<p> A response conditioned to a specific stimulus will generalize to similar stimuli. Various reports have indicated that the slope of the gradient of generalization changes over a period of time. The experiments reported here investigated the generalization of an emotional response, originally conditioned to an 80 db. white noise to a 60 db. white noise. In different groups the tests for generalization were begun immediately after conditioning or after a 4 day waiting period. In further groups the animals' time during the delay period was variously taken up with bar pressing sessions; new conditioning trials with a light CS; unsignalled shock presentations, or they merely stayed in their home cages.</p> / Thesis / Master of Arts (MA)

Considerando a importância de se definir as condições críticas para a deflagração, desenvolvimento e estabilização das voçorocas e outras feições erosivas lineares, e tendo em conta as poucas abordagens nesse sentido realizadas no Brasil, o objetivo geral desta pesquisa é avaliar os limites críticos topográficos para o desenvolvimento das formas resultantes da erosão linear em Piratininga (SP). Os procedimentos metodológicos envolveram a identificação e caracterização das feições erosivas por meio de fotografias aéreas e in situ; a mensuração da declividade (S) em campo e com diversas resoluções de Modelo Digital de Terreno (MDT) e da Área de Contribuição (A) também a partir do MDT; e, a definição dos limites críticos topográficos na forma da seguinte Função Potência: S = a × . A linha do limite crítico foi ajustada no limite inferior dos dados, paralelamente à inclinação linha ortogonal de regressão de S e A. As feições erosivas foram categorizadas em 4 classes e 13 subclasses de acordo com seus aspectos morfométricos e indicadores dos processos atuantes. A mensuração de S e A por diferentes métodos resultou em algumas variações significativas nos dados gerados, sobretudo na comparação entre os valores obtidos em campo e pelo MDT. Foi obtido o seguinte limite crítico topográfico: S = 0,064 × ,, que tem coeficientes semelhantes a outras pesquisas que analisaram feições erosivas resultantes do escoamento subsuperficial. A distinção entre as feições erosivas onde ainda há indícios de recuo das cabeceiras e feições erosivas que já atingiram o equilíbrio, assim como a análise dos pontos de início dos processos erosivos em relação ao limite crítico, forneceu bases empíricas para a discussão da evolução das feições erosivas lineares em relação ao limite crítico topográfico e também para ao entendimento dos estados estável e instável dos voçorocamentos, permitindo a construção de um modelo teórico de previsão da estabilidade e instabilidade das feições erosivas baseado na relação entre a declividade e a área de contribuição em diferentes perfis de encosta. / Considering the importance of defining the critical conditions for the generation, development and stabilization of gullies and other erosional features, and taking into account the few approaches accordingly held in Brazil, the aim of this research is to evaluate the topographical critical limits for developing of gully erosion in Piratininga (SP). The methodological procedures involved: the identification and characterization of the gullies using aerial photographs and in situ; measurement of the slope (S) in the field and with various resolutions of Digital Terrain Model (DTM) and drainage area (A) also from DTM, and the definition of topographical thresholds in the form of following Power Function: S = a × . The topographical threshold line has been set at the lower limit of data, parallel to the slope of orthogonal regression line of S and A. The erosional features were categorized into 4 classes and 13 subclasses according to their morphometric aspects and indicators of active processes. The measurement of S and A by different methods resulted in some significant changes in the data generated, particularly when comparing the values obtained in the field and the DTM. We obtained the following topographic threshold: S = 0,064 × , which has coefficients similar to other studies that analyzed erosional features resulting from subsurface flow. The distinction between active and stabilized gullies, as well as analysis of the starting points of erosion in relation to threshold line, provide an empirical basis for the discussion of the evolution of gullies in relation to the topographic threshold line and also for the understanding of stable and active states of gullies, allowing the construction of a theoretical model for predicting the stability and instability of the gullies based on the relationship between slope and drainage area in different slope profiles.

Considerando a importância de se definir as condições críticas para a deflagração, desenvolvimento e estabilização das voçorocas e outras feições erosivas lineares, e tendo em conta as poucas abordagens nesse sentido realizadas no Brasil, o objetivo geral desta pesquisa é avaliar os limites críticos topográficos para o desenvolvimento das formas resultantes da erosão linear em Piratininga (SP). Os procedimentos metodológicos envolveram a identificação e caracterização das feições erosivas por meio de fotografias aéreas e in situ; a mensuração da declividade (S) em campo e com diversas resoluções de Modelo Digital de Terreno (MDT) e da Área de Contribuição (A) também a partir do MDT; e, a definição dos limites críticos topográficos na forma da seguinte Função Potência: S = a × . A linha do limite crítico foi ajustada no limite inferior dos dados, paralelamente à inclinação linha ortogonal de regressão de S e A. As feições erosivas foram categorizadas em 4 classes e 13 subclasses de acordo com seus aspectos morfométricos e indicadores dos processos atuantes. A mensuração de S e A por diferentes métodos resultou em algumas variações significativas nos dados gerados, sobretudo na comparação entre os valores obtidos em campo e pelo MDT. Foi obtido o seguinte limite crítico topográfico: S = 0,064 × ,, que tem coeficientes semelhantes a outras pesquisas que analisaram feições erosivas resultantes do escoamento subsuperficial. A distinção entre as feições erosivas onde ainda há indícios de recuo das cabeceiras e feições erosivas que já atingiram o equilíbrio, assim como a análise dos pontos de início dos processos erosivos em relação ao limite crítico, forneceu bases empíricas para a discussão da evolução das feições erosivas lineares em relação ao limite crítico topográfico e também para ao entendimento dos estados estável e instável dos voçorocamentos, permitindo a construção de um modelo teórico de previsão da estabilidade e instabilidade das feições erosivas baseado na relação entre a declividade e a área de contribuição em diferentes perfis de encosta. / Considering the importance of defining the critical conditions for the generation, development and stabilization of gullies and other erosional features, and taking into account the few approaches accordingly held in Brazil, the aim of this research is to evaluate the topographical critical limits for developing of gully erosion in Piratininga (SP). The methodological procedures involved: the identification and characterization of the gullies using aerial photographs and in situ; measurement of the slope (S) in the field and with various resolutions of Digital Terrain Model (DTM) and drainage area (A) also from DTM, and the definition of topographical thresholds in the form of following Power Function: S = a × . The topographical threshold line has been set at the lower limit of data, parallel to the slope of orthogonal regression line of S and A. The erosional features were categorized into 4 classes and 13 subclasses according to their morphometric aspects and indicators of active processes. The measurement of S and A by different methods resulted in some significant changes in the data generated, particularly when comparing the values obtained in the field and the DTM. We obtained the following topographic threshold: S = 0,064 × , which has coefficients similar to other studies that analyzed erosional features resulting from subsurface flow. The distinction between active and stabilized gullies, as well as analysis of the starting points of erosion in relation to threshold line, provide an empirical basis for the discussion of the evolution of gullies in relation to the topographic threshold line and also for the understanding of stable and active states of gullies, allowing the construction of a theoretical model for predicting the stability and instability of the gullies based on the relationship between slope and drainage area in different slope profiles.

Accelerated soil erosion is one of the major threats to agricultural production in Ethiopia and the Harerge region is not exceptional. It is estimated that about 1.5 billion tones of soil is being eroded every year in Ethiopia. In the extreme cases, especially for the highlands, the rate of soil loss is estimated to reach up to 300 t ha-1yr-1 with an average of about 70 t ha -1yr-1 which is beyond any tolerable level. The government have made different attempts to avert the situation since 1975 through initiation of a massive program of soil conservation and rehabilitation of severely degraded lands. Despite considerable efforts, the achievements were far bellow expectations. This study was aimed at assessing the effect of some soil properties, rainfall intensity and slope gradients on surface sealing, soil erodibility, runoff and soil loss from selected sites in the Harerge region, eastern Ethiopia, using simulated rainfall. Soil loss was also estimated for the sites using Soil Loss Estimation Model for Southern Africa (SLEMSA) and the Universal soil Loss Equation (USLE). Moreover, the effectiveness of various rates and patterns of wheat residue mulching in controlling soil loss was also evaluated for one of the study sites, (i.e. Regosol of Alemaya University), under both rainfall simulation and field natural rainfall conditions. For most of the erosion parameters, the interaction among soil texture, slope gradient and rainfall intensity was significant. In general however, high rainfall intensity induced high runoff, sediment yield and splash. The effect of slope gradients on most of the erosion parameters was not significant as the slope length was too small to bring about a concentrated flow. The effect of soils dominated by any one of the three soil separates on the erosion parameters was largely dependent on rainfall intensity and slope gradient. The soils form the 15 different sites in Harerge showed different degrees of vulnerability to surface sealing, runoff and sediment yield. These differences were associated with various soil properties. Correlation of soil properties to the erosion parameters revealed that aggregate stability was the main factor that determined the susceptibility of soils to sealing, runoff and soil loss. This was in turn affected by organic carbon content, percent clay and exchangeable sodium percentage (ESP). Soils with relatively high ESP such as those at Babile (13.85) and Gelemso (7.18) were among the lowest in their aggregate stability (percent water stable aggregates of 0.25 –2.0mm diameter); and have highest runoff and sediment yield as compared to other soils in the study. Similarly, most of those soils with relatively low ESP, high organic carbon content (OC%) and high water stable aggregates such as Hamaressa, AU (Alemaya University) vertisol and AU regosol were among the least susceptible to sealing and interrill erosion. Nevertheless, some exceptions include soils like those of Hirna where high runoff was recorded whilst having relatively high OC%, low ESP and high water stable aggregates. Both the SLEMSA and USLE models were able to identify the erosion hazards for the study sites. Despite the differences in the procedures of the two models, significant correlation (r = 0.87) was observed between the values estimated by the two methods. Both models estimated higher soil loss for Gelemso, Babile, Karamara and Hamaressa. Soil loss was lower for Diredawa, AU-vertisol and AU-Alluvial all of which occur on a relatively low slope gradients. The high soil loss for Babile and Gelemso conforms with the relative soil erodibility values obtained under rainfall simulation suggesting that soil erodibility, among others, is the main factor contributing to high soil loss for these soils. The difference in the estimated soil losses for the different sites was a function of the interaction of the various factors involved. Though the laboratory soil erodibility values were low to medium for Hamaressa and Karamara, the estimated soil loss was higher owing to the field topographic situations such as high slope gradient. SLEMSA and USLE showed different degrees of sensitivities to their input variables for the conditions of the study sites. SLEMSA was highly sensitive to changes in rainfall kinetic energy (E) and soil erodibility (F) and less sensitive to the cover and slope length factors. The sensitivity of SLEMSA to changes in the cover factor was higher for areas having initially smaller percentage rainfall interception values. On the other hand, USLE was highly sensitive to slope gradient and less so to slope length as compared to the other input factors. The study on the various rates and application patterns of wheat residue on runoff and soil loss both in the laboratory rainfall simulation and under field natural rainfall conditions revealed that surface application of crop residue is more effective in reducing soil loss and runoff than incorporating the same amount of the residue into the soil. Likewise, for a particular residue application method, runoff and soil loss decreased with increasing application rate of the mulch. However, the difference was not significant between 4 Mg ha-1 and 8 Mg ha-1 wheat straw rates suggesting that the former can effectively control soil loss and can be used in areas where there is limitation of crop residues provided that other conditions are similar to that of the study site (AU Regosols). The effectiveness of lower rates of straw (i.e. less than 4 Mg ha-1 ) should also be studied. It should however be noted that the effectiveness of mulching in controlling soils loss and runoff could be different under various slope gradients, rainfall characteristics and cover types that were not covered in this study. Integrated soil and water conservation research is required to develop a comprehensive database for modelling various soil erosion parameters. Further research is therefore required on the effect of soil properties (with special emphasis to aggregate stability, clay mineralogy, exchangeable cations, soil texture and organic matter), types and rates of crop residues, cropping and tillage systems, mechanical and biological soil conservation measures on soil erosion and its conservation for a better estimation of the actual soil loss in the study sites. Copyright 2004, University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. Please cite as follows: Bobe, BW 2004, Evaluation of soil erosion in the Harerge region of Ethiopia using soil loss models, rainfall simulation and field trials, PhD thesis, University of Pretoria, Pretoria, viewed yymmdd < http://upetd.up.ac.za/thesis/available/etd-08022004-141533 / > / Thesis (PhD (Soil Science))--University of Pretoria, 2004. / Plant Production and Soil Science / unrestricted