Browsing by Author "Lukangu, Gastao."

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Bowen ratio and surface temperature techniques for measuring evaporation from cabbages.
(1997) Lukangu, Gastao.; Savage, Michael John.; Johnston, M. A.
Good irrigation water management requires accurate, automated, non-destructive and simple techniques to measure crop water consumption. The actual evaporation from a cabbage crop was measured using the Bowen ratio energy balance technique (BREB), the surface temperature technique and the Penman-monteith method. All models used the shortened energy balance equation to estimate latent heat in which the advected energy is assumed to be negligible. Four irrigations were applied and 17 rainfall events were recorded during the experiment. The soil at the experimental field was a clay loam. An attempt to detect and reduce measurement error that could result from using inaccurate sensors was performed by calibrating the sensors. Data from inaccurate sensors were not used to compute the latent heat. Error and sensitivity analyse were performed, and the integrity of the weather data using the estimates of weather data from an appropriate model were checked. In addition, a comparative study showed that, for daily totals, there was a very small error in the latent heat calculations when fixed "constants" (density of air, specific heat capacity of air, psychrometric constant, slope of the saturation water vapour pressure vs temperature relationship and specific heat capacity of soil) were used instead of calculated ones. The Bowen ratio (β), a fundamental input of the BREB technique, was estimated accepting the Similarity Principle and excluding nighttime data. However, an error in β was also observed during the daytime measurement of the profiles entities because the sensors were wet and the stability condition was different from neutral conditions under which the Similarity Principle could not be observed. Negative values of β were observed when there were strong winds advecting sensible heat into the field under study. Data were rejected during mornings, and during strong advection periods. Data were also rejected when the sensors were wet because of rain or irrigation. In this experiment, only 35 % of data were valid for determining latent and sensible heat estimated using the BREB technique. Comparative analysis showed that the BREB technique overestimated the latent heat by 17 % compared to the Penman-Monteith method. However, both the Penman-Monteith method and BREB technique could not be trusted because of the presence of advection, a component of the energy balance equation normally assumed to be negligible. Either the surface to air temperature differential or the aerodynamic resistance, or both, were the source of overestimation of latent heat using the surface temperature technique. The surface to air temperature differential was large in magnitude when there were high wind speeds and drier conditions in the upwind field . It was small with lighter wind speeds and wetter surface conditions. An error of less than 5 % was attributed to the use of fixed air density and specific heat capacity and acceptance of 2 % and 20 % error in measuring the net irradiance and soil heat flux density, respectively. A comparative study showed that the surface temperature latent heat was overestimated in relation to the Penman-Monteith and BREB latent heat. Generally, the technique has been reported to overestimate evaporation, although to a lesser extent than the 57 % error reported in this experiment when compared to the BREB technique. An analysis of the energy balance closure, taking the Penman-Monteith and BREB as standards, suggested that the surface temperature technique overestimated the consumption of sensible heat from the air. This observation was also confirmed when the eddy correlation technique was used to compare sensible heat estimated using the surface temperature technique. The effect of placement height of air temperature sensors suggested that the consumption of sensible heat would be overestimated if the sensor was placed far from the crop surface. This overestimation in consumption of sensible heat resulted in an overestimation of latent heat. Irrigation water management was analysed using the crop water stress index (CWSI). The CWSI was calculated using the actual to potential evaporation ratio estimated using the Penman-Monteith method and the surface temperature techniques. The estimated and measured actual surface to air temperature differential, and the estimated potential and non-transpiring surface to air temperature differential were also used to estimate the CWSI using the Penman-Monteith method, the surface temperature technique and empirical method. The estimates of the CWSI using these techniques were inaccurate because of the poor correlation between the surface to air temperature differential and the water vapour pressure deficit (or water vapour pressure deficit and net irradiance). However, use of the CWSI estimated using the actual to potential evaporation ratio (CWSI = 1 - λ(a)/ λE(p) compared well to the standard CWSI determined using the Penman-Monteith approach. The actual canopy resistance was estimated using an empirical equation based on the potential canopy resistance, solar irradiance, soil water content and the shelter factor. A value of 50 s m(-1) was estimated for potential (minimum) canopy resistance of the cabbage crop. The soil water content was poorly correlated to CWSI, while the canopy resistance was well correlated. Comparative analysis showed that the estimated soil water content using the soil water balance equation was underestimated in relation to the soil water content measured using the ThetaProbe (frequency domain reflectometry technique) when the evaporation component was overestimated, and vice versa. Soil water content was underestimated throughout the experiment when evaporation from the surface temperature technique was used. There was an underestimation of soil water content in the early stages and overestimation in later stages of the experiment when the BREB and Penman-Monteith evaporation were used. Use of the estimated soil water content using the soil water balance with the overestimated evaporation would result in an early date of irrigation application, an unnecessarily large irrigation amount and frequent irrigations. More research is needed to find the cause of overestimation of evaporation using the surface temperature technique. The robustness of the equipment allowed a long period of measurement without frequent maintenance, as was required when using the BREB technique. The technique can monitor evaporation and irrigation management aspects at a regional scale. A combination of the Penman-Monteith, surface temperature and empirical method can assist the estimation of the crop water requirement by determining the CWSI. Future research would focus on quantification of sensible and latent heat advection, and analysis of additional resistances to water vapour flow from the surface to the atmosphere. The equipment for the BREB should be refined so that it measures actual latent heat under adverse weather conditions for a protracted period. A precise use of the soil water balance equation for water management should take into consideration runoff, vertical flow of soil water through a profile, intercepted water on plant surfaces and an accurately determined evaporation.
Factors influencing smallholders participation in agricultural markets in Southern Niassa, Mozambique.
(2005) Lukangu, Gastao.; Green, Jannette Maryann.; Worth, Steven Hugh.; Greenfield, Peter L.
Government, donors and NGOs in southern Niassa have been, after the 1992 peace agreement, extensively involved in agricultural development programmes to improve smallholders' food security. A study of the area and literature review revealed that many factors limited the benefits of agricultural market development programmes. Yet, opportunities in southern Niassa suggested that appropriately designed programmes could improve the standard of living of smallholders if these programmes were designed on a solid understanding of factors and strategies influencing agricultural market participation by smallholders. The main research hypothesis of this study was that: smallholders would participate in agricultural markets when their wealth status was high, when they had enough available household labour and when cash crops were profitable." Four main hypotheses were investigated: (i) factors and strategies identified through smallholder perceptions would provide local and time specific information on the constraints and solutions to smallholder market participation; (ii) wealth status and wealth-ranking factors were positively related to market participation where agriculture was the main economic activity as in southern Niassa; (iii) labour aspects such as crop labour requirements (CLR) could be negatively related, while available household labour (AHL) and the ratio AHL/CLR could be positively related to smallholders cultivation of cash crops and subsequent participation in agricultural markets; and (iv) aspects of profitability and indicators could be used to predict smallholder cash crop preferences. Data for this study were collected in Cuamba district of Mozambique from nine focus group discussions (FGDs) with community leaders, 287 household-head questionnaires and staff interviews during September 2002. Nine villages were randomly selected. The leaders' FGDs provided the criteria utilised to rank households according to wealth status and much of the qualitative information of this study. The wealth-ranking tool was used to identify and analyse the socio-economic factors that influenced smallholder market participation. A follow-up interview of managers of promoting institutions also provided greater insight on some aspects raised by smallholders. The study employed (i) descriptive statistics such as means and frequencies; (ii) correlation analysis and standard scores (iii) qualitative analysis was also used for some wealth-ranking, perceived labour demand and aspects of profitability influencing cash crop cultivation, preference and market participation based on information from FGD, farmers and staff; and (iv) simple mathematical expressions for analysis and interpretation of the research findings. This study relied on perceptions, knowledge and experience of smallholders, leaders and leaders of promoting institutions. Smallholder-suggested factors and strategies were in line with the limitations of socio-economic characteristics such as low effective household labour, particularly for females. These strategies included an improvement in outputs and inputs markets, agricultural services and credit at a subsidised prices or low interest rates. Other strategies for improving smallholders' participation in agricultural markets included promotion of profitable cash crops, household food security, provision of extension support services and information about cultivation and agricultural markets. However, smallholders did not identify some factors that have been acknowledged to influence agricultural market participation: ecological and natural resources, policies, institutional infrastructures and physical infrastructures. Smallholders also did not mention socio-economic factors (except household labour) as influencing their decisions to participate in agricultural markets in spite of the fact that researchers assume these factors in almost every study on smallholder market participation. The findings of this research confirmed that a wealth-ranking tool could be used to identify the socio-economic factors affecting smallholders' participation in agricultural markets. The identified wealth-ranking factors such as labour, livestock number, implements and bicycles significantly correlated with wealth status and subsequently to smallholder agricultural market participation. Conversely, household socio-economic characteristics not indicated as wealth-ranking factors such as age and gender related poorly to market participation. The wealth-ranking tool could also be used to identify strategies for improving smallholder participation in agricultural markets, and to evaluate an agricultural market development programme. The study found that, other factors being held constant, CLRs were negatively related to market participation. Weeding was the most labour intensive operation followed by harvesting, soil preparation, transportation, land clearing and seedling preparation. It also found that AHL and the ratio AHL/CLR were positive and significantly related to market participation. The ratio AHL/CLR together with household consumption requirements and yield were used to estimate the total area a household could cultivate, both for food crops for consumption and for cash crops; the proportion of farmers likely to participate in the market; and those unable to cultivate enough for consumption. The research also confirmed that profitability-related aspects correlated to cash crop preferences. Yield was the most important factor that influenced smallholders' preference for cash crops. It was also found that indicators incorporating more aspects of profitability correlated strongly with cash crop preferences. The correlation increased as more aspects were incorporated. A crop, such as tobacco, with a profit of more than twice the profit for food cash crops was preferred more than food cash crops. The indicators and underlying aspects of profitability were used to interpret the current and projected cash crop preference.