Calibration of METRIC Model to Estimate Energy Balance over a Drip-Irrigated Apple Orchard

Abstract

A field experiment was carried out to calibrate and evaluate the METRIC (Mapping EvapoTranspiration at high Resolution Internalized with Calibration) model for estimating the spatial and temporal variability of instantaneous net radiation (Rn i), soil heat flux (G i), sensible heat flux (H i), and latent heat flux (LE i) over a drip-irrigated apple (Malus domestica cv. Pink Lady) orchard located in the Pelarco valley, Maule Region, Chile (35 • 25 20 LS; 71 • 23 57 LW; 189 m.a.s.l.). The study was conducted in a plot of 5.5 hectares using 20 satellite images (Landsat 7 ETM+) acquired on clear sky days during three growing seasons (2012/2013, 2013/2014 and 2014/2015). Specific sub-models to estimate G i , leaf area index (LAI) and aerodynamic roughness length for momentum transfer (Z om) were calibrated for the apple orchard as an improvement to the standard METRIC model. The performance of the METRIC model was evaluated at the time of satellite overpass using measurements of H i and LE i obtained from an eddy correlation system. In addition, estimated values of Rn i , G i and LAI were compared with ground-truth measurements from a four-way net radiometer, soil heat flux plates and plant canopy analyzer, respectively. Validation indicated that LAI, Z om and G i were estimated using the calibrated functions with errors of +2%, +6% and +3% while those were computed using the standard functions with error of +59%, +83%, and +12%, respectively. In addition, METRIC using the calibrated functions estimated H i and LE i with error of +5% and +16%, while using the original functions estimated H i and LE i with error of +29% and +26%, respectively.

Improved Albedo Estimates Implemented in the METRIC Model for Modeling Energy Balance Fluxes and Evapotranspiration over Agricultural and Natural Areas in the Brazilian Cerrado

“…Generally, this flux is the one that presents the biggest modeling errors with satellite data, since it is an empirical approximation specifically calibrated for semi-arid areas and may not accurately represent the local conditions of our study area [1,18,33,70,81], and these errors can be associated with the difference of spatial and temporal scales between satellite data and in situ measurements [23,65]. LE estimates were significantly equal to observations for PDG and RECOR sites, but underestimated for the sugarcane experimental site, although only by 5%.…”

“…According to Reference [23] Kc values for heterogeneous surfaces depend on canopy geometry, plant density, and on adopted agricultural practices, which determine the ET partition between transpiration and evaporation. For sugarcane it was verified that Kc varied in each phenological stage according to increases in leaf area index and water availability.…”

“…As pointed out by Reference [23], variations on surface albedo, as well as on other biophysical parameters, can indirectly affect the accuracy of ET estimations. Therefore, we adjusted the Liang equation [55] for the calculation of broadband surface albedo in the Brazilian Cerrado, a method used by several Land Surface Energy Balance (LSEB) models, in which albedo is calculated by integrating band reflectances across the shortwave spectrum [35,36], as shown in Equation (1)…”

“…For quality control of the validation data in the three experimental sites, the energy balance closure was considered using the ratio of turbulent fluxes (H + LE) to the available energy (Rn − G) at 30 min intervals during the daytime (8:00-18:00 h) [23]. When the 30 min ratios were outside the range between 0.8 and 1.2, the flux data was discarded and replaced by a simple linear interpolation.…”

“…When the 30 min ratios were outside the range between 0.8 and 1.2, the flux data was discarded and replaced by a simple linear interpolation. If the database missed more than 5 of the 30 min intervals during the daytime measurements, the entire day was excluded from the analysis [23,46,47]. LE and H values from the eddy covariance (EC) systems were recalculated using the Bowen ratio method (H/LE), as described by Reference [48].…”