Feasibility of using pyranometers for continuous estimation of ground cover fraction in table grape vineyards

This paper evaluates the feasibility of using pyranometers for continuous estimation of ground cover fraction (GCF) at remote, unattended sites. Photographical techniques were used for measuring GCF (GCFref) at a table grape vineyard grown under a net. Daily pyranometer-driven GCF estimates (GCFpyr) were obtained from solar radiation measurements above and below the canopy. For GCFpyr computation, solar radiation was averaged for two hours around solar noon (midday periods) and for daylight periods (8:00 to 18:00 Universal Time Coordinated). GCFpyr and GCFref (daylight periods) showed a good agreement: mean estimation error, 0.000; root mean square error, 0.113; index of agreement, 0.967. The high GCF attained, the large measurement range for GCF and the presence of the net above the table grape were the likely reasons for the good performance of GCFpyr in this crop despite the short number of pyranometers used. Further research is required to develop more appropriate calibration equations of GCFpyr and for a more detailed evaluation of using a short number of pyranometers to estimate GCF. Additional key words: shading; PAR; Vitis vinifera; global solar radiation. * Corresponding author: [email protected] Received: 12-12-13. Accepted: 04-06-14. Abbreviations used: fPAR (fraction of absorbed PAR); GCF (ground cover fraction); GCFpyr (‘pyranometer-driven’ estimated ground cover fraction); GCFref (measured ground cover fraction used as a reference); IA (index of agreement); LAI (leaf area index); MEE, mean estimation error; MSEs, systematic mean square error; MSEu, random mean square error; PAR (photosynthetically active radiation); RMSE, root mean square error; Rsb (global solar radiation below the canopy); Rsa (global solar radiation above the canopy); UTC (Universal Time Coordinated). Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA) Spanish Journal of Agricultural Research 2014 12(3): 603-610 http://dx.doi.org/10.5424/sjar/2014123-5453 ISSN: 1695-971X eISSN: 2171-9292 RESEARCH ARTICLE OPEN ACCESS 604 A. Martinez-Cob et al. / Span J Agric Res (2014) 12(3): 603-610 use (Goldhamer & Synder, 1989; Allen & Pereira, 2009) or development of crop coefficient (Kc) curves as a function of GCF as the main variable (Williams & Ayars, 2005; López-Urrea et al., 2012; Marsal et al., 2014). Williams & Ayars (2005) stated that the dynamics of GCF along the season explained better the dynamics of Kc than that of LAI. Therefore GCF turns up as a useful and relatively simple variable to be determined in a variety of experimental research works performed at a field plot scale to assist in the interpretation of the experimental results. Several procedures have been used to determine GCF at that scale such as a grid inscribed on a wooden board beneath the crop canopy (Williams & Ayars, 2005), digital photography and digital image processing software (Williams & Ayars, 2005; López-Urrea et al., 2009; Bojaca et al., 2011) and solarimeter bars such as ceptometers due to the close similarity between GCF and fraction of light interception by crop canopies (Ayars et al., 2003; Moratiel & Martínez-Cob, 2012, 2013). Please note that solarimeter bars are also commonly used for LAI determination but this application requires a detailed knowledge of extinction coeff icients appropriate for the studied canopy as well as the separate measurement of direct and diffuse solar radiation thus making LAI determination quite complex. All of these methods are quite accurate and reliable. However they are relatively time consuming, require the work of a qualified technician and, in the case of digital photography, may require an intensive maintenance work. Thus they can be difficult to be applied when the measurements must be performed at remote field sites. In these situations, an automatic relatively cheap and low maintenance instrumentation could be more suitable. A set of pyranometers connected to a datalogger could be adequate at those remote sites. Ideally, the number of pyranometers should be such that the spatial variability of GCF can be adequately sampled. However, setting pyranometers at ground level at commercial farms or unattended sites is quite risky due to the farm activities. Under these situations, the pyranometer sets should include a limited number of sensors and should be placed as close to the rows as possible to avoid any damage by the farm machinery. Such setting may provide biased results and then a complete calibration is absolutely mandatory to correct such bias. The objective of this paper was to evaluate the feasibility of using a set of few pyranometers for continuous estimation of ground cover along the crop season for a table grape vineyard under netting. The estimated GCF values were compared to those derived from a ‘reference’ method, digital photography, in order to check the feasibility of the proposed ‘pyranometerdriven’ method and to eventually calibrate it. Material and methods This work was performed from 2007 to 2009 at a commercial table grape (Vitis vinifera L.) vineyard (2.0 ha) located at the Santa Bárbara Farm (Caspe, Zaragoza, northeast Spain). Following the weather network SIAR (MAGRAMA, 2013), the long-term annual average meteorological conditions (2004-2013) in the area were: annual precipitation, 319 mm; mean air temperature, 15.2°C; mean global solar radiation, 199 W m; and annual reference evapotranspiration (FAO Penman-Monteith method), 1456 mm. The geographical coordinates of the farm were 41°16’N latitude, 0°02’W longitude, and 147 m elevation above the sea level. The 4-year old (in 2007) seedless cultivar ‘Crimson’ was grown in the vineyard; this cultivar was grafted on ‘Richter 110’ rootstock (V. berlandieri × V. rupestris). Row direction was approximately northwest to southeast (about 113° azimuth). Row and vine spacing were 3.5 and 2.5 m, respectively. The vineyard was trained on an overhead trellis system such that the canopy developed within 2.0 to 3.0 m above ground, i.e. the vertical canopy width was about 1.0 m at the maximum development stage. Thus total vine height was about 3.0 m. A net made of a thread warp of high-density white polyethylene (Criado and López, Almería, Spain) covered the vineyard to protect it from hail, birds, and insects. This netting was translucent with individual pores of 12 mm (2.2 mm × 5.4 mm). It was placed at a height of 3.0 m above ground level just above the canopy level (Fig. 1). The vineyard was fully irrigated with a drip irrigation system which included one lateral in each row of vines with integrated self-compensating emitters of a discharge of 2.2 L h spaced 0.5 m. Daily drip irrigation from May to September, herbicide and fertilizer were applied following the farm manager’s criteria. Vines were winter pruned. An additional summer pruning of the shoots in a strip 0.5 m wide between vine rows was performed in 2009 around veraison, to allow a better penetration of light in the canopy thus enhancing berry quality and color uniformity. Suvoc̆arev et al. (2013) provides further details of the vineyard. Continuous estimation of ground cover fraction in vineyards 605 Two methods for obtaining GCF were applied: — Method 1, considered the ‘reference’ method, used digital imagery and post processing (Blanco et al., 2010). Pictures were taken at six different sites in the vineyard by a digital camera (Olympus, model μ810, Tokyo, Japan) that was placed on the ground and focused upwards to a quarter of the whole spacing of a vine (1.25 m × 1.75 m). The images were processed with the GIMP program (available at www.gimp.org), by selecting exactly the quarter of the vine area. The program transformed the picture into black (leaves and branches) and white (clear screen) pixels (Fig. 2). The histogram of the black and white pixels was calculated, giving a value of the fraction of the black pixels which represents the GCF at the site. The digital photography as used here does not require taking the images at a specific time of a day. For each measurement date, the six GCF values at the sites were averaged to get the ‘reference’ GCF value (GCFref) for that date. The total number of available GCFref values was 22 in 2007, 13 in 2008, and 28 in 2009. The images were taken every 7 to 14 days, from 15 February to 26 September in 2007, 26 March to 15 October in 2008, and 23 March to 9 November in 2009. — Method 2 consisted of two pyranometers (Kipp & Zonen, model CM3, Delft, The Netherlands), located one above the canopy just below the net, at about 2.8 m above the ground, and the second one completely below the canopy, at about 2.0 m above the ground and 0.5 m horizontal distance from the vine row (Fig. 1). Both pyranometers were placed over a horizontal plate set at the end of an aluminium bar facing toward southwest (about 223° azimuth). Due to the housing the sensor measured the solar energy received from the whole hemisphere (180° field of view). The spectral sensitivity of the sensor was 300 to 3000 nm. Both sensors were connected to a datalogger (Campbell, model CR10, Shepshed, Leicestershire, UK) that monitored them and continuously recorded hourly averages of global solar radiation above (Rsa) and below (Rsb) the vine canopy from 15 February 2007 to 20 November 2009. The average values of Rsa and Rsb from 11:00 to 13:00 Universal Time Coordinated (UTC) (midday) and from 8:00 to 18:00 UTC (daylight) for each day were used to get the ‘pyranometerFigure 1. View of the trellis system of the table grape vineyard before budbreak and location of the two pyranometers below and above the canopy to calculate the ‘pyranometer-driven’ ground cover fraction (GCFpyr).