is developing countries. Indeed the relative importance o f rainfall and surface characteristics in generating runoff is one of the major research questions posed by this study, listed below. The various issues related to the rainfall characteristics which are most important to accurately simulate are addressed in Chapter 4.
Results from a rainfall simulation campaign carried out by Bryan (1994) in the study area using a five meter spray-type rainfall simulator with approximately 90% reproduction of the kinetic energy of natural rainfall were compared with the results from the rainfall simulations carried out in this study. This was done because it was believed that the rainfall simulations with the highest kinetic energy reproduction could reasonably be considered to be ‘reference values’ of runoff potential in the absence of data on actual rainfall - runoff relations in the study area. As it was discovered that the time to runoff initiation and the steady state infiltration rate were considerably longer and higher, respectively, with the 1.5 meter simulator employed in this study, a new design was developed and built. The results with the original design were still, however, found to be useful in a number of ways in shedding light on the research questions and as such the results from both are reported in Chapter 7. The comparison between the results o f the improved, 3.5 meter drop height simulator and the simple tests are reported in Chapter 8 on a site-by-site basis.
The sampling strategy chosen, which resulted in the selection of the sites, revolved around two criteria:
(a) carrying out work where secondary data were available for comparison, meaning mainly the sites of the University o f Toronto erosion study (Bryan 1994) and the water harvesting fields o f the Rehabilitation o f Arid Environments (RAE) project
(b) ensuring a representative range of surfaces (particularly crust types) found across the study area.
Details of the sampling strategy and of the sites chosen are presented in Chapter 6. The second criteria listed above, regarding the representativity of the surfaces at the study sites, relates to the question of inventorying, classifying and mapping the distribution o f the crusts in an area.
characteristics have been carried out in greatest detail by French researchers in the Sahel. In fact, they have created a ‘catalogue’ of the relationships between the two, based on a long term and extensive rainfall simulation campaign. Indeed, one of the major research questions in this thesis, listed below, was to assess whether and to what degree their methodologies are relevant for the purposes of suitability assessment for water harvesting and whether the same crusts exist in the study area and, if so, whether the runoff characteristics are the same as in the Sahel.
Pursuant to this theme, the question o f how to map the occurrence / distribution of the crusts identified and characterised, for the purpose o f mapping runoff potential, implies extrapolating or interpolating between what are essentially point measures o f runoff potential determined from rainfall simulation. The obvious possibility to achieve this objective is to link hydrological measures to a spectral map available for the entire landscape, in the form of widely available optical remote sensing such as Landsat TM or the French Spot imagery. The resolution of this imagery, however, is far coarser than that at which the hydrological measures are taken, which raises the question of how to bridge this difference in scale (really resolution). Indeed, this is one of the major research questions addressed by this study, listed below, and in particular whether the attempts by the French and other researchers in the Sahel to link the two are applicable to the study area and in the context o f a suitability assessment procedure for water harvesting.
To summarise, the major, often interrelated, research questions posed by this study, all of which are relevant to the environmental aspect of assessing suitability for water harvesting, are:
• Is there a surface state, particular crusting, control on runoff generation in the study area? • What is the relative importance of surface characteristics and rainfall characteristics as
controls on runoff?
• Is it possible, and under what conditions is it appropriate, to substitute rapid, low cost, low energy water acceptance tests and other proxy measures of runoff potential for the higher kinetic energy reproduction but very time consuming rainfall simulations?
• Is, or to what extent is the crust classification system developed by French researchers for the Sahel relevant to the study area in terms of a) principles of classification b) presence of the same crust types and c) if so, whether the hydrological characteristics of these crusts are similar in the study area?
them to a measure available for the entire surface; specifically a spectral ‘map’ in the form of widely available optical remote sensing image products such as Landsat TM and Spot?