Agrometeorology
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Browsing Agrometeorology by Author "Clulow, Alistair David."
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Item Assessment of maize crop health and water stress based on multispectral and thermal infrared unmanned aerial vehicle phenotyping in smallholder farms.(2021) Brewer, Kiara Raynise.; Clulow, Alistair David.; Sibanda, Mbulisi.; Mabhaudhi, Tafadzwanashe.Abstract available in PDF.Item The long-term measurement of total evaporation over Acacia mearnsii using large aperture scintillometry(2007) Clulow, Alistair David.; Savage, Michael John.A large aperture scintillometer (LAS) was operated continuously over a distance of 575 m from 19 August 2006 to 29 September 2007 in the South African KwaZulu-Natal midlands mistbelt area over Wattle (Acacia mearnsii). The LAS measurements of the structure parameter of the refractive index of air ( 2 n C ), were used to calculate the sensible heat flux. The shortened energy balance equation was used to estimate the latent energy flux as a residual from which the total evaporation (ET) was calculated. The LAS estimates of sensible heat flux during the short transition period (1 hour) between stable and unstable conditions were on occasion erroneous and required verification. Advection was also found to affect sensible heat flux estimates. Long-term operation of the LAS was however found to be possible even at remote sites, producing reliable and continuous results. The LAS estimates of sensible heat are sensitive to zero-plane displacement height and wind speed data impact and these should be derived as accurately as possible. Tree heights were measured at monthly intervals and a zero-plane displacement and effective height were calculated every two weeks. The sensible heat flux was thus processed in two week blocks of data corresponding to progressive effective heights. The tree growth rate was consistent over time and was not affected by seasonality, indicating that reduced air temperatures, rainfall and solar irradiance in winter are not limiting growth. The average growth rate was 0.37 m per month or 4.5 m per year. The LAS ET was compared to the American Society of Civil Engineers - Environmental and Water Resources Institute (ASCE-EWRI) short grass reference evaporation (ETsz) for a seven-month period and was found to compare favourably (R2 = 0.78) with outliers caused by advection and rainfall events. Calculations of grass reference evaporation at hourly and daily intervals provide different results. The daily estimates are lower than the hourly estimates by 17 % on average. Where hourly data is summed to calculate a daily ETsz, night-time values should be included. The LAS ET measurements were validated against the Priestley and Taylor (1972) method of estimating ET and found to be in good agreement (R2=0.94). The Priestley and Taylor daily total latent energy flux, from 22 August 2006 to 29 September 2007, was 9 % higher than the LAS results on average. The Bowen ratio for the entire period is less than 1, indicating that the latent energy flux dominates at the site. The ET over the period of measurement (13 months) is 1250 mm and the rainfall is 750 mm. This confirms previous results at the site using the Bowen ratio energy balance method showing that the ET exceeds the rainfall by 45 % and justifies further research into soil water, ground water and root interactions in the deep soil profile.Item Productivity and water use of commercial forestry species and their potential impact on surface water resources in two forestry areas of KwaZulu-Natal, South Africa.(2023) Kaptein, Nkosinathi David.; Clulow, Alistair David.; Toucher, Michelle Lynn.; Everson, Colin Stuart.; Germishuizen, Ilaria.Abstract available in PDF.Item Temperature dynamics within a low-cost aquaponic system and the possibe effects of climate change.(2019) Mkhize, Minenhle Godslove.; Clulow, Alistair David.; Taylor, Simon Michael.; Lagerwall, Gareth.The agricultural sector is facing impeding challenges due to climate change. There is enough evidence showing that climate change has a significant impact on agricultural production. Marginalized communities that lack financial resources and depend on agricultural crop production, are the most vulnerable to climate change effects, which further exacerbates 5 their food insecurity. Existing literature hypothesizes that aquaponics, using Tilapia, has potential in addressing climate change effects in agriculture. However, the low average winter temperature hinders successful adoption of low-cost aquaponic systems using Tilapia fish. The implication of cool conditions (South African temperatures) are more extreme for a low-cost, poorly resourced aquaponic users because they lack temperature regulation 10 systems to maintain optimal temperatures and are simply subject to the surrounding environmental conditions. Therefore, the purpose of this study was to understand the temperature dynamics of a low-cost aquaponic system and the possible effects of future climate change. A study was conducted at KwaDeda, a poor rural community in the Ndwedwe area of 15 KwaZulu-Natal. The two objectives were to (1) understand how the surrounding environmental air temperature affects the water temperature of a low-cost aquaponic system and, to (2) assess the implications of future climate change on a low-cost aquaponic system. Two weather stations (22 km apart) were installed, one to measure hourly environmental air temperature conditions and the other to measure the conditions within the plastic tunnel of 20 a low cost aquaponic system (from June – November 2019). The environmental air temperature had no immediate relationship with water temperature. However, there was an observed lag of 4 hours from the environmental air temperature peak to water temperature peak, which varied slightly with seasonality. The conditions within the tunnel were generally hotter than the outside environmental conditions during the day, 25 however, at night, the tunnel air temperature dropped to be the same and sometimes even lower the outside environmental temperature. The air temperatures in winter and resulting water temperatures of the low-cost aquaponic system was well below the optimum range for Tilapia (22-32 °C). Low-cost systems provide limited means to control water temperature. Therefore, further investigation into low-cost methods to reduce the cooling of the tunnels 30 at night, which later results in cooling of the water, is required. The projected future climate was shown to be both advantageous and disadvantageous for the low-cost aquaponic system. The projected increase in average air temperature due to climate change will be positive for South African conditions, which are generally quite cool for Tilapia. However, extreme weather conditions such as intense storms, high wind speed and hail, that are predicted with climate change, may be a threat to low-cost aquaponic infrastructure. Research into improving the design of low-cost tunnels that can withstand adverse weather conditions is recommended.