Michael J. Savage

Field evaluation of polymer capacitive humidity sensors for Bowen ratio energy balance flux measurements.

The possibility of reliable, reasonably accurate and relatively inexpensive estimates of sensible heat and latent energy fluxes was investigated using a commercial combination thin-film polymer capacitive relative humidity and adjacent temperature sensor instrument. Long-term and unattended water vapour pressure profile difference measurements using low-power combination instruments were compared with those from a cooled dewpoint mirror hygrometer, the latter often used with Bowen ratio energy balance (BREB) systems. An error analysis, based on instrument relative humidity and temperature errors, was applied for various capacitive humidity instrument models. The main disadvantage of a combination capacitive humidity instrument is that two measurements, relative humidity and temperature, are required for estimation of water vapour pressure as opposed to one for a dewpoint hygrometer. In a laboratory experiment using an automated procedure, water vapour pressure differences generated using a reference dewpoint generator were measured using a commercial model (Dew-10) dewpoint hygrometer and a combination capacitive humidity instrument. The laboratory measurement comparisons showed that, potentially, an inexpensive model combination capacitive humidity instrument (CS500 or HMP50), or for improved results a slightly more expensive model (HMP35C or HMP45C), could substitute for the more expensive dewpoint hygrometer. In a field study, in a mesic grassland, the water vapour pressure measurement noise for the combination capacitive humidity instruments was greater than that for the dewpoint hygrometer. The average water vapour pressure profile difference measured using a HMP45C was highly correlated with that from a dewpoint hygrometer with a slope less than unity. Water vapour pressure measurements using the capacitive humidity instruments were not as accurate, compared to those obtained using a dewpoint hygrometer, but the resolution magnitudes for the profile difference measurements were less than the minimum of 0.01 kPa required for BREB measurements when averaged over 20 min. Furthermore, the longer-term capacitive humidity measurements are more reliable and not dependent on a sensor bias adjustment as is the case for the dewpoint hygrometer. A field comparison of CS500 and HMP45C profile water vapour pressure differences yielded a slope of close to unity. However, the CS500 exhibited more variable water vapour pressure measurements mainly due to its increased variation in temperature measurements compared to the HMP45C. Comparisons between 20-min BREB sensible heat fluxes obtained using a HMP45C and a dewpoint hygrometer yielded a slope of almost unity. BREB sensible heat fluxes measured using a HMP45C were reasonably well correlated with those obtained using a surface-layer scintillometer and eddy covariance (slope of 0.9629 and 0.9198 respectively). This reasonable agreement showed that a combination capacitive humidity instrument, with similar relative humidity (RH) and temperature error magnitudes of at most 2% RH and 0.3 °C respectively, and similar measurement time response, would be an adequate and less expensive substitute for a dewpoint hygrometer. Furthermore, a combination capacitive humidity instrument requires no servicing compared to a dewpoint hygrometer which requires a bias adjustment and mirror cleaning each week. These findings make unattended BREB measurements of sensible heat flux and evaporation cheaper and more reliable with the system easier to assemble and service and with reduced instrument power.

The soil water balance of rainfed and irrigated oats, Italian rye grass and rye using the CropSyst model

Crop growth models have been used in simulating the soil water balance for purposes of irrigation management and yield predictions. The application of CropSyst, a cropping systems simulation model, was evaluated for Cedara, South Africa. Simulations included soil water balance of fallow land and rainfed and irrigated winter crops [oats (Avena sativa), Italian ryegrass (Lolium multiflorum) and rye (Secale cereale)]; and irrigation scheduling of the winter crops. Soil, plant, weather and management inputs were used for the soil water balance simulations. Model crop parameters were used from past experiments or obtained from model documentation, with a slight modification to account for varietal differences. The fallow land soil water simulations were more accurate for dry than for wet soil. For all three winter crops, the model consistently over-estimated the soil water content in the upper layers, with a good agreement for the deeper layers until a large precipitation event occurred to which the model responded more slowly than that observed. Simulations using model-scheduled irrigation based on 0.4 and 0.6 maximum allowable depletion criteria indicated that the observed applied irrigation in the field was more than that required. Soil water depletion and accumulated transpiration simulations were similar in both the observed and model-scheduled irrigations, but total soil evaporation and percolation were greater in the case of the observed than the model-scheduled irrigations. Irrigation scheduling using crop growth models may assist in avoiding over- or under-application of irrigation applications by ensuring efficient utilization of rain and irrigation.

Estimation of frost occurrence and duration of frost for a short-grass surface

The estimation of frost duration (FD) was investigated using dielectric, infrared surface temperature and grass temperature subhourly measurements. Near real-time FD data and information displays and alerts were also made available via a web-based system. FD was estimated using a dielectric leaf wetness sensor (LWS) method, for which the sensor voltage was between 274 and 284 mV with a voltage rate of change less than 10 mV h−1 for a 4 min period, and two temperature methods for which infrared thermometer (IRT) and grass temperatures were compared with 0 °C. FD estimation using the LWS method ensured that most of the transitional dry-to-wet and wet-to-dry events were not included in the FD count. Generally, the IRT method yielded the largest estimate of FD, grass temperature method lower and LWS method lowest. Micrometeorological measurements showed consistent air temperature gradients of 2.25 °C m−1 for cloudless nocturnal frosted conditions with few air temperature measurements at 1 m and none above indicating frost occurrence. At the very least, automatic weather station systems should contain a grass thermometer or preferably an IRT for determination of FD with near real-time data and graphics displayed, including timeous alerts of frost occurrence and FD, using the Internet.

Determination of the timing and amount of irrigation of winter cover crops with the use of dielectric constant and capacitance soil water content profile methods

A well-managed irrigation scheduling system needs a rapid, precise, simple, cost-effective and non-destructive soil water sensor that allows for measurements at different depths and different locations across the cropped area. Two soil water content profile measurement methods were used and compared to determine the timing and amount of irrigation of three winter cover crops (Avena sativa, Secale cereale and Lolium multiflorum): the PR1 profile probe dielectric and Diviner 2000 capacitance methods. The laboratory-measured retentivity relationship was used to define the fill (0.39 m3 m−3) and high refill point (0.34 m3 m−3). Soil water content was measured two to three times per week starting from 50 days after planting (DAP) until 164 DAP. After the last rainfall event of 8 mm (142 DAP), the next irrigation was predicted graphically using the PR1 measurements to be five days later and seven days for the Diviner 2000. This difference occurred since the Diviner 2000-measured soil water contents at the rooting depth were slightly higher than those obtained using the PR1 method. The amount of irrigation estimated using graphical and mathematical methods were in good agreement.

Overhead irrigation increased winter chilling and floral bud production in Eucalyptus nitens

Eucalyptus nitens requires a sufficiently cold winter to produce flower buds. In areas in South Africa where E. nitens commercial plantations as well as breeding and production seed orchards are located, winter chilling is often insufficient for floral bud initiation. Hence, under such conditions, E. nitens floral bud and seed crops are poor and inconsistent. The local industry is almost entirely dependent on paclobutrazol (PBZ) applications for encouraging flowering in E. nitens seed orchards. Between 2008 and 2010, an experiment was conducted to investigate the potential of overhead irrigation (sprinkling) as a means of supplementing winter chilling to improve floral bud production in E. nitens. The treatments included three levels of sprinkling (nil, 10 weeks and 16 weeks duration), two levels of PBZ (nil, 0.025 g a.i. per mm basal stem circumference) and two grafted clones (prolific flowerer and shy-flowerer). Sprinkling reduced E. nitens daytime bud temperatures by as much as 16.2 °C on warm, dry winter days. In 2009 (cold winter) and 2010 (warm winter), sprinkling increased chilling accumulation by 44% and 72% (nil versus maximum sprinkling), respectively. In 2009, in the absence of PBZ, sprinkling resulted in a higher percentage of trees of either clone producing umbels (flower buds) compared with the control. In the warmer 2010 winter, sprinkling again increased flowering, with the number of flowering shoots and umbels per tree being significantly higher than the control at p < 0.05. In both 2009 and 2010, PBZ showed a strong additive effect to winter chilling on E. nitens floral bud production. The E. nitens clone × chilling × PBZ flowering interaction was complex and warrants more detailed investigation in future. Overhead sprinkling offers a practical method of supplementing winter chilling and improving floral bud production in high-chill-requiring temperate eucalypt species such as E. nitens.

Web-based teaching, learning and research using accessible real-time data obtained from field-based agrometeorological measurement systems

To enhance teaching and learning in agrometeorology and allied disciplines, a Web-based data and information system was developed. The system uses field-based agrometeorologcial measurements, from automatic weather, radiation and temperature stations, to collect and display near real-time and previous data in graphs/tables. An important feature is the display of agrometeorological data, information and graphics in the lecture room or laboratory with early-warning capability. Examples include hourly short-grass and tall-crop reference evaporation, sunshine duration, grass-surface radiation and energy balances. The system is applicable to the agricultural, earth and environmental sciences. This study presents the rationale, detail, application and evaluation of the system that is currently used by undergraduate and postgraduate students and staff, to access online data and information for lectures, tutorials, practicals, projects and research. For undergraduates, data can be extracted and manipulated, thereby reinforcing computer literacy, numeracy – including statistical ability – and graphical capabilities. The aim is to ensure that these abilities are improved with users obtaining a deeper understanding of the agroenvironment. More than 75% of respondents of an open questionnaire indicated that the graphical display of data had enabled further understanding of agroenvironmental concepts irrespective of language.

Energy and Mass Exchange Over Incomplete Vegetation Cover

Competition for fresh water between agriculture and domestic and industrial uses is increasing worldwide. This is forcing subsistence and commercial agriculture to produce more with less water. Consequently, it is crucial to properly and efficiently manage water resources. This requires accurate determination of crop water loss into the atmosphere, which is greatly influenced by the exchange of energy and mass between the surface and the atmosphere. Measurement of these exchange processes can best be accomplished by micrometeorological methods. However, most micrometeorological methods are very expensive, difficult to set up, require extensive post-data collection corrections and/or involve a high degree of empiricism. This review discusses estimation of evapotranspiration using relatively inexpensive micrometeorological methods in temperature-variance (TV), surface renewal (SR) and mathematical models. The TV and SR methods use high frequency air temperature measurements above a surface to estimate sensible heat flux (H). The latent heat flux (λE), and hence evapotranspiration, is calculated as a residual of the shortened surface energy balance using measured or estimated net radiation and soil heat flux, assuming surface energy balance closure is met. For crops with incomplete cover, the disadvantage of these methods is that they do not allow separation of evapotranspiration into soil evaporation and plant transpiration. The mathematical models (single- and dual-source) involve a combination of radiation and resistance equations to determine evapotranspiration from inputs of automatic weather station observations. Single-source models (Penman-Monteith type equations) are used to determine evapotranspiration over homogeneous surfaces. The dual-source models, basically an extension of single-source models, determine soil evaporation and plant transpiration separately over heterogeneous or sparse vegetation. These mathematical models have also been modified to accommodate inputs of remotely-sensed radiometric surface temperatures that enable estimation of evapotranspiration on a regional and global scale.

Estimation of leaf wetness duration for a short-grass surface

The measurement of leaf wetness duration (LWD) was investigated using subhourly dielectric, infrared surface temperature, dewpoint temperature, grass temperature and relative humidity (RH) measurements. Near real-time LWD data and information displays and alerts were made available timeously via a web-based system. LWD was estimated above a short-grass surface using five methods: dielectric leaf wetness sensors (LWS); a constant RH for which wetness events were registered for RH greater than 87%; RH between 70–87% if RH increased by more than 3% in 30 min; and two dewpoint depression-based methods for which surface-measured temperature, using an infrared thermometer (IRT), and grass temperature were compared with the dewpoint at either 0.1 or 2 m. The RH methods generally overestimated LWD compared to the other methods. There was reasonable agreement between IRT- and grass-temperature methods if rain days were excluded but these methods showed poor agreement with LWS measurements of LWD. Microclimatic and radiative conditions, during nocturnal condensing events, are reported. Automatic weather station data would have more value if grass temperature was included for determination of LWD by comparison of grass temperature with a measured dewpoint, with timeous alerts and web-based display of near real-time LWD data and graphics.

Assessment of fog-water collection on the eastern escarpment of Eritrea

ABSTRACT Fog can be considered a potential water resource for certain semi-arid and arid countries. In Eritrea, a fog-water collection project was implemented in 2007 in the villages of Arborobue and Nefasit. This study presents an assessment of the project after five years of implementation and identifies the main strengths to be technical viability, since it is simple and manageable by communities, and reasonable in areas where conventional water delivery systems are not feasible. The main limitations are that it is not economically viable unless its initial cost is subsidized; moreover, fog capture is seasonal, and net damage occurs during strong winds.

Monitoring eucalypt bud temperature using mobile temperature loggers

Winter chilling is a key environmental trigger of floral induction in temperate Eucalyptus. Over the past two decades, considerable site × eucalypt flowering interaction research has been undertaken over a range of high elevation (>1 100 m asl) forestry sites in the South African summer rainfall area. A practical method of accurately monitoring eucalypt bud temperature at remote sites in these areas was needed for this research. Utilisation of traditional methods of air and bud temperature measurement were not viable, due to significant risks of data and meteorological equipment loss posed by severe weather, vandalism and theft. Between 1996 and 2004, a robust structure for housing the miniature Hobo® temperature logger (hereafter termed ‘Hobo pole’) was designed and utilised for in-field monitoring of air temperature in the research trials. During 2009 and 2010, an experiment was conducted to investigate the relationship between E. nitens bud temperature (BudT), Hobo pole air temperature (HoboAT) and radiation screen air temperature (ScrnAT), and develop appropriate calibration models. Attempts to develop a single model for predicting hourly BudT from hourly HoboAT over the entire annual period yielded unsatisfactory results (maximum R 2 value 0.49). Separate winter and summer regression models were subsequently developed for predicting BudT from HoboAT, ScrnAT from HoboAT, and BudT from ScrnAT. In these regressions, R 2 values were generally slightly higher, and SE values lower, for mid-winter data than for mid-summer data. In mid-winter, BudT on HoboAT gave the highest R 2 value (0.99) and lowest SE value (0.49 °C) of all regressions. The Hobo logger/Hobo pole combination, together with developed regression models (presented in this paper), offers one practical, cost-effective solution for accurately monitoring eucalypt bud temperature at remote, high-elevation forestry sites in South Africa.